Recombinant cholera toxin B subunit in Escherichia coli: high-level secretion, purification, and characterization

Development of Heterologous Expression System and Optimization of the Method of Cholera Toxin β-Subunit Production in E. coli

Cholera is a deadly infection disease, which is usually associated with low hygiene levels and limited access to high-quality drinking water. An effective way to prevent cholera is the use of vaccines. Among active vaccine components there is the CtxB protein (cholera toxin β-subunit). In the current work, we have developed a genetic system for production of the recombinant CtxB in E. coli cells and studied conditions for synthesis and purification of the target product at the laboratory scale. It has been found that the optimal algorithm for isolation of the recombinant protein is to grow E. coli culture in the synthetic M9 medium with glycerol, followed by CtxB purification out of the spent culture medium using Ni2+-chelate affinity chromatography techniques. Forty-eight hours after induction of CtxB expression, concentration of the target product could be up to 50 mg/liter in the culture medium. The CtxB protein retains its pentameric structure during expression and through purification. The latter makes it possible to consider the developed system as a promising tool for the industrial-level production of recombinant CtxB for medical and research purposes.

Cholera toxin B: one subunit with many pharmaceutical applications

Cholera, a waterborne acute diarrheal disease caused by Vibrio cholerae, remains prevalent in underdeveloped countries and is a serious health threat to those living in unsanitary conditions. The major virulence factor is cholera toxin (CT), which consists of two subunits: the A subunit (CTA) and the B subunit (CTB). CTB is a 55 kD homopentameric, non-toxic protein binding to the GM1 ganglioside on mammalian cells with high affinity. Currently, recombinantly produced CTB is used as a component of an internationally licensed oral cholera vaccine, as the protein induces potent humoral immunity that can neutralize CT in the gut. Additionally, recent studies have revealed that CTB administration leads to the induction of anti-inflammatory mechanisms in vivo. This review will cover the potential of CTB as an immunomodulatory and anti-inflammatory agent. We will also summarize various recombinant expression systems available for recombinant CTB bioproduction.

The long term oral regulation of blood glucose in diabetic patients by using of Escherichia coli Nissle 1917 expressing CTB-IGF-1 hybrid protein

Regarding to the high prevalence and comorbidities of chronic high blood glucose in diabetic patients and the limited efficacy and current painful treatments. It is necessary to improve new treatments that are non-invasive and long-term for controlling blood glucose. Recent studies have shown that the healthy microflora in different body organs can perform as the gene vectors for expressing different types of gene therapies in situ. We have proposed that by constructing a recombinant Escherichia coli Nissle 1917 that expresses CTB-IGF-1 hybrid gene under control of ompC glucose sensitive promoter, the intestinal glucose level can be regulated. This method in comparison with other methods is a non-invasive way to control the blood glucose orally and it can be used for all types of diabetes.

Rapid and scalable plant-based production of a cholera toxin B subunit variant to aid in mass vaccination against cholera outbreaks

INTRODUCTION

Cholera toxin B subunit (CTB) is a component of an internationally licensed oral cholera vaccine. The protein induces neutralizing antibodies against the holotoxin, the virulence factor responsible for severe diarrhea. A field clinical trial has suggested that the addition of CTB to killed whole-cell bacteria provides superior short-term protection to whole-cell-only vaccines; however, challenges in CTB biomanufacturing (i.e., cost and scale) hamper its implementation to mass vaccination in developing countries. To provide a potential solution to this issue, we developed a rapid, robust, and scalable CTB production system in plants.

METHODOLOGY/PRINCIPAL FINDINGS

In a preliminary study of expressing original CTB in transgenic Nicotiana benthamiana, the protein was N-glycosylated with plant-specific glycans. Thus, an aglycosylated CTB variant (pCTB) was created and overexpressed via a plant virus vector. Upon additional transgene engineering for retention in the endoplasmic reticulum and optimization of a secretory signal, the yield of pCTB was dramatically improved, reaching >1 g per kg of fresh leaf material. The protein was efficiently purified by simple two-step chromatography. The GM1-ganglioside binding capacity and conformational stability of pCTB were virtually identical to the bacteria-derived original B subunit, as demonstrated in competitive enzyme-linked immunosorbent assay, surface plasmon resonance, and fluorescence-based thermal shift assay. Mammalian cell surface-binding was corroborated by immunofluorescence and flow cytometry. pCTB exhibited strong oral immunogenicity in mice, inducing significant levels of CTB-specific intestinal antibodies that persisted over 6 months. Moreover, these antibodies effectively neutralized the cholera holotoxin in vitro.

