Recombinant Borrelia turicatae Lipoprotein signal peptidase (lspA)

Basic Research Questions

• What is Lipoprotein signal peptidase (LspA) in Borrelia turicatae and what is its function?

  LspA (Lipoprotein signal peptidase or Signal peptidase II) is an essential membrane protein in Borrelia turicatae that plays a critical role in lipoprotein processing. It functions as an aspartic protease (EC 3.4.23.36) that cleaves the signal peptide of intermediate prolipoproteins during the second step of the lipoprotein maturation pathway. The enzyme relies on two catalytic aspartic acid residues to remove the signal peptide after the diacylglyceryl modification of the conserved cysteine residue in the lipobox motif of prolipoproteins . This processing is crucial for proper lipoprotein sorting and localization in B. turicatae, which has unique lipoprotein transport mechanisms compared to other gram-negative bacteria.

• How does LspA fit into the lipoprotein processing pathway in Borrelia species?

  In Borrelia species, the lipoprotein processing pathway involves three sequential steps:

  1. The inner membrane protein diacylglycerol transferase (Lgt) catalyzes the transfer of diacylglyceryl from phosphatidylglycerol to the sulfhydryl group of the lipobox cysteine.

  2. LspA cleaves the signal peptide of the intermediate prolipoprotein.

  3. Apolipoprotein N-acyltransferase (Lnt) catalyzes the transfer of an acyl chain from phosphatidylethanolamine onto the amino terminus of the +1 cysteine, resulting in a triacylated protein .

  Unlike other gram-negative bacteria that follow the '+2' sorting rule via the Lol pathway, Borrelia lipoproteins are targeted to the bacterial surface by default and can be retained in the periplasm by sequence-specific signals . This surface localization is particularly important for Borrelia pathogenesis.

• Why are lipoprotein processing enzymes like LspA important for studying Borrelia turicatae?

  LspA and other lipoprotein processing enzymes are important for studying B. turicatae for several reasons:

  1. B. turicatae causes tick-borne relapsing fever (TBRF), a neglected vector-borne disease that can be misdiagnosed as Lyme disease .

  2. Surface lipoproteins are abundant in Borrelia species and are critical virulence factors during transmission, colonization, and persistence of these pathogens .

  3. Understanding lipoprotein processing can provide insights into how B. turicatae adapts to disparate conditions found in the mammalian host versus the arthropod vector .

  4. LspA is essential in many bacteria, making it a potential target for antimicrobial development against TBRF.

  5. Studying lipoprotein processing helps elucidate pathogenic mechanisms unique to Borrelia species that contribute to their ability to evade host immune responses.

Advanced Research Questions

• How do you design experiments to determine the substrate specificity of B. turicatae LspA compared to other bacterial LspA homologs?

  To determine the substrate specificity of B. turicatae LspA compared to other bacterial homologs, a multi-faceted experimental approach is recommended:

  1. Site-directed mutagenesis: Create chimeric constructs between B. turicatae LspA and homologs from other bacteria (e.g., E. coli, B. burgdorferi) to identify critical regions for substrate recognition.

  2. In vitro enzymatic assays: Express and purify recombinant LspA proteins and test their activity against synthetic peptide substrates derived from various Borrelia lipoprotein signal sequences. Compare kinetic parameters (Km, kcat) to quantify differences in substrate preference.

  3. Complementation studies: Attempt to complement lspA deletion mutants in heterologous systems with the B. turicatae lspA gene to assess functional conservation.

  4. Structural biology approach: Determine the crystal or cryo-EM structure of B. turicatae LspA with bound substrate analogs or inhibitors to identify the structural basis for substrate specificity.

  5. Lipidomic analysis: Compare the lipid modifications of lipoproteins processed by B. turicatae LspA versus other bacterial LspA enzymes using mass spectrometry.

  This comparative approach can reveal unique features of B. turicatae LspA that may explain the distinctive lipoprotein sorting mechanisms in Borrelia species.

• What methodologies are most effective for generating functional recombinant B. turicatae LspA for biochemical studies?

