Recombinant Shigella dysenteriae serotype 1 Lipid A export ATP-binding/permease protein MsbA (msbA)

What is MsbA and what is its fundamental function in Shigella dysenteriae?

MsbA is a multidrug resistance transporter homolog that belongs to a superfamily of transporters containing an adenosine triphosphate (ATP) binding cassette (ABC), also called a nucleotide-binding domain (NBD). In Shigella dysenteriae and other Gram-negative bacteria, MsbA serves the essential function of transporting lipid A, a major component of the bacterial outer cell membrane, from the cytoplasmic leaflet to the periplasmic leaflet of the inner membrane .

This function is critical as MsbA is the only bacterial ABC transporter that is essential for cell viability. Without proper lipid A transport, the outer membrane cannot be properly assembled, leading to bacterial death . Additionally, MsbA has been shown to transport lipopolysaccharide (LPS), further contributing to outer membrane biogenesis .

How is MsbA structurally organized and what conformational states does it adopt?

MsbA consists of two main structural components:

• Transmembrane domains (TMDs) that form the substrate-binding pocket and transport pathway

• Nucleotide-binding domains (NBDs) that bind and hydrolyze ATP to drive conformational changes

The protein undergoes significant conformational changes during its transport cycle, alternating between:

• Inward-facing conformation: Where the TMDs form a cavity open to the cytoplasm, allowing lipid A to enter the transporter

• Outward-facing conformation: Where the TMDs are reoriented to open toward the periplasmic space, facilitating the release of lipid A

These conformational states have been captured through X-ray crystallography and cryo-EM studies. The inward-facing conformation displays a large amplitude opening in the transmembrane portal, which is necessary for the bulky lipid A molecule to enter the transport pathway . When ATP binds, it causes a packing rearrangement of the transmembrane helices and changes the accessibility from cytoplasmic (inward) facing to extracellular (outward) facing .

What laboratory techniques are used to assess MsbA function and activity?

To study MsbA function in the laboratory setting, researchers employ several techniques:

• ATPase activity assays: MsbA exhibits significant ATPase activity that can be measured to assess protein functionality. For example, purified MsbA from E. coli, V. cholerae, and S. typhimurium showed ATPase activities with Vmax values of 1.7, 5.3, and 2.0 μmol/min per milligram, respectively . The general protocol includes:

  • Purification of MsbA to homogeneity

  • Measuring ATP hydrolysis rates under various conditions

  • Determining kinetic parameters (Vmax, Km)

  • Assessing inhibition by compounds like AMPPNP (competitive inhibitor) and vanadate

• Protein purification and reconstitution: MsbA can be expressed in E. coli, purified in detergents like dodecyl maltoside (DDM), and reconstituted in nanodiscs for functional studies .

• X-ray crystallography and cryo-EM: These structural techniques have been crucial for understanding the conformational changes of MsbA during the transport cycle .

Why is MsbA considered a potential antimicrobial target in Shigella dysenteriae?

MsbA represents an attractive antimicrobial target for several reasons:

• Essentiality: MsbA is essential for bacterial viability, making it a critical target for potential antibiotics .

• Growing antimicrobial resistance: S. dysenteriae has acquired resistance to many conventional antibiotics, including ampicillin, trimethoprim, ciprofloxacin, and azithromycin . The CDC considers antibiotic-resistant Shigella a serious threat, with approximately 77,000 resistant infections reported annually in the United States .

• Specificity to Gram-negative bacteria: Since MsbA is specific to Gram-negative bacteria and has structural differences from human ABC transporters, compounds targeting it might have fewer side effects .

• Multidrug resistance role: MsbA is a polyspecific transporter capable of recognizing and transporting various drug molecules, potentially contributing to antibiotic resistance .

How does the differential gene expression of MsbA vary between invasive and non-invasive S. dysenteriae strains?

Transcriptomic analysis of S. dysenteriae isolated from different infection sites has revealed important insights into differential gene expression patterns, including those related to MsbA and other transport systems. A study comparing S. dysenteriae serotype 9 obtained from stool (non-invasive, Sd_FC3355) and blood (invasive, Sd_BA42767) specimens of the same patient showed significant differences in gene expression profiles .

Key findings include:

• 56 genes were differentially expressed between the invasive and non-invasive strains

• The majority of genes (44/56) were highly expressed in the non-invasive isolate from the gut, which is the primary site of invasion for Shigella infection

• Several genes, including cold shock protein (csp), DNA cytosine methyltransferase (dcm), histidine biosynthesis protein (hisE), and enterotoxin genes, were expressed only in the invasive strain, though with reduced expression

• These expression differences highlight the adaptability of S. dysenteriae to different host environments

This differential expression profile suggests that MsbA and related transport systems may be regulated differently depending on the infection stage and site, potentially contributing to the pathogen's ability to adapt to different host environments .