CONCLUSIONS/SIGNIFICANCE

Taken together, these results demonstrated that pCTB has robust producibility in Nicotiana plants and retains most, if not all, of major biological activities of the original protein. This rapid and easily scalable system may enable the implementation of pCTB to mass vaccination against outbreaks, thereby providing better protection of high-risk populations in developing countries.

Lincomycin-induced over-expression of mature recombinant cholera toxin B subunit and the holotoxin in Escherichia coli

Cholera toxin (CT) B subunit (CTB) was overproduced using a novel expression system in Escherichia coli. An expression plasmid was constructed by inserting the gene encoding the full-length CTB and the Shine-Dalgarno (SD) sequence derived from CTB or from the heat-labile enterotoxin B subunit (LTB) of enterotoxigenic E. coli into the lacZalpha gene fragment in the pBluescript SK(+) vector. The E. coli strain MV1184 was transformed with each plasmid and then cultured in CAYE broth containing lincomycin. Recombinant CTB (rCTB) was purified from each cell extract. rCTB was overproduced in both transformants without obvious toxicity and was structurally and biologically identical to that of CT purified from Vibrio cholerae, indicating that the original SD and CTB signal sequences were also sufficient to express rCTB in E. coli. Lincomycin-induced rCTB expression was inhibited by mutating the lac promoter, suggesting that lincomycin affects the lactose operon. Based on these findings, we constructed a plasmid that contained the wild-type CT operon and successfully overproduced CT (rCT) using the same procedure for rCTB. Although rCT had an intact A subunit, the amino-terminal modifications and biological properties of the A and B subunits of rCT were identical to those of CT. These results suggest that this novel rCTB over-expression system would also be useful to generate both wild-type and mutant CT proteins that will facilitate further studies on the characteristics of CT, such as mucosal adjuvant activity.

Construction of cholera toxin B subunit-producing Vibrio cholerae strains using the Mariner-FRT transposon delivery system

The most widely used oral whole-cell-recombinant B subunit cholera vaccine contains the nontoxic cholera toxin B subunit (CTXB) and either heat- or formalin-killed Vibrio cholerae O1 strains. Vibrio cholerae O1 strains in the vaccine provide antibacterial immunity, and CTXB contributes to the vaccine's efficacy by stimulating production of anti-CTXB antibody. Various attempts have been made to increase CTXB production. In this study, the mariner-FRT transposon delivery system developed by Chiang and Mekalanos was used to place the ctxB gene under the control of a strong chromosomal promoter in a nontoxigenic V. cholerae El Tor strain, M7922. The expression level of CTXB in transposon insertion mutant clones was screened by ganglioside-dependent enzyme-linked immunosorbent assay. Among CTXB-producing V. cholerae clones that were isolated, M7922-C1 produced the highest amount of CTXB (3.17+/-1.69 microg mL(-1)). M7922-C1 harbors a single insertion of ctxB into VC0972, which encodes a putative porin protein. Although the level of CTXB expression in this strain was not exceptionally high, this study indicates the possibility of using this delivery system to construct vaccine strains that overexpress specific antigens.

Production and characterization of single chain Fv directed against beta2-agonist clenbuterol

The single chain Fv (scFv) directed against beta2-agonist clenbuterol (CBL) was produced by using phage display technology. The heavy chain and light chain variable region genes (VH) VL) were amplified by the polymerase chain reaction (PCR) from CBL specific hybridoma cell lines 5D1 and assembled as a single chain Fv (scFv) fragment with linker peptide (Gly4Ser)3. Then the scFv DNA fragment was cloned into M13 phagemid vector pCANTAB5E and the anti-CBL antibody libraries were constructed. Phages displaying scFv were enriched by panning with CBL-ovalbumin (CBL-OVA) conjugate. After only one round of panning, antigen-positive recombinant phage clones were successfully selected by ELISA. The positive phage was used to infect Escherichia coli HB2151, and the expression of soluble scFv was then induced by IPTG. The scFv showed an improved sensitivity (with IC50 of 0.78 +/- 0.005 ng/mL (n = 4)) when compared with the parent monoclonal antibody (MAb) (with IC50 of 1.34 +/- 0.006 ng/mL (n = 4)) in competitive indirect ELISA (CI-ELISA). Cross-reactivity studies showed that the specificity of scFv was similar to that of MAb. The recombinant scFv prepared in this study could be potentially used instead of conventional antisera or MAb for development of a rapid and affordable immunoassay for the detection of residual CBL in biological matrices.

