Expression of the truncated tissue plasminogen activator (K2S) gene in tobacco chloroplast

Reteplase Fc-fusions produced in N. benthamiana are able to dissolve blood clots ex vivo

Thrombolytic and fibrinolytic therapies are effective treatments to dissolve blood clots in stroke therapy. Thrombolytic drugs activate plasminogen to its cleaved form plasmin, a proteolytic enzyme that breaks the crosslinks between fibrin molecules. The FDA-approved human tissue plasminogen activator Reteplase (rPA) is a non-glycosylated protein produced in E. coli. rPA is a deletion mutant of the wild-type Alteplase that benefits from an extended plasma half-life, reduced fibrin specificity and the ability to better penetrate into blood clots. Different methods have been proposed to improve the production of rPA. Here we show for the first time the transient expression in Nicotiana benthamiana of rPA fused to the immunoglobulin fragment crystallizable (Fc) domain on an IgG1, a strategy commonly used to improve the stability of therapeutic proteins. Despite our success on the expression and purification of dimeric rPA-Fc fusions, protein instability results in high amounts of Fc-derived degradation products. We hypothesize that the "Y"- shape of dimeric Fc fusions cause steric hindrance between protein domains and leads to physical instability. Indeed, mutations of critical residues in the Fc dimerization interface allowed the expression of fully stable rPA monomeric Fc-fusions. The ability of rPA-Fc to convert plasminogen into plasmin was demonstrated by plasminogen zymography and clot lysis assay shows that rPA-Fc is able to dissolve blood clots ex vivo. Finally, we addressed concerns with the plant-specific glycosylation by modulating rPA-Fc glycosylation towards serum-like structures including α2,6-sialylated and α1,6-core fucosylated N-glycans completely devoid of plant core fucose and xylose residues.

Gene delivery strategies for therapeutic proteins production in plants: Emerging opportunities and challenges

There are sharply rising demands for pharmaceutical proteins, however shortcomings associated with traditional protein production methods are obvious. Genetic engineering of plant cells has gained importance as a new strategy for protein production. But most current genetic manipulation techniques for plant components, such as gene gun bombardment and Agrobacterium mediated transformation are associated with irreversible tissue damage, species-range limitation, high risk of integrating foreign DNAs into the host genome, and complicated handling procedures. Thus, there is urgent expectation for innovative gene delivery strategies with higher efficiency, fewer side effect, and more practice convenience. Materials based nanovectors have established themselves as novel vehicles for gene delivery to plant cells due to their large specific surface areas, adjustable particle sizes, cationic surface potentials, and modifiability. In this review, multiple techniques employed for plant cell-based genetic engineering and the applications of nanovectors are reviewed. Moreover, different strategies associated with the fusion of nanotechnology and physical techniques are outlined, which immensely augment delivery efficiency and protein yields. Finally, approaches that may overcome the associated challenges of these strategies to optimize plant bioreactors for protein production are discussed.

Transient expression of anti-VEFGR2 nanobody in Nicotiana tabacum and N. benthamiana

In human, the interaction between vascular endothelial growth factor (VEGF) and its receptor (VEGFR2) is critical for tumor angiogenesis. This is a vital process for cancer tumor growth and metastasis. Blocking VEGF/VEGFR2 conjugation by antibodies inhibits the neovascularization and tumor metastasis. This investigation designed to use a transient expression platform for production of recombinant anti-VEGFR2 nanobody in tobacco plants. At first, anti-VEGFR2-specific nanobody gene was cloned in a Turnip mosaic virus (TuMV)-based vector, and then, it was expressed in Nicotiana benthamiana and Nicotiana tabacum cv. Xanthi transiently. The expression of nanobody in tobacco plants were confirmed by reverse transcription-polymerase chain reaction (RT-PCR), dot blot, enzyme-linked immunosorbent assays (ELISA), and Western blot analysis. It was shown that tobacco plants could accumulate nanobody up to level 0.45% of total soluble protein (8.3 µg/100 mg of fresh leaf). This is the first report of the successful expression of the camelied anti-VEFGR2 nanobody gene in tobacco plants using a plant viral vector. This system provides a fast solution for production of pharmaceutical and commercial proteins such as anti-cancer nanobodies in tobacco plants.

