HtrA, a Temperature- and Stationary Phase-Activated Protease Involved in Maturation of a Key Microbial Virulence Determinant, Facilitates Borrelia burgdorferi Infection in Mammalian Hosts

HtrA3 paves the way for MSC migration and promotes osteogenesis

Mesenchymal stem cell (MSC) migration determines the healing capacity of bone and is crucial in promoting bone regeneration. Migration of MSCs is highly dependent on degradation of extracellular matrix by proteolytic enzymes. However, the underlying mechanisms of how enzymolysis paves the way for MSCs to migrate from their niche to the defect area is still not fully understood. Here, this study shows that high-temperature requirement A3 (HtrA3) overcomes the physical barrier and provides anchor points through collagen IV degradation, paving the way for MSC migration. HtrA3 is upregulated in MSCs at the leading edge of bone defect during the early stage of healing. HtrA3 degrades the surrounding collagen IV, which increases the collagen network porosity and increases integrin β1 expression. Subsequently, integrin β1 enhances the mechanotransduction of MSCs, thus remodeling the cytoskeleton, increasing cellular stiffness and nuclear translocation of YAP, eventually promoting the migration and subsequent osteogenic differentiation of MSCs. Local administration of recombinant HtrA3 in rat cranial bone defects significantly increases new bone formation and further validates the enhancement of MSC migration. This study helps to reveal the novel roles of HtrA3, explore potential targets for regenerative medicine, and offer new insights for the development of bioactive materials.

Functional characterization and transcriptional analysis of degQ of Xanthomonas campestris pathovar campestris

High-temperature-requirement protein A (HtrA) family proteins play important roles in controlling protein quality and are recognized as virulence factors in numerous animal and human bacterial pathogens. The role of HtrA family proteins in plant pathogens remains largely unexplored. Here, we investigated the HtrA family protein, DegQ, in the crucifer black rot pathogen Xanthomonas campestris pathovar campestris (Xcc). DegQ is essential for bacterial attachment and full virulence of Xcc. Moreover, the degQ mutant strain showed increased sensitivity to heat treatment and sodium dodecyl sulfate. Expressing the intact degQ gene in trans in the degQ mutant could reverse the observed phenotypic changes. In addition, we demonstrated that the DegQ protein exhibited chaperone-like activity. Transcriptional analysis displayed that degQ expression was induced under heat treatment. Our results contribute to understanding the function and expression of DegQ of Xcc for the first time and provide a novel perspective about HtrA family proteins in plant pathogen.

A systemic approach to identify non-abundant immunogenic proteins in Lyme disease pathogens

Borrelia burgdorferi, the pathogen of Lyme disease, differentially produces many outer surface proteins (Osp), some of which represent the most abundant membrane proteins, such as OspA, OspB, and OspC. In cultured bacteria, these proteins can account for a substantial fraction of the total cellular or membrane proteins, posing challenges to the identification and analysis of non-abundant proteins, which could serve as novel pathogen detection markers or as vaccine candidates. Herein, we introduced serial mutations to remove these abundant Osps and generated a B. burgdorferi mutant deficient in OspA, OspB, and OspC in an infectious 297-isolate background, designated as OspABC- mutant. Compared to parental isolate, the mutant did not reflect growth defects in the cultured medium but showed differential mRNA expression of representative tested genes, in addition to gross changes in cellular and membrane protein profiles. The analysis of differentially detectable protein contents of the OspABC- mutant, as compared to the wild type, by two-dimensional gel electrophoresis followed by liquid chromatography-mass spectrometry, identified several spirochete proteins that are dominated by proteins of unknown functions, as well as membrane transporters, chaperons, and metabolic enzymes. We produced recombinant forms of two of these represented proteins, BBA34 and BB0238, and showed that these proteins are detectable during spirochete infection in the tick-borne murine model of Lyme borreliosis and thus serve as potential antigenic markers of the infection.IMPORTANCEThe present manuscript employed a systemic approach to identify non-abundant proteins in cultured Borrelia burgdorferi that are otherwise masked or hidden due to the overwhelming presence of abundant Osps like OspA, OspB, and OspC. As these Osps are either absent or transiently expressed in mammals, we performed a proof-of-concept study in which their removal allowed the analysis of otherwise less abundant antigens in OspABC-deficient mutants and identified several immunogenic proteins, including BBA34 and BB0238. These antigens could serve as novel vaccine candidates and/or genetic markers of Lyme borreliosis, promoting new research in the clinical diagnosis and prevention of Lyme disease.

A chemosensory-like histidine kinase is dispensable for chemotaxis in vitro but regulates the virulence of Borrelia burgdorferi through modulating the stability of RpoS

