Borrelia outer surface protein C is capable of human fibrinogen binding

Multifunctional interaction of CihC/FbpC orthologs of relapsing fever spirochetes with host-derived proteins involved in adhesion, fibrinolysis, and complement evasion

Introduction

Relapsing fever (RF) remains a neglected human disease that is caused by a number of diverse pathogenic Borrelia (B.) species. Characterized by high cell densities in human blood, relapsing fever spirochetes have developed plentiful strategies to avoid recognition by the host defense mechanisms. In this scenario, spirochetal lipoproteins exhibiting multifunctional binding properties in the interaction with host-derived molecules are known to play a key role in adhesion, fibrinolysis and complement activation.

Methods

Binding of CihC/FbpC orthologs to different human proteins and conversion of protein-bound plasminogen to proteolytic active plasmin were examined by ELISA. To analyze the inhibitory capacity of CihC/FbpC orthologs on complement activation, a microtiter-based approach was performed. Finally, AlphaFold predictions were utilized to identified the complement-interacting residues.

Results and discussion

Here, we elucidate the binding properties of CihC/FbpC-orthologs from distinct RF spirochetes including B. parkeri, B. hermsii, B. turicatae, and B. recurrentis to human fibronectin, plasminogen, and complement component C1r. All CihC/FbpC-orthologs displayed similar binding properties to fibronectin, plasminogen, and C1r, respectively. Functional studies revealed a dose dependent binding of plasminogen to all borrelial proteins and conversion to active plasmin. The proteolytic activity of plasmin was almost completely abrogated by tranexamic acid, indicating that lysine residues are involved in the interaction with this serine protease. In addition, a strong inactivation capacity toward the classical pathway could be demonstrated for the wild-type CihC/FbpC-orthologs as well as for the C-terminal CihC fragment of B. recurrentis. Pre-incubation of human serum with borrelial molecules except CihC/FbpC variants lacking the C-terminal region protected serum-susceptible Borrelia cells from complement-mediated lysis. Utilizing AlphaFold2 predictions and existing crystal structures, we mapped the putative key residues involved in C1r binding on the CihC/FbpC orthologs attempting to explain the relatively small differences in C1r binding affinity despite the substitutions of key residues. Collectively, our data advance the understanding of the multiple binding properties of structural and functional highly similar molecules of relapsing fever spirochetes proposed to be involved in pathogenesis and virulence.

The Rrp2-RpoN-RpoS pathway plays an important role in the blood-brain barrier transmigration of the Lyme disease pathogen

Lyme disease, caused by Borrelia (or Borreliella) burgdorferi, is a complex multisystemic disorder that includes Lyme neuroborreliosis resulting from the invasion of both the central and peripheral nervous systems. However, factors that enable the pathogen to cross the blood-brain barrier (BBB) and invade the central nervous system (CNS) are still not well understood. The objective of this study was to identify the B. burgdorferi factors required for BBB transmigration. We utilized a transwell BBB model based on human brain-microvascular endothelial cells and focused on investigating the Rrp2-RpoN-RpoS pathway, a central regulatory pathway that is essential for mammalian infection by B. burgdorferi. Our results demonstrated that the Rrp2-RpoN-RpoS pathway is crucial for BBB transmigration. Furthermore, we identified OspC, a major surface lipoprotein controlled by the Rrp2-RpoN-RpoS pathway, as a significant contributor to BBB transmigration. Constitutive production of OspC in a mutant defective in the Rrp2-RpoN-RpoS pathway did not rescue the impairment in BBB transmigration, indicating that this pathway controls additional factors for this process. Two other major surface lipoproteins controlled by this pathway, DbpA/B and BBK32, appeared to be dispensable for BBB transmigration. In addition, both the surface lipoprotein OspA and the Rrp1 pathway, which are required B. burgdorferi colonization in the tick vector, were found not required for BBB transmigration. Collectively, our findings using in vitro transwell assays uncover another potential role of the Rrp2-RpoN-RpoS pathway in BBB transmigration of B. burgdorferi and invasion into the CNS.

