A novel FK-506-binding-like protein that lacks peptidyl-prolyl isomerase activity is involved in intracellular infection and in vivo virulence of Burkholderia pseudomallei

High Affinity Inhibitors of the Macrophage Infectivity Potentiator Protein from Trypanosoma cruzi, Burkholderia pseudomallei, and Legionella pneumophila─A Comparison

Since Chagas disease, melioidosis, and Legionnaires' disease are all potentially life-threatening infections, there is an urgent need for new treatment strategies. All causative agents, Trypanosoma cruzi, Burkholderia pseudomallei, and Legionella pneumophila, express a virulence factor, the macrophage infectivity potentiator (MIP) protein, emerging as a promising new therapeutic target. Inhibition of MIP proteins having a peptidyl-prolyl isomerase activity leads to reduced viability, proliferation, and cell invasion. The affinity of a series of pipecolic acid-type MIP inhibitors was evaluated against all MIPs using a fluorescence polarization assay. The analysis of structure-activity relationships led to highly active inhibitors of MIPs of all pathogens, characterized by a one-digit nanomolar affinity for the MIPs and a very effective inhibition of their peptidyl-prolyl isomerase activity. Docking studies, molecular dynamics simulations, and quantum mechanical calculations suggest an extended σ-hole of the meta-halogenated phenyl sulfonamide to be responsible for the high affinity.

Analysis of Structure-Activity Relationships of Novel Inhibitors of the Macrophage Infectivity Potentiator (Mip) Proteins of Neisseria meningitidis, Neisseria gonorrhoeae, and Burkholderia pseudomallei

The macrophage infectivity potentiator (Mip) protein is a promising target for developing new drugs to combat antimicrobial resistance. New rapamycin-derived Mip inhibitors have been designed that may be able to combine two binding modes to inhibit the Mip protein of Burkholderia pseudomallei (BpMip). These novel compounds are characterized by an additional substituent in the middle chain linking the lateral pyridine to the pipecoline moiety, constituting different stereoisomers. These compounds demonstrated high affinity for the BpMip protein in the nanomolar range and high anti-enzymatic activity and ultimately resulted in significantly reduced cytotoxicity of B. pseudomallei in macrophages. They also displayed strong anti-enzymatic activity against the Mip proteins of Neisseria meningitidis and Neisseria gonorrhoeae and substantially improved the ability of macrophages to kill the bacteria. Hence, the new Mip inhibitors are promising, non-cytotoxic candidates for further testing against a broad spectrum of pathogens and infectious diseases.

Fluorescent probe for the identification of potent inhibitors of the macrophage infectivity potentiator (Mip) protein of Burkholderia pseudomallei

The macrophage infectivity potentiator (Mip) protein belongs to the immunophilin superfamily. This class of enzymes catalyzes the interconversion between the cis and trans configuration of proline-containing peptide bonds. Mip has been shown to be important for the virulence of a wide range of pathogenic microorganisms, including the Gram-negative bacterium Burkholderia pseudomallei. Small molecules derived from the natural product rapamycin, lacking its immunosuppression-inducing moiety, inhibit Mip's peptidyl-prolyl cis-trans isomerase (PPIase) activity and lead to a reduction in pathogen load in vitro. Here, a fluorescence polarization assay (FPA) to enable the screening and effective development of BpMip inhibitors was established. A fluorescent probe was prepared, derived from previous pipecolic scaffold Mip inhibitors labeled with fluorescein. This probe showed moderate affinity for BpMip and enabled a highly robust FPA suitable for screening large compound libraries with medium- to high-throughput (Z factor ∼ 0.89) to identify potent new inhibitors. The FPA results are consistent with data from the protease-coupled PPIase assay. Analysis of the temperature dependence of the probe's binding highlighted that BpMip's ligand binding is driven by enthalpic rather than entropic effects. This has considerable consequences for the use of low-temperature kinetic assays.