What structural insights have been gained from crystallization studies of MsbA with lipid A?

Crystallization studies have provided valuable insights into the mechanism of lipid A transport by MsbA:

• Substrate binding pathway: X-ray crystallography of MsbA from Salmonella typhimurium at 2.8 Å resolution after co-crystallization with lipid A revealed:

  • A large amplitude opening in the transmembrane portal that allows lipid A to enter the transport pathway

  • Putative lipid A density observed inside the transmembrane cavity, supporting a "trap and flip" model of transport

  • Additional electron density attributed to lipid A near an outer surface cleft at the periplasmic ends of the transmembrane helices, suggesting a potential exit pathway

• Conformational changes: Comparison of different MsbA structures (nucleotide-free and nucleotide-bound) revealed:

  • A flexible hinge formed by extracellular loops 2 and 3 that allows the NBDs to disassociate while the ATP-binding half sites remain facing each other

  • Nucleotide binding causes a packing rearrangement of the transmembrane helices and changes the accessibility from inward-facing to outward-facing

  • The inward and outward openings are mediated by two different sets of transmembrane helix interactions

These structural insights suggest that large ranges of motion are required for substrate transport, with distinct conformational states facilitating the entry, binding, and release of lipid A .

What are the mechanisms of action for MsbA inhibitors and their potential as antimicrobial agents?

Recent structural studies have revealed distinct mechanisms of action for first-generation MsbA inhibitors:

• TBT1 inhibitor mechanism:

  • Unlike typical inhibitors, TBT1 triggers unproductive ATPase activity in MsbA

  • TBT1 binding induces an asymmetric, collapsed inward-facing conformation in MsbA

  • The central substrate-binding pocket becomes more constricted than in drug-free MsbA

  • Upon TBT1 binding, the NBD distance substantially decreases from ~47 to ~20 Å

  • The structural changes provide an explanation for the stimulation of ATPase activity: removal of the TM4-TM5.B bundle and sliding of TM6.A into the central cavity reduce inter-NBD distance, increasing the speed of NBD dimerization and ATP hydrolysis

• G247 inhibitor:

  • Has a distinct binding mode compared to TBT1

  • These differences in binding mechanisms provide valuable information for the design of potential antimicrobial drugs

The study of these inhibitors has already led to the identification of a new lead compound from virtual screening based on the TBT1-induced conformation, suggesting a promising avenue for the development of novel antibiotics targeting MsbA .

What expression systems and purification strategies are optimal for obtaining functional recombinant S. dysenteriae MsbA?

Based on successful studies with MsbA from related species, the following expression and purification strategies are recommended for S. dysenteriae MsbA:

• Expression systems:

  • Heterologous expression in E. coli has been successful for MsbA from various species including E. coli, S. typhimurium, and A. baumannii

  • E. coli expression systems allow for high yield and proper folding of membrane proteins

• Purification protocol:

  • Cell lysis and membrane isolation

  • Solubilization using mild detergents such as dodecyl maltoside (DDM)

  • Affinity chromatography using histidine tags

  • Size exclusion chromatography for further purification

• Functional reconstitution:

  • Reconstitution in nanodiscs using lipids like palmitoyl-oleoyl-phosphatidylglycerol

  • This approach maintains protein activity and stability in a native-like lipid environment

• Activity verification:

  • ATPase assays to confirm functionality (expected Vmax around 1-5 μmol/min per milligram)

  • Inhibition studies using known inhibitors like vanadate (full inhibition at >100 μM) and AMPPNP (Ki range of 10-20 μM)

The purified protein can then be used for structural studies (X-ray crystallography, cryo-EM) and functional assays. For example, A. baumannii MsbA expressed in E. coli cells, purified in DDM, and reconstituted in nanodiscs showed a basal ATPase activity of ~1 μmol ATP per minute per milligram .

How does MsbA contribute to antimicrobial resistance mechanisms in S. dysenteriae?

MsbA contributes to antimicrobial resistance in S. dysenteriae through several mechanisms:

• Direct drug efflux: As a member of the ATP-binding cassette (ABC) antibiotic efflux pump family, MsbA can directly export certain antibiotics from the bacterial cell, particularly nitroimidazole antibiotics . This efflux capacity makes it part of the intrinsic resistance mechanisms in S. dysenteriae.

