Fluorescence lifetime images of green fluorescent protein in HeLa cells during TNF-alpha induced apoptosis

Monitoring cancer treatments in melanoma cells using a fluorescence lifetime nanoprobe based on a CdSe/ZnS quantum dot functionalized with a peptide containing D-penicillamine and histidine

Anticancer therapies with cisplatin and volasertib (BI-6727) were monitored by fluorescence lifetime imaging microscopy (FLIM) in live SK-Mel-2 melanoma cells. A CdSe/ZnS quantum dot functionalized with a peptide containing D-penicillamine and histidine (CdSe/ZnS-PH) was used as intracellular pH fluorescent probe. A faster cytosol acidification was observed for cells treated with cisplatin when compared to volasertib. The first changes in the intracellular pH were found after 2 hours of treatment with cisplatin and 8 hours with volasertib. Additionally, the relationship between cytosol acidification and apoptosis was investigated using an innovative methodology based on time-resolved fluorescence measurements. Similar low percentages of apoptotic cells were observed after short incubation periods (2 - 8 hours) with both drugs. In contrast, late apoptosis and death were found for a large fraction of cells during 24-hour incubation with cisplatin but not volasertib. Thus, the early acidification observed in cisplatin treatment could accelerate apoptosis and cell death. Despite volasertib treatment shows slower mechanism of action than cisplatin, similar inhibitory effects were found for both drugs at longer incubation periods (72 hours). This study proves the potential of CdSe/ZnS-PH nanoparticle as a fluorescence lifetime probe in the study of the mechanism of action of antitumor drugs.

A novel fluorescent protein pair facilitates FLIM-FRET analysis of plant immune receptor interaction under native conditions

Elucidating protein-protein interactions is crucial for our understanding of molecular processes within living organisms. Microscopy-based techniques can detect protein-protein interactions in vivo at the single-cell level and provide information on their subcellular location. Fluorescence lifetime imaging microscopy (FLIM)-Förster resonance energy transfer (FRET) is one of the most robust imaging approaches, but it is still very challenging to apply this method to proteins which are expressed under native conditions. Here we describe a novel combination of fluorescence proteins (FPs), mCitrine and mScarlet-I, which is ideally suited for FLIM-FRET studies of low abundance proteins expressed from their native promoters in stably transformed plants. The donor mCitrine displays excellent brightness in planta, near-mono-exponential fluorescence decay, and a comparatively long fluorescence lifetime. Moreover, the FRET pair has a good spectral overlap and a large Förster radius. This allowed us to detect constitutive as well as ligand-induced interaction of the Arabidopsis chitin receptor components CERK1 and LYK5 in a set of proof-of-principle experiments. Due to the good brightness of the acceptor mScarlet-I, the FP combination can be readily utilized for co-localization studies. The FP pair is also suitable for co-immunoprecipitation experiments and western blotting, facilitating a multi-method approach for studying and confirming protein-protein interactions.

Genetically encoded fluorescence lifetime biosensors: overview, advances, and opportunities

Genetically encoded biosensors based on fluorescent proteins (FPs) are powerful tools for tracking analytes and cellular events with high spatial and temporal resolution in living cells and organisms. Compared with intensiometric readout and ratiometric readout, fluorescence lifetime readout provides absolute measurements, independent of the biosensor expression level and instruments. Thus, genetically encoded fluorescence lifetime biosensors play a vital role in facilitating accurate quantitative assessments within intricate biological systems. In this review, we first provide a concise description of the categorization and working mechanism of genetically encoded fluorescence lifetime biosensors. Subsequently, we elaborate on the combination of the fluorescence lifetime imaging technique and lifetime analysis methods with fluorescence lifetime biosensors, followed by their application in monitoring the dynamics of environment parameters, analytes and cellular events. Finally, we discuss worthwhile considerations for the design, optimization and development of fluorescence lifetime-based biosensors from three representative cases.

