Characterization of fluorescent chimeras of cholera toxin and Escherichia coli heat-labile enterotoxins produced by use of the twin arginine translocation system

Sortase-Modified Cholera Toxoids Show Specific Golgi Localization

Cholera toxoid is an established tool for use in cellular tracing in neuroscience and cell biology. We use a sortase labeling approach to generate site-specific N-terminally modified variants of both the A2-B5 heterohexamer and B5 pentamer forms of the toxoid. Both forms of the toxoid are endocytosed by GM1-positive mammalian cells, and while the heterohexameric toxoid was principally localized in the ER, the B5 pentamer showed an unexpectedly specific localization in the medial/trans-Golgi. This study suggests a future role for specifically labeled cholera toxoids in live-cell imaging beyond their current applications in neuronal tracing and labeling of lipid rafts in fixed cells.

In-Depth Characterization of a Re-Engineered Cholera Toxin Manufacturing Process Using Growth-Decoupled Production in Escherichia coli

Non-toxic derivatives of the cholera toxin are extensively used in neuroscience, as neuronal tracers to reveal the location of cells in the central nervous system. They are, also, being developed as vaccine components and drug-delivery vehicles. Production of cholera-toxin derivatives is often non-reproducible; the quality and quantity require extensive fine-tuning to produce them in lab-scale settings. In our studies, we seek a resolution to this problem, by expanding the molecular toolbox of the Escherichia coli expression system with suitable production, purification, and offline analytics, to critically assess the quality of a probe or drug delivery, based on a non-toxic derivative of the cholera toxin. We present a re-engineered Cholera Toxin Complex (rCTC), wherein its toxic A1 domain was replaced with Maltose Binding Protein (MBP), as a model for an rCTC-based targeted-delivery vehicle. Here, we were able to improve the rCTC production by 11-fold (168 mg/L vs. 15 mg/L), in comparison to a host/vector combination that has been previously used (BL21(DE3) pTRBAB5-G1S). This 11-fold increase in the rCTC production capability was achieved by (1) substantial vector backbone modifications, (2) using Escherichia coli strains capable of growth-decoupling (V strains), (3) implementing a well-tuned fed-batch production protocol at a 1 L scale, and (4) testing the stability of the purified product. By an in-depth characterization of the production process, we revealed that secretion of rCTC across the E. coli Outer Membrane (OM) is processed by the Type II secretion-system general secretory pathway (gsp-operon) and that cholera toxin B-pentamerization is, likely, the rate-limiting step in complex formation. Upon successful manufacturing, we have validated the biological activity of rCTC, by measuring its binding affinity to its carbohydrate receptor GM1 oligosaccharide (K_d = 40 nM), or binding to Jurkat cells (93 pM) and delivering the cargo (MBP) in a retrograde fashion to the cell.

Cell-Free Systems Enable the Production of AB5 Toxins for Diagnostic Applications

Cell-free protein synthesis (CFPS) represents a versatile key technology for the production of toxic proteins. As a cell lysate, rather than viable cells, is used, the toxic effects on the host organism can be circumvented. The open nature of cell-free systems allows for the addition of supplements affecting protein concentration and folding. Here, we present the cell-free synthesis and functional characterization of two AB₅ toxins, namely the cholera toxin (Ctx) and the heat-labile enterotoxin (LT), using two eukaryotic cell-free systems based on Chinese hamster ovary (CHO) and Spodoptera frugiperda (Sf21) cells. Through an iterative optimization procedure, the synthesis of the individual AB₅ toxins was established, and the formation of multimeric structures could be shown by autoradiography. A functional analysis was performed using cell-based assays, thereby demonstrating that the LT complex induced the characteristic cell elongation of target cells after 24 h. The LT complex induced cell death at higher concentrations, starting at an initial concentration of 5 nM. The initial toxic effects of the Ctx multimer could already be detected at 4 nM. The detection and characterization of such AB₅ toxins is of utmost importance, and the monitoring of intracellular trafficking facilitates the further identification of the mechanism of action of these toxins. We showed that the B-subunit of LT (LTB) could be fluorescently labeled using an LTB-Strep fusion protein, which is a proof-of-concept for future Trojan horse applications. Further, we performed a mutational analysis of the CtxA subunit as its template was modified, and an amber stop codon was inserted into CtxA’s active site. Subsequently, a non-canonical amino acid was site-specifically incorporated using bio-orthogonal systems. Finally, a fluorescently labeled CtxA protein was produced using copper-catalyzed click reactions as well as a Staudinger ligation. As expected, the modified Ctx multimer no longer induced toxic effects. In our study, we showed that CFPS could be used to study the active centers of toxins by inserting mutations. Additionally, this methodology can be applied for the design of Trojan horses and targeted toxins, as well as enabling the intracellular trafficking of toxins as a prerequisite for the analysis of the toxin’s mechanism of action.

