Effect of point mutations in translation initiation context on the expression of recombinant human alpha(1)-proteinase inhibitor in transgenic tomato plants

Single-Step Purification and Characterization of A Recombinant Serine Proteinase Inhibitor from Transgenic Plants

Expression of recombinant therapeutic proteins in transgenic plants has a tremendous impact on safe and economical production of biomolecules for biopharmaceutical industry. The major limitation in their production is downstream processing of recombinant protein to obtain higher yield and purity of the final product. In this study, a simple and rapid process has been developed for purification of therapeutic recombinant α1-proteinase inhibitor (rα1-PI) from transgenic tomato plants, which is an abundant serine protease inhibitor in human serum and chiefly inhibits the activity of neutrophil elastase in lungs. We have expressed rα1-PI with modified synthetic gene in transgenic tomato plants at a very high level (≃3.2 % of total soluble protein). The heterologous protein was extracted with (NH4)2SO4 precipitation, followed by chromatographic separation on different matrices. However, only immunoaffinity chromatography resulted into homogenous preparation of rα1-PI with 54 % recovery. The plant-purified rα1-PI showed molecular mass and structural conformation comparable to native serum α1-PI, as shown by mass spectrometry and optical spectroscopy. The results of elastase inhibition assay revealed biological activity of the purified rα1-PI protein. This work demonstrates a simple and efficient one-step purification of rα1-PI from transgenic plants, which is an essential prerequisite for further therapeutic development.

A study on the influence of different promoter and 5'UTR (URM) cassettes from Arabidopsis thaliana on the expression level of the reporter gene β glucuronidase in tobacco and cotton

Several reports of promoters from plants, viral and artificial origin that confer high constitutive expression are known. Among these the CaMV 35S promoter is used extensively for transgene expression in plants. We identified candidate promoters from Arabidopsis based on their transcript levels (meta-analysis of available microarray control datasets) to test their activity in comparison to the CaMV 35S promoter. A set of 11 candidate genes were identified which showed high transcript levels in the aerial tissue (i.e. leaf, shoot, flower and stem). In the initial part of the study binary vectors were developed wherein the promoter and 5'UTR region of these candidate genes (Upstream Regulatory Module, URM) were cloned upstream to the reporter gene β glucuronidase (gus). The promoter strengths were tested in transformed callus of Nicotiana tabacum and Gossypium hirsutum. On the basis of the results obtained from the callus, the influence of the URM cassettes on transgene expression was tested in transgenic tobacco. The URM regions of the genes encoding a subunit of photosystem I (PHOTO) and geranyl geranyl reductase (GGR) in A. thaliana genome showed significantly high levels of GUS activity in comparison to the CaMV 35S promoter. Further, when the 5'UTRs of both the genes were placed downstream to the CaMV 35S promoter it led to a substantial increase in GUS activity in transgenic tobacco lines and cotton callus. The enhancement observed was even higher to that observed with the viral leader sequences like Ω and AMV, known translational enhancers. Our results indicate that the two URM cassettes or the 5'UTR regions of PHOTO and GGR when placed downstream to the CaMV 35S promoter can be used to drive high levels of transgene expression in dicotyledons.

Effect of ATG initiation codon context motifs on the efficiency of translation of mRNA derived from exogenous genes in the transgenic silkworm, Bombyx mori

The context sequence motif surrounding the ATG initiation codon influences mRNA translation efficiency and affects protein production; however, the optimal sequence differs among species. To determine the optimal sequence for production of recombinant proteins in a transgenic silkworm, we compared 14-nucleotide context motifs around the ATG (ATG-context) in 50 silkworm genes and found the following consensus: (A/T)AN(A/T)ATCAAAatgN. We were also able to define the least-common motif: CCN(C/G)CGN(C/T/G)(G/C/T)(T/G)atgC, which served as a negative control. To examine the regulatory role of these motifs in protein expression, we constructed reporter plasmids containing different ATG-context motifs together with either the luciferase gene or an enhanced green fluorescent protein (EGFP) gene. These constructs were then used for comparison of luciferase reporter activity and EGFP production in BmN4 cells in vitro as well as in transgenic silkworms in vivo. We detected 10-fold higher luciferase activity in BmN4 cells transfected with the consensus ATG-context motif construct, compared to the negative control plasmid. ELISA measurements of EGFP translation products with the corresponding constructs in BmN4 cells showed consistently similar results. Interestingly, the translation efficiency of the novel consensus ATG-context motif did not show the highest activity in the transgenic silkworms in vivo, except for the fat body. The highest efficiency in the middle and posterior silk glands was produced by the sericin 1 context. Our results show that the ATG-context motifs differ among silkworm tissues. This result is important for the further improvement of the transgenic silkworm system for the production of recombinant proteins.

