Team:Freiburg Bioware/Project/Results/Modularization Vector Plasmid

Introduction to Modularization of Vectorplasmid. 1

Recombinant and Modular Vectorplasmid Carrying GOI 2

Cloning and Combination Strategies for the Vectorplasmid. 3

Testing functionality of Assembled Vectorplasmid. 7

Fluorescence Microscopy of Target Cells Demonstrates GOI Expression. 7

Analysis of Target Cells by Flow Cytometry demonstrates GOI Expression. 8

Influence of hGH terminator BioBrick on GOI Expression. 8

Influence of Beta-globin intron Biobrick on GOI Expression. 11

Functionality of the Full Assembled Vectorplasmid Demonstrated by GOI Expression. 14

Conclusion. 16

References. 17

 

                        Introduction to Modularization of Vectorplasmid

Producing recombinant virus particles for therapeutical means is, besides specifically target cells, purification and quantification assays of AAV-2, one intention of the Virus Construction Kit provided by the iGEM team Freiburg_Bioware 2010. For obtaining a modular toolkit, the complex components of AAV-2 were extracted and redesigned to match the iGEM standard. Functional activity was tested in cell culture.

Differing from the wildtype AAV-2 genome, the Helper Free System provided by Stratagene comprises three plasmids and a specialized production cell line. AAV-293 cells derived from the HEK cell line express the stably integrated E1A and E1B helper proteins for efficient virus production. The plasmid containing the inverted terminal repeats (ITRs) is encapsidated into the preformed capsids after production of single-stranded DNA therefore also known as vectorplasmid (pGOI). Promoter, beta-globin intron and the hGH terminator signal are flanked by the ITRs and serve in the host cell for regulation of transgene expression. In addition to that, the plasmid coding for the Rep and Cap proteins (pRC) can be provided in trans leading to a layer of specificity due to the fact that the two genes are not packaged into the capsid since lacking of the ITRs impairs encapsidation. Another advantage of the Helper Free System can be attributed to cotransfection of another helper plasmid (pHelper), which provides the necessary proteins normally obtained by superinfection with helper viruses such as adenovirus or herpes simplex virus. These helper genes are required for full viral assembly by regulating gene expression of Rep and Cap proteins.

                        Recombinant and Modular Vectorplasmid Carrying GOI

The iGEM team Freiburg_Bioware 2010 provides a modular Virus Construction Kit for therapeutical applications, quantification assays and purification approaches depending on capsid modifications and the gene of interest flanked by the inverted terminal repeats (ITRs. In order to produce BioBrick-compatible standardized biological parts, we reengineered the plasmids and added new components for gene therapy approaches and analysis of biological activity of assembled BioBrick parts. Each element required for intact and functional plasmids comprising the ITRs, a promoter, a putative enhancer element and the hGH terminator was PCR amplified and fused together de novo. As shown in Figure 1, the vectorplasmid was assembled with the produced BioBricks consisting of the left and right ITR (BBa_K404100 and BBa_K404101), a promoter (pCMV :BBa_K404102 or phTERT: BBa_K404106)) , the beta-globin intron (BBa_K404107), the gene of interests (fluorescent proteins mVenus: BBa_I757008 and mCherry: BBa_J06504, suicide genes mGMK_TK30: BBa_K404112, mGMK_SR39: BBa_K404315 and CD: BBa_K404112) and the hGH terminator (BBa_K404108).

 

Cloning and Combination Strategies for the Vectorplasmid

Organization of the recombinant viral DNA was modified ensuring several layers of specificity to our systems including a tumor-specific promoter and suicide genes encoding prodrug convertases. In order to modularize the rAAV sequence, each plasmid element (Figure 1) was PCR-amplified and cloned into the iGEM standard plasmid pSB1C3. Furthermore, the iGEM team Freiburg_Bioware 2010 performed three site-directed mutagenesis in the gene of interest TK30 (BBa_K404109) and cytosine deaminase (BBa_K404112) for deletion of PstI and NgoMIV iGEM site (for further information see the results page of ‘Arming – Killing the tumor’). Since the inverted terminal repeats (ITRs) are GC-rich regions forming T-shaped hairpins during replication, PCR amplification was not possible. Hence a cloning strategy was developed by the iGEM team Freiburg in order to provide BioBrick-compatible ITRs (see ‘Method Development of Cloning Strategy for ITRs’).

In Figure 2 the schematic overview of the modularization process can be seen which has been followed to conduct the assembly steps required for functional vectorplasmids.

 

The iGEM team Freiburg_Bioware provides two examples demonstrating the assembly procedure for constructing vectorplasmids. The first representative example is the fusion of the BioBrick part beta-globin to the composite parts containing the 5´ elements of the plasmids, which are left ITR and CMV or phTERT promoter, respectively.

As shown in Figure 3 the theoretical cloning performed for assembling the BioBricks beta-globin intron and leftITR_CMV together can be observed.