Pathogenicity and virulence of Burkholderia pseudomallei

The soil saprophyte, Burkholderia pseudomallei, is the causative agent of melioidosis, a disease endemic in South East Asia and northern Australia. Exposure to B. pseudomallei by either inhalation or inoculation can lead to severe disease. B. pseudomallei rapidly shifts from an environmental organism to an aggressive intracellular pathogen capable of rapidly spreading around the body. The expression of multiple virulence factors at every stage of intracellular infection allows for rapid progression of infection. Following invasion or phagocytosis, B. pseudomallei resists host-cell killing mechanisms in the phagosome, followed by escape using the type III secretion system. Several secreted virulence factors manipulate the host cell, while bacterial cells undergo a shift in energy metabolism allowing for overwhelming intracellular replication. Polymerisation of host cell actin into “actin tails” propels B. pseudomallei to the membranes of host cells where the type VI secretion system fuses host cells into multinucleated giant cells (MNGCs) to facilitate cell-to-cell dissemination. This review describes the various mechanisms used by B. pseudomallei to survive within cells.

The molecular interplay of the establishment of an infection - gene expression of Diaphorina citri gut and Candidatus Liberibacter asiaticus

BACKGROUND

Candidatus Liberibacter asiaticus (CLas) is one the causative agents of greening disease in citrus, an unccurable, devastating disease of citrus worldwide. CLas is vectored by Diaphorina citri, and the understanding of the molecular interplay between vector and pathogen will provide additional basis for the development and implementation of successful management strategies. We focused in the molecular interplay occurring in the gut of the vector, a major barrier for CLas invasion and colonization.

RESULTS

We investigated the differential expression of vector and CLas genes by analyzing a de novo reference metatranscriptome of the gut of adult psyllids fed of CLas-infected and healthy citrus plants for 1-2, 3-4 and 5-6 days. CLas regulates the immune response of the vector affecting the production of reactive species of oxygen and nitrogen, and the production of antimicrobial peptides. Moreover, CLas overexpressed peroxiredoxin, probably in a protective manner. The major transcript involved in immune expression was related to melanization, a CLIP-domain serine protease we believe participates in the wounding of epithelial cells damaged during infection, which is supported by the down-regulation of pangolin. We also detected that CLas modulates the gut peristalsis of psyllids through the down-regulation of titin, reducing the elimination of CLas with faeces. The up-regulation of the neuromodulator arylalkylamine N-acetyltransferase implies CLas also interferes with the double brain-gut communication circuitry of the vector. CLas colonizes the gut by expressing two Type IVb pilin flp genes and several chaperones that can also function as adhesins. We hypothesized biofilm formation occurs by the expression of the cold shock protein of CLas.

CONCLUSIONS

The thorough detailed analysis of the transcritome of Ca. L. asiaticus and of D. citri at different time points of their interaction in the gut tissues of the host led to the identification of several host genes targeted for regulation by L. asiaticus, but also bacterial genes coding for potential effector proteins. The identified targets and effector proteins are potential targets for the development of new management strategies directed to interfere with the successful utilization of the psyllid vector by this pathogen.

RegAB Homolog of Burkholderia pseudomallei is the Master Regulator of Redox Control and involved in Virulence

Burkholderia pseudomallei, the etiological agent of melioidosis in humans and animals, often occupies environmental niches and infection sites characterized by limited concentrations of oxygen. Versatile genomic features enable this pathogen to maintain its physiology and virulence under hypoxia, but the crucial regulatory networks employed to switch from oxygen dependent respiration to alternative terminal electron acceptors (TEA) like nitrate, remains poorly understood. Here, we combined a Tn5 transposon mutagenesis screen and an anaerobic growth screen to identify a two-component signal transduction system with homology to RegAB. We show that RegAB is not only essential for anaerobic growth, but also for full virulence in cell lines and a mouse infection model. Further investigations of the RegAB regulon, using a global transcriptomic approach, identified 20 additional regulators under transcriptional control of RegAB, indicating a superordinate role of RegAB in the B. pseudomallei anaerobiosis regulatory network. Of the 20 identified regulators, NarX/L and a FNR homolog were selected for further analyses and a role in adaptation to anaerobic conditions was demonstrated. Growth experiments identified nitrate and intermediates of the denitrification process as the likely signal activateing RegAB, NarX/L, and probably of the downstream regulators Dnr or NsrR homologs. While deletions of individual genes involved in the denitrification process demonstrated their important role in anaerobic fitness, they showed no effect on virulence. This further highlights the central role of RegAB as the master regulator of anaerobic metabolism in B. pseudomallei and that the complete RegAB-mediated response is required to achieve full virulence. In summary, our analysis of the RegAB-dependent modulon and its interconnected regulons revealed a key role for RegAB of B. pseudomallei in the coordination of the response to hypoxic conditions and virulence, in the environment and the host.

