Accepting from the Best Donor; Analysis of Long-Lifetime Donor Fluorescent Protein Pairings to Optimise Dynamic FLIM-based FRET Experiments
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ABSTRACT: FRET biosensors have proven very useful tools for studying the activation of specific signalling pathways in living cells. Most biosensors designed to date have been predicated on fluorescent protein pairs that were identified by, and for use in, intensity based measurements, however fluorescence lifetime provides a more reliable measurement of FRET. Both the technology and fluorescent proteins available for FRET have moved on dramatically in the last decade. Lifetime imaging systems have become increasingly accessible and user-friendly, and there is an entire field of biology dedicated to refining and adapting different characteristics of existing and novel fluorescent proteins. This growing pool of fluorescent proteins includes the long-lifetime green and cyan fluorescent proteins Clover and mTurquoise2, the red variant mRuby2, and the dark acceptor sREACh. Here, we have tested these donors and acceptors in appropriate combinations against the standard or recommended norms (EGFP and mTFP as donors, mCherry and either Ypet or Venus as acceptors) to determine if they could provide more reliable, reproducible and quantifiable FLIM-FRET data to improve on the dynamic range compared to other donors and breadth of application of biosensor technologies. These tests were performed for comparison on both a wide-field, frequency domain system and a multiphoton, TCSPC time domain FLIM system. Clover proved to be an excellent donor with extended dynamic range in combination with mCherry on both platforms, while mRuby2 showed a high degree of variability and poor FRET efficiencies in all cases. mTFP-Venus was the most consistent cyan-yellow pair between the two FLIM methodologies, but mTurquoise2 has better dynamic range and transfers energy consistently over time to the dark acceptor sRCh. Combination of mTFP- sRCh with Clover-mCherry would allow the simultaneous use of two FLIM-FRET biosensors within one sample by eliminating the crosstalk between the yellow acceptor and green donor emissions.
Project description:FRET biosensors have proven very useful tools for studying the activation of specific signalling pathways in living cells. Most biosensors designed to date have been predicated on fluorescent protein pairs that were identified by, and for use in, intensity based measurements, however fluorescence lifetime provides a more reliable measurement of FRET. Both the technology and fluorescent proteins available for FRET have moved on dramatically in the last decade. Lifetime imaging systems have become increasingly accessible and user-friendly, and there is an entire field of biology dedicated to refining and adapting different characteristics of existing and novel fluorescent proteins. This growing pool of fluorescent proteins includes the long-lifetime green and cyan fluorescent proteins Clover and mTurquoise2, the red variant mRuby2, and the dark acceptor sREACh. Here, we have tested these donors and acceptors in appropriate combinations against the standard or recommended norms (EGFP and mTFP as donors, mCherry and either Ypet or Venus as acceptors) to determine if they could provide more reliable, reproducible and quantifiable FLIM-FRET data to improve on the dynamic range compared to other donors and breadth of application of biosensor technologies. These tests were performed for comparison on both a wide-field, frequency domain system and a multiphoton, TCSPC time domain FLIM system. Clover proved to be an excellent donor with extended dynamic range in combination with mCherry on both platforms, while mRuby2 showed a high degree of variability and poor FRET efficiencies in all cases. mTFP-Venus was the most consistent cyan-yellow pair between the two FLIM methodologies, but mTurquoise2 has better dynamic range and transfers energy consistently over time to the dark acceptor sRCh. Combination of mTFP-sRCh with Clover-mCherry would allow the simultaneous use of two FLIM-FRET biosensors within one sample by eliminating the crosstalk between the yellow acceptor and green donor emissions.
Project description:BackgroundDespite the broad use of FRET techniques, available methods for analyzing protein-protein interaction are subject to high labor and lack of systematic analysis. We propose an open source software allowing the quantitative analysis of fluorescence lifetime imaging (FLIM) while integrating the steady-state fluorescence intensity information for protein-protein interaction studies.FindingsOur developed open source software is dedicated to fluorescence lifetime imaging microscopy (FLIM) data obtained from Becker & Hickl SPC-830. FLIM-FRET analyzer includes: a user-friendly interface enabling automated intensity-based segmentation into single cells, time-resolved fluorescence data fitting to lifetime value for each segmented objects, batch capability, and data representation with donor lifetime versus acceptor/donor intensity quantification as a measure of protein-protein interactions.ConclusionsThe FLIM-FRET analyzer software is a flexible application for lifetime-based FRET analysis. The application, the C#. NET source code, and detailed documentation are freely available at the following URL: http://FLIM-analyzer.ip-korea.org.