As an enzootic pathogen, the Lyme disease bacterium Borrelia burgdorferi possesses multiple copies of chemotaxis proteins, including two chemotaxis histidine kinases (CHK), CheA1 and CheA2. Our previous study showed that CheA2 is a genuine CHK that is required for chemotaxis; however, the role of CheA1 remains mysterious. This report first compares the structural features that differentiate CheA1 and CheA2 and then provides evidence to show that CheA1 is an atypical CHK that controls the virulence of B. burgdorferi through modulating the stability of RpoS, a key transcriptional regulator of the spirochete. First, microscopic analyses using green-fluorescence-protein (GFP) tags reveal that CheA1 has a unique and dynamic cellular localization. Second, loss-of-function studies indicate that CheA1 is not required for chemotaxis in vitro despite sharing a high sequence and structural similarity to its counterparts from other bacteria. Third, mouse infection studies using needle inoculations show that a deletion mutant of CheA1 (cheA1mut) is able to establish systemic infection in immune-deficient mice but fails to do so in immune-competent mice albeit the mutant can survive at the inoculation site for up to 28 days. Tick and mouse infection studies further demonstrate that CheA1 is dispensable for tick colonization and acquisition but essential for tick transmission. Lastly, mechanistic studies combining immunoblotting, protein turnover, mutagenesis, and RNA-seq analyses reveal that depletion of CheA1 affects RpoS stability, leading to reduced expression of several RpoS-regulated virulence factors (i.e., OspC, BBK32, and DbpA), likely due to dysregulated clpX and lon protease expression. Bulk RNA-seq analysis of infected mouse skin tissues further show that cheA1mut fails to elicit mouse tnf-α, il-10, il-1β, and ccl2 expression, four important cytokines for Lyme disease development and B. burgdorferi transmigration. Collectively, these results reveal a unique role and regulatory mechanism of CheA1 in modulating virulence factor expression and add new insights into understanding the regulatory network of B. burgdorferi.

A unique borrelial protein facilitates microbial immune evasion

IMPORTANCE

Lyme disease is a major tick-borne infection caused by a bacterial pathogen called Borrelia burgdorferi, which is transmitted by ticks and affects hundreds of thousands of people every year. These bacterial pathogens are distinct from other genera of microbes because of their distinct features and ability to transmit a multi-system infection to a range of vertebrates, including humans. Progress in understanding the infection biology of Lyme disease, and thus advancements towards its prevention, are hindered by an incomplete understanding of the microbiology of B. burgdorferi, partly due to the occurrence of many unique borrelial proteins that are structurally unrelated to proteins of known functions yet are indispensable for pathogen survival. We herein report the use of diverse technologies to examine the structure and function of a unique B. burgdorferi protein, annotated as BB0238-an essential virulence determinant. We show that the protein is structurally organized into two distinct domains, is involved in multiplex protein-protein interactions, and facilitates tick-to-mouse pathogen transmission by aiding microbial evasion of early host cellular immunity. We believe that our findings will further enrich our understanding of the microbiology of B. burgdorferi, potentially impacting the future development of novel prevention strategies against a widespread tick-transmitted infection.

Dome1-JAK-STAT signaling between parasite and host integrates vector immunity and development

Ancestral signaling pathways serve critical roles in metazoan development, physiology, and immunity. We report an evolutionary interspecies communication pathway involving a central Ixodes scapularis tick receptor termed Dome1, which acquired a mammalian cytokine receptor motif exhibiting high affinity for interferon-gamma (IFN-γ). Host-derived IFN-γ facilitates Dome1-mediated activation of the Ixodes JAK-STAT pathway. This accelerates tick blood meal acquisition and development while upregulating antimicrobial components. The Dome1-JAK-STAT pathway, which exists in most Ixodid tick genomes, regulates the regeneration and proliferation of gut cells-including stem cells-and dictates metamorphosis through the Hedgehog and Notch-Delta networks, ultimately affecting Ixodes vectorial competence. We highlight the evolutionary dependence of I. scapularis on mammalian hosts through cross-species signaling mechanisms that dually influence arthropod immunity and development.

Application of biophysical methods for improved protein production and characterization: A case study on an high-temperature requirement A-family bacterial protease

The high-temperature requirement A (HtrA) serine protease family presents an attractive target class for antibacterial therapeutics development. These proteins possess dual protease and chaperone functions and contain numerous binding sites and regulatory loops, displaying diverse oligomerization patterns dependent on substrate type and occupancy. HtrA proteins that are natively purified coelute with contaminating peptides and activating species, shifting oligomerization and protein structure to differently activated populations. Here, a redesigned HtrA production results in cleaner preparations with high yields by overexpressing and purifying target protein from inclusion bodies under denaturing conditions, followed by a high-throughput screen for optimal refolding buffer composition using function-agnostic biophysical techniques that do not rely on target-specific measurements. We use Borrelia burgdorferi HtrA to demonstrate the effectiveness of our function-agnostic approach, while characterization with both new and established biophysical methods shows the retention of proteolytic and chaperone activity of the refolded protein. This systematic workflow and toolset will translate to the production of HtrA-family proteins in higher quantities of pure and monodisperse composition than the current literature standard, with applicability to a broad array of protein purification strategies.

Application of temperature-responsive HIS-tag fluorophores to differential scanning fluorimetry screening of small molecule libraries

Differential scanning fluorimetry is a rapid and economical biophysical technique used to monitor perturbations to protein structure during a thermal gradient, most often by detecting protein unfolding events through an environment-sensitive fluorophore. By employing an NTA-complexed fluorophore that is sensitive to nearby structural changes in histidine-tagged protein, a robust and sensitive differential scanning fluorimetry (DSF) assay is established with the specificity of an affinity tag-based system. We developed, optimized, and miniaturized this HIS-tag DSF assay (HIS-DSF) into a 1536-well high-throughput biophysical platform using the Borrelial high temperature requirement A protease (BbHtrA) as a proof of concept for the workflow. A production run of the BbHtrA HIS-DSF assay showed a tight negative control group distribution of T_m values with an average coefficient of variation of 0.51% and median coefficient of variation of compound T_m of 0.26%. The HIS-DSF platform will provide an additional assay platform for future drug discovery campaigns with applications in buffer screening and optimization, target engagement screening, and other biophysical assay efforts.

