Fluorescent proteins
Fluorescent Protein-Based Assay Development
This blog is a preview of what is to be launched as a new Service Group. Allele Biotech is restructuring its CRO capabilities in the assay development area by combining its fast expanding fluorescent protein portfolio, viral vector and packaging expertise, as well as newly granted patents in shRNA. The focus of this post is fluorescent protein in biosensor and screening assays. A modified version will be used as the landing page for the FB-Based Assay Development Service.
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Overview:
Originally cloned from the jellyfish Aequorea victoria and subsequently from many other marine organisms, fluorescent proteins (FPs) spanning the entire visual spectrum have become some of the most widely used genetically encoded tags. Unlike traditional labeling methods, FPs may be used to specifically label virtually any protein of interest in a living cell with minimal perturbation to its endogenous function. Genes encoding FPs alone or as fusions to a protein of interest may be introduced to cells by a number of different methods, including simple plasmid transfection or viral transduction. Once expressed, FPs are easily detected with standard fluorescence microscopy equipment.
Factors that should be taken into account when designing an FP-based imaging experiment include the desired wavelength(s) for detection, the pH environment of the tagged protein, the total required imaging time, and the expression level or dynamic range required for detection of promoter activity or tagged protein. Individual FPs currently available to the research community vary considerably in their photostability, pH sensitivity, and overall brightness, and so FPs must be chosen with care to maximize the likelihood of success in a particular experimental context.
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FPs as fusion tags:
Use of FPs as fusion tags allows visualization of the dynamic localization of the tagged protein in living cells. For such applications, the cDNA of a protein of interest is attached in-frame to the coding sequence for the desired FP, and both are put under the control of a promoter appropriate to the experimental context (typically CMV for high-level expression, though other promoters may be desirable if overexpression of your protein of interest is suspected of producing artifacts). The most basic uses for fluorescent protein fusions include tracking of specific organelles (fusions to short organelle targeting signals) or cytoskeletal structures (fusions to actin or tubulin, for example). More advanced uses include tracking receptors or exported proteins. In most cases, it is critical that the FP used for fusion tagging be fully monomeric, as any interaction between fusion tags is likely to produce artifacts, some of which may be hard to recognize in the absence of other controls. While in most cases FP fusions do not interfere with normal protein function, whenever possible, FP fusion proteins should be validated by immunostaining the corresponding endogenous protein in non-transfected cells and verifying similar patterns of localization.
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FPs as expression reporters:
FPs are highly useful as quantitative expression reporters. By driving the expression of an FP gene by a specific promoter of interest, it is possible to produce an optical readout of promoter activity. Use of the brightest possible FP ensures the best dynamic range for such an experiment. Because dynamic localization is not generally an issue for expression reporter applications, it is possible to use non-monomeric FPs for this purpose, opening up additional possibilities for multiple wavelength imaging. In order to obtain more reliable quantitative data and to correct for likely variations between individual cells in expression reporter experiments, the use of two spectrally distinct (e.g. green and red) FPs is advisable. By driving expression of one FP with a constitutive promoter and a second FP with the promoter of interest, the ratio of the two signals provides a quantitative readout of relative activity. Averaged over many cells, this technique should provide statistical power necessary for quality expression level experiments. Because FPs normally have a very slow turnover rate in mammalian cells, it may be desirable to add a degradation tag to your FP to enhance temporal resolution when measuring highly dynamic promoter activity.
New Product of the Week 03-08-10 to 03-14-10: mWasabi 2A or IRES dual expression vectors (http://www.allelebiotech.com/shopcart/index.php?c=216&sc=34) ABP-FP-W2A10, orWIRES10
Promotion of the Week 03-08-10 to 03-14-10: for a limited time on Thursday, to be announced on our Facebook page (http://www.facebook.com/pages/San-Diego-CA/Allele-Biotechnology-and-Pharmaceuticals-Inc/78331924957#!/allele.biotech?ref=profile), a strikingly low price will be honored for a commonly used lab reagent or equipment. This is the second week of the follow-us-to-the-basement promotion.
mTFP1 is an excellent FRET donor
Because of its excitation and emission wavelength, sharp excitation and emission peaks, high quantum yield, and exceptional photostability, mTFP1 has always been considered a very good Forster resonance energy transfer (FRET) donor (1). More recently, several groups have investigated the use of mTFP1 in various FRET experiments and imaging modalities and have shown that mTFP1 is indeed one of the best choices (2, 3, 4).
