Recombinant Brucella abortus Type IV secretion system protein virB10 (virB10)

What is the virB10 protein in Brucella abortus?

VirB10 is a critical structural component of the Type IV Secretion System (T4SS) in Brucella abortus, a Gram-negative bacterial pathogen that causes brucellosis. This 388-amino acid protein (aa 1-388) functions as an essential element of the secretion channel that allows Brucella to deliver effector molecules directly into host cells during infection . As part of the virB operon, which consists of 11 genes (virB1 to virB11) and two putative genes (orf12/virB12 and orf13), virB10 plays a fundamental role in bacterial virulence . The protein spans the bacterial cell envelope, creating a bridge between the inner and outer membranes that facilitates the translocation of effector proteins into host cells. Unlike some other components of the virB operon (such as virB12), virB10 is absolutely essential for intracellular replication and persistent infection .

How does the Type IV Secretion System contribute to Brucella virulence?

The Type IV Secretion System (T4SS) encoded by the virB operon represents a major virulence determinant in Brucella abortus. This molecular machinery enables several critical aspects of Brucella pathogenesis through the following mechanisms:

• Intracellular survival: The T4SS enables Brucella to establish a replicative niche within host macrophages by preventing the fusion of Brucella-containing vacuoles with lysosomes.

• Inflammasome modulation: B. abortus induces IL-1β secretion in hepatic stellate cells (HSCs) in a strictly T4SS-dependent manner, as demonstrated by experiments with ΔvirB10 isogenic mutants .

• Profibrotic response induction: The T4SS mediates inhibition of MMP-9 with concomitant collagen deposition and TGF-β1 secretion in HSCs, contributing to liver fibrosis during chronic infection .

• Immune evasion: T4SS-secreted effectors manipulate host cell signaling pathways to create conditions favorable for bacterial persistence.

• Persistent infection: Mutants deficient in T4SS components, including virB10, show severe attenuation in mouse models of infection, highlighting the system's essential role in establishing chronic brucellosis .

The virB10 component specifically contributes to the formation of the secretion channel structure, making it indispensable for T4SS function and Brucella virulence.

What expression systems are suitable for producing recombinant virB10 protein?

The production of recombinant virB10 protein from Brucella abortus can be achieved using several expression systems, each with distinct advantages depending on research objectives. The methodology for each system is outlined below:

The source material for virB10 expression can be obtained from various Brucella abortus strains, with strain 9-941 biovar 1 being commonly used . The choice of expression system should be guided by the intended application, with E. coli being suitable for structural studies and mammalian systems preferable when conformational authenticity is critical for functional analyses.

What strategies can be used to study virB10 function in Brucella pathogenesis?

Investigating virB10 function in Brucella pathogenesis requires a multi-faceted approach combining genetic, cellular, and in vivo methods:

• Genetic Manipulation Techniques:

  • Gene knockout: Creating virB10 deletion mutants through homologous recombination

  • CRISPR-Cas9 targeting: Precise editing of the virB10 gene to create specific mutations or suppression

  • Complementation studies: Reintroducing wild-type or mutated virB10 to confirm phenotype specificity

• Cellular Infection Models:

  • Macrophage survival assays: Comparing intracellular replication of wild-type and ΔvirB10 mutants

  • Inflammasome activation: Measuring IL-1β secretion in hepatic stellate cells infected with wild-type versus ΔvirB10 strains

  • Confocal microscopy: Tracking intracellular trafficking of bacteria using fluorescent markers

• Molecular Assays:

  • Protein-protein interaction studies: Identifying virB10 binding partners using co-immunoprecipitation

  • T4SS effector translocation: Using reporter fusions to track secretion of effector proteins

  • Structural analysis: Determining virB10 conformation in the context of the assembled T4SS

• In vivo Infection Models:

  • Mouse infection: Assessing bacterial loads in spleen and liver after infection with wild-type or mutant strains

  • Tissue pathology: Examining histological changes in infected tissues

  • Immune response characterization: Analyzing cytokine profiles and cellular immunity

Each of these approaches provides complementary information about virB10 function, with the combination yielding a comprehensive understanding of its role in Brucella pathogenesis.

How does virB10 interact with other components of the virB operon?