B subunit of E. coli enterotoxin as adjuvant and carrier in oral and skin vaccination

Mucosal sites are one of the main natural ports of entry into the body. Stimulation of a local response by antibodies as the systemic protection may enhance the efficacy of non-living vaccines, and allow for vaccination by subunit vaccines without the need for injection. Mucosal or skin vaccination necessitates a suitable adjuvant and carrier. Escherichia coli heat-labile enterotoxin (LT) and its B subunit (LTB) have been found to be effective adjuvants. The aim of this study was to efficiently produce and purify recombinant LTB (brLTB), and examine its adjuvant and carrier properties. The gene encoding LTB was cloned and expressed in E. coli, and the product was found to have a pentameric form with the ability to bind the cell receptor, GM1 ganglioside. A one-step method for efficient purification and concentration of brLTB was developed. Both oral and intramuscular vaccination with purified brLTB yielded high antibody titers, which detected the whole toxin. In an attempt to test its adjuvant characteristics, brLTB was mixed with either BSA or a recombinant protein (rKnob of egg drop syndrome adenovirus) and delivered intramuscularly, orally or transcutaneously. The addition of brLTB significantly elevated the antibody response in groups vaccinated orally and transcutaneously, but had no influence in injected groups. Vaccination with another recombinant protein, (viral protein 2 of infectious bursal disease virus) supplemented with brLTB did not elevate the antibody response, as compared to vaccination with the antigen alone. These results demonstrate that the addition of brLTB makes oral and transcutaneous vaccination with protein antigens possible.

Vaccine and adjuvant activity of recombinant subunit B of E. coli enterotoxin produced in yeast

Escherichia coli heat-labile enterotoxin (LT) and cholera toxin (CT) have been studied intensively as vaccines against diseases caused by those bacteria and as adjuvants for mucosal vaccination. Two major problems interfere with the use of these promising adjuvants: their toxicity and the residual bacterial endotoxins mixed with the desired LT. In this study, subunit B of LT was expressed in Pichia pastoris yeast cells (yrLTB) and the recombinant protein was purified and concentrated by ion-exchange chromatography. The final yield of the recombinant protein was 5-8 mg/l induction medium. The molecule is in pentameric form and binds to GM1 gangliosides. When given orally to chickens, anti-LTB antibodies were produced, exhibiting its ability to cross the digestive system and induce an immune response. The adjuvant activity of yrLTB was proven by fusing it to viral protein 2 (VP2) of infectious bursal disease virus. Birds intramuscularly vaccinated with this molecule exhibit 70-100% protection, in a dose-response-dependent manner. This method eliminated the bacterial endotoxins and enabled the production of large quantities of LTB. Expression in a eukaryotic system allows the production of fusion proteins that require post-translational modifications. This may allow oral vaccination with a protein fused to yrLTB. The approach described in this study will enable the efficient production of a non-toxic, eukaryotically expressed enterotoxin as a vaccine against the toxin itself or as a carrier or adjuvant for foreign vaccine molecules.

Advanced genetic strategies for recombinant protein expression in Escherichia coli

Preparations enriched by a specific protein are rarely easily obtained from natural host cells. Hence, recombinant protein production is frequently the sole applicable procedure. The ribosomal machinery, located in the cytoplasm is an outstanding catalyst of recombinant protein biosynthesis. Escherichia coli facilitates protein expression by its relative simplicity, its inexpensive and fast high-density cultivation, the well-known genetics and the large number of compatible tools available for biotechnology. Especially the variety of available plasmids, recombinant fusion partners and mutant strains have advanced the possibilities with E. coli. Although often simple for soluble proteins, major obstacles are encountered in the expression of many heterologous proteins and proteins lacking relevant interaction partners in the E. coli cytoplasm. Here we review the current most important strategies for recombinant expression in E. coli. Issues addressed include expression systems in general, selection of host strain, mRNA stability, codon bias, inclusion body formation and prevention, fusion protein technology and site-specific proteolysis, compartment directed secretion and finally co-overexpression technology. The macromolecular background for a variety of obstacles and genetic state-of-the-art solutions are presented.

