Recombinant Shigella dysenteriae serotype 1 Fumarate reductase subunit C (frdC)

What is the role of fumarate reductase subunit C (frdC) in Shigella dysenteriae serotype 1 metabolism?

Fumarate reductase subunit C (frdC) is a membrane protein component of the fumarate reductase complex that plays a critical role in the anaerobic respiration of Shigella dysenteriae serotype 1 (SD1). This enzyme catalyzes the reduction of fumarate to succinate, serving as a terminal electron acceptor during anaerobic conditions. In SD1, which causes the most severe form of epidemic bacillary dysentery, frdC is particularly important for bacterial survival in the low-oxygen environment of the large intestine.

Proteomic and transcriptomic analyses have shown that SD1 switches to anaerobic energy metabolism in vivo . During this metabolic shift, the fumarate reductase complex becomes crucial for:

• Energy generation via fumarate respiration

• Production of succinate and other essential metabolic intermediates via the reductive branch of the citric acid cycle

• Adaptation to microaerophilic conditions within the host intestinal environment

Studies comparing in vitro and in vivo bacterial growth conditions demonstrate differential expression of anaerobic respiration genes, including the fumarate reductase system .

How do researchers express and purify recombinant Shigella dysenteriae serotype 1 frdC protein?

The expression and purification of recombinant S. dysenteriae serotype 1 frdC typically follows these methodological steps:

• Gene cloning: The frdC gene is amplified from S. dysenteriae serotype 1 (strain Sd197) genomic DNA using PCR with specific primers designed to include appropriate restriction sites.

• Expression vector construction: The amplified gene is cloned into expression vectors such as pET-series vectors or similar systems compatible with E. coli expression.

• Host selection: Common expression hosts include:

  • E. coli BL21(DE3) or its derivatives

  • Yeast expression systems

  • Baculovirus-infected insect cells

  • Mammalian cell expression systems

• Protein expression optimization: Researchers optimize expression conditions including:

  • Induction temperature (typically 16-37°C)

  • IPTG concentration (0.1-1.0 mM)

  • Duration of induction (4-16 hours)

  • Media composition (LB, Terrific Broth, or defined media)

• Purification strategy: As a membrane protein, frdC requires specialized purification procedures:

  • Membrane fraction isolation using ultracentrifugation

  • Solubilization with detergents (e.g., n-dodecyl-β-D-maltoside)

  • Affinity chromatography (His-tag, GST-tag)

  • Size exclusion chromatography

• Protein characterization:

  • SDS-PAGE analysis

  • Western blotting

  • Mass spectrometry

  • Functional assays to confirm enzymatic activity

For membrane proteins like frdC, maintaining native conformation and functionality remains a significant challenge that often requires detergent screening and optimization of solubilization conditions.

What experimental models are used to study the function of Shigella dysenteriae serotype 1 frdC in pathogenesis?

Researchers employ several experimental models to study S. dysenteriae serotype 1 frdC function in pathogenesis:

In vitro models:

• Bacterial anaerobic culture systems: Growth in anaerobic chambers with specific media to simulate intestinal conditions

• Cell invasion assays: Using human colonic epithelial cell lines (HT-29, Caco-2) to evaluate invasion efficiency of wild-type vs. frdC mutants

• Tissue culture infection models: Polarized intestinal epithelial cells grown on transwell filters

Animal models:

• Guinea pig keratoconjunctivitis (Sereny test): Used to evaluate the invasiveness and virulence of S. dysenteriae strains and mutants

• Gnotobiotic piglet model: Allows study of bacterial isolates from large bowel infections, used to compare in vitro and in vivo protein expression

• Mouse pulmonary infection model: Used for assessing virulence and immune responses

• Monkey models: For vaccine strain testing prior to human trials

Human challenge models:

• Controlled human infection models (CHIMs): Under strictly controlled conditions, volunteers receive defined doses of attenuated Shigella strains to evaluate colonization, immune response, and symptoms

Experimental data has shown that fumarate reductase deletion mutants in related bacterial species demonstrate significantly attenuated virulence, suggesting its importance in pathogenesis . Similar studies with S. dysenteriae serotype 1 frdC mutants would likely show comparable effects on bacterial survival and virulence in the intestinal environment.