• Outer membrane integrity: By transporting lipid A and LPS to the outer membrane, MsbA helps maintain the permeability barrier that prevents many antibiotics from entering the cell . This barrier is particularly important for resistance against hydrophobic antibiotics.

• Polyspecific substrate recognition: MsbA has been shown to be a polyspecific transporter capable of recognizing and transporting a wide spectrum of drug molecules, similar to other multidrug resistance transporters like MDR1 and LmrA .

• Association with other resistance mechanisms: In S. dysenteriae clinical isolates, MsbA functions alongside other antimicrobial resistance genes including:

These resistance mechanisms have contributed to the emergence of multidrug-resistant Shigella strains, with pooled values of multidrug-resistant strains generally above 50% for both S. flexneri and S. sonnei .

What are the key considerations for designing structural studies of MsbA-inhibitor complexes?

When designing structural studies of MsbA-inhibitor complexes, researchers should consider:

• Protein stabilization strategies:

  • Use of facial amphiphiles for co-crystallization to stabilize the protein in detergent micelles

  • Selection of appropriate detergents that maintain protein stability without interfering with inhibitor binding

  • Consideration of nanodiscs or other membrane mimetics for maintaining native-like environments

• Crystallization approaches:

  • Co-crystallization with inhibitors rather than soaking, especially for compounds that may induce conformational changes

  • Screening of multiple constructs with various truncations or modifications to improve crystallization properties

  • Use of antibody fragments or nanobodies to stabilize specific conformations

• Data collection and processing:

  • Careful attention to anomalous signal assignment to avoid errors in structure determination

  • Use of single-copy refinement procedures rather than multicopy refinement to improve accuracy

  • Verification of hand assignment and space group determination

• Functional validation:

  • Parallel ATPase assays to confirm that the crystallized protein remains functional

  • Inhibition studies to verify that the observed inhibitor binding correlates with functional effects

  • Mutagenesis of key residues identified in the structures to confirm their role in inhibitor binding

These considerations are crucial for obtaining accurate structural information that can guide the development of new inhibitors targeting MsbA in S. dysenteriae and other pathogenic bacteria.

How can researchers effectively study the in vivo dynamics of MsbA-mediated lipid A transport in S. dysenteriae?

• In vivo fluorescence techniques:

  • Fluorescently labeled lipid A analogs can be used to track transport in live bacteria

  • FRET-based assays can monitor conformational changes of MsbA in real-time

  • Single-molecule tracking can reveal the dynamics of individual MsbA transporters

• Site-specific labeling and EPR spectroscopy:

  • Introduction of cysteine residues at strategic positions for spin labeling

  • Use of electron paramagnetic resonance (EPR) spectroscopy to monitor distance changes during the transport cycle

  • This approach can provide information about conformational dynamics under physiological conditions

• Genetic approaches:

  • Construction of temperature-sensitive or inducible MsbA mutants

  • Monitoring changes in lipid A distribution and membrane composition upon MsbA inactivation

  • Correlation of these changes with bacterial viability and antimicrobial susceptibility

• Mimicking the intracellular environment:

  • Development of in vitro systems that better recreate the native environment of MsbA

  • As noted in one study, "the in vivo mechanism of Shigellae invasion are difficult to fully study until the intracellular environment is mimicked in vitro"

  • This might include reconstitution in complex lipid mixtures that mimic the bacterial inner membrane

• Transcriptomic and proteomic profiling:

  • Analysis of MsbA expression and regulation under different growth conditions and stress responses

  • Comparison between invasive and non-invasive strains to understand adaptation mechanisms

By combining these approaches, researchers can gain a comprehensive understanding of MsbA function in the context of S. dysenteriae pathogenesis and identify potential vulnerabilities for therapeutic intervention.

What are the critical controls needed when assessing the effects of potential MsbA inhibitors on S. dysenteriae?

When evaluating potential MsbA inhibitors against S. dysenteriae, several critical controls must be included to ensure valid and interpretable results:

• Verification of target specificity:

  • Testing inhibitors against MsbA knockout strains complemented with plasmid-encoded MsbA to confirm on-target effects

  • Using site-directed mutagenesis of predicted binding residues to validate the binding mode

  • Comparing effects on related ABC transporters to assess selectivity

• Functional assays:

  • ATPase activity measurements with proper controls:

    • Basal activity (no inhibitor)

    • Known inhibitors as positive controls (e.g., vanadate at >100 μM for full inhibition)

    • ATP-only controls when testing stimulatory compounds like TBT1

  • Transport assays using labeled lipid A or model substrates

  • Membrane integrity assays to distinguish between direct inhibition and membrane disruption