Targeting Protein Folding: A Novel Approach for the Treatment of Pathogenic Bacteria

Infectious diseases are a major cause of morbidity and mortality worldwide, exacerbated by increasing antibiotic resistance in many bacterial species. The development of drugs with new modes of action is essential. A leading strategy is antivirulence, with the aim to target bacterial proteins that are important in disease causation and progression but do not affect growth, resulting in reduced selective pressure for resistance. Immunophilins, a superfamily of peptidyl-prolyl cis-trans isomerase (PPIase) enzymes have been shown to be important for virulence in a broad-spectrum of pathogenic bacteria. This Perspective will provide an overview of the recent advances made in understanding the role of each immunophilin family, cyclophilins, FK506 binding proteins (FKBPs), and parvulins in bacteria. Inhibitor design and medicinal chemistry strategies for development of novel drugs against bacterial FKBPs will be discussed. Furthermore, drugs against human cyclophilins and parvulins will be reviewed in their current indication as antiviral and anticancer therapies.

Mycobacterial transcript cleavage factor Gre, exhibits chaperone-like activity

Gre factors reactivate stalled elongation complexes by enhancing the intrinsic transcript cleavage activity of RNA polymerase. Previous work by us has shown that unlike in Escherichia coli (E.coli), Mycobacterium tuberculosis Gre factor is essential for its survival. Apart from their role in transcription regulation Gre factors have been implicated in stress response. A recent study has shown the role of E.coli GreA as a cellular chaperone, which inhibits aggregation of substrate proteins under heat stress condition. Moreover it was shown that GreA enables E.coli to survive heat shock and oxidative stress. In the current work, we have characterized the moonlighting chaperone activity and its plausible mechanism in Mycobacterium smegmatis Gre (MsGre) factor. We show here that MsGre prevents heat-induced aggregation of the substrate protein and also protects enzymatic activity. Interestingly Gre factor exists as a dimer in solution and does not undergo heat induced oligomerization. From the 8-anilino-1-naphthalene sulfonate (ANS) binding studies MsGre was shown to expose hydrophobic surface upon heat stress that would allow binding to unfolded or partially folded substrate protein. From Circular Dichroism (CD) studies, we also show that MsGre has a stable secondary structure under thermal stress. We propose that the presence of C-terminal FKBP-like fold in MsGre factor that might contribute to its chaperone-like function.

A miniaturized peptidyl-prolyl isomerase enzyme assay

Prolyl-peptidyl isomerases (PPIases) are enzymes that are found in all living organisms. They form an essential part of the cellular protein folding homeostasis machinery. PPIases are associated with many important human diseases, e.g. cardiovascular disease, cancer and Alzheimer's. The development of novel PPIase inhibitors has been limited by the lack of a rapid, laboratory-based assay for these enzymes, as their substrates and products are challenging to distinguish. A well described continuous assay, coupled with the hydrolysis of a peptide by chymotrypsin is highly effective, but comparatively slow. To address this, we developed an improved version of the traditional assay using a temperature controlled plate reader. This assay allows semi-automated medium throughput assays in an academic laboratory for 84 samples per day. The assay shows lower errors, with an average Z' of 0.72. We further developed the assay using a fluorogenic peptide-based FRET probe. This provides an extremely sensitive PPIase assay using substrate at 200 nM, which approaches single turnover conditions. The fluorescent probe achieves an excellent quenching efficiency of 98.6%, and initial experiments showed acceptable Z' of 0.31 and 0.30 for cyclophilin A and hFKBP12 respectively. The assays provide an improved toolset for the quantitative, biochemical analysis of PPIases.

