Increased accumulation and stability of rotavirus VP6 protein in tobacco chloroplasts following changes to the 5' untranslated region and the 5' end of the coding region

Redesigning amino/carboxyl ends of DARPin G3 for high thermostability and production in tobacco transplastomic plants

KEY MESSAGE

Redesigning the N- and C-capping repeats of the native DARPin G3 significantly improved its stability, and may facilitate its purification from the total soluble proteins of high-temperature dried leaf materials of transplastomic plants. Designed ankyrin repeat proteins (DARPins) constitute a promising class of binding molecules that can overcome the limitations of monoclonal antibodies and enable the development of novel therapeutic approaches. Despite their inherent stability, detailed studies have revealed that the original capping repeats derived from natural ankyrin repeat proteins impair the stability of the initial DARPin design. Consequently, the development of thermodynamically stabilized antibody mimetics may facilitate the development of innovative drugs in the future. In this study, we replaced the original N- and C-capping repeats with improved caps to enhance the thermostability of native DARPin G3. Computational analyses suggested that the redesigned thermostable DARPin G3 structure possessed optimal quality and stability. Molecular dynamics simulations verified the stability of the redesigned thermostable DARPin G3 at high temperatures. The redesigned thermostable DARPin G3 was expressed at high levels in tobacco transplastomic plants and subsequently purified from high-temperature dried leaf materials. Thermal denaturation results revealed that the redesigned thermostable DARPin G3 had a higher Tm value than the native DARPin G3, with a Tm of 35.51 °C greater than that of native DARPin G3. The results of the in vitro bioassays confirmed that the purified thermostable DARPin G3 from high-temperature dried leaf materials maintained its binding activity without any loss of affinity and specifically bound to the HER2 receptor on the cell surface. These findings demonstrate the successful improvement in the thermostability of DARPin G3 without compromising its biological activity.

Production of Bovine Rotavirus VP6 Subunit Vaccine in a Transgenic Fodder Crop, Egyptian Clover (Berseem, Trifolium alexandrinum) that Elicits Immune Responses in Rabbit

Group A rotavirus causes acute gastroenteritis in young ones of animals worldwide and is responsible for a high rate of their morbidity and mortality leading to huge economic losses. Developing affordable and safer vaccine on large scale is imperative to reach cattle population worldwide for the long-term control of diarrhea. Rotavirus middle capsid protein layer, VP6, is the most immunogenic and highly conserved protein that induces immune responses against rotavirus. In the present study, bovine group A rotavirus VP6 protein has been expressed for the first time in a highly nutritious and palatable forage crop, Trifolium alexandrinum, using Agrobacterium tumefaciens-mediated stable nuclear transformation. Transgenic nature of the shoots regenerated from cotyledon explants and rooted on hygromycin-containing medium was confirmed by polymerase chain reaction (PCR), Southern blot hybridization, reverse transcription-PCR (RT-PCR) and quantitative real-time PCR (qPCR), and protein expression and quantification by Western blot and enzyme-linked immune-sorbent assay (ELISA), respectively. The transformation efficiency of 2.10% was obtained. The highest amount of VP6 protein produced in a transgenic line was 402 ng/g fresh weights (0.03% of total soluble protein). Oral feeding of transgenic leafy shoots expressing VP6 protein stimulated systemic immunity by inducing significantly higher titers of anti-VP6 serum IgG antibodies in rabbit to reduce rotavirus infection. These transgenic fodder plants offer safer vaccine produced on large scale at low cost with reduced regulatory issues to improve livestock's health and wealth. These plants would be used as alternative to the current live attenuated vaccines to protect young calves against rotavirus infection.

The production of the first functional antibody mimetic in higher plants: the chloroplast makes the DARPin G3 for HER2 imaging in oncology

Background

Designed mimetic molecules are attractive tools in biopharmaceuticals and synthetic biology. They require mass and functional production for the assessment of upcoming challenges in the near future. The DARPin family is considered a mimetic pharmaceutical peptide group with high affinity binding to specific targets. DARPin G3 is designed to bind to the HER2 (human epidermal growth factor receptor 2) tyrosine kinase receptor. Overexpression of HER2 is common in some cancers, including breast cancer, and can be used as a prognostic and predictive tool for cancer. The chloroplasts are cost-effective alternatives, equal to, and sometimes better than, bacterial, yeast, or mammalian expression systems. This research examined the possibility of the production of the first antibody mimetic, DARPin G3, in tobacco chloroplasts for HER2 imaging in oncology.

Results

The chloroplast specific DARPin G3 expression cassette was constructed and transformed into N. tabacum chloroplasts. PCR and Southern blot analysis confirmed integration of transgenes as well as chloroplastic and cellular homoplasmy. The Western blot analysis and ELISA confirmed the production of DARPin G3 at the commercial scale and high dose with the rate of 20.2% in leaf TSP and 33.7% in chloroplast TSP. The functional analysis by ELISA confirmed the binding of IMAC purified chloroplast-made DARPin G3 to the extracellular domain of the HER2 receptor with highly effective picomolar affinities. The carcinoma cellular studies by flow cytometry and immunofluorescence microscopy confirmed the correct functioning by the specific binding of the chloroplast-made DARPin G3 to the HER2 receptor on the surface of HER2-positive cancer cell lines.