The Effect of Methyl Jasmonate and Temperature on the Transient Expression of Recombinant Proteins in Cucurbita pepo L

The aim of this study is to assess the effect of methyl jasmonate (MeJA) and temperature on the valuable pharmaceuticals expression in a virus-mediated transient expression system, and so the Zuchini Yellow Mosaic Virus (ZYMV) based vector was used for transferring the GFP reporter gene and recombinant tissue plasminogen activator (rtPA) gene (K2S) to cucurbit (Cucurbita pepo L.). MeJA, temperature and time (days after inoculation), were evaluated as a factorial experiment in a completely randomized design (CRD). At first, the effect of all treatment combinations on GFP expression was assessed. At this step, the ELISA test was used to select the optimum treatment combination. ELISA method revealed the significant difference between applied treatments. The optimized treatment significantly increased the expression of rtPA compared to the control. The Real-Time PCR reaction for both GFP and rtPA genes showed no significant differences between optimum and control treatments, however, transcripts of the small subunit of RuBisCO were extremely down-regulated in optimum treatment condition. Reduction in RuBisCO expression at protein level was tangible under treatment condition based on the ELISA test. Therefore, it can be inferred that suppressing the expression of RuBisCO, probably resulted in higher access of expression system to free amino acids inside the cell. In this study, MeJA has been shown to be a positive factor, but the low temperature (17 °C), unlike previous studies, suppressed the expression of recombinant protein unexpectedly, probably due to the incompatibility of the viral construct with low temperature. In conclusion, the use of a suitable gene construct, which is not sensitive to temperature, is likely to result in a more favorable outcome.

In silico and in vivo analyses of the mutated human tissue plasminogen activator (mtPA) and the antithetical effects of P19 silencing suppressor on its expression in two Nicotiana species

Human tissue-type plasminogen activator is one of the most important therapeutic proteins involved in the breakdown of blood clots following the stroke. A mutation was found at position 1541 bp (G514E) and the mutated form was cloned into the binary vector pTRAc-ERH. In silico analysis showed that this mutation might have no significant effect on the active site of the tissue plasminogen activator enzyme. Accordingly, zymography assay confirmed the serine protease activity of the mutated form and its derivatives. The expression of the mutated form was verified with/without co-agroinjection of the P19 gene silencing suppressor in both Nicotiana tabacum and N. benthamiana. The ELISA results showed that the concentration of the mutated form in the absence of P19 was 0.65% and 0.74% of total soluble protein versus 0.141% and 1.36% in the presence of P19 in N. benthamiana and N. tabacum, respectively. In N. tabacum, co-agroinjection of P19 had the synergistic effect and increased the mutated tissue plasminogen activator production two-fold higher. However, in N. benthamiana, the presence of P19 had the adverse effect of five-fold reduction in the concentration. Moreover, results showed that the activity of the mutated form and its derivatives was more than that of the purified commercial tissue plasminogen activator.

Expression of VGRNb-PE immunotoxin in transplastomic lettuce (Lactuca sativa L.)

KEY MESSAGE

This research has shown, for the first time, that plant chloroplasts are a suitable compartment for synthesizing recombinant immunotoxins and the transgenic immunotoxin efficiently causes the inhibition of VEGFR2 overexpression, cell growth and proliferation. Angiogenesis refers to the formation of new blood vessels, which resulted in the growth, invasion and metastasis of cancer. The vascular endothelial growth factor receptor 2 (VEGFR2) plays a major role in angiogenesis and blocking of its signaling inhibits neovascularization and tumor metastasis. Immunotoxins are promising therapeutics for targeted cancer therapy. They consist of an antibody linked to a protein toxin and are designed to specifically kill the tumor cells. In our previous study, VGRNb-PE immunotoxin protein containing anti-VEGFR2 nanobody fused to the truncated form of Pseudomonas exotoxin A has been established. Here, we expressed this immunotoxin in lettuce chloroplasts. Chloroplast genetic engineering offers several advantages, including high levels of transgene expression, multigene engineering in a single transformation event and maternal inheritance of the transgenes. Site specific integration of transgene into chloroplast genomes, and homoplasmy were confirmed. Immunotoxin levels reached up to 1.1% of total soluble protein or 33.7 µg per 100 mg of leaf tissue (fresh weight). We demonstrated that transgenic immunotoxin efficiently causes the inhibition of VEGFR2 overexpression, cell growth and proliferation. These results indicate that plant chloroplasts are a suitable compartment for synthesizing recombinant immunotoxins.