  Generating functional recombinant B. turicatae LspA presents challenges due to its membrane-embedded nature. The most effective methodologies include:

  1. Expression system selection: Yeast expression systems have proven successful for B. turicatae LspA , but bacterial systems using E. coli C41(DE3) or C43(DE3) strains designed for membrane protein expression are also viable options.

  2. Construct optimization:

     • Include a cleavable N-terminal tag (His, MBP, or SUMO) for purification

     • Consider expressing a partial construct focusing on the catalytic domain while maintaining critical transmembrane regions

     • Codon-optimize the gene for the expression host

  3. Solubilization strategies:

     • Use mild detergents like DDM, LMNG, or digitonin for extraction

     • Consider nanodiscs or amphipols for maintaining stability in a membrane-like environment

     • Test detergent-free systems using SMALPs (styrene maleic acid lipid particles)

  4. Activity verification:

     • Develop a FRET-based assay using synthetic peptide substrates

     • Test inhibition by globomycin as a positive control for properly folded protein

     • Implement in vitro processing assays using purified B. turicatae prolipoproteins

  5. Stability optimization:

     • Screen various buffer conditions (pH, salt, additives) by thermal shift assays

     • Consider lipid supplementation during purification to maintain native interactions

  The choice between full-length protein versus catalytic domain constructs should be guided by the specific research questions being addressed.

• How can LspA inhibition be leveraged to study lipoprotein trafficking in B. turicatae and what are the experimental considerations?

  Leveraging LspA inhibition to study lipoprotein trafficking in B. turicatae requires careful experimental design:

  1. Inhibitor-based approaches:

     • Globomycin treatment at sub-lethal concentrations can block LspA activity

     • Design a dose-response curve to determine optimal inhibitor concentrations that affect LspA without causing general cellular toxicity

     • Use pulse-chase experiments with radiolabeled amino acids to track accumulated prolipoproteins

  2. Genetic approaches:

     • Develop a conditional knockdown system for lspA using an inducible promoter

     • Create point mutations in the catalytic aspartic acid residues to generate a catalytically inactive but structurally intact LspA

  3. Visualization techniques:

     • Use fluorescently tagged lipoproteins combined with super-resolution microscopy to track trafficking

     • Implement immunofluorescence assays with antibodies against specific lipoproteins of interest

     • Employ immunoelectron microscopy to precisely localize lipoproteins at different stages of processing

  4. Biochemical fractionation:

     • Separate inner membrane, outer membrane, and periplasmic fractions to analyze lipoprotein distribution

     • Use protease accessibility assays to determine surface exposure of lipoproteins

  5. Considerations and controls:

     • Always include wild-type controls processed in parallel

     • Monitor general cell viability and morphology to ensure observations are not due to general stress responses

     • Verify the specificity of inhibition by complementation with an inhibitor-resistant LspA variant if possible

  This approach can reveal how lipoprotein processing affects trafficking pathways unique to Borrelia species.

• What are the differences in LspA function between Borrelia turicatae and Borrelia burgdorferi, and how do these differences impact pathogenesis?

  The differences in LspA function between B. turicatae and B. burgdorferi reflect their distinct pathogenic mechanisms:

  Feature	B. turicatae LspA	B. burgdorferi LspA	Impact on Pathogenesis
  Substrate preference	Processes lipoproteins involved in TBRF pathogenesis (e.g., BipA)	Processes lipoproteins associated with Lyme disease (e.g., OspA, OspC)	Affects the repertoire of surface-exposed virulence factors
  Processing efficiency	May have unique processing kinetics for TBRF-specific lipoproteins	Optimized for Lyme disease-specific lipoproteins	Influences the timing of virulence factor expression during infection
  Regulation	May respond to different environmental cues than B. burgdorferi LspA	Expression regulated during tick feeding and mammalian infection	Determines adaptability to different host environments
  Inhibitor sensitivity	Potential differences in sensitivity to globomycin and other inhibitors	Well-characterized sensitivity to globomycin	Could affect therapeutic approaches for different Borrelia infections
  Structural features	May have unique structural elements related to TBRF lipoprotein processing	Contains standard aspartic protease catalytic domain	Determines interaction with species-specific lipoproteins

  These differences impact pathogenesis in several ways:

  1. B. turicatae causes relapsing fever with neurologic symptoms that can be misdiagnosed as Lyme disease , while B. burgdorferi causes the more persistent Lyme disease.