The virB10 protein functions within a complex network of protein-protein interactions involving other components of the virB operon. These interactions are critical for the assembly and function of the Type IV Secretion System:

• Structural Relationships:

  • VirB10 forms a core complex with VirB7 and VirB9 that spans the bacterial cell envelope

  • The C-terminal domain of VirB10 extends into the outer membrane, while its N-terminal domain anchors in the inner membrane

  • VirB10 undergoes conformational changes during substrate translocation, serving as an energy sensor

• Functional Interactions:

  • VirB10 cooperates with VirB4 and VirB11 (ATPases) to energize the secretion process

  • It forms a secretion channel with other VirB proteins to create a continuous conduit

  • Unlike VirB12, which has been shown to be dispensable for virulence, VirB10 is absolutely essential for T4SS function

• Expression Coordination:

  • The virB operon genes are co-regulated to ensure proper stoichiometry of all components

  • Mutations in upstream virB genes can affect the expression or stability of VirB10

  • Proper assembly of the T4SS requires coordinated expression of all virB genes

Understanding these complex interactions is essential for developing strategies to disrupt T4SS function as potential therapeutic approaches against brucellosis.

How can CRISPR-Cas9 be used to target virB10 for attenuating Brucella virulence?

CRISPR-Cas9 gene editing technology offers a precise approach for targeting the virB10 gene to attenuate Brucella virulence. The methodology involves several key steps:

• sgRNA Design and Vector Construction:

  • Design single guide RNAs (sgRNAs) targeting conserved regions of the virB10 gene

  • Construct a recombinant lentivirus containing the CRISPR-Cas9 system with the virB10-targeting sgRNAs

  • Include appropriate selection markers and promoters for expression in the target system

• Validation in Bacterial Culture:

  • Transform the CRISPR-Cas9 constructs into Brucella abortus

  • Select transformants using appropriate antibiotics

  • Confirm virB10 disruption using PCR, sequencing, and protein expression analysis

• Macrophage Infection Model:

  • Infect bovine macrophages with the B. abortus 2038 strain

  • Microinject the recombinant CRISPR-Cas9 vector into infected macrophages

  • Verify virB10 suppression using Real-Time PCR

• Assessment of Attenuation:

  • Measure bacterial survival and replication within macrophages

  • Evaluate IL-1β secretion and inflammasome activation

  • Compare virulence phenotypes between wild-type and virB10-edited strains

• In vivo Application:

  • Test virB10-targeted Brucella in mouse infection models

  • Assess bacterial loads in spleen and liver

  • Evaluate potential for developing attenuated vaccine strains

  • Consider microinjection of modified blastomeres to transfer passive immunity in livestock

This approach provides several advantages over traditional gene knockout methods, including precision, efficiency, and the potential for generating stable attenuated strains for vaccine development.

What is the role of virB10 in T4SS-dependent IL-1β secretion during Brucella infection?

The virB10 protein plays a crucial role in T4SS-dependent IL-1β secretion during Brucella infection, particularly in hepatic stellate cells (HSCs). This process involves a complex interaction between the bacterial T4SS and host inflammasome components:

• Mechanism of IL-1β Induction:

  • B. abortus infection induces IL-1β secretion in HSCs in a strictly T4SS-dependent manner

  • The ΔvirB10 isogenic mutant shows significantly reduced ability to induce IL-1β compared to wild-type B. abortus

  • At MOI 1000, wild-type B. abortus induces approximately 250-300 pg/ml of IL-1β, while the ΔvirB10 mutant induces only about 50 pg/ml

• Inflammasome Activation Pathway:

  • B. abortus infection increases expression of inflammasome components including caspase-1 (Casp-1), absent in melanoma 2 (AIM2), NLRP3, and ASC

  • This inflammasome activation is dependent on a functional T4SS, as demonstrated by experiments with the ΔvirB10 mutant

  • Inhibitors of inflammasome components (glyburide for NLRP3 and A151 for AIM2) significantly reduce IL-1β secretion during infection

• Connection to Fibrogenesis:

  • The virB10-dependent T4SS contributes to a profibrotic response in HSCs

  • This includes inhibition of MMP-9, collagen deposition, and TGF-β1 secretion

  • Inflammasome inhibitors reverse the fibrotic phenotype induced by B. abortus infection

• In vivo Relevance:

  • Studies with ASC, NLRP3, AIM2, and cCasp-1/11 knockout mice show reduced fibrotic patches in liver after B. abortus infection compared to wild-type mice

  • This confirms the role of inflammasome activation in T4SS-dependent liver fibrosis during brucellosis

This research highlights the critical role of virB10 as part of the T4SS in mediating host inflammasome activation and subsequent inflammatory and fibrotic responses during Brucella infection.