Dual role of an essential HtrA2/Omi protease in the human malaria parasite: Maintenance of mitochondrial homeostasis and induction of apoptosis-like cell death under cellular stress

Members of the HtrA family of serine proteases are known to play roles in mitochondrial homeostasis as well as in programmed cell death. Mitochondrial homeostasis and metabolism are crucial for the survival and propagation of the malaria parasite within the host. Here we have functionally characterized a Plasmodium falciparum HtrA2 (PfHtrA2) protein, which harbours trypsin-like protease activity that can be inhibited by its specific inhibitor, ucf-101. A transgenic parasite line was generated, using the HA-glmS C-terminal tagging approach, for localization as well as for inducible knock-down of PfHtrA2. The PfHtrA2 was localized in the parasite mitochondrion during the asexual life cycle. Genetic ablation of PfHtrA2 caused significant parasite growth inhibition, decreased replication of mtDNA, increased mitochondrial ROS production, caused mitochondrial fission/fragmentation, and hindered parasite development. However, the ucf-101 treatment did not affect the parasite growth, suggesting the non-protease/chaperone role of PfHtrA2 in the parasite. Under cellular stress conditions, inhibition of PfHtrA2 by ucf-101 reduced activation of the caspase-like protease as well as parasite cell death, suggesting the involvement of protease activity of PfHtrA2 in apoptosis-like cell death in the parasite. Under these cellular stress conditions, the PfHtrA2 gets processed but remains localized in the mitochondrion, suggesting that it acts within the mitochondrion by cleaving intra-mitochondrial substrate(s). This was further supported by trans-expression of PfHtrA2 protease domain in the parasite cytosol, which was unable to induce any cell death in the parasite. Overall, we show the specific roles of PfHtrA2 in maintaining mitochondrial homeostasis as well as in regulating stress-induced cell death.

High temperature requirement A3 attenuates hypoxia/reoxygenation induced injury in H9C2 cells via suppressing inflammatory responses

High temperature requirement A3 (HtrA3) belongs to the HtrA family, and its role in inflammation and myocardial ischemia-reperfusion injury remains unknown. Herein, the study aimed to explore the role of HtrA3 in inflammatory cytokine secretion and the nuclear factor kappa B (NF-κB) signaling pathway in hypoxia-reoxygenation (H/R)-induced H9C2 cardiomyoblasts. H9C2 cells were treated with H/R to mimic myocardial ischemia-reperfusion in vitro. Results showed that HtrA3 expression was significantly downregulated and the expression of inflammatory cytokines was regulated in response to H/R. HtrA3 overexpression decreased the secretion of inflammatory cytokines, whereas HtrA3 knockdown led to increase levels of inflammatory cytokines. And H/R-induced inflammation in H9C2 cells was inhibited by the regulation of the NF-κB signaling pathway. Our findings demonstrate that HtrA3 alleviates H/R-induced inflammatory responses in H9C2 cardiomyoblasts, possibly by suppressing the pro-inflammatory NF-κB signaling pathway.

BB0761, a MepM homolog, contributes to Borrelia burgdorferi cell division and mammalian infectivity

M23 family endopeptidases play important roles in cell division and separation in a wide variety of bacteria. Recent studies have suggested that these proteins also contribute to bacterial virulence. However, the biological function of M23 peptidases in pathogenic spirochetes remains unexplored. Here, we describe Borrelia burgdorferi, the bacterial pathogen causing Lyme disease, requires a putative M23 family homolog, BB0761, for spirochete morphology and cell division. Indeed, the inactivation of bb0761 led to an aberrant filamentous phenotype as well as the impairment of B. burgdorferi growth in vitro. These phenotypes were complemented not only with B. burgdorferi bb0761, but also with the mepM gene from E. coli. Moreover, the bb0761 mutant showed a complete loss of infectivity in a murine model of Lyme borreliosis. Resistance of the mutant to osmotic and oxidative stresses was markedly reduced. Our combined results indicate that BB0761 contributes to B. burgdorferi cell division and virulence.

Controlled Proteolysis of an Essential Virulence Determinant Dictates Infectivity of Lyme Disease Pathogens

The Borrelia burgdorferi BB0323 protein undergoes a complex yet poorly defined proteolytic maturation event that generates N-terminal and C-terminal proteins with essential functions in cell growth and infection. Here, we report that a borrelial protease, B. burgdorferi high temperature requirement A protease (BbHtrA), cleaves BB0323 between asparagine (N) and leucine (L) at positions 236 and 237, while the replacement of these residues with alanine in the mutant protein prevents its cleavage, despite preserving its normal secondary structure. The N-terminal BB0323 protein binds BbHtrA, but its cleavage site mutant displays deficiency in such interaction. An isogenic borrelial mutant with NL-to-AA substitution in BB0323 (referred to as Bb^bb0323NL) maintains normal growth yet is impaired for infection of mice or transmission from infected ticks. Notably, the BB0323 protein is still processed in Bb^bb0323NL, albeit with lower levels of mature N-terminal BB0323 protein and multiple aberrantly processed polypeptides, which could result from nonspecific cleavages at other asparagine and leucine residues in the protein. The lack of infectivity of Bb^bb0323NL is likely due to the impaired abundance or stoichiometry of a protein complex involving BB0238, another spirochete protein. Together, these studies highlight that a precise proteolytic event and a particular protein-protein interaction, involving multiple borrelial virulence determinants, are mutually inclusive and interconnected, playing essential roles in the infectivity of Lyme disease pathogens.

