Reliable measurement of the FRET sensitized-quenching transition factor for FRET quantification in living cells

Im-SCC-FRET: improved single-cell-based calibration of a FRET system

We recently developed a SCC-FRET (single-cell-based calibration of a FRET system) method to quantify spectral crosstalk correction parameters (β and δ) and system calibration parameters (G and k) of a Förster resonance energy transfer (FRET) system by imaging a single cell expressing a standard FRET plasmid with known FRET efficiency (E) and donor-acceptor concentration ratio (RC) (Liu et al., Opt. Express30, 29063 (2022)10.1364/OE.459861). Here we improved the SCC-FRET method (named as Im-SCC-FRET) to simultaneously obtain β, δ, G, k and the acceptor-to-donor extinction coefficient ratio (ε A ε D), which is a key parameter to calculate the acceptor-centric FRET efficiency (EA), of a FRET system when the range of β and δ values is set as 0-1. In Im-SCC-FRET, the target function is changed from the sum of absolute values to the sum of squares according to the least squares method, and the initial value of β and δ estimated by the integral but not the maximum value spectral overlap between fluorophore and filter. Compared with SCC-FRET, the experimental results demonstrate that Im-SCC-FRET can obtain more accurate and stable results for β, δ, G, and k, and add the ratio ε A ε D, which is necessary for the FRET hybrid assay. Im-SCC-FRET reduces the complexity of experiment preparation and opens up a promising avenue for developing an intelligent FRET correction system.

Dependence of ghost on the incident light angle into dichroic mirror

The dichroic mirror (DM) is a key component in microscope. We found a ghost in the reflection channel of a dual-channel fluorescence microscope and studied the relationship between the ghost and the incidence angle θ into the DM. The DM emission surface reflection generated ghost if the θ is not45 ° . We analyzed the distance and intensity relationship between the ghost and the primary image, which is θ -dependent and was demonstrated by imaging live cells and a stage micrometer. The ghost can be eliminated by placing the DM between objective and tube lens, but not between tube lens and detector, ensuring that the incident light into the DM is approximately parallel. Furthermore, the transmitted light of the DM is shifted towards a longer wavelength with increasing θ . Collectively, microscopists must carefully optimize the θ when designing a microscope to avoid the ghost.

Internet of Bio Nano Things-based FRET nanocommunications for eHealth

The integration of the Internet of Bio Nano Things (IoBNT) with artificial intelligence (AI) and molecular communications technology is now required to achieve eHealth, specifically in the targeted drug delivery system (TDDS). In this work, we investigate an analytical framework for IoBNT with Forster resonance energy transfer (FRET) nanocommunication to enable intelligent bio nano thing (BNT) machine to accurately deliver therapeutic drug to the diseased cells. The FRET nanocommunication is accomplished by using the well-known pair of fluorescent proteins, EYFP and ECFP. Furthermore, the proposed IoBNT monitors drug transmission by using the quenching process in order to reduce side effects in healthy cells. We investigate the IoBNT framework by driving diffusional rate models in the presence of a quenching process. We evaluate the performance of the proposed framework in terms of the energy transfer efficiency, diffusion-controlled rate and drug loss rate. According to the simulation results, the proposed IoBNT with the intelligent bio nano thing for monitoring the quenching process can significantly achieve high energy transfer efficiency and low drug delivery loss rate, i.e., accurately delivering the desired therapeutic drugs to the diseased cell.

SCC-FRET: single-cell-based calibration of a FRET system

Reliable measurements of calibration parameters are crucial for quantitative three-cube Förster resonance energy transfer (FRET) measurements. Here we have developed a single-cell-based calibration method (SCC-FRET), which can simultaneously obtain spectral crosstalk correction parameters (β and δ) and calibration parameters (G and k) of a quantitative FRET system by imaging a cell expressing one kind of standard FRET plasmid with a known FRET efficiency (E) and the donor-to-acceptor concentration ratio (RC). We performed the SCC-FRET method on a three-cube FRET microscopy for the cells expressing C5V, and obtained β = 0.150 ± 0.000, δ = 0.610 ± 0.000, G = 2.840 ± 0.065, and k = 0.847 ± 0.013. These parameters were used to measure the E and RC values of C17V and C32V constructs in living cells and obtained EC17V = 0.382 ± 0.010 and EC32V = 0.311 ± 0.007, RC17V = 1.010 ± 0.023 and RC32V = 1.050 ± 0.022, consistent with the reported values, demonstrating the effectiveness of the the SCC-FRET method. We also performed the SCC-FRET method for the cells with different S/N levels (S/N > 10, 10 > S/N > 3, 3 > S/N > 1, respectively), and obtained consistent system calibration parameters under different S/N levels, indicating excellent robustness. SCC-FRET requires only imaging a cell expressing one kind of standard FRET plasmid for measuring all calibration parameters under identical imaging conditions, rendering the SCC-FRET method extremely convenient, accurate, and robust. The SCC-FRET provides strong support for expanding the biological application of quantitative FRET analysis in living cells.