In one recent publication, Padilla-Parra et al (2) tested a number of different FRET couples to determine which was the best for fluorescence lifetime imaging (FLIM)-FRET experiments, and found that the mTFP1-EYFP pair was by far the best pair for FLIM-FRET. This group also confirmed that the fluorescence lifetime decay of mTFP1 fits well to a single exponential, and that the time constant for this decay is unaffected by photobleaching, making mTFP1 an excellent choice for any kind of fluorescence lifetime imaging applications, including FLIM-FRET. This group also notes that it is likely that the use of Venus or mCitrine variants in place of EYFP would improve the performance of this FRET pair even further.
In a mathematical analysis of the potential FRET efficiency of mTFP1 with Venus YFP, Day et al. (3) showed that compared with Cerulean (currently the brightest cyan Aequorea GFP variant), one can expect up to 17% better FRET efficiency using mTFP1. This group went on to characterize the mTFP1-Venus pair in live-cell FRET and FLIM-FRET experiments and showed that it worked as predicted in both cases. They also note that mTFP1 has superior brightness and photostability when compared to Cerulean in live cells, which is consistent with all in vitro data reported previously (1). In a related paper, Sun et al. (4) demonstrated that mTFP1 is also an excellent FRET donor for the orange fluorescent protein mKO2.
Together, these recent independent studies confirm that mTFP1 among the best options when choosing a fluorescent protein as a FRET donor. With its proven track record of successful fusions, mTFP1 is also an excellent all-around performer that will enhance almost any live-cell imaging experiment.
(1) Ai et al., (2006) Biochem. J. 400:531-540.
(2) Padilla-Parra et al., (2009) Biophys J. 97(8):2368-76.
(3) Day et al., (2008) J Biomed Opt. 13(3):031203.
(4) Sun et al., (2009) J Biomed Opt. 14(5):054009.
AlleleBlog Admin, by Nathan Shaner
Video of the month (NEW!): Protein Expression Systems on youtube (http://www.youtube.com/watch?v=n81orbUebsQ) and at our protein expression page.
Discount of the week (Dec 14-20): 15% off Phoenix Retrovirus Expression System 2.0 (with selection medium provided)
New product(s) of the week: 48 fluorescent protein fusions on ready-to-infect virus that get into primary mammalian cells as subcellular markers (http://www.allelebiotech.com/shopcart/index.php?c=197&sc=34), 20 infections, only $249 for a limited introduction time.
Construction of An Image Library
The American Society for Cell Biology (ASCB) is “pleased to announce the receipt of a U.S. National Institutes of Health Grand Opportunities (GO) grant to build The Cell: An Image Library. The ASCB will be hiring eight cell biologists or microscopists, each at 25% time,” The job description includes, according to an email job posting, “selecting exemplary images and videos and providing metadata for short tags or descriptions as well as longer annotations including technical details crucial for image interpretations. Annotators will select related key words and note biological source, context, item type, etc., in accordance with set guidelines. Annotators will upload images and videos to the Society’s new image library for research and education.” The grant is in the million dollar range.
The need for creating an extensive image library is deservingly recognized by this “GO” grant from the stimulus program awarded to the NIH by the federal government. The difficult part will be to maintain such an image center once the grant runs out. Will it be kept up-to-date and relevant, or left to collect dust on the old images? We wish that the program would be a great success and that the NIH money well spent.
Allele Biotech has applied to the same round of NIH grants with a related proposal that, rather than cell images in general, focuses more on cell differentiation/dedifferentiation through the use of iPS cells. Title: Foundation for “Subcellular Structureome” as Stem Cell Differentiation Parameters. Summary: The key question to be addressed is how to characterize differentiating stem cells along different lineages definitively and continuously, without disrupting or disturbing the differentiating cells. The broad and long-term goals are to find ways of describing stem cell differentiation in more detailed steps, thereby providing methods to predict and direct cell fate commitment.
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Aim 1 Create a panel of cells that can be reprogrammed into induced pluripotent stem cells (iPSCs) with fluorescent protein (FP) fusion markers for each organelle
.Human fibroblasts and keratinocytes will be selected from a large collection of primary human cells, based on their ease to grow and transfect, number of potential cell passages, and potentials for reprogramming with induction reagents. A group of 24 subcellular localization polypeptides (LP) and FP fusion protein constructs currently offered by Allele Biotech will be stably transfected into the selected cell.
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Aim 2 Characterize the morphological changes of subcellular structures during iPSC differentiation.
