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      Quantitative Co-Expression of Proteins at the Single Cell Level – Application to a Multimeric FRET Sensor

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          Abstract

          Background

          Co-expression of proteins is generally achieved by introducing two (or more) independent plasmids into cells, each driving the expression of a different protein of interest. However, the relative expression levels may vary strongly between individual cells and cannot be controlled. Ideally, co-expression occurs at a defined ratio, which is constant among cells. This feature is of particular importance for quantitative single cell studies, especially those employing bimolecular Förster Resonance Energy Transfer (FRET) sensors.

          Methodology/Principal Findings

          Four co-expression strategies based on co-transfection, a dual promotor plasmid, an internal ribosome entry site (IRES) and a viral 2A peptide were selected. Co-expression of two spectrally separable fluorescent proteins in single living cells was quantified. It is demonstrated that the 2A peptide strategy can be used for robust equimolar co-expression, while the IRES sequence allows expression of two proteins at a ratio of approximately 3:1. Combined 2A and IRES elements were used for the construction of a single plasmid that drives expression of three individual proteins, which generates a FRET sensor for measuring heterotrimeric G-protein activation. The plasmid drives co-expression of donor and acceptor tagged subunits, with reduced heterogeneity, and can be used to measure G-protein activation in single living cells.

          Conclusions/Significance

          Quantitative co-expression of two or more proteins can be achieved with little cell-to-cell variability. This finding enables reliable co-expression of donor and acceptor tagged proteins for FRET studies, which is of particular importance for the development of novel bimolecular sensors that can be expressed from single plasmid.

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          Most cited references 26

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          Fluorescent indicators for Ca2+ based on green fluorescent proteins and calmodulin.

          Important Ca2+ signals in the cytosol and organelles are often extremely localized and hard to measure. To overcome this problem we have constructed new fluorescent indicators for Ca2+ that are genetically encoded without cofactors and are targetable to specific intracellular locations. We have dubbed these fluorescent indicators 'cameleons'. They consist of tandem fusions of a blue- or cyan-emitting mutant of the green fluorescent protein (GFP), calmodulin, the calmodulin-binding peptide M13, and an enhanced green- or yellow-emitting GFP. Binding of Ca2+ makes calmodulin wrap around the M13 domain, increasing the fluorescence resonance energy transfer (FRET) between the flanking GFPs. Calmodulin mutations can tune the Ca2+ affinities to measure free Ca2+ concentrations in the range 10(-8) to 10(-2) M. We have visualized free Ca2+ dynamics in the cytosol, nucleus and endoplasmic reticulum of single HeLa cells transfected with complementary DNAs encoding chimaeras bearing appropriate localization signals. Ca2+ concentration in the endoplasmic reticulum of individual cells ranged from 60 to 400 microM at rest, and 1 to 50 microM after Ca2+ mobilization. FRET is also an indicator of the reversible intermolecular association of cyan-GFP-labelled calmodulin with yellow-GFP-labelled M13. Thus FRET between GFP mutants can monitor localized Ca2+ signals and protein heterodimerization in individual live cells.
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            Correction of multi-gene deficiency in vivo using a single 'self-cleaving' 2A peptide-based retroviral vector.

            Attempts to generate reliable and versatile vectors for gene therapy and biomedical research that express multiple genes have met with limited success. Here we used Picornavirus 'self-cleaving' 2A peptides, or 2A-like sequences from other viruses, to generate multicistronic retroviral vectors with efficient translation of four cistrons. Using the T-cell receptor:CD3 complex as a test system, we show that a single 2A peptide-linked retroviral vector can be used to generate all four CD3 proteins (CD3epsilon, gamma, delta, zeta), and restore T-cell development and function in CD3-deficient mice. We also show complete 2A peptide-mediated 'cleavage' and stoichiometric production of two fluorescent proteins using a fluorescence resonance energy transfer-based system in multiple cell types including blood, thymus, spleen, bone marrow and early stem cell progenitors.
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              Virus free induction of pluripotency and subsequent excision of reprogramming factors

              Reprogramming of somatic cells to pluripotency, thereby creating induced pluripotent stem (iPS) cells, promises to transform regenerative medicine. Most instances of direct reprogramming have been achieved by forced expression of defined factors using multiple viral vectors1-7. However, such iPS cells contain a large number of viral vector integrations1,8, any one of which could cause unpredictable genetic dysfunction. While c-Myc is dispensable for reprogramming9,10, complete elimination of the other exogenous factors is also desired since ectopic expression of either Oct4 or Klf4 can induce dysplasia11,12. Two transient transfection reprogramming methods have been published to address this issue13,14. However, the efficiency of either approach is extremely low, and neither has thus far been applied successfully to human cells. Here we show that non-viral transfection of a single multiprotein expression vector, which comprises the coding sequences of c-Myc​,​ Klf4​,​ Oct4 and Sox2 linked with 2A peptides, can reprogram both mouse and human fibroblasts. Moreover, the transgene can be removed once reprogramming has been achieved. iPS cells produced with this non-viral vector show robust expression of pluripotency markers, indicating a reprogrammed state confirmed functionally by in vitro differentiation assays and formation of adult chimeric mice. When the single vector reprogramming system was combined with a piggyBac transposon15,16 we succeeded in establishing reprogrammed human cell lines from embryonic fibroblasts with robust expression of pluripotency markers. This system minimizes genome modification in iPS cells and enables complete elimination of exogenous reprogramming factors, efficiently providing iPS cells that are applicable to regenerative medicine, drug screening and the establishment of disease models.
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                Author and article information

                Affiliations
                [1 ]Section of Molecular Cytology, van Leeuwenhoek Centre for Advanced Microscopy, Swammerdam Institute for Life Sciences, University of Amsterdam, Amsterdam, The Netherlands
                [2 ]Department of Biochemistry, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, Radboud University, Nijmegen, The Netherlands
                Institut Européen de Chimie et Biologie, France
                Author notes

                Conceived and designed the experiments: JG TWJG. Performed the experiments: JG LvW MJWA IE MAH. Analyzed the data: JG LvW IE MAH. Wrote the paper: JG TWJG.

                Contributors
                Role: Editor
                Journal
                PLoS One
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2011
                17 November 2011
                : 6
                : 11
                3219669
                22114669
                PONE-D-11-09743
                10.1371/journal.pone.0027321
                (Editor)
                Goedhart et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
                Counts
                Pages: 8
                Categories
                Research Article
                Biology
                Biochemistry
                Proteins
                Protein Interactions
                Biotechnology
                Bioengineering
                Biological Systems Engineering
                Molecular Cell Biology
                Signal Transduction
                Signaling Cascades
                Calcium Signaling Cascade
                Signaling in Cellular Processes
                G-Protein Signaling
                Signaling Pathways
                Calcium-Mediated Signal Transduction
                Proteomics
                Protein Interactions
                Synthetic Biology

                Uncategorized

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