High spatio-temporal resolution measurement of A1 R and A2A R interactions combined with Iem-spFRET and E-FRET methods

A₁ R-A_2A R heterodimers regulate striatal glutamatergic neurotransmission. However, few researches about kinetics have been reported. Here, we combined Iem-spFRET and E-FRET to investigate the kinetics of A₁ R and A_2A R interaction. Iem-spFRET obtains the energy transfer efficiency of the whole cell. E-FRET gets energy transfer efficiency with high spatial resolution, whereas, it was prone to biases because background was easily selected due to manual operation. To study the interaction with high spatio-temporal resolution, Iem-spFRET was used to correct the deviation of E-FRET. In this paper, A₁ R and A_2A R interaction was monitored, and the changes of FRET efficiency of the whole or/and partial cell membrane were described. The results showed that activation of A₁ R or A_2A R leads to rapid aggregation, inhibition of A₁ R or A_2A R leads to slow segregation, and the interaction is reversible. These results demonstrated that combination of Iem-spFRET and E-FRET could measure A₁ R and A_2A R interaction with high spatio-temporal resolution.

Optical section structured illumination-based Förster resonance energy transfer imaging

Förster resonance energy transfer (FRET) microscopy is an important tool suitable for studying molecular interactions in living cells. Optical section structured illumination microscopy (OS-SIM), like confocal microscopy, has about 200 nm spatial resolution. In this report, we performed quantitative 3-cube FRET imaging in OS-SIM mode and widefield microscopy (WF) mode, respectively, for living cells expressing FRET constructs consisting of Cerulean (C, donor) and Venus (V, acceptor). OS-SIM images exhibited higher resolution than WF images. Four spectral crosstalk coefficients measured under OS-SIM mode are consistent with those measured under WF mode. Similarly, the system calibration factors G and k measured under OS-SIM mode were consistent with those measured under WF mode. The measured FRET efficiency (E) values of C32V and C17V as well as C5V constructs, standard FRET plasmids, in living Hela cells were E C 32 V OSF = 0.32 ± 0.02 , E C 17 V OSF = 0.38 ± 0.02 , and E C 5 V OSF = 0.45 ± 0.03 , and the measured acceptor-to-donor concentration ratios ( R c ) were R C 32 V OSF = 1.07 ± 0.03 , R C 17 V OSF = 1.09 ± 0.03 , and R C 5 V OSF = 1.02 ± 0.04 , consistent with the reported values. Collectively, our data demonstrates that OS-SIM can be integrated into FRET microscopy to build an OS-SIM-FRET with confocal microscopy-like resolution.

Stoichiometry and regulation network of Bcl-2 family complexes quantified by live-cell FRET assay

The stoichiometry and affinity of Bcl-2 family complexes are essential information for understanding how their interactome network is orchestrated to regulate mitochondrial permeabilization and apoptosis. Based on over-expression model system, FRET analysis was used to quantify the protein-protein interactions among Bax, Bcl-xL, Bad and tBid in healthy and apoptotic cells. Our data indicate that the stoichiometry and affinity of Bcl-2 complexes are dependent on their membrane environment. Bcl-xL, Bad and tBid can form hetero-trimers in mitochondria. Bcl-xL binds preferentially to Bad, then to tBid and Bax in mitochondria, whilst Bcl-xL displays higher affinity to Bad or tBid than to itself. Strikingly, Bax can bind to Bcl-xL in cytosol. In cytosol of apoptotic cells, Bcl-xL associates with Bax to form hetero-trimer with 1:2 stoichiometry, while Bcl-xL associates with Bad to form hetero-trimer with 2:1 stoichiometry and Bcl-xL associates with tBid to form hetero-dimer. In mitochondria, Bcl-xL associates with Bax/Bad to form hetero-dimer in healthy cells, while Bcl-xL associates with Bad to form hetero-tetramer with 3:1 stoichiometry in apoptotic cells.

Automated E-FRET microscope for dynamical live-cell FRET imaging

Acceptor-sensitised 3-cube fluorescence resonance energy transfer (FRET) imaging (also termed as E-FRET imaging) is a popular fluorescence intensity-based FRET quantification method. Here, an automated E-FRET microscope with user-friendly interfaces was set up for dynamical online quantitative live-cell FRET imaging. This microscope reduces the time of a quantitative E-FRET imaging from 12 to 3 s. After locating cells, calibration of the microscope and E-FRET imaging of the cells can be performed automatically by clicking 'Capture' button on interfaces. E-FRET imaging was performed on the microscope for living cells expressing different FRET tandem constructs. Dynamical E-FRET imaging on the microscope for live cells coexpressing CFP-Bax and YFP-Bax treated by staurosporine (STS) revealed three Bax redistribution stages: Bax translocation from cytosol to mitochondria within 10 min, membrane insertion with conformational change on mitochondrial membrane within about 30 min, and subsequent oligomerisation within about 10 min. Because of excellent user-friendly interface and stability, the automated E-FRET microscope is a convenient tool for quantitative FRET imaging of living cell. LAY DESCRIPTION: Acceptor-sensitised 3-cube fluorescence resonance energy transfer (FRET) imaging (also termed as E-FRET) is a popular fluorescence intensity-based FRET quantification methods. E-FRET measurements are currently performed manually, and a complete FRET measurement takes about 12 s. E-FRET measurement necessitates not only a skilled operator and specialised equipment but also expertise in the interpretation of FRET signals, a considerable challenge in the application of FRET technology in living cells. Furthermore, manual E-FRET microscope is hard to perform dynamical quantitative FRET measurement, the ever-increasing applications in mapping the biochemical signal transduction within cells. Here, an automated E-FRET microscope with user-friendly interfaces was set up for dynamical online quantitative live-cell FRET imaging. This microscope reduces the time of a quantitative E-FRET imaging from 12 to 3 s. After locating cells, calibration of the microscope and E-FRET imaging of the cells can be performed automatically by clicking 'Capture' button on interfaces. Because of excellent user-friendly interface and stability, the automated E-FRET microscope is a convenient tool for quantitative FRET imaging of living cell.