Differential Resistance of Borrelia burgdorferi Clones to Human Serum-Mediated Killing Does Not Correspond to Their Predicted Invasiveness

Reservoir host associations have been observed among and within Borrelia genospecies, and host complement-mediated killing is a major determinant in these interactions. In North America, only a subset of Borrelia burgdorferi lineages cause the majority of disseminated infections in humans. We hypothesize that differential resistance to human complement-mediated killing may be a major phenotypic determinant of whether a lineage can establish systemic infection. As a corollary, we hypothesize that borreliacidal action may differ among human subjects. To test these hypotheses, we isolated primary B. burgdorferi clones from field-collected ticks and determined whether the killing effects of human serum differed among those clones in vitro and/or whether these effects were consistent among human sera. Clones associated with human invasiveness did not show higher survival in human serum compared to noninvasive clones. These results indicate that differential complement-mediated killing of B. burgdorferi lineages is not a determinant of invasiveness in humans. Only one significant difference in the survivorship of individual clones incubated in different human sera was detected, suggesting that complement-mediated killing of B. burgdorferi is usually similar among humans. Mechanisms other than differential human complement-mediated killing of B. burgdorferi lineages likely explain why only certain lineages cause the majority of disseminated human infections.

Possible effect of mutations on serological detection of Borrelia burgdorferi sensu stricto ospC major groups: An in-silico study

The outer surface protein C (OspC) of the agent of Lyme disease, Borrelia burgdorferi sensu stricto, is a major lipoprotein surface-expressed during early-phase human infections. Antibodies to OspC are used in serological diagnoses. This study explored the hypothesis that serological test sensitivity decreases as genetic similarity of ospC major groups (MGs) of infecting strains, and ospC A (the MG in the strain B31 used to prepare antigen for serodiagnosis assays) decreases. We used a previously published microarray dataset to compare serological reactivity to ospC A (measured as pixel intensity) versus reactivity to 22 other ospC MGs, within a population of 55 patients diagnosed by two-tier serological testing using B. burgdorferi s.s. strain B31 as antigen, in which the ospC MG is OspC A. The difference in reactivity of sera to ospC A and reactivity to each of the other 22 ospC MGs (termed 'reactivity difference') was the outcome variable in regression analysis in which genetic distance of the ospC MGs from ospC A was the explanatory variable. Genetic distance was computed for the whole ospC sequence, and 9 subsections, from Neighbour Joining phylogenetic trees of the 23 ospC MGs. Regression analysis was conducted using genetic distance for the full ospC sequence, and the subsections individually. There was a significant association between the reactivity difference and genetic distance of ospC MGs from ospC A: increased genetic distance reduced reactivity to OspC A. No single ospC subsection sequence fully explained the relationship between genetic distance and reactivity difference. An analysis of single nucleotide polymorphisms supported a biological explanation via specific amino acid modifications likely to change protein binding affinity. This adds support to the hypothesis that genetic diversity of B. burgdorferi s.s. (here specifically OspC) may impact serological diagnostic test performance. Further prospective studies are necessary to explore the clinical implications of these findings.

Choreography of Lyme Disease Spirochete Adhesins To Promote Vascular Escape

The Lyme disease spirochete Borrelia burgdorferi sensu lato can cause a multitude of clinical manifestations because of its ability to disseminate into any organ system via migration through soft tissue, the lymphatic system, and the circulatory system. The latter is believed to constitute the predominant pathway for dissemination to distal sites from the inoculating tick bite. In spite of its importance, the hematogenous dissemination process remains largely uncharacterized, particularly due to difficulties studying this process in a living host and the lack of an in vitro system that recapitulates animal infection. In the current work, we provide the first information regarding the stage of the vascular transmigration pathway where three important adhesins function during invasion of mouse knee joint peripheral tissue from postcapillary venules. Using intravital imaging coupled with genetic experiments employing sequential double infection, we show a complex temporal choreography of P66, decorin binding proteins (DbpA/B), and outer surface protein C (OspC) at discrete steps along the pathway of vascular escape, underscoring the importance of B. burgdorferi adhesins in hematogenous dissemination in the mouse knee joint and the complexity of vascular transmigration by a disseminating pathogen. IMPORTANCE Lyme disease is caused by the spirochete Borrelia burgdorferi, which is transmitted by a bite from an infected tick. Disease development involves a complex series of host-pathogen interactions as well as dissemination of the infecting organisms to sites distal to the original tick bite. The predominant pathway for this is believed to be hematogenous dissemination. The mechanism by which the spirochetes escape circulation is unknown. Here, using intravital microscopy, where the Lyme spirochete can be observed in a living mouse, we have studied the stage in the vascular escape process where each of three surface adhesins functions to facilitate escape of the spirochete from postcapillary venules to invade mouse knee joint peripheral tissue. A complex pattern of involvement at various locations in the multistage process is described using a unique experimental approach that is applicable to other disseminating pathogens.