How does the expression of frdC change during Shigella dysenteriae serotype 1 infection?

The expression of fumarate reductase, including the frdC subunit, undergoes significant regulation during S. dysenteriae serotype 1 infection cycles:

Expression dynamics during infection:

• Initial colonization phase: Relatively low expression in aerobic environments

• Intestinal adaptation phase: Upregulation as bacteria encounter decreasing oxygen levels

• Intracellular phase: Differential regulation depending on subcellular location

Evidence from transcriptomic and proteomic studies:

Comparative analysis of in vitro versus in vivo bacterial isolates demonstrates that SD1 switches to anaerobic energy metabolism in the host intestinal environment. This metabolic shift includes significant changes in fumarate reductase complex expression .

Key findings from time-course gene expression studies show:

The transcriptional response to acidic environments (similar to those in the stomach before intestinal colonization) involves upregulation of anaerobic respiration genes (GO:0009061) and simultaneous downregulation of aerobic respiration genes (GO:0009060) . This metabolic adaptation includes the fumarate reductase complex (frdABCD), with frdC expression changes corresponding to this pattern.

What is known about the structure-function relationship of Shigella dysenteriae serotype 1 frdC?

The structure-function relationship of S. dysenteriae serotype 1 frdC reveals important insights into its role in bacterial metabolism and pathogenesis:

Structural characteristics:

• frdC is a membrane-anchoring subunit of the fumarate reductase complex

• Contains multiple transmembrane domains that embed the complex in the cytoplasmic membrane

• Forms part of the menaquinol binding site

• Provides the structural framework for optimal positioning of the catalytic subunits

Functional domains:

• Transmembrane helices that anchor the complex to the membrane

• Quinone-binding region that facilitates electron transfer

• Interaction sites with frdA (flavoprotein) and frdB (iron-sulfur protein) subunits

• Proton transfer pathway components

How is frdC expression regulated in Shigella dysenteriae serotype 1 under different environmental conditions?

The regulation of frdC expression in S. dysenteriae serotype 1 is complex and responds to multiple environmental signals:

Oxygen-dependent regulation:

The primary regulator of fumarate reductase expression is the FNR (fumarate and nitrate reduction) protein, which senses oxygen levels and activates transcription of the frd operon under anaerobic conditions. Interestingly, in SD1, the fnr gene is located within a 20-kb chromosomal region that also contains the Shiga toxin genes (stx) .

Acid-induced response:

When exposed to acidic environments (pH 5.5), transcriptomic analysis reveals significant modulation of metabolic pathways including:

• Downregulation of genes coding for the FoF1 ATPase complex

• Upregulation of genes involved in anaerobic respiration

• Coordinated changes in expression patterns with maximal changes occurring approximately 25 minutes post-acid exposure

Additional regulatory mechanisms:

• ArcAB two-component system: Influences metabolic adaptation to anaerobic conditions by negatively regulating genes that consume intermediates during fumarate synthesis while upregulating components of the respiratory chain serving as direct reduction equivalents for fumarate reductase

• Iron availability: Iron-responsive regulators modulate fumarate reductase expression, with an interconnection between iron metabolism and anaerobic respiration

• Small RNA regulation: RyhB, an iron-responsive small RNA molecule, has been shown to regulate gene expression in Shigella dysenteriae

The loss of the fnr gene, as seen in vaccine strains like WRSd1, affects the regulation of nitrate reductase expression and other anaerobic metabolism genes, potentially impacting bacterial colonization capacity .

What are the challenges in producing stable recombinant Shigella dysenteriae serotype 1 frdC for research applications?

