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Year : 2013  |  Volume : 31  |  Issue : 1  |  Page : 10--14

Comparison of two recombinant systems for expression of cholera toxin B subunit from Vibrio cholerae

M Boustanshenas1, B Bakhshi2, M Ghorbani3, D Norouzian4,  
1 Department of Bacteriology, Faculty of Medical Sciences, Tarbiat Modares University, Tehran; Department of Biology, Faculty of Basic Science, Science and Research Branch, Islamic Azad University, Tehran, Iran
2 Department of Bacteriology, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran
3 Department of Research and Development, Research and Production Complex, Pasteur Institute of Iran, Iran
4 Department of Pilot Biotechnology, Pasteur Institute of Iran, Iran

Correspondence Address:
B Bakhshi
Department of Bacteriology, Faculty of Medical Sciences, Tarbiat Modares University, Tehran
Iran

Abstract

Purpose: The aim of this study was to assess the production of recombinant cholera toxin B subunit (rCTB) protein in two different expression systems (pAE_ctxB and pQE_ctxB constructs) in Escherichia coli BL21 (DE3). Materials and Methods: The ctxB fragment was amplified from Vibrio cholerae O 1 ATCC14035 and cloned in pGETM-T easy vector after which it was transformed to E. coli Top 10FSQ and grown on LB-ampicillin agar medium. Sequence analysis confirmed the complete ctxB gene sequence in the construct which was further subcloned to pQE-30 vector. The construct was subsequently transformed to E. coli M15 (pREP4). The recombinant pAE_ctxB and pQE_ctxB were transformed to competent E. coli BL21 (DE3) cells to express CTB protein. Result: Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed the maximum expression of rCTB in both systems at 5 h after induction and western blot analysis confirmed the presence of recombinant CTB in blotting membranes. Conclusion: Expression of rCTB in pAE_ctxB construct was more efficient (15-fold) than pQE_ctxB, and it seems that Lac UV5 in E. coli BL21 (DE3) is more compatible with the former construct. This expression system can be used to produce recombinant CTB in high yield which may enable us to study the oral tolerance or mucosal adjuvant properties of rCTB using animal models.

How to cite this article:
Boustanshenas M, Bakhshi B, Ghorbani M, Norouzian D. Comparison of two recombinant systems for expression of cholera toxin B subunit from Vibrio cholerae.Indian J Med Microbiol 2013;31:10-14

How to cite this URL:
Boustanshenas M, Bakhshi B, Ghorbani M, Norouzian D. Comparison of two recombinant systems for expression of cholera toxin B subunit from Vibrio cholerae. Indian J Med Microbiol [serial online] 2013 [cited 2019 Dec 12 ];31:10-14
Available from: http://www.ijmm.org/text.asp?2013/31/1/10/108705

Full Text

 Introduction



Cholera is a lethal diarrheal disease caused by Vibrio cholerae. The division of V. cholerae into different serogroups is based on its major surface antigen which is called O antigen. Only O 1 and O 139 serotypes are known to cause cholera epidemics. [1] Seven cholera epidemics have occurred all over the world and its outbreaks continue to occur in Iran and other developing countries. [2],[3],[4] Cholera still remains a major problem in countries with poor economic and social conditions in which access to safe water and proper sewage system is not fully provided.

