Using 2A “self-cleaving” peptide in bicistronic mammalian expression

Multiple promoters or internal ribosomal entry sites (IRES) have been used for the production of multiple proteins from the same vector. Potential drawbacks with multiple promoters on viral vectors include unstable genome and interference between promoters. IRES is a relatively large sequence that can cause problems in virus packaging, especially for viruses with very limited genome size such as AAV. In addition, it is required that the start of the second ORF is fairly close to the IRES, adding difficulties to cloning.

2A or 2A-like peptide (collectively called 2A peptide here) is used by several families of viruses, the best known foot-and-mouth disease virus of the Picornaviridae family, for producing multiple polypeptides. Although called a “self-cleaving” peptide or protease site, the mechanism by the 2A sequence for generating two proteins from one transcript is by ribosome skipping–a normal peptide bond is impaired at 2A, resulting in two discontinuous protein fragments from one translation even.

The 2A-based bicistronic expression has been used for several years, but recently gained much more popularity due to its successful use in iPSC generation that required 2 to 4 factors working in concert. Even expression of all factors can be achieved when 2A peptides are used for multiple protein production, due to near 100% efficiency of the 2A “cleavage” at each site, and no interference between multiple 2A sites. Early work used a 36 amino acid sequence as 2A peptide, which was later reduced to about half that size from mutation and screening. Commercial vectors utilizing 2A for co-expression of cDNA and fluorescent protein and/or drug resistance genes have not been available until now. Allele Biotech has introduced a number of such plasmids, establishing another First-to-the-market as it has done many times previously in its 10 year history.

New product of the week 03-29-10 to 04-04-10:

Alleleustrious pmTFP1-2A Bicistronic mammalian expression vector, ABP-FP-T2A10, $399, http://www.allelebiotech.com/shopcart/index.php?c=215&sc=34

Promotion of the week 03-29-10 to 04-04-10:

Buy any GFP-Trap beads or kits, get polyclonal anti-GFP (ABP-PAB-PAGFP10) at half size for FREE!  ***GFP-Trap has been replaced with GFP-nAb, an improved version for the same purposes in the form of agarose beads, magnetic beads, polyacrylamide beads, spin column kits, etc.  Come and take a look at the most comprehensive list of nearly all FPs before purchasing.

Tags: 2A peptide, anti-GFP, bicistronic, drug resistance, GFP-Trap, iPS, iPSCs, IRES, polycistronic, ribosome skip, self-cleaving, transcription, translation

Monitoring the Undifferentiated Stage of Stem Cells—the Pluripotency Markers

Human embryonic stem (ES) cells or induced pluripotent stem (iPS) cells promise to serve as an unlimited source for transplantation or tissue-specific differentiation. However, obtaining and maintaining stem cells are very difficult tasks for multiple reasons. For instance, most stem cell lines tend to spontaneously differentiate in culture, and even if the cells form stem cell-like colonies, they may be of a heterogeneous population.

To identify pluripotency of stem cells, expression of stem cell-specific marker genes (i.e. Oct-3/4, Sox2, Nanog, Rex-1) is monitored by RT-PCR. Alkaline phosphatase activity and methylation profiles of promoters of pluripotency-relevant genes are often analyzed as well. Compared to murine cells, it is noticeably more difficult to obtain human iPSCs, of which stem cell-like colonies sometimes turn out not to be pluripotent cells. We highly recommend testing iPSCs, especially human iPSCs, with antibodies against stage-specific embryonic antigens such as SSEA-3, SSEA-4, TRA-1-60, and TRA-1-81.

However, all of these methods require cell destruction or fixation for analysis, therefore, are inconvenient and costly. Furthermore, many studies using ES or iPS cells involve differentiation of stem cells into different lineages, a method for observing live cells to know their undifferentiation/differentiation stages would be very helpful. There have been a number of publications using murine Oct-4, Nanog, and Rex-1 promoter driven fluorescent proteins as markers for pluripotency tests [1-3]. Allele Biotech provides, under its iPS product line, packaged and validated lentiviral particles that would insert these 3 promoter-FP reporters into the stem cells. Although currently these promoters are of mouse sequences, their use in human stem cells have been reported.

New product of the week 01-25-10 to 01-31-10:

All-In-One-Vector: Human OSKM Lentiviral Paticles, with Oct-4, Sox-2, Klf, and c-Myc all expressed from a single virus, ready-to-use.

• Promotion of the week:

human iPS cell detection primer set, the same as the landmark Yamanaka paper [4] on creating human iPS for the first time.