Gac Is a Transcriptional Repressor of the Lyme Disease Spirochete's OspC Virulence-Associated Surface Protein

The OspC outer-surface lipoprotein is essential for the Lyme disease spirochete's initial phase of vertebrate infection. Bacteria within the midguts of unfed ticks do not express OspC but produce high levels when ticks begin to ingest blood. Lyme disease spirochetes cease production of OspC within 1 to 2 weeks of vertebrate infection, and bacteria that fail to downregulate OspC are cleared by host antibodies. Thus, tight regulation of OspC levels is critical for survival of Lyme borreliae and, therefore, an attractive target for development of novel treatment strategies. Previous studies determined that a DNA region 5' of the ospC promoter, the ospC operator, is required for control of OspC production. Hypothesizing that the ospC operator may bind a regulatory factor, DNA affinity pulldown was performed and identified binding by the Gac protein. Gac is encoded by the C-terminal domain of the gyrA open reading frame from an internal promoter, ribosome-binding site, and initiation codon. Our analyses determined that Gac exhibits a greater affinity for ospC operator and promoter DNAs than for other tested borrelial sequences. In vitro and in vivo analyses demonstrated that Gac is a transcriptional repressor of ospC. These results constitute a substantial advance to our understanding of the mechanisms by which the Lyme disease spirochete controls production of OspC. IMPORTANCE Borrelia burgdorferi sensu lato requires its surface-exposed OspC protein in order to establish infection in humans and other vertebrate hosts. Bacteria that either do not produce OspC during transmission or fail to repress OspC after infection is established are rapidly cleared by the host. Herein, we identified a borrelial protein, Gac, that exhibits preferential affinity to the ospC promoter and 5' adjacent DNA. A combination of biochemical analyses and investigations of genetically manipulated bacteria demonstrated that Gac is a transcriptional repressor of ospC. This is a substantial advance toward understanding how the Lyme disease spirochete controls production of the essential OspC virulence factor and identifies a novel target for preventative and curative therapies.

Beyond cuts and scrapes: plasmin in malaria and other vector-borne diseases

Plasmodium and other vector-borne pathogens have evolved mechanisms to hijack the mammalian fibrinolytic system to facilitate infection of the human host and the invertebrate vector. Plasmin, the effector protease of fibrinolysis, maintains homeostasis in the blood vasculature by degrading the fibrin that forms blood clots. Plasmin also degrades proteins from extracellular matrices, the complement system, and immunoglobulins. Here, we review some of the mechanisms by which vector-borne pathogens interact with components of the fibrinolytic system and co-opt its functions to facilitate transmission and infection in the host and the vector. Further, we discuss innovative strategies beyond conventional therapeutics that could be developed to target the interaction of vector-borne pathogens with the fibrinolytic proteins and prevent their transmission.

Poly(C)-binding Protein 2 Regulates the p53 Expression via Interactions with the 5'-Terminal Region of p53 mRNA

The p53 protein is one of the major transcriptional factors which guards cell homeostasis. Here, we showed that poly(C)-binding protein 2 (PCBP2) can bind directly to the 5′ terminus of p53 mRNA by means of electrophoretic mobility shift assay. Binding sites of PCBP2 within this region of p53 mRNA were mapped using Pb2+-induced cleavage and SAXS methods. Strikingly, the downregulation of PCBP2 in HCT116 cells resulted in a lower level of p53 protein under normal and stress conditions. Quantitative analysis of p53 mRNA in PCBP2-downregulated cells revealed a lower level of p53 mRNA under normal conditions suggesting the involvement of PCBP2 in p53 mRNA stabilisation. However, no significant change in p53 mRNA level was observed upon PCBP2 depletion under genotoxic stress. Moreover, a higher level of p53 protein in the presence of rapamycin or doxorubicin and the combination of both antibiotics was noticed in PCBP2-overexpressed cells compared to control cells. These observations indicate the potential involvement of PCBP2 in cap-independent translation of p53 mRNA especially occurring under stress conditions. It has been postulated that the PCBP2 protein is engaged in the enhancement of p53 mRNA stability, probably via interacting with its 3′ end. Our data show that under stress conditions PCBP2 also modulates p53 translation through binding to the 5′ terminus of p53 mRNA. Thus PCBP2 emerges as a double-function factor in the p53 expression.