Producing stable recombinant S. dysenteriae serotype 1 frdC presents several technical challenges:

Membrane protein expression hurdles:

• Toxicity to expression hosts: Overexpression of membrane proteins can disrupt host cell membrane integrity

• Protein misfolding: Achieving proper folding and insertion into membranes is difficult in heterologous expression systems

• Inclusion body formation: Tendency to aggregate in insoluble form, requiring refolding strategies

• Low yield: Typical membrane protein expression yields remain significantly lower than soluble proteins

Stability challenges:

• Detergent selection: Finding detergents that maintain protein stability while extracting from membranes

• Conformational stability: Preserving native conformation during purification steps

• Functional integrity: Maintaining enzymatic activity throughout the purification process

• Storage conditions: Determining optimal conditions to prevent aggregation or degradation

Methodological approaches to overcome these challenges:

• Expression system optimization:

  • Use of specialized E. coli strains (C41(DE3), C43(DE3)) designed for membrane protein expression

  • Exploration of alternative expression hosts (yeast, insect cells)

  • Controlled expression using tunable promoters

• Fusion tags and constructs:

  • Testing multiple fusion tag combinations (His, MBP, SUMO)

  • Design of truncated constructs focusing on stable domains

  • Co-expression with chaperones to improve folding

• Detergent screening:

  • Systematic testing of various detergent types and concentrations

  • Use of detergent stability assays to identify optimal conditions

  • Application of amphipols or nanodiscs for increased stability

Similar strategies have been successfully applied to other recombinant Shigella proteins, as demonstrated in studies on recombinant invasion plasmid antigen C (IpaC) , which achieved protein stability suitable for vaccine development.

How does the sequence and function of frdC differ between Shigella dysenteriae serotype 1 and other Shigella species?

Comparative analysis of frdC between Shigella dysenteriae serotype 1 and other Shigella species reveals important evolutionary and functional insights:

Sequence conservation:

Genomic studies demonstrate high conservation of the fumarate reductase operon (frdABCD) across Shigella species. This conservation reflects the fundamental importance of anaerobic respiration for these enteric pathogens.

Comparative genomic analysis of complete Shigella genomes has revealed:

• A high degree of uniformity among SD1 genomes

• Conservation of metabolic pathways including anaerobic respiration

• Sequence similarity reflecting the close evolutionary relationship between Shigella species and E. coli

Functional differences:

Despite sequence conservation, functional differences may exist due to:

• Regulatory variations: Different regulatory elements controlling expression

• Metabolic adaptations: Species-specific adaptations to preferred host niches

• Interaction with other pathways: Differential integration with virulence mechanisms

Species-specific contexts:

The four major Shigella species have distinct epidemiological niches and geographical distributions, which may influence the functional importance of anaerobic metabolism:

S. dysenteriae serotype 1, which causes the most severe form of dysentery, may rely more heavily on anaerobic metabolic adaptations, including the fumarate reductase system, for survival in its specific host environment .

What is the significance of frdC in the development of live attenuated Shigella dysenteriae serotype 1 vaccines?

The fumarate reductase system, including the frdC component, has significant implications for live attenuated vaccine development:

Metabolic attenuation strategies:

The deletion or modification of genes involved in anaerobic metabolism represents a potential attenuation strategy for vaccine development. The WRSd1 vaccine strain, derived from S. dysenteriae 1 strain 1617, contains a 20-kb chromosomal deletion that includes not only the Shiga toxin genes but also the fnr gene, which regulates fumarate reductase expression .

Implications for vaccine performance:

• Colonization capacity: The loss of fumarate reductase regulation might impact the ability of vaccine strains to colonize and persist in the intestinal environment. This has been observed with the WRSd1 strain, which showed lower efficacy compared to other Shigella vaccine candidates (WRSS1 and SC602) when administered at the same dose .

• Immune response generation: Lower colonization may lead to reduced antigen presentation and immune stimulation, potentially requiring higher vaccine doses. Research has shown that "the loss of the fnr and associated genes in WRSd1 may be one explanation for the lower efficacy of WRSd1 in guinea pigs compared to WRSS1 and SC602" .