Endogenous and Borrowed Proteolytic Activity in the Borrelia

The Borrelia spp. are tick-borne pathogenic spirochetes that include the agents of Lyme disease and relapsing fever. As part of their life cycle, the spirochetes traffic between the tick vector and the vertebrate host, which requires significant physiological changes and remodeling of their outer membranes and proteome. This crucial proteome resculpting is carried out by a diverse set of proteases, adaptor proteins, and related chaperones. Despite its small genome, Borrelia burgdorferi has dedicated a large percentage of its genome to proteolysis, including a full complement of ATP-dependent proteases. Energy-driven proteolysis appears to be an important physiological feature of this dual-life-cycle bacterium. The proteolytic arsenal of Borrelia is strategically deployed for disposal of proteins no longer required as they move from one stage to another or are transferred from one host to another. Likewise, the Borrelia spp. are systemic organisms that need to break down and move through host tissues and barriers, and so their unique proteolytic resources, both endogenous and borrowed, make movement more feasible. Both the Lyme disease and relapsing fever Borrelia spp. bind plasminogen as well as numerous components of the mammalian plasminogen-activating system. This recruitment capacity endows the spirochetes with a borrowed proteolytic competency that can lead to increased invasiveness.

Borrelia burgdorferi Surface Exposed GroEL Is a Multifunctional Protein

The spirochete, Borrelia burgdorferi, has a large number of membrane proteins involved in a complex life cycle, that includes a tick vector and a vertebrate host. Some of these proteins also serve different roles in infection and dissemination of the spirochete in the mammalian host. In this spirochete, a number of proteins have been associated with binding to plasminogen or components of the extracellular matrix, which is important for tissue colonization and dissemination. GroEL is a cytoplasmic chaperone protein that has previously been associated with the outer membrane of Borrelia. A His-tag purified B. burgdorferi GroEL was used to generate a polyclonal rabbit antibody showing that GroEL also localizes in the outer membrane and is surface exposed. GroEL binds plasminogen in a lysine dependent manner. GroEL may be part of the protein repertoire that Borrelia successfully uses to establish infection and disseminate in the host. Importantly, this chaperone is readily recognized by sera from experimentally infected mice and rabbits. In summary, GroEL is an immunogenic protein that in addition to its chaperon role it may contribute to pathogenesis of the spirochete by binding to plasminogen and components of the extra cellular matrix.

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.

Campylobacter jejuni Serine Protease HtrA Cleaves the Tight Junction Component Claudin-8

Campylobacter jejuni express the high temperature requirement protein A (HtrA), a secreted serine protease, which is implicated in virulence properties of the pathogen. Previous studies have shown that C. jejuni HtrA can cleave the epithelial transmembrane proteins occludin and E-cadherin in the tight and adherens junctions, respectively. In the present report, we studied the interaction of HtrA with another human tight junction protein, claudin-8. Confocal immunofluorescence experiments have shown that C. jejuni infection of the intestinal polarized epithelial cells in vitro leads to a relocation of claudin-8. Wild-type C. jejuni induced the downregulation of claudin-8 signals in the tight junctions and an accumulation of claudin-8 agglomerates in the cytoplasm, which were not seen during infection with isogenic ΔhtrA knockout deletion or protease-inactive S197A point mutants. Western blotting of protein samples from infected vs. uninfected cells revealed that an 18-kDa carboxy-terminal fragment is cleaved-off from the 26-kDa full-length claudin-8 protein, but not during infection with the isogenic ΔhtrA mutant. These results were confirmed by in vitro cleavage assays using the purified recombinant C. jejuni HtrA and human claudin-8 proteins. Recombinant HtrA cleaved purified claudin-8 in vitro giving rise to the same 18-kDa sized carboxy-terminal cleavage product. Mapping studies revealed that HtrA cleavage occurs in the first extracellular loop of claudin-8. Three-dimensional modeling of the claudin-8 structure identified an exposed HtrA cleavage site between the amino acids alanine 58 and asparagine 59, which is in well agreement with the mapping studies. Taken together, HtrA operates as a secreted virulence factor targeting multiple proteins both in the tight and adherens junctions. This strategy may help the bacteria to open the cell-to-cell junctions, and to transmigrate across the intestinal epithelium by a paracellular mechanism and establish an acute infection.

The Lon-1 Protease Is Required by Borrelia burgdorferi To Infect the Mammalian Host

Borrelia burgdorferi encodes a functional homolog of canonical Lon protease termed Lon-2. In addition, B. burgdorferi encodes a second Lon homolog called Lon-1. Recent studies suggest that Lon-1 may function differently from the prototypical Lon protease. However, the function of Lon-1 in B. burgdorferi biology remains virtually unknown. Particularly, the contribution of Lon-1 to B. burgdorferi fitness and infection remains hitherto unexplored. Herein, we show that Lon-1 plays a critical role for the infection of B. burgdorferi in a mammalian host. We found that lon-1 was highly expressed during animal infection, implying an important function of this protein in bacterial infection. We further generated a lon-1 deletion mutant and an isogenic complemented strain. Relative to that of the wild-type strain, the infectivity of the mutant was severely attenuated in a murine infection model. Our data also showed that the mutant displayed growth defects in regular BSK-II medium. Furthermore, bacterial resistance to osmotic stress was markedly reduced when lon-1 was inactivated. When exposed to tert-butyl hydroperoxide, survival of the lon-1 mutant was impaired. In addition, production of several virulence factors, such as BosR, RpoS, and OspC, was elevated in the mutant. These phenotypes were restored when the lon-1 mutation was complemented. Finally, we created a lon-1(S714A) mutant and found that this mutant failed to infect mice, suggesting that the proteolytic activity of Lon-1 is essential for bacterial infection. Taken together, these results demonstrate that Lon-1 is required by B. burgdorferi to infect animal hosts and to cope with environmental stresses.