Conclusion

The efficient functional bioactive production of DARPin G3 in chloroplasts led us to introduce plant chloroplasts as the site of efficient production of the first antibody mimetic molecules. This report, as the first case of the cost-effective production of mimetic molecules, enables researchers in pharmaceuticals, synthetic biology, and bio-molecular engineering to develop tool boxes by producing new molecular substitutes for diverse purposes.

Transient Expression in Cytoplasm and Apoplast of Rotavirus VP6 Protein Fused to Anti-DEC205 Antibody in Nicotiana benthamiana and Nicotiana sylvestris

Rotavirus is the most common cause of severe diarrhea in infants and children worldwide and is responsible for about 215,000 deaths annually. Over 85% of these deaths originate in low-income/developing countries in Asia and Africa. Therefore, it is necessary to explore the development of vaccines that avoid the use of "living" viruses and furthermore, vaccines that have viral antigens capable of generating powerful heterotypic responses. Our strategy is based on the expression of the fusion of the anti-DEC205 single-chain variable fragment (scFv) coupled by an OLLAS tag to a viral protein (VP6) of Rotavirus in Nicotiana plants. It was possible to express transiently in N. benthamiana and N. sylvestris a recombinant protein consisting of the single chain variable fragment linked by an OLLAS tag to the VP6 protein. The presence of the recombinant protein, which had a molecular weight of approximately 75 kDa, was confirmed by immunodetection, in both plant species and in both cellular compartments (cytoplasm and apoplast) where it was expressed. In addition, the recombinant protein was modeled, and it was observed that some epitopes of interest are exposed on the surface, which could favor their immunogenic response.

Improved expression of porcine epidemic diarrhea antigen by fusion with cholera toxin B subunit and chloroplast transformation in Nicotiana tabacum

The porcine epidemic diarrhea virus (PEDV) belongs to the coronavirus family, which causes acute diarrhea in pigs with higher mortality in piglets less than 2 weeks old. The PEDV is one of the major concerns of the pig industry around the world, including Asian countries and Noth America since first identified in Europe. Currently, there is no PEDV licensed vaccine to effectively prevent this disease. This study was performed for the development of a mucosal PEDV vaccine and B subunit of cholera toxin (CTB) as a carrier was employed to surpass the tolerogenic nature of GALT and induce potent immune responses against the target antigen fused to CTB. An epitope (S1D) alone or conjugated with CTB was constructed into the tobacco chloroplasts expression vector which is controlled under the chloroplast rRNA operon promoter with T7g10 5' UTR and the psbA 3'UTR as a terminator. The homoplastomic lines were obtained by third round screening via organogenesis from the leaf tissues which were verified by PCR with antigen and chloroplast specific primers and then confirmed by Southern blot analysis. While the expression level of the S1D alone as detected by Western blotting was approximately 0.07% of total soluble protein, the CTB-S1D fusion protein was expressed up to 1.4%. The fusion protein showed binding to the intestinal membrane GM1-ganglioside receptor, demonstrating its functionality. The result shows that the highest expression of S1D could be achieved by fusion with a stable CTB protein and chloroplast transformation. Furthermore, the CTB-S1D expressed in chloroplasts of Nicotiana tabacum cv. Maryland could be assembled to pentameric form which increases the possibility to develop a mucosal vaccine against PEDV.

Challenges and perspectives in commercializing plastid transformation technology

Plastid transformation has emerged as an alternative platform to generate transgenic plants. Attractive features of this technology include specific integration of transgenes-either individually or as operons-into the plastid genome through homologous recombination, the potential for high-level protein expression, and transgene containment because of the maternal inheritance of plastids. Several issues associated with nuclear transformation such as gene silencing, variable gene expression due to the Mendelian laws of inheritance, and epigenetic regulation have not been observed in the plastid genome. Plastid transformation has been successfully used for the production of therapeutics, vaccines, antigens, and commercial enzymes, and for engineering various agronomic traits including resistance to biotic and abiotic stresses. However, these demonstrations have usually focused on model systems such as tobacco, and the technology per se has not yet reached the market. Technical factors limiting this technology include the lack of efficient protocols for the transformation of cereals, poor transgene expression in non-green plastids, a limited number of selection markers, and the lengthy procedures required to recover fully segregated plants. This article discusses the technology of transforming the plastid genome, the positive and negative features compared with nuclear transformation, and the current challenges that need to be addressed for successful commercialization.