• Vaccine dosing implications: "These results indicate that in a mixture of the three vaccines, WRSd1 may have to be given at a higher dose than SC602 and WRSS1" .

Balance between attenuation and immunogenicity:

The ideal live attenuated vaccine must balance safety (sufficient attenuation) with effective immunogenicity (adequate colonization). Understanding the role of metabolic genes like frdC in pathogenesis helps in rational vaccine design that maintains this balance.

The development of WRSd1 demonstrates that while removing virulence factors like Shiga toxin is essential for safety, the coincidental loss of metabolic regulators like fnr has consequences for vaccine performance that must be addressed through dosing adjustments or further strain engineering.

How can researchers use recombinant frdC protein to study host immune responses to Shigella dysenteriae serotype 1?

Recombinant frdC protein offers valuable research applications for studying host immune responses to S. dysenteriae serotype 1:

Immunological research applications:

• Antibody response characterization:

  • ELISA-based detection of anti-frdC antibodies in patient sera

  • Mapping of B-cell epitopes using synthetic peptide arrays

  • Analysis of antibody subclass distribution (IgG, IgA, IgM)

  • Temporal monitoring of antibody responses in infection and convalescence

• T-cell response studies:

  • In vitro stimulation of peripheral blood mononuclear cells (PBMCs) with recombinant frdC

  • Characterization of CD4+ and CD8+ T-cell responses

  • Cytokine profiling (IFN-γ, IL-17, etc.) by ELISPOT or flow cytometry

  • Identification of T-cell epitopes through peptide libraries

• Vaccine immunogenicity assessment:

  • Evaluation of vaccine-induced immune responses to metabolic antigens

  • Comparative analysis of natural infection versus vaccination responses

  • Correlation of anti-frdC responses with protection in challenge models

Methodological approaches:

Experimental systems using recombinant frdC can be designed following approaches similar to those used for other Shigella antigens. For example, studies with recombinant invasion plasmid antigen C (IpaC) have demonstrated that:

• Single intranasal doses of nanoparticle-encapsulated protein can elicit robust immune responses

• These responses include temporal increases in antibody production and improved cytokine profiles

• Challenge experiments with heterologous Shigella strains can assess cross-protection

Similar methodologies could be applied to study the immunological significance of metabolic antigens like frdC, potentially revealing new insights into host recognition of bacterial metabolic machinery during infection.

What role does frdC play in the acid stress response of Shigella dysenteriae serotype 1 during infection?

The fumarate reductase system, including frdC, contributes significantly to S. dysenteriae serotype 1's response to acid stress during infection:

Acid stress encounter during infection:

S. dysenteriae encounters various acidic environments during its infection cycle:

• Gastric acid (pH ~2) during initial ingestion

• Phagosomal acidification (pH ~4.5-5.5) during macrophage interaction

• Volatile fatty acids in the intestinal lumen (pH ~5.5-6.5)

• Acidic microenvironments at inflammation sites

Transcriptional adaptation to acid stress:

Gene expression analysis reveals that exposure to acid stress triggers a coordinated metabolic response, with significant changes occurring in anaerobic metabolism pathways:

• Early response (5 min post-acid exposure): Initial adjustment to acid stress includes downregulation of aerobic respiration genes and preliminary upregulation of anaerobic pathways

• Later response (25 min post-acid exposure): More pronounced metabolic shift with maximal expression changes in genes involved in anaerobic respiration, including fumarate reductase components

This metabolic remodeling helps the bacterium maintain pH homeostasis and energy production under acidic conditions. Specifically, the upregulation of fumarate reductase activity may contribute to:

• Proton consumption: The reduction of fumarate to succinate consumes protons, potentially helping to buffer against acidity

• Alternative energy generation: Shifting to anaerobic respiration with fumarate as terminal electron acceptor provides energy when aerobic respiration is compromised

• Metabolic intermediate production: Generation of succinate and other metabolites needed for adaptation

Integrated acid stress response:

The acid stress response in S. dysenteriae serotype 1 involves multiple mechanisms working in concert:

These integrated responses allow S. dysenteriae to survive the various acid challenges encountered during infection, with the fumarate reductase system playing a supportive role in this adaptation .