Piggybacking on the Cholera Toxin: Identification of a CTB-Binding Protein as an Approach for Targeted Delivery of Proteins to Motor Neurons

A significant unmet need exists for the delivery of biologic drugs such as polypeptides or nucleic acids to the central nervous system for the treatment and understanding of neurodegenerative diseases. Naturally occurring bacterial toxins have been considered as tools to meet this need. However, due to the complexity of tethering macromolecular drugs to toxins and the inherent dangers of working with large quantities of recombinant toxins, no such route has been successfully exploited. Developing a method where a bacterial toxin's nontoxic targeting subunit can be assembled with a drug immediately prior to in vivo administration has the potential to circumvent some of these issues. Using a phage-display screen, we identified two antibody mimetics, anticholera toxin Affimer (ACTA)-A2 and ACTA-C6 that noncovalently associate with the nonbinding face of the cholera toxin B-subunit. In a first step toward the development of a nonviral motor neuron drug-delivery vehicle, we show that Affimers can be selectively delivered to motor neurons in vivo.

Directed Assembly of Homopentameric Cholera Toxin B-Subunit Proteins into Higher-Order Structures Using Coiled-Coil Appendages

The self-assembly of proteins into higher order structures is ubiquitous in living systems. It is also an essential process for the bottom-up creation of novel molecular architectures and devices for synthetic biology. However, the complexity of protein–protein interaction surfaces makes it challenging to mimic natural assembly processes in artificial systems. Indeed, many successful computationally designed protein assemblies are prescreened for “designability”, limiting the choice of components. Here, we report a simple and pragmatic strategy to assemble chosen multisubunit proteins into more complex structures. A coiled-coil domain appended to one face of the pentameric cholera toxin B-subunit (CTB) enabled the ordered assembly of tubular supra-molecular complexes. Analysis of a tubular structure determined by X-ray crystallography has revealed a hierarchical assembly process that displays features reminiscent of the polymorphic assembly of polyomavirus proteins. The approach provides a simple and straightforward method to direct the assembly of protein building blocks which present either termini on a single face of an oligomer. This scaffolding approach can be used to generate bespoke supramolecular assemblies of functional proteins. Additionally, structural resolution of the scaffolded assemblies highlight “native-state” forced protein–protein interfaces, which may prove useful as starting conformations for future computational design.

Monitoring Protein Secretion in Streptomyces Using Fluorescent Proteins

Fluorescent proteins are a major cell biology tool to analyze protein sub-cellular topology. Here we have applied this technology to study protein secretion in the Gram-positive bacterium Streptomyces lividans TK24, a widely used host for heterologous protein secretion biotechnology. Green and monomeric red fluorescent proteins were fused behind Sec (SP^Sec) or Tat (SP^Tat) signal peptides to direct them through the respective export pathway. Significant secretion of fluorescent eGFP and mRFP was observed exclusively through the Tat and Sec pathways, respectively. Plasmid over-expression was compared to a chromosomally integrated sp^Sec-mRFP gene to allow monitoring secretion under high and low level synthesis in various media. Fluorimetric detection of SP^Sec-mRFP recorded folded states, while immuno-staining detected even non-folded topological intermediates. Secretion of SP^Sec-mRFP is unexpectedly complex, is regulated independently of cell growth phase and is influenced by the growth regime. At low level synthesis, highly efficient secretion occurs until it is turned off and secretory preforms accumulate. At high level synthesis, the secretory pathway overflows and proteins are driven to folding and subsequent degradation. High-level synthesis of heterologous secretory proteins, whether secretion competent or not, has a drastic effect on the endogenous secretome, depending on their secretion efficiency. These findings lay the foundations of dissecting how protein targeting and secretion are regulated by the interplay between the metabolome, secretion factors and stress responses in the S. lividans model.