Cholera symptoms are mainly produced by cholera toxin (CT), an enterotoxin which is coded by ctxAB operon that is part of a filamentous bacteriophage genome and composed of two subunits. Subunit A (CTA) is a 28-kDa protein with an enzymatic toxic ability which can increase cellular adenylate cyclase level in intestinal cells and accelerates the secretion of chloride and bicarbonate from the mucosal cells into the intestinal lumen and eventually cause diarrhoea. Subunit B (CTB) is a pentameric protein responsible for binding to GM 1 ganglioside receptor on the surface of host intestinal epithelial cells and facilitates endocytosis of CTA to cells. Molecular weight of each CTB monomer is approximately 11.6 kDa and is recognised as a potent immunogen in intestinal and nasal mucosal sites. [5] The studies indicated that this protein can significantly stimulate the immune system and create immunity against V. cholerae infection. On the other hand, CTB has been reported to stimulate serum secretory antibodies against antigens that are usually poor immunogens; specially when presented together in chemically conjugated or genetically fused forms. [6] It is now accepted that V. cholerae cannot be eradicated from its natural aqueous reservoirs. [7] According to studies, development of an effective vaccine is one of the ways to prevent or control cholera. A single oral clinical infection caused by V. cholerae can induce long-term immunity and adequate immune response can be achieved if appropriate antigens are delivered in combination with V. cholerae. Based on this understanding, variety of vaccines against cholera were developed; vaccines are divided into two principal kinds, the killed- and live-attenuated V. cholerae vaccines. [8] Produced vaccines against cholera have been developed over time and with the advent of recombinant DNA knowledge, different types of potent recombinant strains of V. cholerae were emerged to be used for producing vaccines. [9] In this study, two different expression vectors (pAE_ctxB and pQE_ctxB) were evaluated to express recombinant CTB in E. coli BL21 (DE3) strain and the production process was optimised and characterised.

 Materials and Methods



Selection and construction of recombinant vectors

Two different plasmid vectors were selected for CTB production. pQE plasmid was purchased from Qiagen (Valencia, CA). ctxB gene was amplified by PCR using V. cholerae O 1 ATCC14035 DNA as template and ctxB-F (5'GCG TCATGA TTA AAT TAA AAT TTG GTGTTT TTT TTA CAG TTT/TAC3'/ctxB-R (5'CGCTCGAGGGAACCGCGTGGCACCAGATT TGCCATAGTAATTG 3') as primers, whereas BspHI and XhoI restriction sites have been designed in primer sequences. Amplified ctxB was cloned in pGETM-T easy vector (Promega) and transformed to Escherichia coli Top 10F' (Invitrogen, Carlsbad, CA, USA) grown on LB agar medium containing ampicillin, 100 μg/l; IPTG (Isopropyl β-D-1-thiogalactopyranoside) 40 μg/l; and X-gal. 30 μg/l. White colonies were selected and grown in ampicillin-containing LB broth for 4 h and plasmids were extracted using QIAprep Spin Miniprep Kit (Qiagen). Sequence analysis (Genfanavaran, Macrogen, Seoul, Korea) of extracted plasmids with SP6 and T7-promoter universal primers was performed to confirm the ctxB sequence. The pGEM-T_ctxB construct was then digested with BspHI and XhoI restriction enzymes and subjected to subcloning to NcoI and XhoI digested pQE-30 vector. The construct was transformed to E. coli M15 (pREP4) (Qiagen). The second construct (pAE_ctxB) was kindly provided by Dr. Arκas from Institute of Butantan of Brazil.

Expression of recombinant CTB protein in Escherichia coli

E. coli BL21 (DE3) was used as an expression host in this study (Invitrogen, Carlsbad, CA). As both plasmid vectors used in this study have the T 7 RNA polymerase gene as a promoter for the ctxB gene and the induction of this gene in E. coli BL21 (DE3) is affected by IPTG, this substance (1 mol/l) was used as inducer for LB broth cultures supplemented with ampicillin (1 mmol/l) at OD 600 . The cells were harvested after 5 h and subjected to centrifugation at 4°C, 4000 g for 20 min.

Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis and Western blotting

Lysis buffer (100 mmol/l, NaCl'; 50 mmol/l, Tris-Cl; 1 mmol/l, EDTA; and 0.1 v/w %, Triton X-100) in combination with sonication was used for lysing the cells. Each sample was boiled for 10 min and subjected to electrophoresis on two separate 15% (v/w) sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) gels under the same running conditions.