Development, synthesis and structure-activity-relationships of inhibitors of the macrophage infectivity potentiator (Mip) proteins of Legionella pneumophila and Burkholderia pseudomallei

The bacteria Burkholderia pseudomallei and Legionella pneumophila cause severe diseases like melioidosis and Legionnaire's disease with high mortality rates despite antibiotic treatment. Due to increasing antibiotic resistances against these and other Gram-negative bacteria, alternative therapeutical strategies are in urgent demand. As a virulence factor, the macrophage infectivity potentiator (Mip) protein constitutes an attractive target. The Mip proteins of B. pseudomallei and L. pneumophila exhibit peptidyl-prolyl cis/trans isomerase (PPIase) activity and belong to the PPIase superfamily. In previous studies, the pipecolic acid moiety proved to be a valuable scaffold for inhibiting this PPIase activity. Thus, a library of pipecolic acid derivatives was established guided by structural information and computational analyses of the binding site and possible binding modes. Stability and toxicity considerations were taken into account in iterative extensions of the library. Synthesis and evaluation of the compounds in PPIase assays resulted in highly active inhibitors. The activities can be interpreted in terms of a common binding mode obtained by docking calculations.

Inhibitors of macrophage infectivity potentiator-like PPIases affect neisserial and chlamydial pathogenicity

The pathogenic bacteria Chlamydia trachomatis, Neisseria gonorrhoeae and Neisseria meningitidis express the surface-exposed macrophage infectivity potentiator (MIP)-like protein, which plays a role in their pathogenicity. MIP exhibits a peptidyl-prolyl isomerase (PPIase) activity that is inhibited by rapamycin and FK506. In this study, pipecolic acid derivatives were tested for their activity against the chlamydial and neisserial MIP. Two MIP inhibitors were identified, PipN3 and PipN4, that affected the developmental cycle of C. trachomatis in HeLa cells. Furthermore, we could show that deletion of neisserial MIP or addition of the two MIP inhibitors affected the survival of N. gonorrhoeae in the presence of neutrophils. Furthermore, both compounds inhibited the adherence, invasion and/or survival of N. meningitidis in epithelial cells. These results confirm the importance of MIP-like proteins in infection and indicate the relevance of pipecolic acid derivatives as antimicrobials against C. trachomatis, N. gonorrhoeae and N. meningitidis.

Using multispectral imaging flow cytometry to assess an in vitro intracellular Burkholderia thailandensis infection model

The use of in vitro models to understand the interaction of bacteria with host cells is well established. In vitro bacterial infection models are often used to quantify intracellular bacterial load by lysing cell populations and subsequently enumerating the bacteria. Modern established techniques employ the use of fluorescence technologies such as flow cytometry, fluorescent microscopy, and/or confocal microscopy. However, these techniques often lack either the quantification of large data sets (microscopy) or use of gross fluorescence signal which lacks the visual confirmation that can provide additional confidence in data sets. Multispectral imaging flow cytometry (MIFC) is a novel emerging field of technology. This technology captures a bright field and fluorescence image of cells in a flow using a charged coupled device camera. It allows the analysis of tens of thousands of single cell images, making it an extremely powerful technology. Here MIFC was used as an alternative method of analyzing intracellular bacterial infection using Burkholderia thailandensis E555 as a model organism. It has been demonstrated that the data produced using traditional enumeration is comparable to data analyzed using MIFC. It has also been shown that by using MIFC it is possible to generate other data on the dynamics of the infection model rather than viable counts alone. It has been demonstrated that it is possible to inhibit the uptake of bacteria into mammalian cells and identify differences between treated and untreated cell populations. The authors believe this to be the first use of MIFC to analyze a Burkholderia bacterial species during intracellular infection. © 2016 Crown copyright. Published by Wiley Periodicals Inc. on behalf of ISAC.

Vaccine potential of bacterial macrophage infectivity potentiator (MIP)-like peptidyl prolyl cis/trans isomerase (PPIase) proteins

Peptidyl prolyl cis/trans isomerases (PPIases) are a superfamily of proteins ubiquitously distributed among living organisms, which function primarily to assist the folding and structuring of unfolded and partially folded polypeptide chains and proteins. In this review, we focus specifically on the Macrophage Infectivity Potentiator (MIP)-like PPIases, which are members of the immunophilin family of FK506-binding proteins (FKBP). MIP-like PPIases have accessory roles in virulence and are candidates for inclusion in vaccines protective against both animal and human bacterial pathogens. A structural vaccinology approach obviates any issues over molecular mimicry and potential cross-reactivity with human FKBP proteins and studies with a representative antigen, the Neisseria meningitidis-MIP, support this strategy. Moreover, a dual approach of vaccination and drug targeting could be considered for controlling bacterial infectious diseases of humans and animals.