A 28 nt long synthetic 5'UTR (synJ) as an enhancer of transgene expression in dicotyledonous plants

BACKGROUND

A high level of transgene expression is required, in several applications of transgenic technology. While use of strong promoters has been the main focus in such instances, 5'UTRs have also been shown to enhance transgene expression. Here, we present a 28 nt long synthetic 5'UTR (synJ), which enhances gene expression in tobacco and cotton.

RESULTS

The influence of synJ on transgene expression was studied in callus cultures of cotton and different tissues of transgenic tobacco plants. The study was based on comparing the expression of reporter gene gus and gfp, with and without synJ as its 5'UTR. Mutations in synJ were also analyzed to identify the region important for enhancement. synJ, enhances gene expression by 10 to 50 fold in tobacco and cotton depending upon the tissue studied. This finding is based on the experiments comparing the expression of gus gene, encoding the synJ as 5'UTR under the control of 35S promoter with expression cassettes based on vectors like pBI121 or pRT100. Further, the enhancement was in most cases equivalent to that observed with the viral leader sequences known to enhance translation like Ω and AMV. In case of transformed cotton callus as well as in the roots of tobacco transgenic plants, the up-regulation mediated by synJ was much higher than that observed in the presence of both Ω as well as AMV. The enhancement mediated by synJ was found to be at the post-transcriptional level. The study also demonstrates the importance of a 5'UTR in realizing the full potential of the promoter strength. synJ has been utilized to design four cloning vectors: pGEN01, pBGEN02, pBGEN02-hpt and pBGEN02-ALSdm each of which can be used for cloning the desired transgene and achieving high level of expression in the resulting transgenic plants.

CONCLUSIONS

synJ, a synthetic 5'UTR, can enhance transgene expression under a strong promoter like 35S as well as under a weak promoter like nos in dicotyledonous plants. synJ can be incorporated as the 5'UTR of transgenes, especially in cases where high levels of expression is required. A set of vectors has also been designed to facilitate this process.

Differential subcellular targeting of recombinant human α₁-proteinase inhibitor influences yield, biological activity and in planta stability of the protein in transgenic tomato plants

The response of protein accumulation site on yield, biological activity and in planta stability of therapeutic recombinant human proteinase inhibitor (α₁-PI) was analyzed via targeting to different subcellular locations, like endoplasmic reticulum (ER), apoplast, vacuole and cytosol in leaves of transgenic tomato plants. In situ localization of the recombinant α₁-PI protein in transgenic plant cells was monitored by immunohistochemical staining. Maximum accumulation of recombinant α₁-PI in T₀ and T₁ transgenic tomato plants was achieved from 1.5 to 3.2% of total soluble protein (TSP) by retention in ER lumen, followed by vacuole and apoplast, whereas cytosolic targeting resulted into degradation of the protein. The plant-derived recombinant α₁-PI showed biological activity for elastase inhibition, as monitored by residual porcine pancreatic elastase (PPE) activity assay and band-shift assay. Recombinant α₁-PI was purified from transgenic tomato plants with high yield, homogeneity and biological activity. Purified protein appeared as a single band of ∼48-50 kDa on SDS-PAGE with pI value ranging between 5.1 and 5.3. Results of mass spectrometry and optical spectroscopy of purified recombinant α₁-PI revealed the structural integrity of the recombinant protein comparable to native serum α₁-PI. Enzymatic deglycosylation and lectin-binding assays with the purified recombinant α₁-PI showed compartment-specific N-glycosylation of the protein targeted to ER, apoplast and vacuole. Conformational studies based on urea-induced denaturation and circular dichroism (CD) spectroscopy revealed relatively lower stability of the recombinant α₁-PI protein, compared to its serum counterpart. Pharmacokinetic evaluation of plant derived recombinant and human plasma-purified α₁-PI in rat, by intravenous route, revealed significantly faster plasma clearance and lower area under curve (AUC) of recombinant protein. Our data suggested significance of protein sorting sequences and feasibility to use transgenic plants for the production of stable, glycosylated and biologically active recombinant α₁-PI for further therapeutic applications.

In vivo analysis of translation initiation sites in Plasmodium falciparum

Regulation of gene expression in the malaria parasite Plasmodium falciparum is tightly controlled and little is known about the many steps involved. One step i.e. translation initiation is also poorly understood and in P. falciparum, choice of the translation initiation site (TIS) is a critical decision largely due to the high frequency of AUGs in the relatively long 5' untranslated regions of parasite mRNAs. The sequences surrounding the TIS have a major role to play in translation initiation and this report evaluates these sequences by mutational analysis of the heat shock protein 86 gene, transient transfection and reporter assays in the parasite. We find that purines at the -3 and +4 positions are essential for efficient translation in P. falciparum, similar to other eukaryotes. Interestingly, a U at the -1 position results in 2.5-fold higher reporter activity compared to wild type. Certain classes of protein biosynthetic genes show higher frequencies of U at the -1 position, suggesting that these genes may exhibit higher levels of translation. This work defines the optimal sequences for TIS choice and has implications for the design of efficient expression vectors in an important human pathogen.