Quantitative dual-channel FRET microscopy

Acceptor-sensitized quantitative Förster resonance energy transfer (FRET) measurement (E-FRET) is mainly impeded by donor emission crosstalk and acceptor direct excitation crosstalk. In this paper, we develop a novel E-FRET approach (Lux-E-FRET) based on linear unmixing (Lux) of the fluorescence intensity ratio between two detection channels with each excitation of two different wavelengths. The two detection channels need not to selectively collect the emission of donor or acceptor, and the excitation wavelengths need not to selectively excite donor or acceptor. For a tandem FRET sensor, Lux-E-FRET only needs single excitation wavelength. We performed Lux-E-FRET measurements on our dual-channel wide-field fluorescence microscope for FRET constructs in living cells, and obtained consistent FRET efficiencies with those measured by other methods. Collectively, Lux-E-FRET completely overcomes all spectral crosstalks and thus is applicable to the donor-acceptor pair with larger spectral overlapping.

Mechanism of hepatic targeting via oral administration of DSPE-PEG-cholic acid-modified nanoliposomes

In oral administration, gastrointestinal physiological environment, gastrointestinal epithelial cell membranes, and blood circulation are typical biological barriers to hepatic delivery of ligand-modified nanoparticle drug delivery systems. To elucidate the mechanism of oral hepatic targeting of cholic acid receptor-mediated nanoliposomes (LPs) (distearoyl phosphatidylethanolamine–polyethylene glycol–cholic acid-modified LPs, CA-LPs), evaluations were performed on colon cancer Caco-2 cell monolayers, liver cancer HepG2 cells, and a rat intestinal perfusion model. CA-LPs, ~100 nm in diameter, exhibited sustained-release behavior and had the greatest stability in rat gastrointestinal fluid and serum for both size and entrapment efficiency. CA-LPs demonstrated highest transport across Caco-2 cells and highest cellular uptake by HepG2 cells. The enhanced endocytosis of CA-LPs was found to be mediated by Na⁺/taurocholate cotransporting polypeptide and involved the caveolin-mediated endocytosis pathway. Further, we used fluorescence resonance energy transfer (FRET) technology to show that the CA-LPs maintained their structural integrity in part during the transport across the Caco-2 cell monolayer and uptake by HepG2 cells.

Farnesylthiosalicylic acid sensitizes hepatocarcinoma cells to artemisinin derivatives

Dihydroartemisinin (DHA) and artesunate (ARS), two artemisinin derivatives, have efficacious anticancer activities against human hepatocarcinoma (HCC) cells. This study aims to study the anticancer action of the combination treatment of DHA/ARS and farnesylthiosalicylic acid (FTS), a Ras inhibitor, in HCC cells (Huh-7 and HepG2 cell lines). FTS pretreatment significantly enhanced DHA/ARS-induced phosphatidylserine (PS) externalization, Bak/Bax activation, mitochondrial membrane depolarization, cytochrome c release, and caspase-8 and -9 activations, characteristics of the extrinsic and intrinsic apoptosis. Pretreatment with Z-IETD-FMK (caspase-8 inhibitor) potently prevented the cytotoxicity of the combination treatment of DHA/ARS and FTS, and pretreatment with Z-VAD-FMK (pan-caspase inhibitor) significantly inhibited the loss of ΔΨm induced by DHA/ARS treatment or the combination treatment of DHA/ARS and FTS in HCC cells. Furthermore, silencing Bak/Bax modestly but significantly inhibited the cytotoxicity of the combination treatment of DHA/ARS and FTS. Interestingly, pretreatment with an antioxidant N-Acetyle-Cysteine (NAC) significantly prevented the cytotoxicity of the combination treatment of DHA and FTS instead of the combination treatment of ARS and FTS, suggesting that reactive oxygen species (ROS) played a key role in the anticancer action of the combination treatment of DHA and FTS. Similar to FTS, DHA/ARS also significantly prevented Ras activation. Collectively, our data demonstrate that FTS potently sensitizes Huh-7 and HepG2 cells to artemisinin derivatives via accelerating the extrinsic and intrinsic apoptotic pathways.