Solution structure of decorin-binding protein A from Borrelia burgdorferi

Pathogenicity and virulence of Borrelia burgdorferi

Infection with Borrelia burgdorferi often triggers pathophysiologic perturbations that are further augmented by the inflammatory responses of the host, resulting in the severe clinical conditions of Lyme disease. While our apprehension of the spatial and temporal integration of the virulence determinants during the enzootic cycle of B. burgdorferi is constantly being improved, there is still much to be discovered. Many of the novel virulence strategies discussed in this review are undetermined. Lyme disease spirochaetes must surmount numerous molecular and mechanical obstacles in order to establish a disseminated infection in a vertebrate host. These barriers include borrelial relocation from the midgut of the feeding tick to its body cavity and further to the salivary glands, deposition to the skin, haematogenous dissemination, extravasation from blood circulation system, evasion of the host immune responses, localization to protective niches, and establishment of local as well as distal infection in multiple tissues and organs. Here, the various well-defined but also possible novel strategies and virulence mechanisms used by B. burgdorferi to evade obstacles laid out by the tick vector and usually the mammalian host during colonization and infection are reviewed.

Serological Analysis Identifies Consequential B Cell Epitopes on the Flexible Linker and C-Terminus of Decorin Binding Protein A (DbpA) from Borrelia burgdorferi

Decorin binding protein A (DbpA) is a surface adhesin of Borrelia burgdorferi, the causative agent of Lyme disease. While DbpA is one of the most immunogenic of B. burgdorferi’s nearly 100 lipoproteins, the B cell epitopes on DbpA recognized by humans following B. burgdorferi infection have not been fully elucidated. In this report we profiled ~270 B. burgdorferi-seropositive human serum samples for IgM and IgG reactivity with a tiled DbpA 18-mer peptide array derived from B. burgdorferisensu stricto strains B31 and 297. Using enzyme-linked immunosorbent assays (ELISA) and multiplex immunoassays (MIA), we identified 12 DbpA-derived peptides whose antibody reactivities were significantly elevated (generally <10-fold) in B. burgdorferi-seropositive sera, compared to those measured in a healthy cohort. The most reactive peptide (>80-fold IgG, 10-fold IgM) corresponded to residues 64 to 81, which map to an exposed flexible loop between DbpA’s α-helix 1 and α-helix 2. This loop, whose sequence is identical between strains B31 and 297, overhangs DbpA’s substrate binding pocket. A second strongly reactive antibody target (>80-fold IgG, 3 to 5-fold IgM) mapped to DbpA’s C-terminus, a lysine rich tail implicated in attachment to glycosaminoglycans. We postulate that antibody responses against these two targets on DbpA could limit B.burgdorferi’s ability to attach to and colonize distal tissues during the early stages of infection.

IMPORTANCE The bacterium, Borrelia burgdorferi, is the causative agent of Lyme disease, the most reported tick-borne illness in the United States. In humans, clinical manifestations of Lyme disease are complex and can persist for months, even in the face of a robust antibody response directed against numerous B. burgdorferi surface proteins, including decorin binding protein A (DbpA), which is involved in the early stages of infection. In this study we employed ~270 serum samples from B. burgdorferi-seropositive individuals to better understand human antibody reactivity to specific regions (called epitopes) of DbpA and how such antibodies may function in limiting B. burgdorferi dissemination and tissue colonization.

Conserved lysine residues in decorin binding proteins of Borrelia garinii are critical in adhesion to human brain microvascular endothelial cells

Lyme borreliosis is a tick-borne disease caused by Borrelia burgdorferi sensu lato spirochetes (Lyme borreliae). When the disease affects the central nervous system, it is referred to as neuroborreliosis. In Europe, neuroborreliosis is most often caused by Borrelia garinii. Although it is known that in the host Lyme borreliae spread from the tick bite site to distant tissues via the blood vasculature, the adherence of Lyme borreliae to human brain microvascular endothelial cells has not been studied before. Decorin binding proteins are adhesins expressed on Lyme borreliae. They mediate the adhesion of Lyme borreliae to decorin and biglycan, and the lysine residues located in the binding site of decorin binding proteins are important to the binding activity. In this study, we show that lysine residues located in the canonical binding site can also be found in decorin binding proteins of Borrelia garinii, and that these lysines contribute to biglycan and decorin binding. Most importantly, we show that the lysine residues are crucial for the binding of Lyme borreliae to decorin and biglycan expressing human brain microvascular endothelial cells, which in turn suggests that they are involved in the pathogenesis of neuroborreliosis.