The Role of Fibrin(ogen) in Wound Healing and Infection Control

Fibrinogen, one of the most abundant plasma proteins playing a key role in hemostasis, is an important modulator of wound healing and host defense against microbes. In the current review, we address the role of fibrin(ogen) throughout the process of wound healing and subsequent tissue repair. Initially fibrin(ogen) acts as a provisional matrix supporting incoming leukocytes and acting as reservoir for growth factors. It later goes on to support re-epithelialization, angiogenesis, and fibroplasia. Importantly, removal of fibrin(ogen) from the wound is essential for wound healing to progress. We also discuss how fibrin(ogen) functions through several mechanisms to protect the host against bacterial infection by providing a physical barrier, entrapment of bacteria in fibrin(ogen) networks, and by directing immune cell function. The central role of fibrin(ogen) in defense against bacterial infection has made it a target of bacterial proteins, evolved to interact with fibrin(ogen) to manipulate clot formation and degradation for the purpose of promoting microbial virulence and survival. Further understanding of the dual roles of fibrin(ogen) in wound healing and infection could provide novel means of therapy to improve recovery from surgical or chronic wounds and help to prevent infection from highly virulent bacterial strains, including those resistant to antibiotics.

Borrelia burgdorferi infection modifies protein content in saliva of Ixodes scapularis nymphs

BACKGROUND

Lyme disease (LD) caused by Borrelia burgdorferi is the most prevalent tick-borne disease. There is evidence that vaccines based on tick proteins that promote tick transmission of B. burgdorferi could prevent LD. As Ixodes scapularis nymph tick bites are responsible for most LD cases, this study sought to identify nymph tick saliva proteins associated with B. burgdorferi transmission using LC-MS/MS. Tick saliva was collected using a non-invasive method of stimulating ticks (uninfected and infected: unfed, and every 12 h during feeding through 72 h, and fully-fed) to salivate into 2% pilocarpine-PBS for protein identification using LC-MS/MS.

RESULTS

We identified a combined 747 tick saliva proteins of uninfected and B. burgdorferi infected ticks that were classified into 25 functional categories: housekeeping-like (48%), unknown function (18%), protease inhibitors (9%), immune-related (6%), proteases (8%), extracellular matrix (7%), and small categories that account for <5% each. Notably, B. burgdorferi infected ticks secreted high number of saliva proteins (n=645) than uninfected ticks (n=376). Counter-intuitively, antimicrobial peptides, which function to block bacterial infection at tick feeding site were suppressed 23-85 folds in B. burgdorferi infected ticks. Similar to glycolysis enzymes being enhanced in mammalian cells exposed to B. burgdorferi : eight of the 10-glycolysis pathway enzymes were secreted at high abundance by B. burgdorferi infected ticks. Of significance, rabbits exposed to B. burgdorferi infected ticks acquired potent immunity that caused 40-60% mortality of B. burgdorferi infected ticks during the second infestation compared to 15-28% for the uninfected. This might be explained by ELISA data that show that high expression levels of immunogenic proteins in B. burgdorferi infected ticks.

CONCLUSION

Data here suggest that B. burgdorferi infection modified protein content in tick saliva to promote its survival at the tick feeding site. For instance, enzymes; copper/zinc superoxide dismutase that led to production of H2O2 that is toxic to B. burgdorferi were suppressed, while, catalase and thioredoxin that neutralize H2O2, and pyruvate kinase which yields pyruvate that protects Bb from H2O2 killing were enhanced. We conclude data here is an important resource for discovery of effective antigens for a vaccine to prevent LD.

Lyme Disease Pathogenesis

Lyme disease Borrelia are obligately parasitic, tick- transmitted, invasive, persistent bacterial pathogens that cause disease in humans and non-reservoir vertebrates primarily through the induction of inflammation. During transmission from the infected tick, the bacteria undergo significant changes in gene expression, resulting in adaptation to the mammalian environment. The organisms multiply and spread locally and induce inflammatory responses that, in humans, result in clinical signs and symptoms. Borrelia virulence involves a multiplicity of mechanisms for dissemination and colonization of multiple tissues and evasion of host immune responses. Most of the tissue damage, which is seen in non-reservoir hosts, appears to result from host inflammatory reactions, despite the low numbers of bacteria in affected sites. This host response to the Lyme disease Borrelia can cause neurologic, cardiovascular, arthritic, and dermatologic manifestations during the disseminated and persistent stages of infection. The mechanisms by which a paucity of organisms (in comparison to many other infectious diseases) can cause varied and in some cases profound inflammation and symptoms remains mysterious but are the subjects of diverse ongoing investigations. In this review, we provide an overview of virulence mechanisms and determinants for which roles have been demonstrated in vivo, primarily in mouse models of infection.