Transfected primary cells that stably express subcellular localization marker proteins will be induced with either current retroviral/lentiviral vectors based reprogramming cDNAs, or a non-integrating baculoviral vector under development at Allele Biotech. These cells, 48 lines in total, will be maintained and expanded under stem cell culture conditions, then induced to differentiate into chondracytes or keratinocytes as examples of cell fate. Morphology data will be analyzed and recorded at each known stage and additional “substage” to be defined in the process.
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Aim 3 Correlate morphological changes to known molecular properties of each stage and provide a “signature” set of morphological changes for each stage of each lineage
Signature morphological changes, i.e. significantly different shape, location, sub-type, and copies of organelles in a cell compared to its immediate upstream stage, will be correlated to results obtained by standard expression assays at the RNA and protein levels.
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Aim 4 Use the morphology parameters to establish more defined stages of cell fate commitments
Data points will be used to create a novel morphology-based cell fate commitment atlas, which will be very helpful in guiding the stem cell and regenerate medicine research at molecular biology, cell biology and physiology levels.
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Aim 5 Construct more FP fusions as organelle-specific markers and combine with stage specific gene promoter driven markers
If necessary, we plan to identify more LPs as fusion marker partners after obtaining the initial set of data, and to expand the signature morphology image database. The database can be further complemented with stage-specific gene promoter driven FP images.
Weekly Promotion of Nov 30-Dec 6: 15% off luciferase assay kit ABP-PA-ABLA011 1000 reactions at only $250.00 212.50. Compare it to what you normally pay for firefly luciferase assays and find out how much you are saving.
Reminder: Allele Biotech Spotlight Promo for ASCB Dec 09 Meeting is still on, order by Dec 9th on iPS and FP groups!
New Product of the Week of Nov 30-Dec 6: Allele Biotech’s ProperFold expression vector with fluorescent protein as indicator for proper protein folding, tracking, and purification. pORB-mWasabi+-sIRES-VSVG
Allele Biotech Spotlight Promo for ASCB Dec 09 Meeting!
This year our President and CEO, Dr. Jiwu Wang Ph.D., will be presenting at the American Society for Cell Biology meeting in San Diego, December 5th through 9th. Dr. Wang will be presenting results of two studies that involved the Allele Biotech Fluorescent Proteins and iPSC product lines:
Monomeric photoconvertable fluorescent protein variants produced by directed evolution for brightness and efficient photoconversion – a collaborative effort with the Campbell lab at the University of Alberta
Increased efficiency and speed of reprogramming of human cells into induced stem cells using high-titer lentiviral vectors encoding cell cycle progression and survival genes – a collaborative effort with the Chang lab at the University of Florida
In honor of this prestigious occasion Allele Biotech is having a Spotlight Promotion on all Fluorescent Protein and iPSC Products! The promotions, which will vary from product to product, will include 10% and 20% off price reductions, FREE shipping, and even “Buy 2 get one Free” deals!
Products eligible for the Spotlight Promotions begin with:
ABP-FP-____ Catalog
ABP-SC-____ Catalog
To qualify for these promotions you must be attending the ASCB meeting in San Diego and provide us with a copy of your registration form or be one of our loyal facebook, twitter, or myspace friends. Any questions can go to oligo@allelebiotech.com
Call for details and ask for info on the Spotlight Promotions! Offers good now through December, 9th 2009!
New Product of the Month 11/23-29/09: ThermoExp500 PCR machine (thermocycler) $4,250.00, with almost twice as fast temperature ramping as MJ’s TC1000, and more reliability.
RNAi Design, Validation and Target Screening
Since Tuschl et al. published the first empirical guidelines on how to design effective siRNA [1], the most significant advancement (based on the understanding of biochemical mechanisms of RNAi such as how RISC is assembled) is the recognition of asymmetric thermostability of the 5’ end of the antisense strand (AS) relative to that of the sense strand (SS) [2, 3]. siRNAs with an A/T-rich AS 5’ end can be more easily integrated into RISC. By biasing against the sense strand for RISC loading, the off-target effects due to the presence of the SS (as one of the sources of off-targets effects) can also be minimized. In recent years datasets of increased number of siRNAs and shRNAs became available and statistical analysis suggested additional rules for RNAi design. These newer rules in general define the siRNA prediction parameters in more detail, for instance, the number of bases of the 5’ ends that should be included when calculating asymmetric thermostability, base preferences at each particular position, and the identity of the 2 nt 3’ overhang [4, 5]. Computer programs and websites are developed based on these features also resulting from NIH funded research through universities and organizations. Among the well-known ones, Design of SIRna (DSIR at biodev.extra.cea.fr/DSIR/DSIR.html) and the shRNA search program at the Broad Institute (broadinstitute.org/genome_bio/trc/publicSearchForHairpinsForm.php) are freely available.