Cancer-on-a-Chip for Modeling Immune Checkpoint Inhibitor and Tumor Interactions

Cancer immunotherapies, including immune checkpoint inhibitor (ICI)-based therapies, have revolutionized cancer treatment. However, patient response to ICIs is highly variable, necessitating the development of methods to quickly assess efficacy. In this study, an array of miniaturized bioreactors has been developed to model tumor-immune interactions. This immunotherapeutic high-throughput observation chamber (iHOC) is designed to test the effect of anti-PD-1 antibodies on cancer spheroid (MDA-MB-231, PD-L1+) and T cell (Jurkat) interactions. This system facilitates facile monitoring of T cell inhibition and reactivation using metrics such as tumor infiltration and interleukin-2 (IL-2) secretion. Status of the tumor-immune interactions can be easily captured within the iHOC by measuring IL-2 concentration using a micropillar array where sensitive, quantitative detection is allowed after antibody coating on the surface of array. The iHOC is a platform that can be used to model and monitor cancer-immune interactions in response to immunotherapy in a high-throughput manner.

FLIM-FRET-Based Structural Characterization of a Class-A GPCR Dimer in the Cell Membrane

Class-A G protein-coupled receptors (GPCRs) are known to homo-dimerize in the membrane. Yet, methods to characterize the structure of GPCR dimer in the native environment are lacking. Accordingly, the molecular basis and functional relevance of the class-A GPCR dimerization remain unclear. Here, we present the dimeric structural model of GPR17 in the cell membrane. The dimer mainly involves transmembrane helix 5 (TM5) at the interface, with F229 in TM5, a critical residue. An F229A mutation makes GPR17 monomeric regardless of the expression level of the receptor. Monomeric mutants of GPR17 display impaired ERK1/2 activation and cannot be properly internalized upon agonist treatment. Conversely, the F229C mutant is cross-linked as a dimer and behaves like wild-type. Importantly, the GPR17 dimer structure has been modeled using sparse inter-protomer FRET distance restraints obtained from fluorescence lifetime imaging microscopy. The same approach can be applied to characterizing the interactions of other important membrane proteins in the cell.

Development of a Robust Autofluorescence Lifetime Sensing Method for Use in an Endoscopic Application

Endoscopic autofluorescence lifetime imaging is a promising technique for making quantitative and non-invasive diagnoses of abnormal tissue. However, motion artifacts caused by vibration in the direction perpendicular to the tissue surface in a body makes clinical diagnosis difficult. Thus, this paper proposes a robust autofluorescence lifetime sensing technique with a lens tracking system based on a laser beam spot analysis. Our optical setup can be easily mounted on the head of an endoscope. The variation in distance between the optical system and the target surface is tracked by the change in the spot size of the laser beam captured by the camera, and the lens actuator is feedback-controlled to suppress motion artifacts. The experimental results show that, when using a lens tracking system, the standard deviation of fluorescence lifetime is dramatically reduced. Furthermore, the validity of the proposed method is experimentally confirmed by using a bio-mimicking phantom that replicates the shape, optical parameters, and chemical component distribution of the cancerous tissue.

Effects of Nanosecond Pulsed Electric Fields on the Intracellular Function of HeLa Cells As Revealed by NADH Autofluorescence Microscopy

The fluorescence lifetime of the endogenous fluorophore of reduced nicotinamide adenine dinucleotide (NADH) in HeLa cells is affected by the application of nanosecond pulsed electric fields (nsPEFs). In this study, we found that after nsPEF application, the fluorescence lifetime became longer and then decreased in a stepwise manner upon further application, irrespective of the pulse width in the range of 10-50 ns. This application time dependence of the NADH fluorescence lifetime is very similar to the time-lapse dependence of the NADH fluorescence lifetime following the addition of an apoptosis inducer, staurosporine. These results, as well as the membrane swelling and blebbing after the application of nsPEFs, indicate that apoptosis is also induced by the application of nsPEFs in HeLa cells. In contrast to the lifetime, the fluorescence intensity remarkably depended on the pulse width of the applied nsPEF. When the pulse width was as large as 50 ns, the intensity monotonically increased and was distributed over the entire cell as the application duration became longer. As the pulse width of the applied electric field became smaller, the magnitude of the field-induced increase in NADH fluorescence intensity decreased; the intensity was reduced by the electric field when the pulse width was as small as 10 ns. These results suggest that the mechanism of electric-field-induced apoptosis depends on the pulse width of the applied nsPEF.