Microbial peptidyl-prolyl cis/trans isomerases (PPIases): virulence factors and potential alternative drug targets

Initially discovered in the context of immunomodulation, peptidyl-prolyl cis/trans isomerases (PPIases) were soon identified as enzymes catalyzing the rate-limiting protein folding step at peptidyl bonds preceding proline residues. Intense searches revealed that PPIases are a superfamily of proteins consisting of three structurally distinguishable families with representatives in every described species of prokaryote and eukaryote and, recently, even in some giant viruses. Despite the clear-cut enzymatic activity and ubiquitous distribution of PPIases, reports on solely PPIase-dependent biological roles remain scarce. Nevertheless, they have been found to be involved in a plethora of biological processes, such as gene expression, signal transduction, protein secretion, development, and tissue regeneration, underscoring their general importance. Hence, it is not surprising that PPIases have also been identified as virulence-associated proteins. The extent of contribution to virulence is highly variable and dependent on the pleiotropic roles of a single PPIase in the respective pathogen. The main objective of this review is to discuss this variety in virulence-related bacterial and protozoan PPIases as well as the involvement of host PPIases in infectious processes. Moreover, a special focus is given to Legionella pneumophila macrophage infectivity potentiator (Mip) and Mip-like PPIases of other pathogens, as the best-characterized virulence-related representatives of this family. Finally, the potential of PPIases as alternative drug targets and first tangible results are highlighted.

FKBPs in bacterial infections

BACKGROUND

FK506-binding proteins (FKBPs) contain a domain with peptidyl-prolyl-cis/trans-isomerase (PPIase) activity and bind the immunosuppressive drugs FK506 and rapamycin. FKBPs belong to the immunophilin family and are found in eukaryotes and bacteria.

SCOPE OF REVIEW

In this review we describe two major groups of bacterial virulence-associated FKBPs, the trigger factor and Mip-like PPIases. Moreover, we discuss the contribution of host FKBPs in bacterial infection processes.

MAJOR CONCLUSIONS

Since PPIases are regarded as alternative antiinfective drug targets we highlight current research strategies utilizing pipecolinic acid and cycloheximide derivatives as well as substrate based inhibitors.

GENERAL SIGNIFICANCE

The current research strategies suggest a beneficial synergism of drug development and basic research. This article is part of a Special Issue entitled Proline-directed Foldases: Cell Signaling Catalysts and Drug Targets.

Melioidosis: molecular aspects of pathogenesis

Burkholderia pseudomallei is a gram-negative bacterium that causes melioidosis, a multifaceted disease that is highly endemic in southeast Asia and northern Australia. This facultative intracellular pathogen possesses a large genome that encodes a wide array of virulence factors that promote survival in vivo by manipulating host cell processes and disarming elements of the host immune system. Antigens and systems that play key roles in B. pseudomallei virulence include capsular polysaccharide, lipopolysaccharide, adhesins, specialized secretion systems, actin-based motility and various secreted factors. This review provides an overview of the current and steadily expanding knowledge regarding the molecular mechanisms used by this organism to survive within a host and their contribution to the pathogenesis of melioidosis.

Proteomic analysis of the Burkholderia pseudomallei type II secretome reveals hydrolytic enzymes, novel proteins, and the deubiquitinase TssM

