N-glycosylation and N-glycan moieties of CTB expressed in rice seeds

Effects of Kifunensine on Production and N-Glycosylation Modification of Butyrylcholinesterase in a Transgenic Rice Cell Culture Bioreactor

The production and N-glycosylation of recombinant human butyrylcholinesterase (BChE), a model highly glycosylated therapeutic protein, in a transgenic rice cell suspension culture treated with kifunensine, a strong α-mannosidase I inhibitor, was studied in a 5 L bioreactor. A media exchange was performed at day 7 of cultivation by removing spent sugar-rich medium (NB+S) and adding fresh sugar-free (NB-S) medium to induce the rice α-amylase 3D (RAmy3D) promoter to produce rice recombinant human BChE (rrBChE). Using a 1.25X-concentrated sugar-free medium together with an 80% reduced working volume during the media exchange led to a total active rrBChE production level of 79 ± 2 µg (g FW)^-1 or 7.5 ± 0.4 mg L^-1 in the presence of kifunensine, which was 1.5-times higher than our previous bioreactor runs using normal sugar-free (NB-S) media with no kifunensine treatment. Importantly, the amount of secreted active rrBChE in culture medium was enhanced in the presence of kifunensine, comprising 44% of the total active rrBChE at day 5 following induction. Coomassie-stained SDS-PAGE gel and Western blot analyses revealed different electrophoretic migration of purified rrBChE bands with and without kifunensine treatment, which was attributed to different N-glycoforms. N-Glycosylation analysis showed substantially increased oligomannose glycans (Man5/6/7/8) in rrBChE treated with kifunensine compared to controls. However, the mass-transfer limitation of kifunensine was likely the major reason for incomplete inhibition of α-mannosidase I in this bioreactor study.

High expression of GUS activities in sweet potato storage roots by sucrose-inducible minimal promoter

We developed transgenic sweet potato with Spo^min (sucrose-inducible minimal promoter)-GUS gene-fused constructs. Induced GUS activities by Spo^min were higher than those by CaMV 35S promoter. We developed transgenic sweet potato (Ipomoea batatas L. Lam. cv. Kokei no. 14) plants with Spo^min (sucrose-inducible minimal promoter)-GUS gene-fused constructs with signal peptides for sorting to cytosol, apoplast and ER, and we analyzed the GUS expression pattern of cut tissue after sucrose treatment. Induced GUS activities by Spo^min were several hundred times higher than those by the CaMV 35S promoter. Also, GUS activities in storage roots induced with a Spo^min-cytosol-GUS construct were higher than those with either Spo^min-apoplast or -ER-GUS constructs. The induced GUS activities by Spo^min were higher in storage roots without sucrose treatment than those with sucrose treatment. Chilling (4 °C) storage roots with Spo^min constructs for 4 weeks produced higher GUS activities than in storage roots stored at 25 °C for 4 weeks. The calculated maximum GUS content in the storage roots was up to about 224.2 μg/g fresh weight. The chilling treatment increased the free sucrose content in the storage roots, and this increase in endogenous sugar levels induced increased GUS activities in the storage roots. Therefore, Spo^min appears to be a useful promoter to develop protein production systems using sweet potato variety Kokei no. 14 storage roots by postharvest treatment.

Comparison of the N-glycosylation on recombinant miraculin expressed in tomato plants with native miraculin

Miraculin is a promising protein with taste-modifying properties. Focusing on the unique function and potential of miraculin, recombinant miraculin production has been explored with the use of heterologous expression systems, but the activities of recombinant miraculins were much lower than those of native miraculin, probably due to the difference in post-translational modification, especially N-glycosylation. For practical use therefore, the differences between N-glycan of recombinant miraculin compared to that of native miraculin should be minimized. Here, to establish the platform for functional miraculin production, we expressed miraculin in tomato plants with the same taste-modifying activity as native miraculin purified from miracle fruit, and we compared the N-glycan structures with those of native miraculin. Our N-glycan structural analysis using purified miraculin, followed by hydrazynolysis, 2-pyridylamine (PA)-labeling, high-performance liquid chromatography, and a liquid chromatography tandem-mass spectrometry analysis revealed that both the native and recombinant miraculins carried an M3 structure as a predominant structure and that most of the N-glycan structures on the miraculins were pauci-mannosidic structures with a smaller amount of plant-specific α1,3-fucosylated and/or β1,2-xylosylated N-glycans and without a Lewis a epitope. These results indicate that the N-glycoform of native miraculin from miracle fruit and recombinant miraculin expressed in tomato plants are almost identical to each other with similar ratios and that, therefore, plant-specific N-glycans are essential for showing the full taste-modifying activity of miraculin.