A joint effort: The interplay between the innate and the adaptive immune system in Lyme arthritis

Articular joints are a major target of Borrelia burgdorferi, the causative agent of Lyme arthritis. Despite antibiotic treatment, recurrent or persistent Lyme arthritis is observed in a significant number of patients. The host immune response plays a crucial role in this chronic arthritic joint complication of Borrelia infections. During the early stages of B. burgdorferi infection, a major hinder in generating a proper host immune response is the lack of induction of a strong adaptive immune response. This may lead to a delayed hyperinflammatory reaction later in the disease. Several mechanisms have been suggested that might be pivotal for the development of Lyme arthritis and will be highlighted in this review, from molecular mimicry of matrix metallopeptidases and glycosaminoglycans, to autoimmune responses to live bacteria, or remnants of Borrelia spirochetes in joints. Murine studies have suggested that the inflammatory responses are initiated by innate immune cells, but this does not exclude the involvement of the adaptive immune system in this dysregulated immune profile. Genetic predisposition, via human leukocyte antigen-DR isotype and microRNA expression, has been associated with the development of antibiotic-refractory Lyme arthritis. Yet the ultimate cause for (antibiotic-refractory) Lyme arthritis remains unknown. Complex processes of different immune cells and signaling cascades are involved in the development of Lyme arthritis. When these various mechanisms are fully been unraveled, new treatment strategies can be developed to target (antibiotic-refractory) Lyme arthritis more effectively.

Characterization of 6S RNA in the Lyme disease spirochete

6S RNA binds to RNA polymerase and regulates gene expression, contributing to bacterial adaptation to environmental stresses. In this study, we examined the role of 6S RNA in murine infectivity and tick persistence of the Lyme disease spirochete Borrelia (Borreliella) burgdorferi. B. burgdorferi 6S RNA (Bb6S RNA) binds to RNA polymerase, is expressed independent of growth phase or nutrient stress in culture, and is processed by RNase Y. We found that rny (bb0504), the gene encoding RNase Y, is essential for B. burgdorferi growth, while ssrS, the gene encoding 6S RNA, is not essential, indicating a broader role for RNase Y activity in the spirochete. Bb6S RNA regulates expression of the ospC and dbpA genes encoding outer surface protein C and decorin binding protein A, respectively, which are lipoproteins important for host infection. The highest levels of Bb6S RNA are found when the spirochete resides in unfed nymphs. ssrS mutants lacking Bb6S RNA were compromised for infectivity by needle inoculation, but injected mice seroconverted, indicating an ability to activate the adaptive immune response. ssrS mutants were successfully acquired by larval ticks and persisted through fed nymphs. Bb6S RNA is one of the first regulatory RNAs identified in B. burgdorferi that controls the expression of lipoproteins involved in host infectivity.

Assessing the Contribution of an HtrA Family Serine Protease During Borrelia turicatae Mammalian Infection

Tick-borne relapsing fever (TBRF), characterized by recurring febrile episodes, is globally distributed and among the most common bacterial infections in some African countries. Despite the public health concern that this disease represents, little is known regarding the virulence determinants required by TBRF Borrelia during infection. Because the chromosomes of TBRF Borrelia show extensive colinearity with those of Lyme disease (LD) Borrelia, the exceptions represent unique genes encoding proteins that are potentially essential to the disparate enzootic cycles of these two groups of spirochetes. One such exception is a gene encoding an HtrA family protease, BtpA, that is present in TBRF Borrelia, but not in LD spirochetes. Previous work suggested that btpA orthologs may be important for resistance to stresses faced during mammalian infection. Herein, proteomic analyses of the TBRF spirochete, Borrelia turicatae, demonstrated that BtpA, as well as proteins encoded by adjacent genes in the B. turicatae genome, were produced in response to culture at mammalian body temperature, suggesting a role in mammalian infection. Further, transcriptional analyses revealed that btpA was expressed with the genes immediately upstream and downstream as part of an operon. To directly assess if btpA is involved in resistance to environmental stresses, btpA deletion mutants were generated. btpA mutants demonstrated no growth defect in response to heat shock, but were more sensitive to oxidative stress produced by t-butyl peroxide compared to wild-type B. turicatae. Finally, btpA mutants were fully infectious in a murine relapsing fever (RF) infection model. These results indicate that BtpA is either not required for mammalian infection, or that compensatory mechanisms exist in TBRF spirochetes to combat environmental stresses encountered during mammalian infection in the absence of BtpA.