  2. The different sets of lipoproteins processed by these LspAs contribute to the distinct clinical manifestations and host immune responses between TBRF and Lyme disease.

  3. B. turicatae lipoproteins like BipA are important for TBRF diagnosis and pathogenesis , while B. burgdorferi relies on different sets of lipoproteins like OspA and OspC.

  4. Understanding these differences can help develop species-specific diagnostic tests and targeted therapeutic approaches.

• How can recombinant LspA be used to develop novel diagnostic tools for differentiating Borrelia turicatae infection from other Borrelia species infections?

  Recombinant LspA can contribute to novel diagnostic tools for differentiating Borrelia infections through several approaches:

  1. Development of LspA-specific antibody panels:

     • Generate monoclonal antibodies against species-specific epitopes of LspA

     • Create an antibody-based assay to detect LspA variants in patient samples

     • Use these antibodies in immunohistochemistry to identify the infecting species in tissue samples

  2. LspA substrate profiling:

     • Develop assays based on the differential processing of synthetic peptide substrates by LspA from different species

     • Create a panel of fluorogenic peptides representing signal sequences from different Borrelia species

     • The pattern of peptide cleavage could indicate the species present in a sample

  3. Anti-LspA antibody detection in patient sera:

     • Express recombinant LspA from multiple Borrelia species

     • Develop an ELISA or protein microarray to detect species-specific antibodies in patient samples

     • Analyze the antibody binding pattern to differentiate between infections

  4. Methodological considerations:

     • Ensure recombinant LspA maintains proper folding to preserve epitope integrity

     • Validate with known positive and negative samples, including potential cross-reactive species

     • Combine with other Borrelia antigens like BipA for improved diagnostic accuracy

  This approach complements existing diagnostic methods based on BipA, which has shown promise in differentiating between TBRF and Lyme disease as well as between different TBRF species infections .

Experimental Applications and Challenges

• What are the technical challenges in studying the interaction between LspA and other components of the lipoprotein processing machinery in Borrelia turicatae?

  Studying the interactions between LspA and other components of the lipoprotein processing machinery in B. turicatae presents several technical challenges:

  1. Membrane protein complexes:

     • LspA, Lgt, and Lnt are all membrane proteins, making them difficult to study in their native states

     • Traditional pull-down assays may disrupt important interactions during solubilization

     • The interactions may be transient and depend on specific lipid environments

  2. Specialized techniques required:

     • Crosslinking approaches need optimization to capture protein-protein interactions in the membrane

     • Blue native PAGE can preserve membrane protein complexes but requires extensive optimization

     • Proximity labeling methods (BioID, APEX) may introduce artifacts in the small Borrelia cells

  3. Reconstitution challenges:

     • Recreating the native membrane environment for in vitro studies is difficult

     • The lipid composition of Borrelia membranes differs from model systems

     • Co-expression and co-purification of multiple components may yield low quantities

  4. Visualization difficulties:

     • The small size of Borrelia cells (0.2-0.5 μm diameter) makes high-resolution imaging challenging

     • Discriminating between inner and outer membrane proteins requires specialized techniques

     • Fluorescent protein fusions may interfere with native interactions

  5. Methodological approaches to overcome these challenges:

     • Implement advanced membrane protein structural biology techniques like cryo-electron tomography

     • Develop nanodiscs with Borrelia-specific lipid compositions

     • Use genetic approaches like bacterial two-hybrid systems adapted for membrane proteins

     • Apply microfluidic techniques to enhance the sensitivity of interaction detection

  Understanding these interactions is crucial for elucidating the unique lipoprotein sorting mechanisms in Borrelia species compared to other bacteria.

• How does environmental regulation affect LspA expression and activity in Borrelia turicatae during its life cycle between ticks and mammals?