N-glycan structures and downstream mannose-phosphorylation of plant recombinant human alpha-L-iduronidase: toward development of enzyme replacement therapy for mucopolysaccharidosis I

Arabidopsis N-glycan processing mutants provide the basis for tailoring recombinant enzymes for use as replacement therapeutics to treat lysosomal storage diseases, including N-glycan mannose phosphorylation to ensure lysosomal trafficking and efficacy. Functional recombinant human alpha-L-iduronidase (IDUA; EC 3.2.1.76) enzymes were generated in seeds of the Arabidopsis thaliana complex-glycan-deficient (cgl) C5 background, which is deficient in the activity of N-acetylglucosaminyl transferase I, and in seeds of the Arabidopsis gm1 mutant, which lacks Golgi α-mannosidase I (GM1) activity. Both strategies effectively prevented N-glycan maturation and the resultant N-glycan structures on the consensus sites for N-glycosylation of the human enzyme revealed high-mannose N-glycans of predominantly Man₅ (cgl-IDUA) or Man_6-8 (gm1-IDUA) structures. Both forms of IDUA were equivalent with respect to their kinetic parameters characterized by cleavage of the artificial substrate 4-methylumbelliferyl-iduronide. Because recombinant lysosomal enzymes produced in plants require the addition of mannose-6-phosphate (M6P) in order to be suitable for lysosomal delivery in human cells, we characterized the two IDUA proteins for their amenability to downstream in vitro mannose phosphorylation mediated by a soluble form of the human phosphotransferase (UDP-GlcNAc: lysosomal enzyme N-acetylglucosamine [GlcNAc]-1-phosphotransferase). Gm1-IDUA exhibited a slight advantage over the cgl-IDUA in the in vitro M6P-tagging process, with respect to having a better affinity (i.e. lower K _m) for the soluble phosphotransferase. This may be due to the greater number of mannose residues comprising the high-mannose N-glycans of gm1-IDUA. Our elite cgl- line produces IDUA at > 5.7% TSP (total soluble protein); screening of the gm1 lines showed a maximum yield of 1.5% TSP. Overall our findings demonstrate the relative advantages and disadvantages associated with the two platforms to create enzyme replacement therapeutics for lysosomal storage diseases.

Good manufacturing practices production of a purification-free oral cholera vaccine expressed in transgenic rice plants

The first Good Manufacturing Practices production of a purification-free rice-based oral cholera vaccine (MucoRice-CTB) from transgenic plants in a closed cultivation system yielded a product meeting regulatory requirements. Despite our knowledge of their advantages, plant-based vaccines remain unavailable for human use in both developing and industrialized countries. A leading, practical obstacle to their widespread use is producing plant-based vaccines that meet governmental regulatory requirements. Here, we report the first production according to current Good Manufacturing Practices of a rice-based vaccine, the cholera vaccine MucoRice-CTB, at an academic institution. To this end, we established specifications and methods for the master seed bank (MSB) of MucoRice-CTB, which was previously generated as a selection-marker-free line, evaluated its propagation, and given that the stored seeds must be renewed periodically. The production of MucoRice-CTB incorporated a closed hydroponic system for cultivating the transgenic plants, to minimize variations in expression and quality during vaccine manufacture. This type of molecular farming factory can be operated year-round, generating three harvests annually, and is cost- and production-effective. Rice was polished to a ratio of 95 % and then powdered to produce the MucoRice-CTB drug substance, and the identity, potency, and safety of the MucoRice-CTB product met pre-established release requirements. The formulation of MucoRice-CTB made by fine-powdering of drug substance and packaged in an aluminum pouch is being evaluated in a physician-initiated phase I study.