Cooperation between the chloroplast psbA 5'-untranslated region and coding region is important for translational initiation: the chloroplast translation machinery cannot read a human viral gene coding region

Chloroplast mRNA translation is regulated by the 5'-untranslated region (5'-UTR). Chloroplast 5'-UTRs also support translation of the coding regions of heterologous genes. Using an in vitro translation system from tobacco chloroplasts, we detected no translation from a human immunodeficiency virus tat coding region fused directly to the tobacco chloroplast psbA 5'-UTR. This lack of apparent translation could have been due to rapid degradation of mRNA templates or synthesized protein products. Replacing the psbA 5'-UTR with the E. coli phage T7 gene 10 5'-UTR, a highly active 5'-UTR, and substituting synonymous codons led to some translation of the tat coding region. The Tat protein thus synthesized was stable during translation reactions. No significant degradation of the added tat mRNAs was observed after translation reactions. These results excluded the above two possibilities and confirmed that the tat coding region prevented its own translation. The tat coding region was then fused to the psbA 5'-UTR with a cognate 5'-coding segment. Significant translation was detected from the tat coding region when fused after 10 or more codons. That is, translation could be initiated from the tat coding region once translation had started, indicating that the tat coding region inhibits translational initiation but not elongation. Hence, cooperation/compatibility between the 5'-UTR and its coding region is important for translational initiation.

Plastids: The Green Frontiers for Vaccine Production

Infectious diseases pose an increasing risk to health, especially in developing countries. Vaccines are available to either cure or prevent many of these diseases. However, there are certain limitations related to these vaccines, mainly the costs, which make these vaccines mostly unaffordable for people in resource poor countries. These costs are mainly related to production and purification of the products manufactured from fermenter-based systems. Plastid biotechnology has become an attractive platform to produce biopharmaceuticals in large amounts and cost-effectively. This is mainly due to high copy number of plastids DNA in mature chloroplasts, a characteristic particularly important for vaccine production in large amounts. An additional advantage lies in the maternal inheritance of plastids in most plant species, which addresses the regulatory concerns related to transgenic plants. These and many other aspects of plastids will be discussed in the present review, especially those that particularly make these green biofactories an attractive platform for vaccine production. A summary of recent vaccine antigens against different human diseases expressed in plastids will also be presented.

Rotavirus VP6 preparations as a non-replicating vaccine candidates

Rotavirus (RV) structural proteins VP4 and VP7, located on the surface of viral particles, elicit neutralizing antibodies (Abs) and are therefore considered to be important components of RV vaccines. However, despite inducing neutralizing Abs, limits of cross-neutralizing activity and lack of full correlation with protection limit the usefulness of these proteins as protective agents against RV disease. VP6 protein, which forms the middle layer of RV particles, is discussed as an alternative vaccine candidate since it can induce cross-protective immune responses against different RV strains although the Ab raised is not neutralizing. This report reviews different functions of VP6 that can lead to considering it as an alternative vaccine against RV disease.

A modular cloning toolbox for the generation of chloroplast transformation vectors

Plastid transformation is a powerful tool for basic research, but also for the generation of stable genetically engineered plants producing recombinant proteins at high levels or for metabolic engineering purposes. However, due to the genetic makeup of plastids and the distinct features of the transformation process, vector design, and the use of specific genetic elements, a large set of basic transformation vectors is required, making cloning a tedious and time-consuming effort. Here, we describe the adoption of standardized modular cloning (GoldenBraid) to the design and assembly of the full spectrum of plastid transformation vectors. The modular design of genetic elements allows straightforward and time-efficient build-up of transcriptional units as well as construction of vectors targeting any homologous recombination site of choice. In a three-level assembly process, we established a vector fostering gene expression and formation of griffithsin, a potential viral entry inhibitor and HIV prophylactic, in the plastids of tobacco. Successful transformation as well as transcript and protein production could be shown. In concert with the aforesaid endeavor, a set of modules facilitating plastid transformation was generated, thus augmenting the GoldenBraid toolbox. In short, the work presented in this study enables efficient application of synthetic biology methods to plastid transformation in plants.

Transgenic plants: a 5-year update on oral antipathogen vaccine development

The progressive interest in transgenic plants as advantageous platforms for the production and oral delivery of vaccines has led to extensive research and improvements in this technology over recent years. In this paper, the authors examine the most significant advances in this area, including novel approaches for higher yields and better containment, and the continued evaluation of new vaccine prototypes against several infectious diseases. The use of plants to deliver vaccine candidates against viruses, bacteria, and eukaryotic parasites within the last 5 years is discussed, focusing on innovative expression strategies and the immunogenic potential of new vaccines. A brief section on the state of the art in mucosal immunity is also included.

Unsolved problems in plastid transformation

Plants have been proved as a novel production platform for a wide range of biologically important compounds such as enzymes, therapeutic proteins, antibiotics, and proteins with immunological properties. In this context, plastid genetic engineering can be potentially used to produce recombinant proteins. However, several challenges still remain to be overcome if the full potential of plastid transformation technology is to be realized. They include the development of plastid transformation systems for species other than tobacco, the expression of transgenes in non-green plastids, the increase of protein accumulation and the appearance of pleiotropic effects. In this paper, we discuss the novel tools recently developed to overcome some limitations of chloroplast transformation.