  The environmental regulation of LspA expression and activity in B. turicatae throughout its life cycle involves complex mechanisms:

  1. Temperature-dependent regulation:

     • Like other Borrelia proteins, LspA expression may be regulated by temperature shifts between tick (approximately 22°C) and mammalian host (37°C) environments

     • Quantitative RT-PCR should be performed to measure lspA transcript levels under different temperature conditions

     • Protein levels should be monitored by western blotting using anti-LspA antibodies

  2. Tick-mammal transition adaptations:

     • During the transition from tick to mammalian host, B. turicatae undergoes significant transcriptional reprogramming

     • LspA activity may be modulated to process specific sets of lipoproteins required for each environment

     • For example, BrpA (Borrelia repeat protein A) shows differential expression during tick colonization , and its processing may depend on LspA activity

  3. Experimental approaches to study regulation:

     • Use reporter gene fusions (e.g., lspA promoter-GFP) to track expression in real-time

     • Implement RNA-seq to identify co-regulated genes under different environmental conditions

     • Compare LspA activity using fluorogenic substrate assays under different pH, temperature, and ionic strength conditions

     • Develop a tick feeding model to analyze LspA expression during natural infection cycles

  4. Methodological considerations:

     • Controls should include house-keeping genes that maintain stable expression across conditions

     • Multiple time points should be sampled to capture the dynamic nature of regulation

     • Both in vitro models and in vivo infection studies should be performed for comprehensive analysis

  Understanding this regulation is critical given that B. turicatae must adapt to the disparate conditions found in the mammalian host and arthropod vector , and LspA likely plays a key role in processing the lipoproteins necessary for these adaptations.

• What methods can be used to determine whether LspA is essential for Borrelia turicatae survival and infection, and what are the experimental limitations?

  Determining whether LspA is essential for B. turicatae survival and infection requires sophisticated genetic and infection model approaches:

  1. Genetic approaches to test essentiality:

     • Attempt direct gene knockout using homologous recombination, which would likely fail if the gene is essential

     • Implement conditional knockdown systems such as:

       • Tetracycline-inducible promoter control of lspA expression

       • CRISPR interference (CRISPRi) to reduce transcription

       • Degron-tagged LspA for controlled protein degradation

     • Create merodiploid strains with an extra copy of lspA before attempting deletion of the native gene

  2. Methodological considerations:

     • Design careful controls including attempts to delete known essential and non-essential genes

     • Include complementation controls to verify phenotypes are specifically due to lspA deficiency

     • Monitor for suppressor mutations that may arise to compensate for LspA deficiency

  3. In vitro and in vivo phenotypic analysis:

     • Evaluate growth curves, morphology, and membrane integrity under LspA depletion conditions

     • Assess lipoprotein processing using immunoblotting against known B. turicatae lipoproteins

     • Test infectivity in mouse models of TBRF using conditional knockdown strains

     • Evaluate colonization of Ornithodoros turicata ticks after feeding on infected mice

  4. Experimental limitations:

     • Genetic manipulation of B. turicatae is challenging compared to model organisms

     • Conditional systems may have leaky expression, complicating interpretation

     • Essential genes may appear non-essential in laboratory conditions but be required in natural environments

     • Tick feeding models are complex and may introduce variables difficult to control

     • LspA depletion may have pleiotropic effects making it difficult to distinguish direct from indirect impacts

  5. Alternative approaches:

     • Use chemical genetics with globomycin to inhibit LspA activity rather than genetic deletion

     • Develop LspA variants with temperature-sensitive mutations

     • Create partial loss-of-function mutations in catalytic residues to evaluate dose-dependent phenotypes

  These approaches can help determine whether LspA is essential and elucidate its specific roles in B. turicatae pathogenesis.

• How can structural studies of B. turicatae LspA inform the development of species-specific inhibitors for potential therapeutic applications?