Expression and functional validation of heat-labile enterotoxin B (LTB) and cholera toxin B (CTB) subunits in transgenic rice (Oryza sativa)

We expressed the heat-labile enterotoxin B (LTB) subunit from enterotoxigenic Escherichia coli and the cholera toxin B (CTB) subunit from Vibrio cholerae under the control of the rice (Oryza sativa) globulin (Glb) promoter. Binding of recombinant LTB and CTB proteins was confirmed based on G_M1-ganglioside binding enzyme-linked immunosorbent assays (G_M1-ELISA). Real-time PCR of three generations (T₃, T₄, and T₅) in homozygous lines (LCI-11) showed single copies of LTB, CTB, bar and Tnos. LTB and CTB proteins in rice transgenic lines were detected by Western blot analysis. Immunogenicity trials of rice-derived CTB and LTB antigens were evaluated through oral and intraperitoneal administration in mice, respectively. The results revealed that LTB- and CTB-specific IgG levels were enhanced in the sera of intraperitoneally immunized mice. Similarly, the toxin-neutralizing activity of CTB and LTB in serum of orally immunized mice was associated with elevated levels of both IgG and IgA. The results of the present study suggest that the combined expression of CTB and LTB proteins can be utilized to produce vaccines against enterotoxigenic strains of Escherichia coli and Vibrio cholera, for the prevention of diarrhea.

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.

N-glycosylation of cholera toxin B subunit in Nicotiana benthamiana: impacts on host stress response, production yield and vaccine potential

Plant-based transient overexpression systems enable rapid and scalable production of subunit vaccines. Previously, we have shown that cholera toxin B subunit (CTB), an oral cholera vaccine antigen, is N-glycosylated upon expression in transgenic Nicotiana benthamiana. Here, we found that overexpression of aglycosylated CTB by agroinfiltration of a tobamoviral vector causes massive tissue necrosis and poor accumulation unless retained in the endoplasmic reticulum (ER). However, the re-introduction of N-glycosylation to its original or an alternative site significantly relieved the necrosis and provided a high CTB yield without ER retention. Quantitative gene expression analysis of PDI, BiP, bZIP60, SKP1, 26Sα proteasome and PR1a, and the detection of ubiquitinated CTB polypeptides revealed that N-glycosylation significantly relieved ER stress and hypersensitive response, and facilitated the folding/assembly of CTB. The glycosylated CTB (gCTB) was characterized for potential vaccine use. Glycan profiling revealed that gCTB contained approximately 38% plant-specific glycans. gCTB retained nanomolar affinity to GM1-ganglioside with only marginal reduction of physicochemical stability and induced an anti-cholera holotoxin antibody response comparable to native CTB in a mouse oral immunization study. These findings demonstrated gCTB's potential as an oral immunogen and point to a potential role of N-glycosylation in increasing recombinant protein yields in plants.

Novel transgenic rice-based vaccines

Oral vaccination can induce both systemic and mucosal antigen-specific immune responses. To control rampant mucosal infectious diseases, the development of new effective oral vaccines is needed. Plant-based vaccines are new candidates for oral vaccines, and have some advantages over the traditional vaccines in cost, safety, and scalability. Rice seeds are attractive for vaccine production because of their stability and resistance to digestion in the stomach. The efficacy of some rice-based vaccines for infectious, autoimmune, and other diseases has been already demonstrated in animal models. We reported the efficacy in mice, safety, and stability of a rice-based cholera toxin B subunit vaccine called MucoRice-CTB. To advance MucoRice-CTB for use in humans, we also examined its efficacy and safety in primates. The potential of transgenic rice production as a new mucosal vaccine delivery system is reviewed from the perspective of future development of effective oral vaccines.