Probing of protein localization and shuttling in mitochondrial microcompartments by FLIM with sub-diffraction resolution

The cell is metabolically highly compartmentalized. Especially, mitochondria host many vital reactions in their different microcompartments. However, due to their small size, these microcompartments are not accessible by conventional microscopy. Here, we demonstrate that time-correlated single-photon counting (TCSPC) fluorescence lifetime-imaging microscopy (FLIM) classifies not only mitochondria, but different microcompartments inside mitochondria. Sensor proteins in the matrix had a different lifetime than probes at membrane proteins. Localization in the outer and inner mitochondrial membrane could be distinguished by significant differences in the lifetime. The method was sensitive enough to monitor shifts in protein location within mitochondrial microcompartments. Macromolecular crowding induced by changes in the protein content significantly affected the lifetime, while oxidizing conditions or physiological pH changes had only marginal effects. We suggest that FLIM is a versatile and completive method to monitor spatiotemporal events in mitochondria. The sensitivity in the time domain allows for gaining substantial information about sub-mitochondrial localization overcoming diffraction limitation. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Sensing of intracellular environments by fluorescence lifetime imaging of exogenous fluorophores

Fluorescence lifetime imaging (FLIM) has been recognized as a powerful microscopy technique to examine environments in living systems. The fluorescence lifetime does not depend on the photobleaching and optical conditions, which allows us to obtain quantitative information on intracellular environments by analyzing the fluorescence lifetime. A variety of exogenous fluorophores have been applied in FLIM measurements to examine cellular processes. Information on the correlation between the fluorescence lifetime and the physiological parameters is essential to elucidate the cellular environments from the fluorescence lifetime measurements of exogenous fluorophores. In this review, exogenous fluorophores used for lifetime-based sensing are summarized, with the expectation that it becomes a basis for selecting the fluorophore used to investigate the intracellular environment with FLIM. Experimental results of the intracellular sensing of pH, metal ions, oxygen, viscosity, and other physiological parameters on the basis of the FLIM measurements are described along with a brief explanation of the mechanism of the change in the fluorescence lifetime.

Applying fluorescence lifetime imaging microscopy to evaluate the efficacy of anticancer drugs

Fluorescence lifetime imaging microscopy was applied to evaluate the efficacy of anticancer drugs. A decrease in the fluorescence lifetime of the nucleus in apoptotic cancer cells stained by SYTO 13 dye was detected after treatment with antitumor antibiotics such as doxorubicin or epirubicin. It was confirmed that the change in fluorescence lifetime occurred earlier than morphological changes in the cells. We found that the fluorescence lifetime of the nucleus in the cells treated with epirubicin decreased more rapidly than that of the cells treated with doxorubicin. This implies that epirubicin was more efficacious than doxorubicin in the treatment of cancer cells. The change in fluorescence lifetime was, however, not indicated when the cells were treated with cyclophosphamide. The decrease in fluorescence lifetime was associated with the processes involving caspase activation and chromatin condensation. Therefore, this technique would provide useful information about apoptotic cells, particularly in the early stages.

Differential sub-cellular processing of single-wall carbon nanotubes via interfacial modifications

Real-space and fluorescence lifetime imaging reveal that non-covalently attached dispersing agents influence sub-cellular trafficking and localization of carbon nanotubes.

Electric field effect on the ground state proton transfer in the H-bonded HBDI complex: an implication of the green fluorescent protein

In this paper, first-principles calculations were performed regarding the electric field effect on the ground state proton transfer (GSPT) in the H-bonded p-hydroxybenzylideneimidazolidinone (HBDI) network that represents the active site of the green fluorescent protein (GFP).

Subcellular localization-dependent changes in EGFP fluorescence lifetime measured by time-resolved flow cytometry

Intracellular protein transport and localization to subcellular regions are processes necessary for normal protein function. Fluorescent proteins can be fused to proteins of interest to track movement and determine localization within a cell. Currently, fluorescence microscopy combined with image processing is most often used to study protein movement and subcellular localization. In this contribution we evaluate a high-throughput time-resolved flow cytometry approach to correlate intracellular localization of human LC3 protein with the fluorescence lifetime of enhanced green fluorescent protein (EGFP). Subcellular LC3 localization to autophagosomes is a marker of the cellular process called autophagy. In breast cancer cells expressing native EGFP and EGFP-LC3 fusion proteins, we measured the fluorescence intensity and lifetime of (i) diffuse EGFP (ii) punctate EGFP-LC3 and (iii) diffuse EGFP-ΔLC3 after amino acid starvation to induce autophagy-dependent LC3 localization. We verify EGFP-LC3 localization with low-throughput confocal microscopy and compare to fluorescence intensity measured by standard flow cytometry. Our results demonstrate that time-resolved flow cytometry can be correlated to subcellular localization of EGFP fusion proteins by measuring changes in fluorescence lifetime.