How does the fumarate reductase complex (including frdC) contribute to antibiotic resistance in Shigella dysenteriae serotype 1?

The fumarate reductase complex, including the frdC subunit, may contribute to antibiotic resistance in S. dysenteriae serotype 1 through several mechanisms:

Metabolic adaptation and persistence:

• Anaerobic survival: By enabling energy generation under anaerobic conditions, fumarate reductase helps bacteria persist in microaerophilic niches where antibiotics may penetrate poorly

• Metabolic state influence: Bacteria in alternative metabolic states (e.g., anaerobic metabolism) often exhibit altered susceptibility to antibiotics that target active growth processes

• Persister cell formation: Metabolic adaptations facilitated by anaerobic respiratory enzymes can contribute to persister cell formation, a phenotypic state highly tolerant to antibiotics

Emerging research on drug-resistant strains:

Studies in Kolkata, India documented the emergence of multidrug-resistant S. dysenteriae serotype 1 strains showing resistance to multiple antibiotics:

These strains exhibited alarming fluoroquinolone resistance patterns , though the direct relationship between fumarate reductase and these resistance phenotypes requires further investigation.

Mechanistic hypotheses:

Several mechanisms might link fumarate reductase function to antibiotic resistance:

• Membrane permeability effects: As a membrane protein, frdC may influence membrane composition or organization, potentially affecting the entry of certain antibiotics

• Redox balance maintenance: The electron transport capabilities of the fumarate reductase complex could help mitigate oxidative stress induced by some antibiotics

• Metabolic bypass pathways: Bacteria may utilize alternative metabolic pathways involving fumarate reductase to circumvent antibiotic-targeted metabolic steps

Understanding these relationships could inform new therapeutic strategies to combat increasingly resistant S. dysenteriae strains, which pose significant clinical challenges worldwide.

What structural and functional similarities exist between Shigella dysenteriae serotype 1 frdC and homologous proteins in other enteric pathogens?

Structural and functional similarities between S. dysenteriae serotype 1 frdC and homologous proteins in other enteric pathogens reveal evolutionary conservation of this important metabolic component:

Structural conservation:

The fumarate reductase complex, including the membrane anchor subunit frdC, shows significant structural conservation across enteric bacteria:

• Transmembrane domain organization: Similar arrangement of membrane-spanning helices

• Quinone binding sites: Conservation of residues involved in menaquinol interaction

• Subunit interaction interfaces: Preserved contact regions between frdC and the catalytic subunits (frdA and frdB)

Functional homology:

Comparative studies indicate similar functional roles for frdC across different bacterial species:

Evolutionary insights:

The high degree of conservation suggests that:

• Core metabolic function: Fumarate reductase represents a fundamental component of bacterial energy metabolism that has been maintained through evolution

• Specialization within constraints: Despite different host niches and pathogenesis strategies, the basic structure and function remain conserved

• Horizontal gene transfer potential: The similarity across species could reflect historical gene transfer events among enteric bacteria

This conservation makes frdC an interesting target for comparative studies and potentially for broad-spectrum therapeutic approaches targeting multiple enteric pathogens simultaneously.

How can researchers use genetic modification of frdC to study Shigella dysenteriae serotype 1 pathogenesis?