Burkholderia pseudomallei, the etiologic agent of melioidosis, is an opportunistic pathogen that harbors a wide array of secretion systems, including a type II secretion system (T2SS), three type III secretion systems (T3SS), and six type VI secretion systems (T6SS). The proteins exported by these systems provide B. pseudomallei with a growth advantage in vitro and in vivo, but relatively little is known about the full repertoire of exoproducts associated with each system. In this study, we constructed deletion mutations in gspD and gspE, T2SS genes encoding an outer membrane secretin and a cytoplasmic ATPase, respectively. The secretion profiles of B. pseudomallei MSHR668 and its T2SS mutants were noticeably different when analyzed by SDS-PAGE. We utilized liquid chromatography-tandem mass spectrometry (LC-MS/MS) to identify proteins present in the supernatants of B. pseudomallei MSHR668 and B. pseudomallei ΔgspD grown in rich and minimal media. The MSHR668 supernatants contained 48 proteins that were either absent or substantially reduced in the supernatants of ΔgspD strains. Many of these proteins were putative hydrolytic enzymes, including 12 proteases, two phospholipases, and a chitinase. Biochemical assays validated the LC-MS/MS results and demonstrated that the export of protease, phospholipase C, and chitinase activities is T2SS dependent. Previous studies had failed to identify the mechanism of secretion of TssM, a deubiquitinase that plays an integral role in regulating the innate immune response. Here we present evidence that TssM harbors an atypical signal sequence and that its secretion is mediated by the T2SS. This study provides the first in-depth characterization of the B. pseudomallei T2SS secretome.

Caspase-1-dependent and -independent cell death pathways in Burkholderia pseudomallei infection of macrophages

The cytosolic pathogen Burkholderia pseudomallei and causative agent of melioidosis has been shown to regulate IL-1β and IL-18 production through NOD-like receptor NLRP3 and pyroptosis via NLRC4. Downstream signalling pathways of those receptors and other cell death mechanisms induced during B. pseudomallei infection have not been addressed so far in detail. Furthermore, the role of B. pseudomallei factors in inflammasome activation is still ill defined. In the present study we show that caspase-1 processing and pyroptosis is exclusively dependent on NLRC4, but not on NLRP3 in the early phase of macrophage infection, whereas at later time points caspase-1 activation and cell death is NLRC4- independent. In the early phase we identified an activation pathway involving caspases-9, -7 and PARP downstream of NLRC4 and caspase-1. Analyses of caspase-1/11-deficient infected macrophages revealed a strong induction of apoptosis, which is dependent on activation of apoptotic initiator and effector caspases. The early activation pathway of caspase-1 in macrophages was markedly reduced or completely abolished after infection with a B. pseudomallei flagellin FliC or a T3SS3 BsaU mutant. Studies using cells transfected with the wild-type and mutated T3SS3 effector protein BopE indicated also a role of this protein in caspase-1 processing. A T3SS3 inner rod protein BsaK mutant failed to activate caspase-1, revealed higher intracellular counts, reduced cell death and IL-1β secretion during early but not during late macrophage infection compared to the wild-type. Intranasal infection of BALB/c mice with the BsaK mutant displayed a strongly decreased mortality, lower bacterial loads in organs, and reduced levels of IL-1β, myeloperoxidase and neutrophils in bronchoalveolar lavage fluid. In conclusion, our results indicate a major role for a functional T3SS3 in early NLRC4-mediated caspase-1 activation and pyroptosis and a contribution of late caspase-1-dependent and -independent cell death mechanisms in the pathogenesis of B. pseudomallei infection.

Inflammasome activation is important for host defence against bacterial infection. Many gram-negative pathogens use secretion systems to inject bacterial proteins such as flagellin or structural components of the secretion machinery itself into the host cytosol leading to caspase-1 activation and pyroptotic cell death. However, little is known about the B. pseudomallei factors that trigger caspase-1 activation as well as downstream signalling pathways and effector mechanisms of caspase-1. Here, we identified the B. pseudomallei T3SS3 inner rod protein BsaK as an early activator of caspase-1-dependent cell death and IL-1β secretion in primary macrophages and as a virulence factor in murine melioidosis. We could show that upon infection of macrophages, caspase-7 is activated downstream of the NLRC4/caspase-1 inflammasome and requires caspase-9 processing. Although caspase-7 was essential for cleavage of the DNA damage sensor PARP during pyroptosis, it did neither contribute to cytokine production nor B. pseudomallei growth restriction by promoting early macrophage death. In addition to a rapid NLRC4/caspase-1- dependent induction of pyroptosis in wild-type macrophages, we observed a delayed activation of classical apoptosis in macrophages lacking caspase-1/11. Thus, initiation of different cell death pathways seems to be an effective strategy to limit intracellular B. pseudomallei infection.