Strong interactions between Salp15 homologues from the tick I. ricinus and distinct types of the outer surface OspC protein from Borrelia

Ticks belonging to the genus Ixodes are parasites feeding on vertebrate blood and vectors for many pathogenic microbes, including Borrelia burgdorferi sensu lato spirochetes, the causative agent of Lyme borreliosis. The tick saliva contains a mixture of bioactive molecules showing a wide range of properties for efficient engorgement. One of the most extensively studied components of tick saliva is a 15-kDa salivary gland protein (Salp15) from Ixodes scapularis. This multifunctional protein suppresses the immune response of hosts through pleiotropic action on a few crucial defense pathways. Salp15 and its homologue from I. ricinus Iric1 have been also shown to bind to Borrelia burgdorferi sensu stricto outer surface protein C (OspC) permitting the spirochetes to evade antibody-mediated killing in the human host. Further studies revealed that Salp15 and Iric1 protected B. burgdorferi s. s. and B. garinii expressing OspC against the complement system. OspC is the most variable protein on the outer surface of Borrelia, which in addition to Salp15 can also bind other ligands, such as plasminogen, fibrinogen, fibronectin or complement factor 4. So far several OspC variants produced by B. burgdorferi s. l. spirochetes were shown to be capable of binding Salp15 or its homologue, but the protection against borreliacidal antibodies has only been proven in the case of B. burgdorferi s. s. The question of Salp15 contribution to Borrelia survival during the infection has been comprehensively studied during the last decades. In contrast, the organization of the OspC-Salp15 complex has been poorly explored. This report describes the binding between three Salp15 homologues from the tick Ixodes ricinus (Iric1, Iric2 and Iric3) and OspC from four B. burgdorferi sensu lato strains in terms of the binding parameters, analyzed with two independent biophysical methods - Microscale thermophoresis (MST) and Biolayer interferometry (BLI). The results of both experiments show a binding constant at the nanomolar level, which indicates very strong interactions. While the Iric1-OspC binding has been reported before, we show in this study that also Iric2 and Iric3 are capable of OspC binding with high affinity. This observation suggests that these two Salp15 homologues might be used by B. burgdorferi s. l. in a way analogous to Iric1. A comparison of the results from the two methods let us propose that N-terminal immobilization of OspC significantly increases the affinity between the two proteins. Finally, our results indicate that the Iric binding site is located in close proximity of the OspC epitopes recognized by human antibodies, which may have important biological and medical implications.

Transgenic functional complementation with a transmission -associated protein restores spirochete infectivity by tick bite

The relapsing fever spirochete Borrelia hermsii and the Lyme disease spirochete Borrelia burgdorferi sensu stricto each produces an abundant, orthologous, outer membrane protein, Vtp and OspC, respectively, when transmitted by tick bite. Gene inactivation studies have shown that both proteins are essential for spirochete infectivity when transmitted by their respective tick vectors. Therefore, we transformed a vtp-minus mutant of B. hermsii with ospC from B. burgdorferi and examined the behavior of this transgenic spirochete in its soft tick vector Ornithodoros hermsi. IFA staining indicated up to 97.8 % of the transgenic B. hermsii upregulated OspC in the ticks' salivary glands compared to no more than 12.8 % in the midgut, similar to our previous findings with wild-type B. hermsii producing Vtp. Transformation with ospC also restored B. hermsii infectivity to mice when fed upon by infected ticks. Previous sequence analysis of Vtp for 79 isolates and DNA samples of B. hermsii in our laboratory showed this protein is highly polymorphic with 9 divergent amino acid types, yet strikingly the signal peptide is identical among all samples and the same for all OspC signal peptides for B. burgdorferi and related species examined to date. Searches in multiple genome sequences for other species of relapsing fever spirochetes failed to find the same signal peptide sequence to help identify potential transmission-associated proteins. However, some candidate signal peptides with highly similar sequences were found and worthy of future efforts with other species. While OspC of B. burgdorferi restored infectivity to a Vtp-minus mutant of B. hermsii, the functions of these proteins are not known. Our results should stimulate investigators to search for orthologous transmission-associated proteins in other tick-borne spirochetes to better understand how this group of pathogens has coevolved with diverse tick vectors.