Genetic modification of frdC provides valuable approaches to investigate S. dysenteriae serotype 1 pathogenesis:

Genetic modification strategies:

• Gene deletion/knockout:

  • Complete deletion of the frdC gene using homologous recombination

  • CRISPR-Cas9 mediated gene disruption

  • Transposon mutagenesis screening to identify essential functions

• Site-directed mutagenesis:

  • Modification of specific residues involved in menaquinone binding

  • Alteration of transmembrane domains to affect membrane anchoring

  • Mutation of residues at subunit interfaces (frdA-frdC interaction)

• Reporter gene fusions:

  • Transcriptional fusions (promoter-reporter) to study expression regulation

  • Translational fusions to track protein localization and abundance

  • Conditional expression systems to control frdC levels

Phenotypic analysis approaches:

Modified strains can be characterized using various experimental systems:

• In vitro phenotyping:

  • Growth curve analysis under aerobic versus anaerobic conditions

  • Fumarate reductase activity assays using biochemical methods

  • Membrane potential measurements to assess energy conservation

• Cell culture infection models:

  • Invasion efficiency in epithelial cell lines

  • Intracellular survival and replication rates

  • Cell-to-cell spread capabilities

• Animal model testing:

  • Colonization ability in appropriate animal models

  • Competitive index assays comparing wild-type and mutant strains

  • Virulence assessment using pathology scoring and survival analysis

Methodological considerations:

When constructing frdC mutants, researchers should consider:

• Polar effects: Ensuring mutations don't disrupt downstream genes in the frd operon

• Complementation studies: Restoring the wild-type phenotype by providing frdC in trans

• Marker selection: Using appropriate antibiotic resistance markers for selection

Similar approaches have been successfully applied in related bacteria, as demonstrated in a study where "an isogenic A. pleuropneumoniae fumarate reductase deletion mutant was constructed and studied by using a pig aerosol infection model. The mutant was shown to be significantly attenuated, thereby strongly supporting a crucial role for fumarate reductase in the pathogenesis" . Analogous methods would be valuable for dissecting the role of fumarate reductase in S. dysenteriae virulence.

What techniques can researchers use to assess the enzymatic activity of recombinant Shigella dysenteriae serotype 1 frdC in experimental settings?

Assessing the enzymatic activity of recombinant S. dysenteriae serotype 1 frdC requires specialized techniques due to its membrane protein nature and its function as part of the fumarate reductase complex:

Activity assay methodologies:

• Reconstitution in liposomes or nanodiscs:

  • Incorporation of purified recombinant frdC along with other fumarate reductase subunits into artificial membrane systems

  • Creation of proteoliposomes with defined lipid composition

  • Reconstitution in nanodiscs for a more native-like membrane environment

• Electron transport measurements:

  • Artificial electron donor oxidation assays (e.g., using reduced benzyl viologen)

  • Quinone reduction monitoring by spectrophotometric methods

  • Oxygen consumption measurements in inverted membrane vesicles

• Substrate conversion assays:

  • Monitoring fumarate to succinate conversion using HPLC

  • Coupled enzyme assays tracking NAD(P)H oxidation

  • Isotope-labeled substrate tracking to measure conversion rates

Advanced biophysical techniques:

• Membrane potential measurements:

  • Fluorescent dye-based assays (e.g., using DiSC3(5))

  • Patch-clamp techniques on proteoliposomes

  • Ion-selective electrode measurements

• Structural analysis coupled to function:

  • Hydrogen-deuterium exchange mass spectrometry to probe conformational changes during catalysis

  • Site-directed spin labeling with electron paramagnetic resonance (EPR) spectroscopy

  • Solid-state NMR techniques for membrane protein structural analysis

• Single-molecule approaches:

  • Fluorescence resonance energy transfer (FRET) to monitor conformational changes

  • Atomic force microscopy to assess protein-protein interactions

  • Total internal reflection fluorescence (TIRF) microscopy to study dynamics

Experimental controls and considerations:

When designing enzymatic assays for membrane proteins like frdC, researchers should include:

• Appropriate controls:

  • Empty liposomes/nanodiscs without protein

  • Heat-inactivated enzyme preparations

  • Site-directed mutants with predicted loss of function

• Optimization parameters:

  • Detergent type and concentration for solubilization

  • Lipid composition in reconstitution systems

  • pH and buffer conditions that mimic physiological environment

  • Temperature and ionic strength optimization