Project description:BackgroundIn spite of a great number of monomeric fluorescent proteins developed in the recent years, the reported fluorescent protein-based FRET pairs are still characterized by a number of disadvantageous features, complicating their use as reporters in cell biology and for high-throughput cell-based screenings.ResultsHere we screened some of the recently developed monomeric protein pairs to find the optimal combination, which would provide high dynamic range FRET changes, along with high pH- and photo-stability, fast maturation and bright fluorescence, and reliable detection in any fluorescent imaging system. Among generated FRET pairs, we have selected TagGFP-TagRFP, combining all the mentioned desirable characteristics. On the basis of this highly efficient FRET pair, we have generated a bright, high contrast, pH- and photo-stable apoptosis reporter, named CaspeR3 (Caspase 3 Reporter).ConclusionThe combined advantages suggest that the TagGFP-TagRFP is one of the most efficient green/red couples available to date for FRET/FLIM analyses to monitor interaction of proteins of interest in living cells and to generate FRET-based sensors for various applications. CaspeR3 provides reliable detection of apoptosis, and should become a popular tool both for cell biology studies and high throughput screening assays.
Project description:Fluorescence Resonance Energy Transfer (FRET) using fluorescent protein variants is widely used to study biochemical processes in living cells. FRET detection by fluorescence lifetime measurements is the most direct and robust method to measure FRET. The traditional cyan-yellow fluorescent protein based FRET pairs are getting replaced by green-red fluorescent protein variants. The green-red pair enables excitation at a longer wavelength which reduces cellular autofluorescence and phototoxicity while monitoring FRET. Despite the advances in FRET based sensors, the low FRET efficiency and dynamic range still complicates their use in cell biology and high throughput screening. In this paper, we utilized the higher lifetime of NowGFP and screened red fluorescent protein variants to develop FRET pairs with high dynamic range and FRET efficiency. The FRET variations were analyzed by proteolytic activity and detected by steady-state and time-resolved measurements. Based on the results, NowGFP-tdTomato and NowGFP-mRuby2 have shown high potentials as FRET pairs with large fluorescence lifetime dynamic range. The in vitro measurements revealed that the NowGFP-tdTomato has the highest Förster radius for any fluorescent protein based FRET pairs yet used in biological studies. The developed FRET pairs will be useful for designing FRET based sensors and studies employing Fluorescence Lifetime Imaging Microscopy (FLIM).
Project description:Fluorescence Lifetime Imaging (FLIM) is an intrinsically quantitative method to screen for protein-protein interactions and is frequently used to record the outcome of signal transduction events. With new highly sensitive and photon efficient FLIM instrumentation, the technique also becomes attractive to screen, with high temporal resolution, for fast changes in Förster Resonance Energy Transfer (FRET), such as those occurring upon activation of cell signaling. The second messenger cyclic adenosine monophosphate (cAMP) is rapidly formed following activation of certain cell surface receptors. cAMP is subsequently degraded by a set of phosphodiesterases (PDEs) which display cell-type specific expression and may also affect baseline levels of the messenger. To study which specific PDEs contribute most to cAMP regulation, we knocked down individual PDEs and recorded breakdown rates of cAMP levels following transient stimulation in HeLa cells stably expressing the FRET/FLIM sensor, Epac-SH189. Many hundreds of cells were recorded at 5 s intervals for each condition. FLIM time traces were calculated for every cell, and decay kinetics were obtained. cAMP clearance was significantly slower when PDE3A and, to a lesser amount, PDE10A were knocked down, identifying these isoforms as dominant in HeLa cells. However, taking advantage of the quantitative FLIM data, we found that knockdown of individual PDEs has a very limited effect on baseline cAMP levels. By combining photon-efficient FLIM instrumentation with optimized sensors, systematic gene knockdown and an automated open-source analysis pipeline, our study demonstrates that dynamic screening of transient cell signals has become feasible. The quantitative platform described here provides detailed kinetic analysis of cellular signals in individual cells with unprecedented throughput.
Project description:Fluorescence lifetime imaging microscopy (FLIM)-based Förster resonance energy transfer (FRET) measurement (FLIM-FRET) is one of the powerful methods for imaging of intracellular protein activities such as protein-protein interactions and conformational changes. Here, using saturation mutagenesis, we developed a dark yellow fluorescent protein named ShadowY that can serve as an acceptor for FLIM-FRET. ShadowY is spectrally similar to the previously reported dark YFP but has a much smaller quantum yield, greater extinction coefficient, and superior folding property. When ShadowY was paired with mEGFP or a Clover mutant (CloverT153M/F223R) and applied to a single-molecule FRET sensor to monitor a light-dependent conformational change of the light-oxygen-voltage domain 2 (LOV2) in HeLa cells, we observed a large FRET signal change with low cell-to-cell variability, allowing for precise measurement of individual cell responses. In addition, an application of ShadowY to a separate-type Ras FRET sensor revealed an EGF-dependent large FRET signal increase. Thus, ShadowY in combination with mEGFP or CloverT153M/F223R is a promising FLIM-FRET acceptor.