BPSS1504, a cluster 1 type VI secretion gene, is involved in intracellular survival and virulence of Burkholderia pseudomallei

Burkholderia pseudomallei is a Gram-negative rod and the causative agent of melioidosis, an emerging infectious disease of tropical and subtropical areas worldwide. B. pseudomallei harbors a remarkable number of virulence factors, including six type VI secretion systems (T6SS). Using our previously described plaque assay screening system, we identified a B. pseudomallei transposon mutant defective in the BPSS1504 gene that showed reduced plaque formation. The BPSS1504 locus is encoded within T6SS cluster 1 (T6SS1), which is known to be involved in the pathogenesis of B. pseudomallei in mammalian hosts. For further analysis, a B. pseudomallei BPSS1504 deletion (BpΔBPSS1504) mutant and complemented mutant strain were constructed. B. pseudomallei lacking the BPSS1504 gene was highly attenuated in BALB/c mice, whereas the in vivo virulence of the complemented mutant strain was fully restored to the wild-type level. The BpΔBPSS1504 mutant showed impaired intracellular replication and formation of multinucleated giant cells in macrophages compared with wild-type bacteria, whereas the induction of actin tail formation within host cells was not affected. These observations resembled the phenotype of a mutant lacking hcp1, which is an integral component of the T6SS1 apparatus and is associated with full functionality of the T6SS1. Transcriptional expression of the T6SS components vgrG, tssA, and hcp1, as well as the T6SS regulators virAG, bprC, and bsaN, was not dependent on BPSS1504 expression. However, secretion of Hcp1 was not detectable in the absence of BPSS1504. Thus, BPSS1504 seems to serve as a T6SS component that affects Hcp1 secretion and is therefore involved in the integrity of the T6SS1 apparatus.

Intracellularly induced cyclophilins play an important role in stress adaptation and virulence of Brucella abortus

Brucella is an intracellular bacterial pathogen that causes the worldwide zoonotic disease brucellosis. Brucella virulence relies on its ability to transition to an intracellular lifestyle within host cells. Thus, this pathogen must sense its intracellular localization and then reprogram gene expression for survival within the host cell. A comparative proteomic investigation was performed to identify differentially expressed proteins potentially relevant for Brucella intracellular adaptation. Two proteins identified as cyclophilins (CypA and CypB) were overexpressed in the intracellular environment of the host cell in comparison to laboratory-grown Brucella. To define the potential role of cyclophilins in Brucella virulence, a double-deletion mutant was constructed and its resulting phenotype was characterized. The Brucella abortus ΔcypAB mutant displayed increased sensitivity to environmental stressors, such as oxidative stress, pH, and detergents. In addition, the B. abortus ΔcypAB mutant strain had a reduced growth rate at lower temperature, a phenotype associated with defective expression of cyclophilins in other microorganisms. The B. abortus ΔcypAB mutant also displays reduced virulence in BALB/c mice and defective intracellular survival in HeLa cells. These findings suggest that cyclophilins are important for Brucella virulence and survival in the host cells.

Trypanosoma brucei FKBP12 differentially controls motility and cytokinesis in procyclic and bloodstream forms

FKBP12 proteins are able to inhibit TOR kinases or calcineurin phosphatases upon binding of rapamycin or FK506 drugs, respectively. The Trypanosoma brucei FKBP12 homologue (TbFKBP12) was found to be a cytoskeleton-associated protein with specific localization in the flagellar pocket area of the bloodstream form. In the insect procyclic form, RNA interference-mediated knockdown of TbFKBP12 affected motility. In bloodstream cells, depletion of TbFKBP12 affected cytokinesis and cytoskeleton architecture. These last effects were associated with the presence of internal translucent cavities limited by an inside-out configuration of the normal cell surface, with a luminal variant surface glycoprotein coat lined up by microtubules. These cavities, which recreated the streamlined shape of the normal trypanosome cytoskeleton, might represent unsuccessful attempts for cell abscission. We propose that TbFKBP12 differentially affects stage-specific processes through association with the cytoskeleton.