Recombinant protein secretion in Escherichia coli

The secretory production of recombinant proteins by the Gram-negative bacterium Escherichia coli has several advantages over intracellular production as inclusion bodies. In most cases, targeting protein to the periplasmic space or to the culture medium facilitates downstream processing, folding, and in vivo stability, enabling the production of soluble and biologically active proteins at a reduced process cost. This review presents several strategies that can be used for recombinant protein secretion in E. coli and discusses their advantages and limitations depending on the characteristics of the target protein to be produced.

Evaluation of bottlenecks in proinsulin secretion by Escherichia coli

This work evaluates three potential bottlenecks in recombinant human proinsulin secretion by Escherichia coli: protein stability, secretion capacity and the effect of molecular size on secretion efficiency. A maximum secretion level of 7.2 mg g(-1) dry cell weight was obtained in the periplasm of E. coli JM109(DE3) host cells. This value probably represents an upper limit in the transport capacity of E. coli cells secreting ZZ-proinsulin and similar proteins with the protein A signal peptide. A selective deletion study was performed in the fusion partner and no effect of the molecular size (17-24 kDa) was detected on secretion efficiency. The protective effect against proteolysis provided by the ZZ domain was thoroughly demonstrated in the periplasm of E. coli and it was also shown that a single Z domain is able to provide the same protection level without compromising the downstream processing. The use of this shorter fusion partner enables a 1.6-fold increase in the recovery of the target protein after cleavage of the affinity handle.

Production of salmon calcitonin by direct expression of a glycine-extended precursor in Escherichia coli

The export of heterologous products into the conditioned medium of an Escherichia coli culture offers the advantages of a higher product yield, an increased probability of recovering an intact recombinant protein, proper folding for biological activity, and greater stability of a secreted product. In this report, we describe the development of an optimized direct expression system, designed to maximize the extracellular accumulation of recombinant glycine-extended salmon calcitonin peptide (sCTgly). We have used dual promoters, an ompA signal sequence, co-expression of homologous secretion factor genes, and multiple gene cartridges to express the sCTgly. High-density fermentation conditions have been developed that allow for the selective secretion and accumulation of the expressed sCTgly at very high levels. Purification and in vitro enzymatic conversion by peptidylglycine alpha-amidating monooxygenase yields authentic, biologically active salmon calcitonin. This recombinant production technology is applicable to a variety of amidated peptide hormones.

Fiberless recombinant adenoviruses: virus maturation and infectivity in the absence of fiber

In vivo targeting of therapeutic genes to specific tissues has become a major issue in gene therapy, in particular when recombinant adenovirus vectors are used. Restriction of the viral tropism to selected cell types requires the abrogation of the interaction between the viral fiber and its natural cellular receptors and the introduction of a new binding specificity into the virion. In this context, fiberless adenoviruses are attractive vectors, since they may be used as substrates for the insertion of a new ligand in other capsid proteins. In this study, we confirm by using cloned full-length adenovirus genomes with the fiber gene deleted that efficient virus particle formation can occur in the absence of fiber. As expected, the infectivity of such fiberless viruses was severely reduced, but it could be only partially restored when the viruses were produced in cells stably providing the fiber in trans. Although incorporation of penton base into the fiberless particles was normal and binding of the particles to the cellular integrins was functional, several pieces of experimental evidence suggest that later steps in the cell entry process are impaired in correlation with an incorrect maturation of several structural proteins of the fiberless particles. These observations support the hypothesis that the fiber protein may have additional biological functions besides its role in cell binding. Together with the fiber complementation cells, such fiberless vectors constitute unique tools to investigate the role of the fiber in virus assembly, maturation, and cell entry and to explore the possibility of deriving gene transfer vectors with novel target specificities.