1. Da Yong WU, Zhen YAO (2005). Isolation and characterization of the murine Nanog gene promoter. Cell Research, 15 (5): 317–324.
2. Rachel Eiges, Maya Schuldiner..et.al (2001). Establishment of human embryonic stem cell?transfected clones carrying a marker for undifferentiated cell. Current Biology 11: 514–518.
3. Guangjin Pan, Jun Li, Yali Zhou, Hui Zheng, and Duanqing pei (2006). A negative feedback loop of transcription factors that control stem cell pluripotency and self?renewal. ASEB Journal 20: E1094? E1102
4. Takahashi et al, Induction of Pluripotent Stem Cell from Adult Human Fibroblasts by Defined Factors (2007). Cell 131, 861-872

Tags: differentiation, fluorescent protein, iPS, iPS lentivirus, iPSC, iPSCs, lentivirus, pluripotency reporter, promoter, promoter-FP, stem cells, undifferentiation stage

GPCR: problems and resolutions in high content screening, Part II

2) Without a mark (continued from Part I by Niels Yuhui Ni, MD Ph.D. of Allele Biotech)

In this section, the author focused on the “Label-Free” system. “A newer alternative that has given hope” because these systems are closer to the real cellular conditions most GPCR studies are meant to address. However, label-free systems must depend on complicated detection system for high content analysis. As commented, “For some scientist though, this technology is simply still too new and for now, too expensive for many categorical assessments…” While I am a believer in label-free detection, however, here is my question, for this group of scientists, are there any good alternatives now, before the label-free systems become more accessible? Using the traditional over-expression cell lines seems less and less attractive (see Part I under the same blog topic here)? A system that combines immortalized primary cells and a non-integrated expression system could be a nice system that does not require high end equipment or heavy commitment in technology development especially if the components were commercially available. The third, critical component of this system could be the application of newer, brighter and monomeric and thus less toxic fluorescent proteins or FRET pairs as sensors. Obviously part of the reason that I thought about such a system was because our research team at Allele Biotech has the background in all 3 components, whereas others might have their own preferred methods. To me, it seems that immortalized primary cells present a renewable cell source, and for non-integration delivery, we prefer Baculo viral delivery vehicles for mammalian infection (called BacMam by some). Both platforms are being offered as products or services already or in the pipeline teed up for launching.

Baculo2Mam non-intergrated viral delivery system:
Many people may know about Baculovirus such as in the Bac-to-Bac system from Invotrogen or the Sapphire baculovirus system from Orbigen (acquired by Allele Biotech). But how many of us know that even though mammalian cells are not the nature host of baculovirus, they still can be infected with modest modifications on the virus. Both the safety and efficiency of Baculovirus for mammalian use are superb. Based on the data from our customers’ projects, most protein expression require baculoviral protein expression in insect cells, only about 10% require Baculo2Mam. We actually feel a sense of responsibility for introducing this technology to as many as researchers as possible.

The following list shows some of the advantages of Baculo2Mam I can list right now, for more details check back on our blog articles in coming days or contact us at any time for discussion.
1) Baculoviruses are Risk Group 1 or biosafety 1 agents.
They are produced in insect cells and can not replicate in mammalian cells. They express genes in human or mouse cells in non-integrated state for about 2 weeks (varies in different cells).
2) Baculoviruses can be easily generated in high titer and production rapidly scaled-up.
That is when compared with other viral systems. For example, baculovirus is a budding virus that is released into cell medium, unlike adenovirus that requires lysing cells during productions. Allele Biotech now provides Baculo2Mam viral packaging service at an affordable price for routine use. Your viral clones can be stored in Allele Biotech’s Baculo2Mam virus bank; if you need the virus again, you can just order a production service at an even lower price.
3) Broad host cell range including many primary cells.
Many terminally differentiated primary cells such as neuron, adipocytes have been tested in Allele Biotech’s lab as target for modified Boculovirus. To assess the infection efficiency, you can order a pre-made Baculo2Mam-mWasabi GFP or Baculo2Mam-LanRFP control for a test run. Once you order custom or regular Baculo2Mam products, the cost of the control will be credited back.
4) Up to now, little or no cytopathic effects were observed of using baculovirus in mammalian cell cultures.
5) Other points that may be related to GPCR assays in relevance to mimicking natural cellular environment:
a) Delivery of biosensor to cells just prior to assay without establishing cell lines
b) Large insert capacity for expressing long cDNAs.
c) Multiple virus transductions, simultaneous delivery of multiple genes
d) Expression level can be adjusted by viral titer
e) Finally, Baculo2Mam Viruses can be stably stored at 4oC for up to 3 months, and even longer as seed stocks (i.e. titer will drop but still amplifiable).