A bite so sweet: the glycobiology interface of tick-host-pathogen interactions

Vector-borne diseases constitute 17% of all infectious diseases in the world; among the blood-feeding arthropods, ticks transmit the highest number of pathogens. Understanding the interactions between the tick vector, the mammalian host and the pathogens circulating between them is the basis for the successful development of vaccines against ticks or the tick-transmitted pathogens as well as for the development of specific treatments against tick-borne infections. A lot of effort has been put into transcriptomic and proteomic analyses; however, the protein-carbohydrate interactions and the overall glycobiology of ticks and tick-borne pathogens has not been given the importance or priority deserved. Novel (bio)analytical techniques and their availability have immensely increased the possibilities in glycobiology research and thus novel information in the glycobiology of ticks and tick-borne pathogens is being generated at a faster pace each year. This review brings a comprehensive summary of the knowledge on both the glycosylated proteins and the glycan-binding proteins of the ticks as well as the tick-transmitted pathogens, with emphasis on the interactions allowing the infection of both the ticks and the hosts by various bacteria and tick-borne encephalitis virus.

Spirochetal Lipoproteins in Pathogenesis and Immunity

Lipoproteins are lipid-modified proteins that dominate the spirochetal proteome. While found in all bacteria, spirochetal lipoproteins have unique features and play critical roles in spirochete biology. For this reason, considerable effort has been devoted to determining how the lipoproteome is generated. Essential features of the structural elements of lipoproteins are now understood with greater clarity, enabling greater confidence in identification of lipoproteins from genomic sequences. The journey from the ribosome to the outer membrane, and in some cases, to the cellular surface has been defined, including secretion, lipidation, sorting, and export across the outer membrane. Given their abundance and importance, it is not surprising that spirochetes have developed a number of strategies for regulating the spatiotemporal expression of lipoproteins. In some cases, lipoprotein expression is tied to various environmental cues, while in other cases, it is linked to growth rate. This regulation enables spirochetes to express certain lipoproteins at high levels in one phase of the spirochete lifecycle, while dramatically downregulating the same lipoproteins in other phases. The mammalian host has developed specialized mechanisms for recognizing lipoproteins and triggering an immune response. Evasion of that immune response is essential for spirochete persistence. For this reason, spirochetes have developed mechanisms for altering lipoproteins. Lipoproteins recognized by antibodies formed during infection are key serodiagnostic antigens. In addition, lipoprotein vaccines have been developed for generating an immune response to control or prevent a spirochete infection. This chapter summarizes our current understanding of lipoproteins in interactions of spirochetes with their hosts.

Structure of decorin binding protein B from Borrelia burgdorferi and its interactions with glycosaminoglycans

Decorin-binding proteins (DBPs), DBPA and DBPB, are surface lipoproteins on Borrelia burgdorferi, the causative agent of Lyme disease. DBPs bind to the connective tissue proteoglycan decorin and facilitate tissue colonization by the bacterium. Although structural and biochemical properties of DBPA are well understood, little is known about DBPB. In current work, we determined the solution structure of DBPB from strain B31 of B. burgdorferi and characterized its interactions with glycosaminoglycans (GAGs). Our structure shows that DBPB adopts the same topology as DBPA, but possesses a much shorter terminal helix, resulting in a longer unstructured C-terminal tail, which is also rich in basic amino acids. Characterization of DBPB-GAG interactions reveals that, despite similar GAG affinities of DBPA and DBPB, the primary GAG-binding sites in DBPB are different from DBPA. In particular, our results indicate that lysines in the C-terminus of DBPB are vital to DBPB's ability to bind GAGs whereas C-terminal tail for DBPA from strain B31 only plays a minor role in facilitating GAG bindings. Furthermore, the traditional GAG-binding pocket important to DBPA-GAG interactions is only secondary to DBPB's GAG-binding ability.

Flexible Linker Modulates Glycosaminoglycan Affinity of Decorin Binding Protein A

Decorin binding protein A (DBPA) is a glycosaminoglycan (GAG)-binding adhesin found on the surface of the bacterium Borrelia burgdorferi (B. burgdorferi), the causative agent of Lyme disease. DBPA facilitates bacterial adherence to extracellular matrices of human tissues and is crucial during the early stage of the infection process. Interestingly, DBPA from different strains (B31, N40, and PBr) show significant differences in GAG affinities, but the structural basis for the differences is not clear. In this study, we show that GAG affinity of N40 DBPA is modulated in part by flexible segments that control access to the GAG binding site, such that shortening of the linker leads to higher GAG affinity when analyzed using ELISA, gel mobility shift assay, solution NMR, and isothermal titration calorimetry. Our observation that GAG affinity differences among different B. burgdorferi strains can be attributed to a flexible linker domain regulating access to the GAG-binding domain is novel. It also provides a rare example of how neutral amino acids and dynamic segments in GAG binding proteins can have a large influence on GAG affinity and provides insights into why the number of basic amino acids in the GAG-binding site may not be the only factor determining GAG affinity of proteins.