Contribution of the serine protease HtrA in Escherichia coli to infection in foxes

Escherichia coli can cause severe, acute hemorrhagic pneumonia and systemic infection in farmed foxes, raccoon dogs and minks, leading to considerable economic losses to the farmers. It is well established that the htrA-encoded serine protease HtrA is critical for bacterial growth and survival under stress, and HtrA has been determined to be a potential vaccine target. However, the roles of HtrA in E. coli pathogenesis remain unknown. In this study, we generated an htrA-deletion mutant of the E. coli protype strain HBCLE-12 that causes pneumonia in silver foxes and then evaluated the changes in bacterial physiological characteristics in the absence of HtrA. The data show that knockout of the htrA gene did not affect growth and biochemical characteristics but led to impaired virulence of the strain. Increased susceptibility to environmental stresses, impaired survival in serum, and reduced biofilm formation may contribute to the virulence attenuation of the mutant. Furthermore, the HtrA-deficient mutant was subjected to RNA-seq analysis, and 16 differentially expressed genes were determined. This study provided insight that HtrA plays a definitive role in E. coli-induced infection.

Analysis of a flagellar filament cap mutant reveals that HtrA serine protease degrades unfolded flagellin protein in the periplasm of Borrelia burgdorferi

Unlike external flagellated bacteria, spirochetes have periplasmic flagella (PF). Very little is known about how PF are assembled within the periplasm of spirochaetal cells. Herein, we report that FliD (BB0149), a flagellar cap protein (also named hook-associated protein 2), controls flagellin stability and flagellar filament assembly in the Lyme disease spirochete Borrelia burgdorferi. Deletion of fliD leads to non-motile mutant cells that are unable to assemble flagellar filaments and pentagon-shaped caps (10 nm in diameter, 12 nm in length). Interestingly, FlaB, a major flagellin protein of B. burgdorferi, is degraded in the fliD mutant but not in other flagella-deficient mutants (i.e., in the hook, rod, or MS-ring). Biochemical and genetic studies reveal that HtrA, a serine protease of B. burgdorferi, controls FlaB turnover. Specifically, HtrA degrades unfolded but not polymerized FlaB, and deletion of htrA increases the level of FlaB in the fliD mutant. Collectively, we propose that the flagellar cap protein FliD promotes flagellin polymerization and filament growth in the periplasm. Deletion of fliD abolishes this process, which leads to leakage of unfolded FlaB proteins into the periplasm where they are degraded by HtrA, a protease that prevents accumulation of toxic products in the periplasm.

Campylobacter jejuni enters gut epithelial cells and impairs intestinal barrier function through cleavage of occludin by serine protease HtrA

Campylobacter jejuni secretes HtrA (high temperature requirement protein A), a serine protease that is involved in virulence. Here, we investigated the interaction of HtrA with the host protein occludin, a tight junction strand component. Immunofluorescence studies demonstrated that infection of polarized intestinal Caco-2 cells with C. jejuni strain 81-176 resulted in a redistribution of occludin away from the tight junctions into the cytoplasm, an effect that was also observed in human biopsies during acute campylobacteriosis. Occludin knockout Caco-2 cells were generated by CRISPR/Cas9 technology. Inactivation of this gene affected the polarization of the cells in monolayers and transepithelial electrical resistance (TER) was reduced, compared to wild-type Caco-2 cells. Although tight junctions were still being formed, occludin deficiency resulted in a slight decrease of the tight junction plaque protein ZO-1, which was redistributed off the tight junction into the lateral plasma membrane. Adherence of C. jejuni to Caco-2 cell monolayers was similar between the occludin knockout compared to wild-type cells, but invasion was enhanced, indicating that deletion of occludin allowed larger numbers of bacteria to pass the tight junctions and to reach basal membranes to target the fibronectin receptor followed by cell entry. Finally, we discovered that purified C. jejuni HtrA cleaves recombinant occludin in vitro to release a 37 kDa carboxy-terminal fragment. The same cleavage fragment was observed in Western blots upon infection of polarized Caco-2 cells with wild-type C. jejuni, but not with isogenic ΔhtrA mutants. HtrA cleavage was mapped to the second extracellular loop of occludin, and a putative cleavage site was identified. In conclusion, HtrA functions as a secreted protease targeting the tight junctions, which enables the bacteria by cleaving occludin and subcellular redistribution of other tight junction proteins to transmigrate using a paracellular mechanism and subsequently invade epithelial cells.

HtrA of Borrelia burgdorferi Leads to Decreased Swarm Motility and Decreased Production of Pyruvate

Borrelia burgdorferi HtrA (HtrABb) is a serine protease that targets damaged or improperly folded proteins. In our previous studies, HtrABb specifically degraded basic membrane protein BmpD, chemotaxis phosphatase CheX, and outer membrane protein P66. In addition, HtrABb degrades virulence factor BB0323 and components of the extracellular matrix fibronectin and aggrecan. A proteomics-based analysis (two-dimensional difference gel electrophoresis [2-D DIGE], liquid chromatography-mass spectrometry [LC-MS]) of an HtrABb-overexpressing strain of B. burgdorferi (A3HtrAOE) revealed that protein levels of P66 were reduced in comparison to wild-type B. burgdorferi, confirming its status as an HtrABb substrate. Hbb, a P66-DNA-binding transcription factor, was specifically degraded by HtrABb, providing supportive evidence for a role for both in the regulation of P66. A3HtrAOE exhibited reduced motility in swarm assays, a possible link between overabundance of HtrABb and its enzymatic specificity for P66. However, the ΔP66 strain did not have reduced motility in the swarm assays, negating a role for this protein. The proteomics analyses also identified three enzymes of the glycolytic pathway, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), glycerol-3-phosphate dehydrogenase (GPDH), and glycerol kinase (GK), and one enzyme involved in carbohydrate metabolism, diphosphate-fructose-6-phosphate 1-phosphotransferase, which were reduced in A3HtrAOE. Consistent with its reduced protein levels of these glycolytic enzymes, A3HtrAOE was also deficient in production of pyruvate. We propose a model for a role for HtrABb in contributing to a decrease in metabolic activity of B. burgdorferi.