pH dependence of the fluorescence lifetime of FAD in solution and in cells

We have studied physiological parameters in a living cell using fluorescence lifetime imaging of endogenous chromophores. In this study, pH dependence of the fluorescence lifetime of flavin adenine dinucleotide (FAD), that is a significant cofactor exhibiting autofluorescence, has been investigated in buffer solution and in cells. The fluorescence lifetime of FAD remained unchanged with pH 5 to 9 in solution. However, the fluorescence lifetime in HeLa cells was found to decrease with increasing intracellular pH, suggesting that pH in a single cell can be estimated from the fluorescence lifetime imaging of FAD without adding exogenous fluorescent probes.

Probing the dynamics of doxorubicin-DNA intercalation during the initial activation of apoptosis by fluorescence lifetime imaging microscopy (FLIM)

Doxorubicin is a potent anthracycline antibiotic, commonly used to treat a wide range of cancers. Although postulated to intercalate between DNA bases, many of the details of doxorubicin's mechanism of action remain unclear. In this work, we demonstrate the ability of fluorescence lifetime imaging microscopy (FLIM) to dynamically monitor doxorubicin-DNA intercalation during the earliest stages of apoptosis. The fluorescence lifetime of doxorubicin in nuclei is found to decrease rapidly during the first 2 hours following drug administration, suggesting significant changes in the doxorubicin-DNA binding site's microenvironment upon apoptosis initiation. Decreases in doxorubicin fluorescence lifetimes were found to be concurrent with increases in phosphorylation of H2AX (an immediate signal of DNA double-strand breakage), but preceded activation of caspase-3 (a late signature of apoptosis) by more than 150 minutes. Time-dependent doxorubicin FLIM analyses of the effects of pretreating cells with either Cyclopentylidene-[4-(4-chlorophenyl)thiazol-2-yl)-hydrazine (a histone acetyltransferase inhibitor) or Trichostatin A (a histone deacetylase inhibitor) revealed significant correlation of fluorescence lifetime with the stage of chromatin decondensation. Taken together, our findings suggest that monitoring the dynamics of doxorubicin fluorescence lifetimes can provide valuable information during the earliest phases of doxorubicin-induced apoptosis; and implicate that FLIM can serve as a sensitive, high-resolution tool for the elucidation of intercellular mechanisms and kinetics of anti-cancer drugs that bear fluorescent moieties.

Application of nanosecond pulsed electric fields into HeLa cells expressing enhanced green fluorescent protein and fluorescence lifetime microscopy

An electrode microchamber has been constructed for applying nanosecond pulsed strong electric fields to living cells, and fluorescence lifetime microscopy (FLIM) has been used to investigate the effects of external electric fields on dynamics and function of HeLa cells expressing enhanced green fluorescent protein (EGFP). Both morphological change in cells and reduction of the fluorescence lifetime of EGFP have been observed after application of electric fields having a pulsed width of 50 ns and a strength of 4 MV m(-1), indicating that apoptosis, which is a programmed cell death, was induced by nanosecond pulsed electric fields and that fluorescence lifetime of EGFP decreased along with the induction of apoptosis. The reduction of the fluorescence lifetime occurred before the morphological change, indicating that FLIM provides a sensitive and noninvasive detection of the progress of apoptosis induced by application of nanosecond pulsed electric fields.