• Cytotoxicity and selectivity controls:

  • Testing on mammalian cells to assess selective toxicity

  • Hemolysis assays to detect membrane-damaging effects

  • Growth inhibition of bacteria lacking MsbA (Gram-positive) to identify off-target effects

• Resistance development:

  • Serial passage experiments to monitor resistance development

  • Sequencing of resistant mutants to confirm whether resistance mutations map to msbA

  • Cross-resistance testing with known antibiotics

• In vivo efficacy controls:

  • Comparison with standard-of-care antibiotics

  • Pharmacokinetic/pharmacodynamic studies to ensure adequate exposure

  • Multiple S. dysenteriae strains including clinical isolates with different resistance profiles

These controls are essential for developing MsbA inhibitors as potential therapeutic agents against S. dysenteriae infections, particularly given the increasing prevalence of multidrug-resistant strains.

How might MsbA inhibitors be integrated into combination therapies to combat multidrug-resistant S. dysenteriae?

The integration of MsbA inhibitors into combination therapies represents a promising approach to address multidrug-resistant S. dysenteriae infections:

• Synergistic combinations with existing antibiotics:

  • MsbA inhibitors could potentiate the activity of antibiotics that are typically effluxed or blocked by the outer membrane

  • For example, combining MsbA inhibitors with β-lactams, fluoroquinolones, or macrolides might overcome existing resistance mechanisms

  • This strategy is particularly relevant given the high prevalence of resistance genes like bla OXA, bla CTX-M-1, and qnrS in clinical S. dysenteriae isolates

• Targeting multiple steps in LPS biogenesis:

  • Combining MsbA inhibitors with compounds targeting other steps in LPS synthesis or transport

  • This multi-target approach could reduce the likelihood of resistance development

  • Potential complementary targets include LpxC (lipid A synthesis) and LptD (outer membrane insertion)

• Sensitization to host immune defenses:

  • MsbA inhibition may alter the outer membrane composition, potentially increasing bacterial susceptibility to host antimicrobial peptides

  • Combination with immunomodulatory agents could enhance clearance of S. dysenteriae infections

• Adjuvant therapy with bioconjugate vaccines:

  • MsbA inhibitors could be used alongside emerging Shigella bioconjugate vaccines

  • While vaccines like Flexyn2a have shown promise (51.7% vaccine efficacy against severe shigellosis), they don't provide complete protection

  • Combining vaccination with targeted antibiotic therapy could improve outcomes in high-risk populations

• Delivery system considerations:

  • Development of nanoparticle formulations that co-deliver MsbA inhibitors with other antibiotics

  • Site-specific delivery systems to target the gastrointestinal tract, the primary site of S. dysenteriae infection

These integrated approaches could address the growing challenge of antibiotic resistance in S. dysenteriae, which has become a serious public health concern, particularly in low and middle-income countries where pooled values of multidrug-resistant strains generally exceed 50% .

What comparative insights can be gained by studying MsbA across different pathogenic species?

Comparative analysis of MsbA across different pathogenic species provides valuable insights into both conserved and species-specific aspects of this essential transporter:

• Structural conservation and variation:

  • MsbA orthologs from E. coli, V. cholerae, and S. typhimurium show high structural homology but exhibit differences in ATPase activity (Vmax values of 1.7, 5.3, and 2.0 μmol/min per milligram, respectively)

  • These differences may reflect adaptations to specific membrane compositions or environmental niches

  • Comparative structural analysis can identify conserved regions essential for function versus variable regions that might be targeted for species-specific inhibition

• Species-specific inhibitor responses:

  • Different bacterial species show varying sensitivity to MsbA inhibitors

  • For example, TBT1 has been studied in A. baumannii MsbA, where it induces a specific collapsed inward-facing conformation

  • Understanding these differences can guide the development of narrow-spectrum or broad-spectrum inhibitors

• Evolutionary adaptations in pathogenic contexts:

  • Comparison of MsbA between pathogenic and non-pathogenic bacteria may reveal adaptations related to virulence

  • For S. dysenteriae, which causes severe dysentery, MsbA may have specific properties related to survival in the human gut and bloodstream

  • These adaptations could be targeted for therapeutic intervention

• Resistome analysis:

  • MsbA is found in various resistomes, including perfect matches in Escherichia coli, Shigella boydii, S. dysenteriae, S. flexneri, and S. sonnei

  • Sequence variants are present in numerous other species including Citrobacter, Cronobacter, Enterobacter, Klebsiella, and Salmonella

  • This distribution information helps predict the potential spectrum of activity for MsbA-targeting therapeutics