Engineering an AB5 Protein Carrier

The promise of biologic therapeutics is hindered by the challenge to deliver their activity to biochemically relevant sites within diseased cells. The favourable application of the natural protein carriers of the AB₅ toxin family to this challenge has been restricted owing to still unresolved requirements for assembling non-native cargo into carrier complexes. Here, we clarify the properties of fusion peptides which allow co-assembly of a selected fluorescent protein cargo with the non-toxic B subunit of a heat-labile enterotoxin. We establish the influence of sequence length, sequence identity and secondary structure of these linking domains on the assembly and disassembly of the complexes. Through our engineering framework we identify several non-native, reduced length fusion sequences that robustly assemble with the native carriers, maintain their ability to deliver protein cargo to cells, and demonstrate substantially refined in vitro properties. Constructs based upon these sequences should prove directly applicable to a variety of protein delivery challenges, and the described design framework should find immediate application to other members of the AB₅ protein carrier family.

Immunogenicity of a Staphylococcus aureus-cholera toxin A2/B vaccine for bovine mastitis

Staphylococcus aureus causes a chronic, contagious disease of the udder, or mastitis, in dairy cows. This infection is often refractory to antibiotic treatment, and has a significant economic impact on milk production worldwide. An effective vaccine to prevent S. aureus mastitis would improve animal health, reduce antibiotic dependence and inform human vaccine approaches. The iron-regulated surface determinant A (IsdA) and clumping factor A (ClfA) are conserved S. aureus extracellular-matrix adhesins and target vaccine antigens. Here we report the results of two bovine immunogenicity trials using purified IsdA and ClfA-cholera toxin A₂/B chimeras (IsdA-CTA₂/B and ClfA-CTA₂/B). Cows were intranasally inoculated with IsdA-CTA₂/B + ClfA-CTA₂/B at dry off and followed for 70 days. Trial 1 utilized three groups with one or two booster doses at a total concentration of 600 or 900 μg. Trial 2 utilized two groups with one booster at a total concentration of 1200 μg. Humoral immune responses in serum and milk were examined by ELISA. Responses in serum were significant between groups and provide evidence of antigen-specific IgG induction after vaccination in both trials. Cellular proliferation was detected by flow cytometry using antigen-stimulated PBMCs from day 60 of Trial 2 and revealed an increase in CD4+ T cells from vaccinated cows. IsdA and ClfA stimulation induced IL-4 expression, but not IFN-γ or IL-17, in PBMCs from day 60 as determined by cytokine expression analysis. Opsonophagocytosis of S. aureus confirmed the functional in vitro activity of anti-IsdA antibodies from Trial 2 serum and milk. The vaccine was well tolerated and safe, and results support the potential of mucosally-delivered CTA₂/B chimeras to protect cows from mastitis caused by S. aureus.

Cholera Toxin Subunit B as Adjuvant--An Accelerator in Protective Immunity and a Break in Autoimmunity

Cholera toxin subunit B (CTB) is the nontoxic portion of cholera toxin. Its affinity to the monosialotetrahexosylganglioside (GM1) that is broadly distributed in a variety of cell types including epithelial cells of the gut and antigen presenting cells, macrophages, dendritic cells, and B cells, allows its optimal access to the immune system. CTB can easily be expressed on its own in a variety of organisms, and several approaches can be used to couple it to antigens, either by genetic fusion or by chemical manipulation, leading to strongly enhanced immune responses to the antigens. In autoimmune diseases, CTB has the capacity to evoke regulatory responses and to thereby dampen autoimmune responses, in several but not all animal models. It remains to be seen whether the latter approach translates to success in the clinic, however, the versatility of CTB to manipulate immune responses in either direction makes this protein a promising adjuvant for vaccine development.