 

The plasmids were digested with both XbaI and PstI (beta-globin intron: BBa_K404107) or SpeI and PstI (leftITR_CMV) and loaded on an agarose gel. As demonstrated in the preparative gel in Figure 4, the expected bands could be detected under UV light and the extracted DNA could be successfully ligated. Each assembly step for producing BioBrick intermediates was conducted following the same strategy.

 

Separated fragments were extracted using the Gel Extraction Kit provided by Qiagen (Hilden, Germany) and ligated with T4-ligase. After ligation has been carried out, E. coli XL-1B cells were transformed and incubated over night at 37°C. Picking clones from the transformation plate was performed the following day and DYT medium was inoculated incubating overnight. Plasmid DNA was isolated and test digestion revealed that cloning was successful obtaining the composite part leftITR_CMV_beta-globin intron (BBa_K404117).

Plasmid production incorporating all required elements for transgene expression and genome encapsidation into empty viral capsids was performed by fusing the downstream elements consisting of the hGH terminator and right ITR to the intermediate part providing the gene of interest and the promoter fused to the left ITR. Figure 5 demonstrates the assembly performed with pSB1C3_leftITR_phTERT_beta-globin intron_mVenus and pSB1C3_hGH_rightITR (BBa_K404116). The fragment obtained after digestion on the left lane fits to the hGH-terminator_rightITR length. The isolated fragments were ligated and successful assembly was confirmed by test digestion obtaining the vectorplasmid pSB1C3_leftITR_phTERT_beta-globin intron_mVenus_hGH_rightITR (BBa_K404124).

 

Since cloning does not confirm biological activity, we analyzed the plasmids and their functional components, hGH terminator and beta-globin intron, in cell culture. Assembled plasmids have been cotransfected, using AAV-293 cells, which provide the stable integrated E1A and E1B genes, with helper plasmids required for capsid assembly  and genome encapsidation (pRC and pHelper) in a molar ratio of 1:1:1 (pGOI:pRC:pHelper). Virus particles containing the single stranded DNA were harvested 72-hours post transfection and HT1080 cells transduced with constant volumes of viral vectors. 48-hours post infection; transduced cells expressing the gene of interest were analyzed by flow cytometry. Facilitating and demonstrating the analysis of functionality of the assembled plasmid, mVenus was used in first place since fluorescent proteins enable facile visualization using fluorescent microscopy and flow cytometry analysis.

                        Testing functionality of Assembled Vectorplasmid

Fluorescence Microscopy of Target Cells Demonstrates GOI Expression

Qualitative analysis of mVenus expression by fluorescence microscopy was conducted using Axio Observer Z1 showing that transduced HT1080 cells and non-transduced cells could be easily distinguished. In Figure 6 cells were excited with 505nm and fluorescence emission at 536nm was detected. Therefore, successful infection of tumor cells by recombinant viral particles carrying the assembled vectorplasmid coding for mVenus could be demonstrated.

 

Analysis of Target Cells by Flow Cytometry demonstrates GOI Expression

Characterizing the function of the hGH terminator, the beta-globin intron and the complete plasmid, several approaches were conducted followed by analysis via flow cytometry.

Influence of hGH terminator BioBrick on GOI Expression

The iGEM team Freiburg provides the hGH plolyadenylation sequence within the ‘Virus Construction Kit’ due to the fact that almost every eukaryotic mRNA is processed at their 3´ and 5´end except for histone mRNAs (Millevoi et al. 2006). Pre-mRNAs contain two canonical conserved sequences. First, the polyadenylation signal “AATAAA” which is recognized by the multiprotein complex and second the GT-rich region (downstream sequence element, DSE) which is located 30 nucleotides downstream of the cleavage site. The assembled 3´end-processing machinery cleaves the mRNA transcript immediately after a CA-nucleotide therefore defining the cleavage site (Danckwardt et al. 2008). Recombinant vectorplasmids were engineered containing the inverted terminal repeats (ITRs), a strong eukaryotic promoter (CMV promoter: BBa_K404102) and mVenus as gene of interest with and without the hGH terminator signal. Transduction of HT1080 cells with constant volume of viral particles containing the vectorplasmids and measuring mVenus expression 24-hours post infection by flow cytometry demonstrated that transgene expression of the constructs lacking the hGH termination signal is significantly reduced as shown in Figure 7 and Figure 8 confirming the expected results that hGH is essential for mRNA processing. The iGEM team Freiburg_Bioware 2010 therefore suggests using the provided hGH termination signal within the Virus Construction Kit for optimal gene expression.