Project description:The fluorescent-protein based fluorescence resonance energy transfer (FRET) approach is a powerful method for quantifying protein-protein interactions in living cells, especially when combined with fluorescence lifetime imaging microscopy (FLIM). To compare the performance of different FRET couples for FRET-FLIM experiments, we first tested enhanced green fluorescent protein (EGFP) linked to different red acceptors (mRFP1-EGFP, mStrawberry-EGFP, HaloTag (TMR)-EGFP, and mCherry-EGFP). We obtained a fraction of donor engaged in FRET (f(D)) that was far from the ideal case of one, using different mathematical models assuming a double species model (i.e., discrete double exponential fixing the donor lifetime and double exponential stretched for the FRET lifetime). We show that the relatively low f(D) percentages obtained with these models may be due to spectroscopic heterogeneity of the acceptor population, which is partially caused by different maturation rates for the donor and the acceptor. In an attempt to improve the amount of donor protein engaged in FRET, we tested mTFP1 as a donor coupled to mOrange and EYFP, respectively. mTFP1 turned out to be at least as good as EGFP for donor FRET-FLIM experiments because 1), its lifetime remained constant during light-induced fluorescent changes; 2), its fluorescence decay profile was best fitted with a single exponential model; and 3), no photoconversion was detected. The f(D) value when combined with EYFP as an acceptor was the highest of all tandems tested (0.7). Moreover, in the context of fast acquisitions, we obtained a minimal f(D) (mf(D)) for mTFP1-EYFP that was almost two times greater than that for mCherry-EGFP (0.65 vs. 0.35). Finally, we compared EGFP and mTFP1 in a biological situation in which the fusion proteins were highly immobile, and EGFP and mTFP1 were linked to the histone H4 (EGFP-H4 and mTFP1-H4) in fast FLIM acquisitions. In this particular case, the fluorescence intensity was more stable for EGFP-H4 than for mTFP1-H4. Nevertheless, we show that mTFP1/EYFP stands alone as the best FRET-FLIM couple in terms of f(D) analysis.
Project description:We present a high content multiwell plate cell-based assay approach to quantify protein interactions directly in cells using Förster resonance energy transfer (FRET) read out by automated fluorescence lifetime imaging (FLIM). Automated FLIM is implemented using wide-field time-gated detection, typically requiring only 10 s per field of view (FOV). Averaging over biological, thermal and shot noise with 100's to 1000's of FOV enables unbiased quantitative analysis with high statistical power. Plotting average donor lifetime vs. acceptor/donor intensity ratio clearly identifies protein interactions and fitting to double exponential donor decay models provides estimates of interacting population fractions that, with calibrated donor and acceptor fluorescence intensities, can yield dissociation constants. We demonstrate the application to identify binding partners of MST1 kinase and estimate interaction strength among the members of the RASSF protein family, which have important roles in apoptosis via the Hippo signalling pathway. KD values broadly agree with published biochemical measurements.
Project description:We report a new technique to detect enzyme activity inside cells. The method based on Fluorescence Lifetime Imaging (FLIM) technology allows one to follow sensor cleavage by proteolytic enzyme caspase-3. Specifically, we use the FLIM FRET of living cells via the confocal fluorescence microscopy. A specially designed lentivector pLVT with the DNA fragment of TagRFP-23-KFP was applied for transduction of A549 cell lines. Computer simulations are carried out to estimate FRET efficiency and to analyze possible steric restrictions of the reaction between the substrate TagRFP-23-KFP and caspase-3 dimer. Successful use of the fuse protein TagRFP-23-KFP to register the caspase-3 activation based on average life-time measurements is demonstrated. We show that the average life-time distribution is dramatically changed for cells with the modified morphology that is typical for apoptosis. Namely, the short-lived component at 1.8-2.1 ns completely disappears and the long-lived component appears at 2.4-2.6 ns. The latter is a fingerprint of the TagRFP molecule released after cleavage of the TagRFP-23-KFP complex by caspase-3. Analysis of life-time distributions for population of cells allows us to discriminate apoptotic and surviving cells within single frame and to peform statistical analysis of drug efficiency. This system can be adjusted for HTS by using special readers oriented on measurements of fluorescence life-time.
Project description:A bright and photostable fluorescent dye with a disulfide (S-S) linker and maleimide group (Rho594-S2-mal), as cleavable and reactive sites, was synthesized and conjugated with anti-GFP nanobodies (NB). The binding of EGFP (FRET donor) with anti-GFP NB labeled with one or two Rho594-S2-mal residues was studied in vitro and in cellulo. The linker was cleaved with dithiothreitol recovering the donor (FP) signal. The bioconjugates (FP-NB-dye) were applied in FRET-FLIM assays, confocal imaging, and superresolution STED microscopy.