Biotechnological production of recombinant tissue plasminogen activator protein (reteplase) from transplastomic tobacco cell cultures

Transplastomic plants are a system of choice for the mass production of biopharmaceuticals due to the polyploidy of the plastid genome and the low risk of pollen-mediated outcrossing because of maternal inheritance. However, as field-grown plants, they can suffer contamination by agrochemicals and fertilizers, as well as fluctuations in yield due to climatic changes and infections. Tissue-type plasminogen activator (tPA), a protein used to treat heart attacks, converts plasminogen into plasmine, which digests fibrin and induces the dissolution of fibrin clots. Recently, we obtained transplastomic tobacco plants carrying the K2S gene encoding truncated human tPA (reteplase) with improved biological activity, and confirmed the presence of the target protein in the transgenic plant leaves. Considering the advantages of plant cell cultures for biopharmaceutical production, we established a cell line derived from the K2S tobacco plants. The active form of reteplase was quantified in cultures grown in light or darkness, with production 3-fold higher in light.

Transient expression of recombinant tissue plasminogen activator (rt-PA) gene in cucurbit plants using viral vector

OBJECTIVE

To use a transient expression system to express a truncated human tissue plasminogen activator (K2S) gene in cucurbit plants.

RESULTS

The recombinant tissue plasminogen activator protein (K2S form) was expressed in active form in cucurbit plants. Its molecular weight was 43 kDa. The plant-derived rt-PA was determined using goat anti-rabbit antibody by western blotting. Among the infected lines, the highest expression of rt-PA was 62 ng/100 mg per leaf tissue as measured by ELISA. The enzymatic activity of the plant-derived rt-PA was 0.8 IU/ml.

CONCLUSIONS

The K25 form of rt-PA was expressed for the first time using the viral expression system. Plant-derived rt-PA showed similar potency to commercially-available PA.

Purification of recombinant tissue plasminogen activator (rtPA) protein from transplastomic tobacco plants

Plants are low cost platforms for the production of recombinant proteins, but their complexity renders the purification of plant recombinant proteins more difficult than proteins expressed in yeast or bacteria. Plastid transformation enables high-level expression of foreign genes and the accumulation of recombinant proteins in plastid organelles. Histidine (His) tags are widely used for affinity purification of recombinant proteins in a nickel column. The human tissue-type plasminogen activator (tPA) is one of the most important pharmaceutical recombinant proteins involved in the breakdown of blood clots in different parts of the body. The truncated form of the tissue plasminogen activator (K2S) has a longer plasma half-life, better diffusion into the clot, and higher fibrinolytic activity. In a construct designed to insert the K2S gene in the tobacco chloroplast, the sequence of six histidines and a factor Xa protease site was fused to the C-terminus of the K2S protein. The presence and amount of tPA recombinant protein in transplastomic tobacco plants was estimated by ELISA analysis using a specific antibody. The protein was purified from total soluble protein, insoluble protein aggregates and the protein was extracted from the isolated chloroplast using nickel resin and a chromatography column. After digestion of the purified protein with factor Xa, the presence of the purified tPA protein was confirmed by western blot analysis.

Dicistronic expression of human proinsulin-protein A fusion in tobacco chloroplast

Different expression systems such as bacteria and mammalian cells have been used to produce pharmaceutical proteins. In recent years, the use of plants as bioreactors offers efficient and economical systems in recombinant protein production. Furthermore, because of the large number of plastid copies in plants, chloroplast engineering functions as an effective method to increase recombinant protein expression. Because the commercially available insulin for treatment does not contain C-peptide, which is of great importance for type 1 diabetic patients, the current study introduces the human proinsulin gene fused with protein A into the tobacco chloroplast genome using the biolistic method. To achieve homoplasmy, three rounds of selection and regeneration of transforming cells were performed on the medium that contained spectinomycin antibiotic and hormones. The PCR analysis indicated the presence of the proinsulin gene in transplastomic plants. The reverse-transcription PCR analysis confirmed the expression of the proinsulin-protein A fusion at the transcription level. Immunoblot assays of leaf-derived protein extracts confirmed that the target gene expression is up to 0.2% of the total soluble protein. Our study showed that protein A fusion is not as efficient as other reported fusions. The transplastomic plants were also confirmed for homoplasmy using Southern blot analysis.