Production of cholera toxin B subunit in Lactobacillus

The intracellular expression of the B subunit of cholera toxin (CTB) was first achieved in Lactobacillus paracasei LbTGS1.4 with an expression cassette including the P25 promoter of Streptococcus thermophilus combined with the translation initiation region from the strongly expressed L. pentosus D-lactate dehydrogenase gene (ldhD). Secretion of CTB was next attempted in L. paracasei LbTGS1.4 and L. plantarum NCIMB8826 with four different signal sequences from exported proteins of lactic acid bacteria (Lactococcus lactis Usp45 and PrtP, Enterococcus faecalis unknown protein and S. pyogenes M6 protein). Host-dependent secretion of CTB was clearly observed: whereas none of the secretion cassettes led to detectable CTB in the extracellular fraction of L. paracasei LbTGS1.4, secretion of CTB molecules was clearly achieved with three of the selected signal sequences in L. plantarum NCIMB8826.

Affinity purification of recombinant cholera toxin B subunit oligomer expressed in Bacillus brevis for potential human use as a mucosal adjuvant

For use as a mucosal adjuvant for human vaccines, a simple method has been developed for the affinity purification of recombinant cholera toxin B subunit which had been expressed in a safe host, Bacillus brevis. Recombinant cholera toxin B subunit, adsorbed quantitatively to a D-galactose-agarose column, was eluted with an 0.1-0.4 M D-galactose gradient with a yield of > 90%. The cholera toxin B subunit preparation was similar to the native cholera toxin B subunit with respect to GM1 binding ability, remarkable stability of the pentamer, and the dissociation-reassociation property by shifting pHs. Cross-linking experiments with glutaraldehyde demonstrated that the pentameric form was predominant; tetrameric, trimeric, dimeric and monomeric forms were detected to a lesser extent, and additionally 10- and 15-mers were observed depending on the concentration of the cholera toxin B subunit.

Construction and characterization of versatile cloning vectors for efficient delivery of native foreign proteins to the periplasm of Escherichia coli

Induction of the wild type cholera toxin operon (ctxAB) from multicopy clones in Escherichia coli inhibited growth and resulted in low yields of cholera toxin (CT). We found that production of wild type CT or its B subunit (CT-B) as a periplasmic protein was toxic for E. coli, but by replacing the native signal sequences of both CT-A and CT-B with the signal sequence from the B subunit of E. coli heat-labile enterotoxin LTIIb we succeeded for the first time in producing CT holotoxin in high yield in E. coli. Based on these findings, we designed and constructed versatile cloning vectors that use the LTIIb-B signal sequence to direct recombinant native proteins with high efficiency to the periplasm of E. coli. We confirmed the usefulness of these vectors by producing two other secreted recombinant proteins. First, using phoA from E. coli, we demonstrated that alkaline phosphatase activity was 17-fold greater when the LTIIb-B signal sequence was used than when the native leader for alkaline phosphatase was used. Second, using the pspA gene that encodes pneumococcal surface protein A from Streptococcus pneumoniae, we produced a 299-residue amino-terminal fragment of PspA in E. coli in large amounts as a soluble periplasmic protein and showed that it was immunoreactive in Western blots with antibodies against native PspA. The vectors described here will be useful for further studies on structure-function relationships and vaccine development with CT and PspA, and they should be valuable as general tools for delivery of other secretion-competent recombinant proteins to the periplasm in E. coli.

Regulation and expression of human Fabs under the control of the Escherichia coli arabinose promoter, PBAD

BACKGROUND

The L-arabinose operon from E. coli contains an inducible promoter PBAD which has been extensively studied for the control of gene expression. PBAD has a number of potential advantages over Plac, and has been used successfully to promote high level expression of recombinant proteins.

OBJECTIVES

The aim of this study was to investigate PBAD as an alternative system to Plac for the bacterial expression of recombinant Fabs.

STUDY DESIGN

The promoter PBAD from the E. coli arabinose operon araBAD and the gene encoding the regulator of this promoter, were cloned into the phagemid expression vector MCO1. Expression of human recombinant tetanus toxoid (TT) and c-erbB2 Fabs under the control of PBAD was compared at two induction temperatures with the same Fabs produced under the control of Plac.

RESULTS

Expression of TT and c-erbB2 Fabs under the control of PBAD was comparable to Fab expression from Plac. However, highly expressed TT Fab under the control of PBAD was localised to the soluble periplasmic fraction whereas under the control of Plac, there was greater leakage of Fab into the culture supernatant. In addition, Fab expression from PBAD could be more tightly repressed than from Plac.

CONCLUSION

PBAD is a useful and cheaply inducible alternative to the more commonly used Plac for the rapid expression of soluble recombinant human antibody fragments.