Promotion of the first week of 2010:

in the spirit of celebrating Allele’s 10th anniversary and in line with the ongoing “get oligos free for a month” program, we offer $20 off for oligos on 3’ TAMRA or FAM modifications.

New product of the week:

iPS specific gene promoter-fluorescent protein reporter lentiviruses.

Tags: bacmam, baculovirus for mammalian cells, biosensors, cell based assay, cell imortalization service, Fluorescent proteins, GPCR, GPCR assays, immortalizaed primary cells, label-free detection, membrane based assay, viral packaging service

GPCR: problems and resolutions in hihg content screening

A review in the December, 2009 issue of Nature Methods, “GPCR: insane in the membrane” by Michael Eisenstein impressed me greatly. It presents a clear and wide view of the GPCR research fields, from the basic background knowledge, to difficulties and current solutions in real cases, to future development. After careful reading and thinking, I felt that there were still several questions left to for me to pounder.

1) The Plot Thickens

“In many cases, you identify a GPCR as a target based on physiological data, and the receptor might be expressed in the brain in a particular neuron, but then you perform a high-throughput screen in over expression, immortalized cell lines that are nothing like the cell in which the receptor normally resides,”This is the inherent problem for all the engineered and artificial cell based assays most widely used in GPCR research right now. As stated in this review, “There are so many ways to be misled by using an over-expressed receptor in a non-native cell line”, it was shocking to me, making me realize that cell line screening does not only have a problem of getting raw data of low quality, but also a problem of being seriously misleading, a problem of direction rather than efficiency.

Directed differentiation of iPS cells and ESCs might be one of the potential solutions to this problem. However, from our experience in iPS cells dedifferentiation and differentiation at Allele Biotech, as a main vendor of iPS cell reagents, it is hard to make this platform in current situation suitable for high content analysis of GPCR. As much as we wish and believe, like many other researchers and the general society, that stem cells will contribute significantly also to the drug screening field, much work needs to be done before that happens.

Primary cells could be a good alternative. Based on data from Allele Biotech’s trade partners in the primary cell business, the primary cells market has been expanding dramatically since late last year. I believe that the utility and advantage of primary cells have been increasingly appreciated. However, there are still some problems with using primary cells in GPCR studies. For example, neurons are hard to obtain and can not amplify in vitro, in which cases there seems to be no chance to satisfy the need by screening. In fact, neuron research is one of the main GPCR research fields. Therefore, this is a must-fix problem.

How about “Immortalized Primary Cells”? The immortalized primary cells maintain the properties of primary cells well, relative to established cell lines. We have built our own advanced immortalization technology, based on which new products and services will be released in the first season of next year. During the last several years, we have successfully immortalized immunological cells and cancer cells. WE have yet to have the experience of working with neurons. Anyone who has interests of cell immortalization is welcome to contact us for collaboration or custom service. “Introducing Cost Effectiveness to your research”, just as the slogan of Allele Biotech, we will be proud to serve you with cutting-edge technology and cost effectiveness!

2) Without a mark to be continued…

By Niels Yuhui Ni, researcher at Allelle Biotech

ASCB Abstract: Increased rate of reprogramming of induced pluripotent stem cells using high-titer lentiviral vectors encoding multiple cell growth and survival regulatory genes

Liang Yin, Jiwu Wang, Lung-Ji Chang and Yichen Wang

Objective: Differentiated cells can be reprogrammed into induced pluripotent stem cells (iPSC) with enforced expression of multiple transcription factors. We aim to improve the reprogramming efficiency using high titer lentiviral vectors encoding additional cell growth and survival regulatory genes.

Methods: Lentiviral vectors encoding multiple cell cycle and apoptosis genes in addition to c-Myc, Klf4, Oct4 and Sox2 were constructed and used to generate iPSC. The iPSC were extensively characterized by immunohistochemical staining and flow cytometry.

Results: Human mesenchymal stem cells can be efficiently transduced and reprogrammed into iPSC using high-titer lentiviral vectors encoding the four known transcription factors. The addition of siRNA suppressing p53 and cell cycle and survival genes including telomerase and BclXL significantly increased the efficiency and rate of iPSC generation. Human iPSC colonies were formed within a week after lentiviral gene transfer.

Conclusions: The protocol for iPSC generation has been improved with high titer lentiviral vectors encoding additional immortalization cellular factors regulating cell cycle progression, senescence and apoptosis. Deletion of the integrated lentiviral genomes using Cre-loxP recombination could increase the safety profile of the reprogrammed iPSC.

Flow cytometry analysis of reprogrammed human marrow stromal cells.