How do virB10 mutations affect intracellular survival of Brucella in different cell types?

Mutations in virB10 have profound and cell type-specific effects on Brucella intracellular survival:

In mouse models, virB10 mutants show dramatically reduced bacterial loads in spleen and other organs compared to wild-type strains. Unlike virB12, which has been shown to be dispensable for virulence, virB10 is absolutely essential for persistent infection in vivo .

The cell type-specific effects of virB10 mutation highlight the central role of the T4SS in Brucella's ability to adapt to different intracellular environments and manipulate various host cell types during infection.

What are the challenges in developing virB10-based vaccines against brucellosis?

Developing virB10-based vaccines against brucellosis presents several significant challenges that must be addressed through methodical research approaches:

• Antigen Design Challenges:

  • Maintaining proper protein conformation when expressed as a recombinant antigen

  • Identifying the most immunogenic epitopes within the 388 amino acid sequence

  • Determining optimal delivery format (protein subunit, DNA vaccine, live attenuated)

  • Addressing potential conformational changes when virB10 is expressed outside the T4SS complex

• Immunological Challenges:

  • Inducing both humoral and cell-mediated immunity, which are both essential for protection

  • Ensuring long-lasting immune memory against a pathogen known for persistence

  • Addressing strain variations in virB10 across different Brucella species

  • Preventing immune evasion mechanisms employed by the pathogen

• Technical Production Challenges:

  • Optimizing expression systems for consistent, high-yield production

  • Ensuring proper protein folding during recombinant production

  • Developing stable formulations that maintain immunogenicity

  • Creating effective adjuvant combinations for optimal immune stimulation

• Validation Challenges:

  • Demonstrating efficacy in appropriate animal models (mice, natural hosts)

  • Developing correlates of protection for brucellosis vaccines

  • Comparing efficacy with existing vaccine strains (S19, RB51)

  • Ensuring safety, particularly for zoonotic pathogens

• Regulatory and Implementation Challenges:

  • Meeting regulatory requirements for veterinary and potentially human vaccines

  • Developing DIVA (Differentiating Infected from Vaccinated Animals) capability

  • Addressing thermostability for field deployment in endemic regions

  • Cost-effective production for use in livestock vaccination programs

While virB10 represents a promising vaccine target due to its essential role in virulence, these challenges necessitate a comprehensive research approach combining structural biology, immunology, and vaccinology to develop an effective vaccine candidate.

What methodologies can be used to identify virB10 interaction with host proteins?

Investigating interactions between virB10 and host cell proteins requires specialized methodologies that can detect protein-protein interactions across bacterial and host cell boundaries:

• In vitro Approaches:

  • Affinity Purification with Mass Spectrometry:

    • Express recombinant virB10 with affinity tags (His, GST)

    • Immobilize on appropriate matrix and incubate with host cell lysates

    • Identify bound proteins using LC-MS/MS

    • Validate top candidates through reverse pull-downs

  • Surface Plasmon Resonance (SPR):

    • Immobilize purified virB10 on sensor chips

    • Flow candidate host proteins across the surface

    • Measure binding kinetics (kon, koff) and affinity (KD)

    • Quantify interaction strength under varying conditions

• Cellular Approaches:

  • Proximity Labeling Techniques:

    • Express virB10 fused to BioID, TurboID, or APEX2 enzymes

    • Allow proximal proteins to become biotinylated during infection

    • Purify biotinylated proteins and identify by mass spectrometry

    • Map the virB10 "interactome" within specific cellular compartments

  • Co-immunoprecipitation from Infected Cells:

    • Express epitope-tagged virB10 in Brucella

    • Infect host cells and perform crosslinking at various timepoints

    • Immunoprecipitate virB10 complexes

    • Identify co-precipitating host proteins

• Advanced Imaging Methods:

  • Förster Resonance Energy Transfer (FRET):