Each sample of bacteria from pervious step was resuspended in 160 μl lysis buffer. One of the gels was stained with 1% (v/w) Coomassie brilliant blue R-250 to be visualised and the other gel was subjected to protein blotting onto polyvinylidene difluoride (PVDF) membrane (Hi-bond Amersham Biosciences, Piscataway, NJ, USA) using semi-dry blotting system (BioRad, Hercules, CA, USA). Membranes were blocked in blocking buffer containing 1% non-fat milk powder in phosphate buffer saline (PBS) at 4°C overnight after which membranes were incubated sequentially with 1:1000 dilution of rabbit polyclonal anti-CT antibody (Sigma-Aldrich, St. Louis, MO, Germany) and 1:10,000 dilution of horseradish peroxidase (HRP)-conjugated goat anti-rabbit IgG (Sigma-Aldrich, St. Louis, MO, Germany), followed by washing steps. Producing of recombinant CTB was detected with bound antibodies using chemiluminescent substrate, ECL (Enhanced chemiluminescence) (Hi-bond Amersham Biosciences, Piscataway, NJ, USA). Chemiluminescence on PVDF membrane was detected by exposure to Kodak X-Omat Blue Autoradiography Film. Protein expression in both constructs was evaluated using FireReader D56 software (UVI Tec, UK).

 Results



Extracted plasmids from E. coli Top10F' transformed with pGEM-T_ctxB were digested with EcoRI and two bands of 3000 bp and 390 bp were obtained which corresponded to pGEM-T vector and ctxB gene, respectively [Figure 1]a. Sequence analysis confirmed the complete ctxB gene sequence in the construct.{Figure 1}

The pGEM-T_ctxB construct digested with BspHI and XhoI restriction enzymes is shown in [Figure 1]b. The approximately 392-bp band which corresponds to the CTB fragment was further purified and applied for subcloning to XhoI and NcoI digested pQE plasmid [Figure 1]b. The identity of pQE_ctxB construct was confirmed by double digestion with XbaI and XhoI [Figure 1]c. The second construct (pAE_ctxB) was confirmed by double digestion with BamHI and HindIII restriction enzymes [Figure 1]d.{Figure 1}

Expression of recombinant CTB

Time gradient was applied to recognise the optimum time period for maximum expression of CTB after induction by IPTG. This experiment showed that 5 h after induction with IPTG at a final concentration of 1mM was the best condition for producing CTB in E. coli BL21 (DE3) [Figure 2]. For both constructs, protein expression was conducted at temperature of 37°C, 1 mM IPTG and stopped at the 5 th h after induction, which was the best condition for producing recombinant CTB, followed by SDS-PAGE analysis [Figure 2]a and c. Immunoblotting was performed to characterise recombinant CTB. Western blot analysis of first system (pQE_ctxB) confirmed the expression of rCTB as the foremost 14.5-kDa band on SDS-PAGE analysis and chemiluminescent visualised immunoblotting [Figure 2]b and d.{Figure 2}

The oligomeric CTB protein dissociated into monomers of approximately 14 kDa when the cell extract was boiled for 10 min. Monomeric form of rCTB had slightly higher molecular weight than the monomeric form of CTB from commercial Cholera toxin (CT) (11.6 kDa). This discrepancy was due to extra six amino acids (6XHis-tag) that were added to the N-terminus and C-terminus of ctxB gene in pAE_ctxB and pQE_ctxB systems, respectively. The molecular weight of expressed CTB from pQE_ctxB system was ~ 14.5 kDa in comparison to 14-kDa CTB from pAE_ctxB system due to its thrombin sequence which has been added to its C-terminus for removal of His-tag from the expressed protein. The total amount of expressed rCTB with pAE_ctxB in BL21 (DE3) strain before purification was assessed to be approximately 1.2 g/l. while pQE_ctxB produced 0.08 g/l of rCTB in the same host.

 Discussion



CTB is recognised as a potent immunogen in the intestinal and nasal mucosal sites which can stimulate immune systems in animal models. [10] In addition, this protein can be used as an adjuvant in vaccines for oral or nasal immunisations in human. [7] Based on previous studies, CTB can be used as an immunogen protein to develop immunity against toxigenic strains of V. cholerae or as an adjuvant to increase immune responses against other antigens which caused many groups to express CTB in several bacterial systems, such as E. coli, [11] Lactobacillus and Bacillus brevis[12],[13] or even V. cholerae strains lacking the CTA gene. [6] Yeasts (Saccharomyces cerevisiae) and plants such as tomato, potato and tobacco have been used to produce recombinant CTB but the amount of expressed protein was too low in these systems. [14]

In this study, two different expression vectors (pAE_ctxB and pQE_ctxB) were evaluated for producing recombinant CTB in E. coli BL21 (DE3) cells.