Immunogenicity of a West Nile virus DIII-cholera toxin A2/B chimera after intranasal delivery

West Nile virus (WNV) causes potentially fatal neuroinvasive disease and persists at endemic levels in many parts of the world. Despite advances in our understanding of WNV pathogenesis, there remains a significant need for a human vaccine. The domain III (DIII) region of the WNV envelope protein contains epitopes that are the target of neutralizing antibodies. We have constructed a chimeric fusion of the non-toxic cholera toxin (CT) CTA2/B domains to DIII for investigation as a novel mucosally-delivered WNV vaccine. Purification and assembly of the chimera, as well as receptor-binding and antigen delivery, were verified by western blot, GM1 ELISA and confocal microscopy. Groups of BALB/c mice were immunized intranasally with DIII-CTA2/B, DIII, DIII mixed with CTA2/B, or CTA2/B control, and boosted at 10 days. Analysis of serum IgG after 14 and 45 days revealed that mucosal immunization with DIII-CTA2/B induced significant DIII-specific humoral immunity and drove isotype switching to IgG2a. The DIII-CTA2/B chimera also induced antigen-specific IgM and IgA responses. Bactericidal assays indicate that the DIII-CTA2/B immunized mice produced DIII-specific antibodies that can trigger complement-mediated killing. A dose escalation resulted in increased DIII-specific serum IgG titers on day 45. DIII antigen alone, in the absence of adjuvant, also induced significant systemic responses after intranasal delivery. Our results indicate that the DIII-CTA2/B chimera is immunogenic after intranasal delivery and merits further investigation as a novel WNV vaccine candidate.

Cholera toxin subunit B peptide fusion proteins reveal impaired oral tolerance induction in diabetes-prone but not in diabetes-resistant mice

The cholera toxin B subunit (CTB) has been used as adjuvant to improve oral vaccine delivery in type 1 diabetes. The effect of CTB/peptide formulations on Ag-specific CD4(+) T cells has remained largely unexplored. Here, using tetramer analysis, we investigated how oral delivery of CTB fused to two CD4(+) T-cell epitopes, the BDC-2.5 T-cell 2.5 mi mimotope and glutamic acid decarboxylase (GAD) 286-300, affected diabetogenic CD4(+) T cells in nonobese diabetic (NOD) mice. When administered i.p., CTB-2.5 mi activated 2.5 mi(+) T cells and following intragastric delivery generated Ag-specific Foxp3(+) Treg and Th2 cells. While 2.5 mi(+) and GAD-specific T cells were tolerized in diabetes-resistant NODxB6.Foxp3(EGFP) F1 and nonobese resistant (NOR) mice, this did not occur in NOD mice. This indicated that NOD mice had a recessive genetic resistance to induce oral tolerance to both CTB-fused epitopes. In contrast to NODxB6.Foxp3(EGFP) F1 mice, oral treatment in NOD mice lead to strong 2.5 mi(+) T-cell activation and the sequestration of these cells to the effector-memory pool. Oral treatment of NOD mice with CTB-2.5 mi failed to prevent diabetes. These findings underline the importance of investigating the effect of oral vaccine formulations on diabetogenic T cells as in selected cases they may have counterproductive consequences in human patients.

Mucosal immunization with a Staphylococcus aureus IsdA-cholera toxin A2/B chimera induces antigen-specific Th2-type responses in mice