Vectorplasmid lacking hGH termination signal

Vectorplasmid containing hGH terminator signal

Figure 8: Flow cytometry analysis of vectorplasmids with and without hGH terminator. A: Gating non transduced cells (control); subcellular debris and clumps can be distinguished from single cells by size, estimated forward scatter (FS Lin) and granularity, estimated side scatter (SS Lin) B: Non transduced cells applied against mVenus (Analytical gate was set such that 1% or fewer of negative control cells fell within the positive region (R5). C: Gating transduced cells (R2 ≙R14) (used plasmids for transfection: GOI: pSB1C3_lITR_CMV_beta-globin intron_mVenus_rITR (BBa_K404127), pHelper, pRC). D: Transduced cells plotted against mVenus, R10 comprises transduced cells by detecting mVenus expression. E: Overlay of non-transduced (red) and transduced (green) cells applied against mVenus.F: Gating non-transduced cells (control) G: Non-transduced cells applied against mVenus. H: Gating transduced cells (R2 ≙R14) (used plasmids for transfection: GOI: reassembled pSB1C3_lITR_CMV_beta-globin_mVenus_hGH_rITR (BBa_K404119), pHelper, pRC). I: Transduced cells applied against mVenus, R10 comprised transduced cells, by detecting mVenus expression. J: Overlay of non-transduced (red) and transduced (green) cells applied against mVenus.

 

 

Influence of Beta-globin intron Biobrick on GOI Expression

Providing an element assumed to be an enhancer of transgene expression (Nott et al. 2003), the iGEM team Freiburg tested a beta-globin intron derived from the human beta globin gene which can be fused upstream of the desired gene of interest. The beta-globin intron BioBrick consists of a partial chimeric CMV promoter followed by the intron II of the beta-globin gene. The 3´end of the intron is fused to the first 25 bases of human beta globin gene exon 3. The beta globin intron BioBrick is assumed to enhance eukaryotic gene expression (Nott et al. 2003). Analysis was conducted as described for the hGH terminator experiment (see above). As shown in Figure 9 and Figure 10 the vectorplasmid missing the beta-globin intron showed a negligible difference in mVenus expression compared to viral genomes containing the beta-globin intron. Considering these results and taking into account that a constant volume of viral particles has been used for transduction, the difference between the construct containing and lacking the beta-globin intron is minimal. Since packaging efficiency of the AAV-2 decreases with increasing sizes of the insert (Dong et al. 1996), the iGEM team Freiburg_Bioware suggests using the beta-globin intron in dependence on the size of your transgene.

 

 

Functionality of the Full Assembled Vectorplasmid Demonstrated by GOI Expression

After assembly of plasmids containing all required elements (see Figure 1), functionality was tested in cell culture. AAV-293 cells stably expressing E1A and E1B proteins were transfected with three plasmids  (pHelper, pRC, pGOI). Virus particles were harvested 72-hours post-transfection and the tumor cell line HT1080 was transduced with the recombinant viral vectors encapsidating the gene of interest mVenus (BBa_I757008).

The iGEM team Freiburg_Bioware 2010 compared the standard-plasmid containing a subcloned mVenus (pAAV_mVenus, derived from the Stratagene system) with the assembled plasmid pSB1C3_lITR_CMV_beta-globin_mVenus_hGH_rITR (pSB1C3_mVenus: BBa_K404119). Fluorescence expression data obtained by flow cytometry analysis are shown in Figure 11 and Figure 12. Comparing mVenus expression of the standard plasmid and the modified, assembled plasmid reveals that biological functionality of the reassembled plasmid was confirmed.

 

 

                        Conclusion

Idea of the modular ‘Virus Construction Kit’ is to provide all required elements for producing recombinant, functional virus particles delivering encapsidated genes of interest to specific cells. First step was  to modify and modularize the vectorplasmid comprising basically the cis-elements for replication (ITRs), a strong eukaryotic or tissue specific promoter (pCMV or phTERT), the gene of interest (fluorescent proteins or suicide genes) and the hGH termination signal. Each element was successfully cloned and reassembled resulting in functional vectorplasmids determined by flow cytometry and fluorescence microscopy analyses. Experiments have been performed with mVenus since measurement of fluorescent proteins can be easily performed and visualized. Considering the results, the iGEM team Freiburg_Bioware 2010 then tested the construct containing the suicide genes thymidine kinase and cytosine deaminase. Further details demonstrating efficient tumor killing, using prodrug-activating systems, see results page ‘Arming – Killing the tumor’.

                        References

Danckwardt, S., Hentze, M.W. & Kulozik, A.E., 2008. 3' end mRNA processing: molecular mechanisms and implications for health and disease. The EMBO journal, 27(3), 482-98. Available at: http://www.ncbi.nlm.nih.gov/pubmed/18256699.

Dong, J.Y., Fan, P.D. & Frizzell, R.a., 1996. Quantitative analysis of the packaging capacity of recombinant adeno-associated virus. Human gene therapy, 7(17), 2101-12. Available at: http://www.ncbi.nlm.nih.gov/pubmed/8934224.

Millevoi, S. et al., 2006. An interaction between U2AF 65 and CF I(m) links the splicing and 3' end processing machineries. The EMBO journal, 25(20), 4854-64. Available at: http://www.ncbi.nlm.nih.gov/pubmed/17024186.

Nott, A., Meislin, S.H. & Moore, M.J., 2003. A quantitative analysis of intron effects on mammalian gene expression. RNA (New York, N.Y.), 9(5), 607-17. Available at: http://www.ncbi.nlm.nih.gov/pubmed/12702819.