Current structural biology of the heparin interactome

Heparin is the best known therapeutically active carbohydrate. It can bind and regulate multiple functional proteins such as coagulation cofactors, chemokines, and growth factors. This versatility has led to the recently developed concept of the heparin interactome--a group of proteins that, as the name implies, interact with heparin. The heparin interactome is structurally and functionally diverse. Though natural ligands of this class of proteins may be any of the glycosaminoglycans however, their structural biology is generally studied using heparin as a model compound. NMR spectroscopy contributes significantly to structural investigations of the resultant complexes in solution. This review aims therefore at discussing the current status in structural biology of the molecular complexes formed between heparin and its protein partners through the current concept of the heparin interactome.

Structural mechanisms underlying sequence-dependent variations in GAG affinities of decorin binding protein A, a Borrelia burgdorferi adhesin

Decorin-binding protein A (DBPA) is an important surface adhesin of the bacterium Borrelia burgdorferi, the causative agent of Lyme disease. DBPA facilitates the bacteria's colonization of human tissue by adhering to glycosaminoglycan (GAG), a sulfated polysaccharide. Interestingly, DBPA sequence variation among different strains of Borrelia spirochetes is high, resulting in significant differences in their GAG affinities. However, the structural mechanisms contributing to these differences are unknown. We determined the solution structures of DBPAs from strain N40 of B. burgdorferi and strain PBr of Borrelia garinii, two DBPA variants whose GAG affinities deviate significantly from strain B31, the best characterized version of DBPA. Our structures revealed that significant differences exist between PBr DBPA and B31/N40 DBPAs. In particular, the C-terminus of PBr DBPA, unlike C-termini from B31 and N40 DBPAs, is positioned away from the GAG-binding pocket and the linker between helices one and two of PBr DBPA is highly structured and retracted from the GAG-binding pocket. The repositioning of the C-terminus allowed the formation of an extra GAG-binding epitope in PBr DBPA and the retracted linker gave GAG ligands more access to the GAG-binding epitopes than other DBPAs. Characterization of GAG ligands' interactions with wild-type (WT) PBr and mutants confirmed the importance of the second major GAG-binding epitope and established the fact that the two epitopes are independent of one another and the new epitope is as important to GAG binding as the traditional epitope.

Strain-specific variation of the decorin-binding adhesin DbpA influences the tissue tropism of the lyme disease spirochete

Lyme disease spirochetes demonstrate strain- and species-specific differences in tissue tropism. For example, the three major Lyme disease spirochete species, Borrelia burgdorferi sensu stricto, B. garinii, and B. afzelii, are each most commonly associated with overlapping but distinct spectra of clinical manifestations. Borrelia burgdorferi sensu stricto, the most common Lyme spirochete in the U.S., is closely associated with arthritis. The attachment of microbial pathogens to cells or to the extracellular matrix of target tissues may promote colonization and disease, and the Lyme disease spirochete encodes several surface proteins, including the decorin- and dermatan sulfate-binding adhesin DbpA, which vary among strains and have been postulated to contribute to strain-specific differences in tissue tropism. DbpA variants differ in their ability to bind to its host ligands and to cultured mammalian cells. To directly test whether variation in dbpA influences tissue tropism, we analyzed murine infection by isogenic B. burgdorferi strains that encode different dbpA alleles. Compared to dbpA alleles of B. afzelii strain VS461 or B. burgdorferi strain N40-D10/E9, dbpA of B. garinii strain PBr conferred the greatest decorin- and dermatan sulfate-binding activity, promoted the greatest colonization at the inoculation site and heart, and caused the most severe carditis. The dbpA of strain N40-D10/E9 conferred the weakest decorin- and GAG-binding activity, but the most robust joint colonization and was the only dbpA allele capable of conferring significant joint disease. Thus, dbpA mediates colonization and disease by the Lyme disease spirochete in an allele-dependent manner and may contribute to the etiology of distinct clinical manifestations associated with different Lyme disease strains. This study provides important support for the long-postulated model that strain-specific variations of Borrelia surface proteins influence tissue tropism.