Staphylococcus aureus is a leading cause of opportunistic infection worldwide and a significant public health threat. The iron-regulated surface determinant A (IsdA) adhesin is essential for S. aureus colonization on human nasal epithelial cells and plays an important role in iron acquisition and resistance to human skin defenses. Here we investigated the murine immune response to intranasal administration of a cholera toxin A(2)/B (CTA(2)/B) chimera containing IsdA. Plasmids were constructed to express the IsdA-CTA(2)/B chimera and control proteins in Escherichia coli. Proper construction of the chimera was verified by SDS-PAGE, Western blotting, GM1 enzyme-linked immunosorbent assay (ELISA), and confocal microscopy. Groups of female BALB/c mice were mock immunized or immunized with IsdA-CTA(2)/B, IsdA mixed with CTA(2)/B, or IsdA alone, followed by one booster immunization at 10 days postpriming. Analysis of serum IgG and nasal, intestinal, and vaginal IgA suggested that mucosal immunization with IsdA-CTA(2)/B induces significant IsdA-specific humoral immunity. Functional in vitro assays revealed that immune serum significantly blocks the adherence of S. aureus to human epithelial cells. Splenocytes from mice immunized with IsdA-CTA(2)/B showed specific cellular proliferation and production of interleukin-4 (IL-4) after in vitro stimulation. Immunization with IsdA-CTA(2)/B drove isotype switching to IgG1, indicative of a Th2-type response. Our results suggest that the immunogenicity of the S. aureus IsdA-CTA(2)/B chimera merits further investigation as a potential mucosal vaccine candidate.

Purification and characterization of Yersinia enterocolitica and Yersinia pestis LcrV-cholera toxin A(2)/B chimeras

Yersinia pestis is a virulent human pathogen and potential biological weapon. Despite a long history of research on this organism, there is no licensed vaccine to protect against pneumonic forms of Y. pestis disease. In the present study, plasmids were constructed to express cholera toxin A(2)/B chimeric molecules containing the LcrV protective antigen from Yersinia enterocolitica and Y. pestis. These chimeras were expressed and purified to high yields from the supernatant of transformed Escherichia coli. Western and GM(1) ELISA assays were used to characterize the composition, receptor-binding and relative stability of the LcrV-CTA(2)/B chimera in comparison to cholera toxin. In addition, we investigated the ability of the Y. pestis LcrV-CTA(2)/B chimera to bind to and internalize into cultured epithelial cells and macrophages by confocal microscopy. These studies indicate that the uptake and trafficking of the LcrV antigen from the chimera is comparable to the trafficking of native toxin. Together these findings report that stable, receptor-binding, non-toxic LcrV-cholera toxin A(2)/B chimeras can be expressed at high levels in E. coli and purified from the supernatant. In addition, the internalization of antigen in vitro reported here supports the development of these molecules as novel mucosal vaccine candidates.

Type II secretion system secretin PulD localizes in clusters in the Escherichia coli outer membrane

The cellular localization of a chimera formed by fusing a monomeric red fluorescent protein to the C terminus of the Klebsiella oxytoca type II secretion system outer membrane secretin PulD (PulD-mCherry) in Escherichia coli was determined in vivo by fluorescence microscopy. Like PulD, PulD-mCherry formed sodium dodecyl sulfate- and heat-resistant multimers and was functional in pullulanase secretion. Chromosome-encoded PulD-mCherry formed fluorescent foci on the periphery of the cell in the presence of high (plasmid-encoded) levels of its cognate chaperone, the pilotin PulS. Subcellular fractionation demonstrated that the chimera was located exclusively in the outer membrane under these circumstances. A similar localization pattern was observed by fluorescence microscopy of fixed cells treated with green fluorescent protein-tagged affitin, which binds with high affinity to an epitope in the N-terminal region of PulD. At lower levels of (chromosome-encoded) PulS, PulD-mCherry was less stable, was located mainly in the inner membrane, from which it could not be solubilized with urea, and did not induce the phage shock response, unlike PulD in the absence of PulS. The fluorescence pattern of PulD-mCherry under these conditions was similar to that observed when PulS levels were high. The complete absence of PulS caused the appearance of bright and almost exclusively polar fluorescent foci.