    • Tag virB10 and candidate host proteins with appropriate fluorophores

    • Measure energy transfer indicating close proximity (<10 nm)

    • Quantify interaction dynamics in living cells during infection

  • Super-resolution Microscopy:

    • Visualize co-localization of virB10 and host proteins

    • Track temporal changes in protein distribution

    • Correlate with stages of intracellular infection

• Functional Validation:

  • siRNA Knockdown of Candidate Interactors:

    • Silence expression of identified host proteins

    • Assess impact on Brucella intracellular survival

    • Determine functional significance of identified interactions

  • Domain Mapping through Mutagenesis:

    • Generate virB10 variants with specific domains altered

    • Identify regions necessary for host protein interactions

    • Correlate with virulence phenotypes

These complementary approaches provide a comprehensive strategy for identifying and characterizing virB10-host protein interactions, offering insights into the molecular mechanisms of Brucella pathogenesis.

What are the optimal conditions for expressing recombinant virB10 protein?

Optimal conditions for expressing recombinant virB10 protein vary depending on the expression system. Based on experimental data and standard practices in recombinant protein production, the following optimized protocols are recommended:

E. coli Expression System Protocol:

• Vector Selection and Cloning:

  • Vector: pET28a(+) with N-terminal 6xHis-tag

  • Insert: Full-length virB10 gene (coding for aa 1-388)

  • Restriction sites: NdeI and XhoI

  • Transform into cloning strain (DH5α) for verification

• Expression Conditions:

  • Host strain: BL21(DE3) or Rosetta(DE3) for rare codon optimization

  • Culture medium: LB or 2xYT with 50 μg/ml kanamycin

  • Growth temperature: 37°C until OD600 reaches 0.6-0.8

  • Induction: 0.5 mM IPTG

  • Post-induction: 18°C for 16-20 hours (critical for proper folding)

  • OD monitoring: Sample at 0, 2, 4, and 16 hours post-induction

• Cell Harvest and Lysis:

  • Centrifugation: 5,000 × g for 15 minutes at 4°C

  • Resuspension: 50 mM Tris-HCl pH 8.0, 300 mM NaCl, 10% glycerol, 0.1% Triton X-100

  • Lysis: Sonication (6 cycles of 30 sec ON, 30 sec OFF) at 40% amplitude

  • Clarification: 15,000 × g for 30 minutes at 4°C

• Purification Strategy:

  • IMAC: Ni-NTA resin with gradient elution (20-250 mM imidazole)

  • Buffer exchange: PD-10 columns or dialysis into storage buffer

  • Storage: 20 mM Tris-HCl pH 7.5, 150 mM NaCl, 10% glycerol at -80°C

Alternative Expression Systems:

For projects requiring properly folded protein with native-like conformation, insect cell expression systems offer advantages:

• Baculovirus Expression:

  • Construct: virB10 gene in pFastBac1 with honeybee melittin signal peptide

  • Cell line: Sf9 or High Five insect cells

  • Infection: MOI of 5, 27°C for 72 hours

  • Purification: Secreted protein from medium using affinity chromatography

Validation of proper expression should include Western blotting, mass spectrometry, and functional assays to confirm the integrity and activity of the recombinant virB10 protein.

How can virB10 knockout mutants be generated and validated?

Generating and validating virB10 knockout mutants involves a systematic approach combining molecular genetics and functional assays:

Generation Methods:

• Homologous Recombination:

  • Design a construct containing an antibiotic resistance cassette (kanamycin) flanked by 500-1000 bp regions homologous to sequences upstream and downstream of virB10

  • Transform into Brucella by electroporation (2.5 kV, 25 μF, 400 Ω)

  • Select on Brucella broth agar with kanamycin (50 μg/ml)

  • Screen colonies by PCR using primers that flank the targeted region

• CRISPR-Cas9 System:

  • Design sgRNAs targeting conserved regions of virB10 with minimal off-target effects

  • Construct a recombinant lentivirus containing the CRISPR-Cas9 system with virB10-targeting sgRNAs

  • Introduce the system into Brucella cells

  • Screen for mutations using T7 endonuclease assay or direct sequencing

Validation Methods:

• Genetic Validation:

  • PCR Analysis: Using primers that amplify across the targeted region

  • Sequencing: Confirming the exact genetic modification

  • Southern Blotting: Using probes specific to virB10 and the antibiotic resistance gene

• Transcriptional Validation:

  • RT-PCR: Confirming absence of virB10 mRNA using gene-specific primers

  • qRT-PCR: Quantitative assessment of knockout efficiency

  • RNA-Seq: Assessing broader transcriptional changes in the mutant

• Protein Validation:

  • Western Blotting: Using anti-virB10 antibodies to confirm absence of the protein

  • Mass Spectrometry: Proteomic analysis of wild-type versus mutant bacteria

• Functional Validation:

  • Macrophage Survival Assay: Infect J774 cells with wild-type and mutant strains at MOI 100; harvest at 0, 24, and 48 hours; plate lysates for CFU counting

  • IL-1β Secretion Assay: Measure IL-1β in supernatants of infected hepatic stellate cells by ELISA

  • Mouse Infection: Inoculate mice with 105 CFU intraperitoneally; harvest spleens at 2 weeks; compare bacterial loads

• Complementation Studies:

  • Reintroduce wild-type virB10 gene on a plasmid

  • Confirm restoration of wild-type phenotypes

  • Include appropriate controls (empty vector)

This comprehensive validation approach ensures that observed phenotypes are specifically due to virB10 disruption rather than polar effects or secondary mutations.

What assays are most effective for measuring virB10-dependent virulence traits?

Several specialized assays can effectively measure virB10-dependent virulence traits in Brucella abortus:

1. Intracellular Survival and Replication Assay:

Procedure:

• Culture J774.A1 macrophages in DMEM + 10% FBS to 80% confluence in 24-well plates

• Infect with wild-type and ΔvirB10 Brucella at MOI 100

• Centrifuge at 400 × g for 10 min to synchronize infection

• Incubate for 1 hour, then add gentamicin (50 μg/ml) for 30 min to kill extracellular bacteria

• Maintain cells in medium with 20 μg/ml gentamicin

• At timepoints (0, 24, 48 hrs), lyse cells with 0.1% Triton X-100

• Plate serial dilutions on Brucella agar

• Calculate replication index: CFU(48h)/CFU(0h)

2. T4SS-Dependent IL-1β Secretion Assay:

Procedure:

• Culture hepatic stellate cells (LX-2 cell line) in appropriate medium

• Infect with wild-type B. abortus and ΔvirB10 mutant at MOI 100-1000

• Collect supernatants at 24 hours post-infection

• Measure IL-1β concentration by ELISA

• Compare levels between wild-type (typically 250-300 pg/ml) and ΔvirB10 mutant (approximately 50 pg/ml)

• Include controls with inflammasome inhibitors (glyburide, A151)

3. Phagolysosome Fusion Assay:

Procedure:

• Label lysosomes with LysoTracker Red

• Infect macrophages with GFP-expressing Brucella (wild-type and ΔvirB10)

• Fix cells at different timepoints (1, 6, 24 hrs)

• Analyze by confocal microscopy

• Calculate percentage of bacteria co-localizing with lysosomal marker

• Wild-type Brucella typically shows reduced co-localization compared to ΔvirB10 mutants

4. Fibrosis Induction Assay:

Procedure:

• Infect hepatic stellate cells with wild-type and ΔvirB10 Brucella

• At 72 hours post-infection, measure:

  • Collagen deposition (Sirius Red staining)

  • MMP-9 activity (zymography)

  • TGF-β1 secretion (ELISA)

• Compare profibrotic responses between strains

5. In Vivo Virulence Assessment:

Procedure:

• Infect mice (BALB/c, 6-8 weeks) intraperitoneally with 105 CFU

• Harvest spleens and livers at 1, 2, and 4 weeks post-infection

• Homogenize tissues and plate serial dilutions

• Compare bacterial loads between wild-type and ΔvirB10 strains

• Process tissue sections for histopathology to assess inflammation and fibrosis

These assays provide complementary data on the various aspects of virB10-dependent virulence, from cellular invasion to persistent infection and host response modulation.

What in vivo models are most appropriate for studying virB10 function?