  Structural studies of B. turicatae LspA can drive the development of species-specific inhibitors through a structure-based drug design approach:

  1. Structural determination methods:

     • X-ray crystallography of recombinant LspA, potentially in complex with globomycin or substrate analogs

     • Cryo-EM to determine the structure in a more native membrane-like environment

     • NMR studies of specific domains to understand dynamic interactions

     • Computational modeling based on homologous structures if experimental structures prove challenging

  2. Key structural features to target:

     • Identify unique features of the catalytic site that differ from human aspartic proteases

     • Map species-specific surface loops or pockets absent in mammalian homologs

     • Characterize the substrate binding groove to identify distinctive elements for B. turicatae lipoproteins

     • Analyze the membrane-embedded regions for potential allosteric targeting

  3. Structure-based inhibitor design workflow:

     • Perform in silico screening against identified species-specific pockets

     • Design peptidomimetic inhibitors based on natural substrates but incorporating non-cleavable bonds

     • Implement fragment-based approaches to identify initial chemical matter

     • Optimize lead compounds for selectivity against other bacterial LspA enzymes and human proteases

  4. Validation and refinement methodologies:

     • Develop enzymatic assays using fluorogenic substrates to quantify inhibition

     • Determine co-crystal structures with lead compounds to guide optimization

     • Test cellular activity using B. turicatae growth inhibition assays

     • Evaluate off-target effects against human cell lines and other bacterial species

  5. Therapeutic potential considerations:

     • Assess blood-brain barrier penetration for treating neurological manifestations of TBRF

     • Evaluate pharmacokinetics in animal models of infection

     • Consider combination therapy approaches with other antibiotics

  This structure-guided approach can lead to selective inhibitors with potential therapeutic applications for TBRF caused by B. turicatae.

• What is the relationship between LspA function and the unique surface lipoprotein display mechanisms in Borrelia turicatae compared to other bacteria?

  The relationship between LspA function and the unique surface lipoprotein display in B. turicatae involves several distinctive mechanisms:

  1. Default surface localization:

     • Unlike most Gram-negative bacteria where the Lol system directs lipoproteins to specific locations, Borrelia lipoproteins are targeted to the bacterial surface by default

     • LspA processing is a prerequisite for this default pathway, as it removes the signal peptide to allow further trafficking

     • The '+2' sorting rule that applies to many Gram-negative bacteria does not function in Borrelia

  2. Retention signals instead of export signals:

     • In Borrelia, sequence-specific signals act to retain certain lipoproteins in the periplasm, rather than signals that direct export

     • For example, negative charges can act as partial subsurface retention signals in certain peptide contexts

     • LspA must properly process lipoproteins for these retention signals to function correctly

  3. Experimental evidence from model systems:

     • Studies with chimeras between the outer surface lipoprotein OspA, the periplasmic oligopeptide-binding lipoprotein OppAIV, and reporter proteins have elucidated these mechanisms

     • Mutagenesis of the OspA N-terminus defined less than five N-terminal amino acids as the minimal secretion-facilitating signal

     • With the exception of negative charges, lipoprotein secretion occurs independent of N-terminal sequence

  4. Methodological approaches to study this relationship:

     • Construct LspA variants with altered specificity and analyze their impact on lipoprotein sorting

     • Create reporter systems with fluorescent lipoproteins to track trafficking in real-time

     • Perform site-directed mutagenesis of LspA processing sites in various lipoproteins to determine the effect on localization

     • Implement proteomic approaches to identify the complete repertoire of surface versus periplasmic lipoproteins

  5. Implications for pathogenesis:

     • This unique system allows Borrelia to display an abundance of surface lipoproteins, some of which play important roles in the pathogenesis of relapsing fever

     • Surface-displayed lipoproteins like BipA are important for diagnosis and potentially for vaccine development

     • The surface localization of lipoproteins facilitates interactions with host factors and immune evasion

  Understanding this relationship is crucial for developing targeted approaches to prevent or treat TBRF by interfering with the pathogen's unique lipoprotein display system.

Research Applications and Future Directions

• How can the study of B. turicatae LspA contribute to understanding the evolutionary divergence between relapsing fever and Lyme disease Borrelia species?