Tryptophan-based chromophore in fluorescent proteins can be anionic

Cyan fluorescent proteins (CFP) with tryptophan66-based chromophore are widely used for live cell imaging. In contrast to green and red fluorescent proteins, no charged states of the CFP chromophore have been described. Here, we studied synthetic CFP chromophore and found that its indole group can be deprotonated rather easily (pKa 12.4).We then reproduced this effect in the CFP mCerulean by placing basic amino acids in the chromophore microenvironment. As a result, green-emitting variant with an anionic chromophore and key substitution Val61Lys was obtained. This is the first evidence strongly suggesting that tryptophan-based chromophores in fluorescent proteins can exist in an anionic charged state. Switching between protonated and deprotonated Trp66 in fluorescent proteins represents a new unexplored way to control their spectral properties.

Single wall carbon nanotubes enter cells by endocytosis and not membrane penetration

Background

Carbon nanotubes are increasingly being tested for use in cellular applications. Determining the mode of entry is essential to control and regulate specific interactions with cells, to understand toxicological effects of nanotubes, and to develop nanotube-based cellular technologies. We investigated cellular uptake of Pluronic copolymer-stabilized, purified ~145 nm long single wall carbon nanotubes (SWCNTs) through a series of complementary cellular, cell-mimetic, and in vitro model membrane experiments.

Results

SWCNTs localized within fluorescently labeled endosomes, and confocal Raman spectroscopy showed a dramatic reduction in SWCNT uptake into cells at 4°C compared with 37°C. These data suggest energy-dependent endocytosis, as shown previously. We also examined the possibility for non-specific physical penetration of SWCNTs through the plasma membrane. Electrochemical impedance spectroscopy and Langmuir monolayer film balance measurements showed that Pluronic-stabilized SWCNTs associated with membranes but did not possess sufficient insertion energy to penetrate through the membrane. SWCNTs associated with vesicles made from plasma membranes but did not rupture the vesicles.

Conclusions

These measurements, combined, demonstrate that Pluronic-stabilized SWCNTs only enter cells via energy-dependent endocytosis, and association of SWCNTs to membrane likely increases uptake.

Intracellular pH sensing using autofluorescence lifetime microscopy

Fluorescence lifetime images of reduced nicotinamide adenine dinucleotide (NADH) that is a key cofactor in cellular metabolism were obtained in a cell at various values of intracellular pH. The average fluorescence lifetime of NADH is found to become shorter monotonically with increasing pH, indicating that pH in a single cell can be determined by fluorescence lifetime imaging of NADH without adding exogenous fluorescent probes. The magnitude of the pH-induced lifetime change is higher in cells than that in buffer solution. The fluorescence lifetime of NADH is not uniform inside a cell; the fluorescence lifetime of nuclear NADH is shorter than that of mitochondrial NADH at each pH, and the magnitude of the pH-induced change is larger in nuclei than in other areas. The local electric field effect on the fluorescence lifetime is discussed since this effect may be one of the strong possibilities for the nonuniformity of the autofluorescence lifetime of NADH in cells.

Fluorescence decay dynamics of flavin adenine dinucleotide in a mixture of alcohol and water in the femtosecond and nanosecond time range

Fluorescence decays of flavin adenine dinucleotide (FAD) that is a typical autofluorescent species in cells and tissues have been measured in a mixture of alcohol and water in the femtosecond and nanosecond time range. The fluorescence lifetimes of both the stacked conformation between the isoalloxazine and adenine moieties in close proximity and the extended open conformation in water are affected by the addition of alcohol. The nanosecond fluorescence lifetime of the open conformation increases with decreasing dielectric constant of the medium, contributing to the enhancement of the fluorescence intensity of FAD in less dielectric media. The fluorescence lifetime of the open conformation is also affected by medium viscosity, suggesting that the photoexcited open conformation is quenched by the dynamic interaction between the two aromatic rings. The fluorescence component decaying in tens of picoseconds is attributed to the stacked conformation that shows the efficient fluorescence quenching due to the intramolecular electron transfer. The picosecond fluorescence lifetime of the stacked conformation increases with decreasing dielectric constant, suggesting the shift of the distribution of the stacked conformation to a longer intramolecular distance between the two aromatic rings in less dielectric media. The pre-exponential factor of the picosecond decaying component relative to that of the nanosecond one decreases with the increase of the alcohol concentration in the femtosecond time-resolved fluorescence, which demonstrates the increase in the population of the open conformation with the reduction of the dielectric constant. The possibility to evaluate the polar environment in a cell by the fluorescence lifetime of FAD is discussed based on the results obtained.