Several in vivo models offer distinct advantages for studying virB10 function in Brucella pathogenesis:

1. Mouse Infection Models:

Standard Mouse Model:

• Animals: BALB/c or C57BL/6 mice (6-8 weeks old)

• Infection route: Intraperitoneal injection with 105 CFU

• Timepoints: Acute (1-2 weeks) and chronic (4-12 weeks)

• Assessments: Bacterial burden in spleen and liver, splenomegaly, histopathology

• Applications: Compare colonization and persistence between wild-type and ΔvirB10 mutants

Inflammasome Component Knockout Mice:

• Strains: ASC-/-, NLRP3-/-, AIM2-/-, Casp-1/11-/- mice

• Protocol: Same as standard model

• Specific readout: Liver fibrosis assessment

• Key finding: Reduced fibrotic patches in knockout mice infected with wild-type Brucella compared to wild-type mice

• Application: Dissect the interaction between T4SS and specific host immune pathways

Comparative Assessment of Mouse Models:

2. Large Animal Models:

Cattle Model:

• Animals: Holstein-Friesian cattle (preferred age: 6-12 months)

• Infection: Conjunctival inoculation (107-108 CFU)

• Duration: 3-6 months

• Assessments: Bacteremia, seroconversion, lymph node colonization

• Applications: Study virB10 role in natural host, vaccine testing

Goat Model:

• More manageable alternative to cattle

• Similar protocol and assessments

• Particularly relevant for B. melitensis research

• Useful for reproductive pathology studies

3. Ex Vivo and Specialized Models:

Placental Explant Culture:

• Bovine placental tissue maintained in culture

• Direct infection with wild-type and ΔvirB10 mutants

• Assessment of bacterial replication and tissue damage

• Application: Study reproductive pathology without whole animal use

Humanized Mouse Models:

• NOD-SCID-γc-/- mice reconstituted with human immune cells

• Allows study of human-specific immune interactions

• Useful for translational research relevant to human brucellosis

The choice of model should be guided by the specific research question, ethical considerations, available facilities, and relevance to the natural disease process. For initial mechanistic studies of virB10 function, the mouse model provides the most practical and well-characterized system, while validation in natural hosts is essential for translational applications.

How do experimental design principles apply to virB10 research?

Effective investigation of virB10 function requires careful application of experimental design principles to ensure valid and reproducible results:

1. Experimental Design Framework:

A well-designed virB10 study should follow these methodological principles:

• Clear hypothesis formulation: Define specific questions about virB10 function

• Control implementation: Include appropriate positive controls (wild-type Brucella), negative controls (ΔvirB10), and relevant reference strains

• Variable manipulation: Isolate the effect of virB10 by controlling other variables

• Randomization: Assign subjects randomly to experimental groups

• Blinding: Perform analyses without knowledge of sample identity

• Replication: Include biological replicates (minimum n=3) and technical replicates

Example Experimental Design: virB10 and Inflammasome Activation Study

Research Question: Does virB10-dependent T4SS function directly activate the NLRP3 inflammasome?

Experimental Groups:

• Wild-type B. abortus infection (MOI 100, 500, 1000)

• ΔvirB10 mutant infection (MOI 100, 500, 1000)

• Complemented ΔvirB10 strain

• Positive control (LPS + ATP for inflammasome activation)

• Negative control (uninfected cells)

Treatment Variables:

• Addition of inflammasome inhibitors (glyburide, YVAD, A151)

• Time points (6, 12, 24 hours post-infection)

• Cell types (macrophages, hepatic stellate cells)

Measured Outcomes:

• IL-1β secretion (ELISA)

• Caspase-1 activation (Western blot for cleaved caspase-1)

• ASC speck formation (immunofluorescence)

• NLRP3 and AIM2 expression (qRT-PCR)

Statistical Analysis:

• ANOVA with appropriate post-hoc tests

• Sample size determination based on power analysis

• Data normalization strategy defined a priori

3. Avoiding Common Pitfalls in virB10 Research:

4. Data Analysis and Interpretation:

• Use appropriate statistical tests based on data distribution

• Control for multiple comparisons

• Report effect sizes alongside p-values

• Interpret results in context of existing literature

• Distinguish between correlation and causation

• Consider alternative explanations for observed phenotypes

By applying these experimental design principles, researchers can generate robust and reliable data on virB10 function, contributing to our understanding of Brucella pathogenesis and potentially identifying new therapeutic targets.