Differential analysis of protein expression in RNA-binding-protein transgenic and parental rice seeds cultivated under salt stress

Transgenic plants tolerant to various environmental stresses are being developed to ensure a consistent food supply. We used a transgenic rice cultivar with high saline tolerance by introducing an RNA-binding protein (RBP) from the ice plant (Mesembryanthemum crystallinum); differences in salt-soluble protein expression between nontransgenic (NT) and RBP rice seeds were analyzed by 2D difference gel electrophoresis (2D-DIGE), a gel-based proteomic method. To identify RBP-related changes in protein expression under salt stress, NT and RBP rice were cultured with or without 200 mM sodium chloride. Only two protein spots differed between NT and RBP rice seeds cultured under normal conditions, one of which was identified as a putative abscisic acid-induced protein. In NT rice seeds, 91 spots significantly differed between normal and salt-stress conditions. Two allergenic proteins of NT rice seeds, RAG1 and RAG2, were induced by high salt. In contrast, RBP rice seeds yielded seven spots and no allergen spots with significant differences in protein expression between normal and salt-stress conditions. Therefore, expression of fewer proteins was altered in RBP rice seeds by high salt than those in NT rice seeds.

Inhibition of human MCF-7 breast cancer cells and HT-29 colon cancer cells by rice-produced recombinant human insulin-like growth binding protein-3 (rhIGFBP-3)

Background

Insulin-like growth factor binding protein-3 (IGFBP-3) is a multifunctional molecule which is closely related to cell growth, apoptosis, angiogenesis, metabolism and senescence. It combines with insulin-like growth factor-I (IGF-I) to form a complex (IGF-I/IGFBP-3) that can treat growth hormone insensitivity syndrome (GHIS) and reduce insulin requirement in patients with diabetes. IGFBP-3 alone has been shown to have anti-proliferation effect on numerous cancer cells.

Methodology/Principal Findings

We reported here an expression method to produce functional recombinant human IGFBP-3 (rhIGFBP-3) in transgenic rice grains. Protein sorting sequences, signal peptide and endoplasmic reticulum retention tetrapeptide (KDEL) were included in constructs for enhancing rhIGFBP-3 expression. Western blot analysis showed that only the constructs with signal peptide were successfully expressed in transgenic rice grains. Both rhIGFBP-3 proteins, with or without KDEL sorting sequence inhibited the growth of MCF-7 human breast cancer cells (65.76 ± 1.72% vs 45.00 ± 0.86%, p < 0.05; 50.84 ± 1.97% vs 45.00 ± 0.86%, p < 0.01 respectively) and HT-29 colon cancer cells (65.14 ±3.84% vs 18.01 ± 13.81%, p < 0.05 and 54.7 ± 9.44% vs 18.01 ± 13.81%, p < 0.05 respectively) when compared with wild type rice.

Conclusion/Significance

These findings demonstrated the feasibility of producing biological active rhIGFBP-3 in rice using a transgenic approach, which will definitely encourage more research on the therapeutic use of hIGFBP-3 in future.

MucoRice-cholera toxin B-subunit, a rice-based oral cholera vaccine, down-regulates the expression of α-amylase/trypsin inhibitor-like protein family as major rice allergens

To develop a cold chain- and needle/syringe-free rice-based cholera vaccine (MucoRice-CTB) for human use, we previously advanced the MucoRice system by introducing antisense genes specific for endogenous rice storage proteins and produced a molecularly uniform, human-applicable, high-yield MucoRice-CTB devoid of plant-associated sugar. To maintain the cold chain-free property of this vaccine for clinical application, we wanted to use a polished rice powder preparation of MucoRice-CTB without further purification but wondered whether this might cause an unexpected increase in rice allergen protein expression levels in MucoRice-CTB and prompt safety concerns. Therefore, we used two-dimensional fluorescence difference gel electrophoresis and shotgun MS/MS proteomics to compare rice allergen protein expression levels in MucoRice-CTB and wild-type (WT) rice. Both proteomics analyses showed that the only notable change in the expression levels of rice allergen protein in MucoRice-CTB, compared with those in WT rice, was a decrease in the expression levels of α-amylase/trypsin inhibitor-like protein family such as the seed allergen protein RAG2. Real-time PCR analysis showed mRNA of RAG2 reduced in MucoRice-CTB seed. These results demonstrate that no known rice allergens appear to be up-reregulated by genetic modification of MucoRice-CTB, suggesting that MucoRice-CTB has potential as a safe oral cholera vaccine for clinical application.