Production of active eukaryotic proteins through bacterial expression systems: a review of the existing biotechnology strategies

Among the various expression systems employed for the over-production of proteins, bacteria still remains the favorite choice of a Protein Biochemist. However, even today, due to the lack of post-translational modification machinery in bacteria, recombinant eukaryotic protein production poses an immense challenge, which invariably leads to the production of biologically in-active protein in this host. A number of techniques are cited in the literature, which describe the conversion of inactive protein, expressed as an insoluble fraction, into a soluble and active form. Overall, we have divided these methods into three major groups: Group-I, where the factors influencing the formation of insoluble fraction are modified through a stringent control of the cellular milieu, thereby leading to the expression of recombinant protein as soluble moiety; Group-II, where protein is refolded from the inclusion bodies and thereby target protein modification is avoided; Group-III, where the target protein is engineered to achieve soluble expression through fusion protein technology. Even within the same family of proteins (e.g., tyrosine kinases), optimization of standard operating protocol (SOP) may still be required for each protein's over-production at a pilot-scale in Escherichia coli. However, once standardized, this procedure can be made amenable to the industrial production for that particular protein with minimum alterations.

Production of active eukaryotic proteins through bacterial expression systems: a review of the existing biotechnology strategies

Among the various expression systems employed for the over-production of proteins, bacteria still remains the favorite choice of a Protein Biochemist. However, even today, due to the lack of post-translational modification machinery in bacteria, recombinant eukaryotic protein production poses an immense challenge, which invariably leads to the production of biologically in-active protein in this host. A number of techniques are cited in the literature, which describe the conversion of inactive protein, expressed as an insoluble fraction, into a soluble and active form. Overall, we have divided these methods into three major groups: Group-I, where the factors influencing the formation of insoluble fraction are modified through a stringent control of the cellular milieu, thereby leading to the expression of recombinant protein as soluble moiety; Group-II, where protein is refolded from the inclusion bodies and thereby target protein modification is avoided; Group-III, where the target protein is engineered to achieve soluble expression through fusion protein technology. Even within the same family of proteins (e.g., tyrosine kinases), optimization of standard operating protocol (SOP) may still be required for each protein's over-production at a pilot-scale in Escherichia coli. However, once standardized, this procedure can be made amenable to the industrial production for that particular protein with minimum alterations.

The bacterial twin-arginine translocation pathway

The twin-arginine translocation (Tat) pathway is responsible for the export of folded proteins across the cytoplasmic membrane of bacteria. Substrates for the Tat pathway include redox enzymes requiring cofactor insertion in the cytoplasm, multimeric proteins that have to assemble into a complex prior to export, certain membrane proteins, and proteins whose folding is incompatible with Sec export. These proteins are involved in a diverse range of cellular activities including anaerobic metabolism, cell envelope biogenesis, metal acquisition and detoxification, and virulence. The Escherichia coli translocase consists of the TatA, TatB, and TatC proteins, but little is known about the precise sequence of events that leads to protein translocation, the energetic requirements, or the mechanism that prevents the export of misfolded proteins. Owing to the unique characteristics of the pathway, it holds promise for biotechnological applications.

Functional analysis of the twin-arginine translocation pathway in Corynebacterium glutamicum ATCC 13869

Compared to those of other gram-positive bacteria, the genetic structure of the Corynebacterium glutamicum Tat system is unique in that it contains the tatE gene in addition to tatA, tatB, and tatC. The tatE homologue has been detected only in the genomes of gram-negative enterobacteria. To assess the function of the C. glutamicum Tat pathway, we cloned the tatA, tatB, tatC, and tatE genes from C. glutamicum ATCC 13869 and constructed mutants carrying deletions of each tat gene or of both the tatA and tatE genes. Using green fluorescent protein (GFP) fused with the twin-arginine signal peptide of the Escherichia coli TorA protein, we demonstrated that the minimal functional Tat system required TatA and TatC. TatA and TatE provide overlapping function. Unlike the TatB proteins from gram-negative bacteria, C. glutamicum TatB was dispensable for Tat function, although it was required for maximal efficiency of secretion. The signal peptide sequence of the isomaltodextranase (IMD) of Arthrobacter globiformis contains a twin-arginine motif. We showed that both IMD and GFP fused with the signal peptide of IMD were secreted via the C. glutamicum Tat pathway. These observations indicate that IMD is a bona fide Tat substrate and imply great potential of the C. glutamicum Tat system for industrial production of heterologous folded proteins.