In a study by Haryanti et al. (2008), pBAD plasmid was used to produce rCTB in E. coli BL21 (DE3) pLysS strain and the recombinant CTB was characterised by immunoblotting using anti His-tag antibody. [15] Specificity of anti His-tag antibodies cannot warranty the prevention of non-specific bindings during immunoblotting, whereas the rabbit polyclonal anti-CT antibody used in our study had more specificity to binding to CTB for that reason there is no non-specific bindings were observed in immunoblotting results.

Arκas et al. (2002) designed CTB subunit gene (ctxB), inserted into pAE plasmid and produced rCTB in E. coli BL21 (SI). Expression of rCTB was induced by NaCl in this system and the basal amount of produced rCTB was high. [16] The production of E. coli BL21 (SI) strain was discontinued recently, so this is important to develop a new efficient expression system for producing recombinant CTB by means of pAE plasmid as an expression vector.

In this study, SDS-PAGE and western blot analysis of recombinant CTB production in both pAE_ctxB and pQE_ctxB plasmids demonstrated that these two systems are highly efficient for producing rCTB. There are some advantages in using these systems which include (i) Target genes in pAE plasmid are expressed under the control of transcription and termination signals of T 7 RNA polymerase are under the control of the lacUV5 promoter, which is induced by IPTG, (ii) the histidine sequence which was added to the recombinant protein in this system makes it possible to purify the produced recombinant proteins through Ni 2+-charged column chromatography and did not represent any apparent limitation for the protein refolding and activity. One additional advantage of pAE_ctxB is deletion of signal peptide sequence from ctxB gene pAE_ctxB plasmid which prevents protein from secretion into culture medium.

In a study by Haryanti (2008), the reaction between anti His-tag antibody and rCTB was detected using 3,3'- diamino benzidine tetrahydrochloride (DAB) as a substrate of HRP-conjugated goat anti-mouse antibody. [15] Sensitivity of DAB in this system cannot assure accurate demonstration of expression, whereas ECL used in our study is more sensitive and can provide a precise detection of proteins from western blots more than chromogenic indicator systems. [17]

Recombinant CTB expression in pAE_ctxB construct in BL21 (DE3) strain was almost 15-fold higher than pQE_ctxB plasmid using the same host; it means that pAE_ctxB is a more efficient construct to produce rCTB in this system. Arκas et al. (2002) used this construct to express rCTB in BL21 (SI) and the yield of produced recombinant CTB before purification was about 0.12 g/l which is 10-fold less than the rCTB yield in BL21 (DE3) used in this study. This disparity may occur because of some differences between two systems including (i) in this study, BL21 (DE3) was used to express rCTB which is induced by IPTG, whereas Arκas et al. (2002) have used BL21 (SI) in which expression of rCTB is induced by NaCl, (ii) the use of LB broth in our system in comparison with the medium used by them (LBON medium, LB broth without NaCl) which may affect the growth of bacteria and consequently their protein expression, (iii) in the present system. The volume of medium used to produce rCTB was 100 ml, whereas Arκas et al. (2002) produced rCTB in larger volume (1 L) which may affect bacterial growth due to the aeration limitations.

 Conclusion



pAE plasmid can express a high level of recombinant CTB in E. coli BL21 (DE3), which is almost 15-fold higher than pQE_ctxB. E. coli still remains one of the most useful organisms to produce recombinant proteins compared to other bacteria. Lac UV5 in BL21 (DE3) is compatible with pAE_ctxB plasmid and this expression system can be used to produce recombinant CTB in high yield. This enables us to study the oral tolerance or mucosal adjuvant properties of rCTB using animal models.

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