Being a vector-borne bacterium, B. burgdorferi must remodel its protein content as it transfers from tick to mammal. Proteolysis is a mechanism whereby remodeling can be accomplished. HtrABb degrades a number of proteins whose disappearance may help in preparing this organism for a stage of low metabolic activity.

Borrelia burgdorferi protein interactions critical for microbial persistence in mammals

Borrelia burgdorferi is the causative agent of Lyme disease that persists in a complex enzootic life cycle, involving Ixodes ticks and vertebrate hosts. The microbe invades ticks and vertebrate hosts in spite of active immune surveillance and potent microbicidal responses, and establishes long-term infection utilising mechanisms that are yet to be unravelled. The pathogen can cause multi-system disorders when transmitted to susceptible mammalian hosts, including in humans. In the past decades, several studies identified a limited number of B. burgdorferi gene-products critical for pathogen persistence, transmission between the vectors and the host, and host-pathogen interactions. This review will focus on the interactions between B. burgdorferi proteins, as well as between microbial proteins and host components, protein and non-protein components, highlighting their roles in pathogen persistence in the mammalian host. A better understanding of the contributions of protein interactions in the microbial virulence and persistence of B. burgdorferi would support development of novel therapeutics against the infection.

Borrelia burgdorferi surface-located Lmp1 protein processed into region-specific polypeptides that are critical for microbial persistence

One of the Borrelia burgdorferi virulence determinants, annotated as Lmp1, is a surface-exposed, conserved, and potential multi-domain protein involved in various functions in spirochete infectivity. Lmp1 contributes to host-pathogen interactions and evasion of host adaptive immunity by spirochetes. Here, we show that in diverse B. burgdorferi species, Lmp1 exists as distinct, region-specific, and lower molecular mass polypeptides encompassing 1 or more domains, including independent N-terminal and middle regions and a combined middle and C-terminal region. These polypeptides originate from complex posttranslational maturation events, partly supported by a periplasmic serine protease termed as BbHtrA. Although spirochete persistence in mice is independently supported by domain-specific Lmp1 polypeptides, transmission of B. burgdorferi from ticks to mammals requires essential contributions from both N-terminal and middle regions. Interference with the functions of Lmp1 domains or their complex posttranslational maturation events may aid in development of novel therapeutic strategies to combat infection and transmission of pathogens.

Extracellular HtrA serine proteases: An emerging new strategy in bacterial pathogenesis

The HtrA family of chaperones and serine proteases is important for regulating stress responses and controlling protein quality in the periplasm of bacteria. HtrA is also associated with infectious diseases since inactivation of htrA genes results in significantly reduced virulence properties by various bacterial pathogens. These virulence features of HtrA can be attributed to reduced fitness of the bacteria, higher susceptibility to environmental stress and/or diminished secretion of virulence factors. In some Gram-negative and Gram-positive pathogens, HtrA itself can be exposed to the extracellular environment promoting bacterial colonisation and invasion of host tissues. Most of our knowledge on the function of exported HtrAs stems from research on Helicobacter pylori, Campylobacter jejuni, Borrelia burgdorferi, Bacillus anthracis, and Chlamydia species. Here, we discuss recent progress showing that extracellular HtrAs are able to cleave cell-to-cell junction factors including E-cadherin, occludin, and claudin-8, as well as extracellular matrix proteins such as fibronectin, aggrecan, and proteoglycans, disrupting the epithelial barrier and producing substantial host cell damage. We propose that the export of HtrAs is a newly discovered strategy, also applied by additional bacterial pathogens. Consequently, exported HtrA proteases represent highly attractive targets for antibacterial treatment by inhibiting their proteolytic activity or application in vaccine development.

Artificial Infection of Ticks with Borrelia burgdorferi Using a Microinjection Method and Their Detection In Vivo Using Quantitative PCR Targeting flaB RNA

Borrelia burgdorferi is maintained in nature by a tick-rodent infection cycle where it traverses and colonizes a variety of host and vector tissues. A tick-borne murine model has been developed to study Lyme disease in the laboratory, which has a substantial impact in advancing our knowledge of spirochete infectivity and pathogenesis. Here, we detail a microinjection-based method for rapid and efficient infection of ticks with B. burgdorferi. While laboratory generation of B. burgdorferi-infected nymphs via natural larval engorgement on infected hosts and subsequent molting could take several weeks to months, the microinjection-based infection procedure requires only a few hours to generate infected ticks and allows introduction of defined quantities of spirochetes, including mutant isolates that are attenuated for infection in mice and thus cannot be naturally acquired by ticks. We also describe a quantitative PCR-based protocol for the measurement of B. burgdorferi in tick and murine hosts targeting spirochete RNA that is highly efficient, reproducible, and a better surrogate of active infection.