  Studying B. turicatae LspA can provide valuable insights into the evolutionary divergence between relapsing fever (RF) and Lyme disease (LD) Borrelia species through several approaches:

  1. Comparative genomic analysis:

     • Perform phylogenetic analysis of LspA sequences across RF Borrelia (B. turicatae, B. hermsii, B. parkeri) and LD Borrelia (B. burgdorferi, B. afzelii, B. garinii)

     • Calculate selection pressures (dN/dS ratios) on different regions of LspA to identify adaptively evolving sites

     • Analyze synteny and genomic context of the lspA gene across species

  2. Functional substrate specificity analysis:

     • Compare the repertoire of lipoproteins processed by LspA across RF and LD species

     • Identify conserved and divergent lipoprotein signal sequences

     • Create chimeric LspA enzymes to determine which domains contribute to species-specific functions

  3. Vector-host adaptation signatures:

     • Analyze how LspA function relates to adaptation to different tick vectors (soft ticks for RF vs. hard ticks for LD)

     • Investigate how LspA processing contributes to different host ranges and tissue tropisms

     • Examine temperature-dependent regulation and activity across species adapted to different niches

  4. Methodological approaches:

     • Implement ancestral sequence reconstruction to infer the properties of LspA in the common ancestor

     • Use heterologous expression to test functional complementation between species

     • Employ experimental evolution under selective pressures to observe LspA adaptations in real-time

  5. Evolutionary implications:

     • The putative recent evolutionary separation of B. parkeri and B. turicatae provides an opportunity to study early divergence events

     • The differential ability of these closely related species to bind host factors like factor H (mediated by surface lipoproteins processed by LspA) demonstrates how processing of surface proteins influences pathogen-host interactions

     • Understanding these differences helps explain how these pathogens have adapted to cause distinct clinical manifestations

  This research can provide a molecular framework for understanding the evolutionary forces driving the divergence of these bacterial pathogens and their associated diseases.

• What role might LspA play in the host immune evasion strategies of Borrelia turicatae, and how can this be experimentally investigated?

  LspA likely plays an indirect but crucial role in the host immune evasion strategies of B. turicatae through its processing of key surface lipoproteins. This role can be experimentally investigated through several approaches:

  1. Relationship to antigenic variation:

     • B. turicatae undergoes antigenic variation of surface proteins like variable small proteins (Vsp) to evade host immunity

     • LspA processes these variable surface lipoproteins, making it essential for this immune evasion mechanism

     • Experimental approach: Create an LspA conditional knockdown system and monitor the ability of B. turicatae to establish persistent infection and undergo antigenic switching

  2. Processing of immune evasion lipoproteins:

     • Surface lipoproteins like VspA and VspB in B. turicatae are part of a larger family that includes the Vsp proteins of B. hermsii and OspC proteins of Lyme disease agents

     • These proteins are associated with immune evasion and influence infection outcome

     • Experimental approach: Perform pulse-chase experiments to track LspA-dependent processing of these proteins during immune challenge

  3. Complement resistance mechanisms:

     • Some Borrelia surface lipoproteins (like BpcA in B. parkeri) bind human complement regulators factor H and factor H-related protein 1, providing resistance to complement-mediated killing

     • Although the homologous B. turicatae protein failed to bind these factors , other surface lipoproteins may have similar functions

     • Experimental approach: Screen for complement resistance factors processed by LspA using a CRISPR interference library targeting LspA substrates

  4. Host protease interaction:

     • Surface lipoproteins processed by LspA can bind host plasminogen, which when activated to plasmin can degrade extracellular matrix components

     • This interaction may facilitate tissue invasion and dissemination

     • Experimental approach: Use fluorescently labeled plasminogen to track its interaction with B. turicatae surface proteins in wild-type versus LspA-depleted conditions

  5. Comprehensive methodological workflow:

     • Create a library of reporter constructs with various lipoprotein signal sequences fused to fluorescent proteins

     • Track the processing and localization of these reporters under normal and immune challenge conditions

     • Correlate processing efficiency with survival in the presence of immune components

     • Use immunoprecipitation to identify host immune factors that interact with properly processed surface lipoproteins

  These studies would elucidate how LspA contributes to immune evasion and could identify novel targets for therapeutic intervention in TBRF.