Lyme disease, the most common vector-borne disease in the United States, is caused by a bacterium, Borrelia burgdorferi. This bacterium infects the skin at the site of the tick bite and then can spread to other tissues, such as the heart, joints or nervous system, causing carditis, arthritis or neurologic disease. To colonize human tissues, the pathogen produces surface proteins that promote bacterial attachment to these sites. For example, DbpA binds to decorin, a component of human tissue. Different Lyme disease strains differ in the particular tissues they colonize and the disease they cause, but we do not understand why. Different strains also make distinct versions of DbpA that bind decorin differently, so variation of DbpA might contribute to strain-to-strain variation in clinical manifestations. To test this, we infected mice with Lyme disease strains that were identical except for the particular DbpA variant they produced. We found that the strains colonized different tissues and caused different diseases, such as arthritis or carditis. These results provide the first solid evidence that variation of an outer surface protein, in this case DbpA, influences what tissues are most affected during Lyme disease.

Identification of lysine residues in the Borrelia burgdorferi DbpA adhesin required for murine infection

Decorin-binding protein A (DbpA) of Borrelia burgdorferi mediates bacterial adhesion to heparin and dermatan sulfate associated with decorin. Lysines K82, K163, and K170 of DbpA are known to be important for in vitro interaction with decorin, and the DbpA structure, initially solved by nuclear magnetic resonance (NMR) spectroscopy, suggests these lysine residues colocalize in a pocket near the C terminus of the protein. In the current study, we solved the structure of DbpA from B. burgdorferi strain 297 using X-ray crystallography and confirmed the existing NMR structural data. In vitro binding experiments confirmed that recombinant DbpA proteins with mutations in K82, K163, or K170 did not bind decorin, which was due to an inability to interact with dermatan sulfate. Most importantly, we determined that the in vitro binding defect observed upon mutation of K82, K163, or K170 in DbpA also led to a defect during infection. The infectivity of B. burgdorferi expressing individual dbpA lysine point mutants was assessed in mice challenged via needle inoculation. Murine infection studies showed that strains expressing dbpA with mutations in K82, K163, and K170 were significantly attenuated and could not be cultured from any tissue. Proper expression and cellular localization of the mutated DbpA proteins were examined, and NMR spectroscopy determined that the mutant DbpA proteins were structurally similar to wild-type DbpA. Taken together, these data showed that lysines K82, K163, and K170 potentiate the binding of DbpA to dermatan sulfate and that an interaction(s) mediated by these lysines is essential for B. burgdorferi murine infection.

That's my story, and I'm sticking to it--an update on B. burgdorferi adhesins

Adhesion is the initial event in the establishment of any infection. Borrelia burgdorferi, the etiological agent of Lyme disease, possesses myriad proteins termed adhesins that facilitate contact with its vertebrate hosts. B. burgdorferi adheres to host tissues through interactions with host cells and extracellular matrix, as well as other molecules present in serum and extracellular fluids. These interactions, both general and specific, are critical in the establishment of infection. Modulation of borrelial adhesion to host tissues affects the microorganisms's ability to colonize, disseminate, and persist. In this review, we update the current knowledge on structure, function, and role in pathogenesis of these “sticky” B. burgdorferi infection-associated proteins.

The novel heparin-binding motif in decorin-binding protein A from strain B31 of Borrelia burgdorferi explains the higher binding affinity

Decorin-binding protein A (DBPA), a glycosaminoglycan (GAG)-binding lipoprotein found in Borrelia burgdorferi, is crucial to the transmission of Lyme disease in its earliest stages. Because of its role in the initial transmission of the disease, DBPA is an ideal target for vaccine development. DBPA sequences from different strains also contain considerable heterogeneity, leading to differing affinities for GAGs and proteoglycans among different DBPA sequences. Through biophysical and structural analysis of DBPA from strain B31, we have discovered a novel and important GAG-binding epitope in B31 DBPA. Removal of the epitope greatly attenuated its affinity for DBPA and may explain the differential GAG affinities seen in DBPAs from other strains of B. burgdorferi. Paramagnetic perturbation of the protein with TEMPO-labeled heparin fragments showed bound GAGs are located close to the linker region containing the BXBB motif that plays a significant role in determining the specific affinity and orientation of binding of GAG to DBPA. Thermodynamic contributions of the new motif to GAG binding were also characterized by both nuclear magnetic resonance and isothermal titration calorimetry and compared with those of other DBPA residues previously known to be involved in GAG interactions. These analyses showed the motif is as important as other known binding epitopes. The discovery of the motif offers a possible structural explanation for the previously observed differences in GAG affinities of DBPA variants from different Borrelia strains and improves our understanding of DBPA-GAG interactions.