Borrelia burgdorferi glycosaminoglycan-binding proteins: a potential target for new therapeutics against Lyme disease

The spirochete bacterium Borrelia burgdorferi sensu lato is the causative agent of Lyme disease, the most common vector-borne disease in Europe and the United States. The spirochetes can be transmitted to humans via ticks, and then spread to different tissues, leading to arthritis, carditis and neuroborreliosis. Although antibiotics have commonly been used to treat infected individuals, some treated patients do not respond to antibiotics and experience persistent, long-term arthritis. Thus, there is a need to investigate alternative therapeutics against Lyme disease. The spirochete bacterium colonization is partly attributed to the binding of the bacterial outer-surface proteins to the glycosaminoglycan (GAG) chains of host proteoglycans. Blocking the binding of these proteins to GAGs is a potential strategy to prevent infection. In this review, we have summarized the recent reports of B. burgdorferi sensu lato GAG-binding proteins and discussed the potential use of synthetic and semi-synthetic compounds, including GAG analogues, to block pathogen interaction with GAGs. Such information should motivate the discovery and development of novel GAG analogues as new therapeutics for Lyme disease. New therapeutic approaches should eventually reduce the burden of Lyme disease and improve human health.

Overexpression of serine protease HtrA enhances disruption of adherens junctions, paracellular transmigration and type IV secretion of CagA by Helicobacter pylori

BACKGROUND

The serine protease HtrA is an important factor for regulating stress responses and protein quality control in bacteria. In recent studies, we have demonstrated that the gastric pathogen Helicobacter pylori can secrete HtrA into the extracellular environment, where it cleaves-off the ectodomain of the tumor suppressor and adherens junction protein E-cadherin on gastric epithelial cells.

RESULTS

E-cadherin cleavage opens cell-to-cell junctions, allowing paracellular transmigration of the bacteria across polarized monolayers of MKN-28 and Caco-2 epithelial cells. However, rapid research progress on HtrA function is mainly hampered by the lack of ΔhtrA knockout mutants, suggesting that htrA may represent an essential gene in H. pylori. To circumvent this major handicap and to investigate the role of HtrA further, we overexpressed HtrA by introducing a second functional htrA gene copy in the chromosome and studied various virulence properties of the bacteria. The resulting data demonstrate that overexpression of HtrA in H. pylori gives rise to elevated rates of HtrA secretion, cleavage of E-cadherin, bacterial transmigration and delivery of the type IV secretion system (T4SS) effector protein CagA into polarized epithelial cells, but did not affect IL-8 chemokine production or the secretion of vacuolating cytotoxin VacA and γ-glutamyl-transpeptidase GGT.

CONCLUSIONS

These data provide for the first time genetic evidence in H. pylori that HtrA is a novel major virulence factor controlling multiple pathogenic activities of this important microbe.

Investigation of ospC Expression Variation among Borrelia burgdorferi Strains

Outer surface protein C (OspC) is the most studied major virulence factor of Borrelia burgdorferi, the causative agent of Lyme disease. The level of OspC varies dramatically among B. burgdorferi strains when cultured in vitro, but little is known about what causes such variation. It has been proposed that the difference in endogenous plasmid contents among strains contribute to variation in OspC phenotype, as B. burgdorferi contains more than 21 endogenous linear (lp) and circular plasmids (cp), and some of which are prone to be lost. In this study, we analyzed several clones isolated from B. burgdorferi strain 297, one of the most commonly used strains for studying ospC expression. By taking advantage of recently published plasmid sequence of strain 297, we developed a multiplex PCR method specifically for rapid plasmid profiling of B. burgdorferi strain 297. We found that some commonly used 297 clones that were thought having a complete plasmid profile, actually lacked some endogenous plasmids. Importantly, the result showed that the difference in plasmid profiles did not contribute to the ospC expression variation among the clones. Furthermore, we found that B. burgdorferi clones expressed different levels of BosR, which in turn led to different levels of RpoS and subsequently, resulted in OspC level variation among B. burgdorferi strains.

Borrelia burgdorferi Keeps Moving and Carries on: A Review of Borrelial Dissemination and Invasion

Borrelia burgdorferi is the etiological agent of Lyme disease, a multisystemic, multistage, inflammatory infection resulting in patients experiencing cardiac, neurological, and arthritic complications when not treated with antibiotics shortly after exposure. The spirochetal bacterium transmits through the Ixodes vector colonizing the dermis of a mammalian host prior to hematogenous dissemination and invasion of distal tissues all the while combating the immune response as it traverses through its pathogenic lifecycle. The innate immune response controls the borrelial burden in the dermis, but is unable to clear the infection and thereby prevent progression of disease. Dissemination in the mammalian host requires temporal regulation of virulence determinants to allow for vascular interactions, invasion, and colonization of distal tissues. Virulence determinants and/or adhesins are highly heterogenetic among environmental B. burgdorferi strains with particular genotypes being associated with the ability to disseminate to specific tissues and the severity of disease, but fail to generate cross-protective immunity between borrelial strains. The unique motility of B. burgdorferi rendered by the endoflagella serves a vital function for dissemination and protection from immune recognition. Progress has been made toward understanding the chemotactic regulation coordinating the activity of the two polar localized flagellar motors and their role in borrelial virulence, but this regulation is not yet fully understood. Distinct states of motility allow for dynamic interactions between several B. burgdorferi adhesins and host targets that play roles in transendothelial migration. Transmigration across endothelial and blood-brain barriers allows for the invasion of tissues and elicits localized immune responses. The invasive nature of B. burgdorferi is lacking in proactive mechanisms to modulate disease, such as secretion systems and toxins, but recent work has shown degradation of host extracellular matrices by B. burgdorferi contributes to the invasive capabilities of the pathogen. Additionally, B. burgdorferi may use invasion of eukaryotic cells for immune evasion and protection against environmental stresses. This review provides an overview of B. burgdorferi mechanisms for dissemination and invasion in the mammalian host, which are essential for pathogenesis and the development of persistent infection.