Several companies such as Open Biosystems, System Biosciences, Dharmacon/ThermoFisher, Sigma-Aldrich, Invitrogen/LifeTech, provide premade RNAi reagents against various numbers of human and rodent genes. Although some product lines from these suppliers are labeled as validated RNAi reagents, apparently only one reveals clone sequences and only a few hundred among the claimed 4,500 shRNA clones. It is not possible to find what shRNAs are used against any target gene from most companies even though many of them claim to have a few hundred pre-validated constructs. Some of them may provide additional information upon purchase.
Even with the recent advancement of RNAi design technologies, prediction of effective RNAi is still far from accurate. Depending on the datasets used to score the success rates of the programs at DSIR, Broad or any other software, the general consensus is that about 50% of predicted RNAi target sequences will be effective, resulting in better than 70% gene knockdown. Allele Biotech uses a software that was trained with known RNAi results to predict siRNA target candidates on a given mRNA, and then applies an additional set of rules to pick the most promising candidates. Off-target effects caused by partial-matching between AS strand and untended targets are reduced by searching the chosen site against the NCBI gene base. The basic rules Allele Biotech uses include most currently known ones and are similar to what are listed by The RNAi Consortium (TRC) program at the Broad Institute.
Criteria for RNAi design:
(1) Overall GC content is between 30-55%
(2) The 4 bases at the 5’ of AS is more AT-rich than those of the SS
(3) The first base of AS and SS 5’ is preferably A/T and G/C, respectively
(4) “U” is preferred at the 10th position of the antisense from the 5’ end
(5) “C” is to be avoided as the last base of an overhang
(6) Avoid 4-nt mono-nucleotide regions
(7) Avoid 6-nt GC-rich regions
(8) If possible, do not include those with apparent secondary structures
These selected rules are based on a number of publications (for example, [4-6]), but it is impossible to include all known rules, many of which conflict with each other. In case of conflicting rules we rely more on recent discoveries and our own experience from providing RNAi service during the past 8 years.
Allele Biotech provides RNAi validation and screening services to customers using synthetic siRNA, linear DNA cassettes with engineered Pol III promoter, and shRNA expressing lentiviral vectors in high throughput formats. In a unique design, all RNAi target candidate sequences of a gene transcript are fused consecutively to a bright green fluorescent protein, mWasabi, on a lentiviral vector. Instead of analyzing gene silencing by QPCR, the initial selection of effective RNAi can be performed by measuring fluorescence.
RNAi screening has been conducted to identify correlations between gene functions and cellular phenotypes such as synthetic lethality among DNA damage signaling and repair pathway factors. Successfully performing high throughput screenings requires capabilities of efficient RNAi design, viral packaging, fluorescent proteins, and advanced cell culture and analysis techniques. In addition to these capabilities, Allele’s RNAi services are provided with access to commercial use of Allele’s own patents on Pol III promoter driven shRNA expression, and licensed patents on lentiviral vector, packaging, and fluorescent proteins.
New Product/Service of week Nov 16-22, 09:
RNAi validation/screening service.
1. Tuschl, T., P.D. Zamore, R. Lehmann, D.P. Bartel, and P.A. Sharp, Targeted mRNA degradation by double-stranded RNA in vitro. Genes Dev, 1999. 13(24): p. 3191-7.
2. Khvorova, A., A. Reynolds, and S.D. Jayasena, Functional siRNAs and miRNAs exhibit strand bias. Cell, 2003. 115(2): p. 209-16.
3. Schwarz, D.S., G. Hutvagner, T. Du, Z. Xu, N. Aronin, and P.D. Zamore, Asymmetry in the assembly of the RNAi enzyme complex. Cell, 2003. 115(2): p. 199-208.
4. Vert, J.P., N. Foveau, C. Lajaunie, and Y. Vandenbrouck, An accurate and interpretable model for siRNA efficacy prediction. BMC Bioinformatics, 2006. 7: p. 520.
5. Zhou, H. and X. Zeng, Energy profile and secondary structure impact shRNA efficacy. BMC Genomics, 2009. 10 Suppl 1: p. S9.
6. Ui-Tei, K., Y. Naito, F. Takahashi, T. Haraguchi, H. Ohki-Hamazaki, A. Juni, R. Ueda, and K. Saigo, Guidelines for the selection of highly effective siRNA sequences for mammalian and chick RNA interference. Nucleic Acids Res, 2004. 32(3): p. 936-48.
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