Recent developments in monitoring calcium and protein interactions in cells using fluorescence lifetime microscopy

Live-cell biosensors based on the fluorescence lifetime of environment-sensing dyes

In this work, we examine the use of environment-sensitive fluorescent dyes in fluorescence lifetime imaging microscopy (FLIM) biosensors. We screened merocyanine dyes to find an optimal combination of environment-induced lifetime changes, photostability, and brightness at wavelengths suitable for live-cell imaging. FLIM was used to monitor a biosensor reporting conformational changes of endogenous Cdc42 in living cells. The ability to quantify activity using phasor analysis of a single fluorophore (e.g., rather than ratio imaging) eliminated potential artifacts. We leveraged these properties to determine specific concentrations of activated Cdc42 across the cell.

A turquoise fluorescence lifetime-based biosensor for quantitative imaging of intracellular calcium

The most successful genetically encoded calcium indicators (GECIs) employ an intensity or ratiometric readout. Despite a large calcium-dependent change in fluorescence intensity, the quantification of calcium concentrations with GECIs is problematic, which is further complicated by the sensitivity of all GECIs to changes in the pH in the biological range. Here, we report on a sensing strategy in which a conformational change directly modifies the fluorescence quantum yield and fluorescence lifetime of a circular permutated turquoise fluorescent protein. The fluorescence lifetime is an absolute parameter that enables straightforward quantification, eliminating intensity-related artifacts. An engineering strategy that optimizes lifetime contrast led to a biosensor that shows a 3-fold change in the calcium-dependent quantum yield and a fluorescence lifetime change of 1.3 ns. We dub the biosensor Turquoise Calcium Fluorescence LIfeTime Sensor (Tq-Ca-FLITS). The response of the calcium sensor is insensitive to pH between 6.2-9. As a result, Tq-Ca-FLITS enables robust measurements of intracellular calcium concentrations by fluorescence lifetime imaging. We demonstrate quantitative imaging of calcium concentrations with the turquoise GECI in single endothelial cells and human-derived organoids.

Multiparametric Time-Correlated Single Photon Counting Luminescence Microscopy

Classic time-correlated single photon counting (TCSPC) technique involves detection of single photons of a periodic optical signal, registration of the photon arrival time in respect to the reference pulse, and construction of photon distribution with regard to the detection times. This technique achieves extremely high time resolution and near-ideal detection efficiency. Modern TCSPC is multi-dimensional, i.e., in addition to the photon arrival time relative to the excitation pulse, spatial coordinates within the image area, wavelength, time from the start of the experiment, and many other parameters are determined for each photon. Hence, the multi-dimensional TCSPC allows generation of photon distributions over these parameters. This review describes both classic and multi-dimensional types of TCSPC microscopy and their application for fluorescence lifetime imaging in different areas of biological studies.

Photoluminescence imaging of Zn2+in living systems

Advances in PL imaging techniques, such as confocal microscopy, two photon microscopy, lifetime and optical imaging techniques, have made remarkable contributions in Zn²⁺tracking.

Fluorescence lifetime imaging microscopy of intracellular glucose dynamics

BACKGROUND

One of the major hurdles in studying diabetes pathophysiology is the lack of adequate methodology that allows for direct and real-time determination of glucose transport and metabolism in cells and tissues. In this article, we present a new methodology that adopts frequency-domain fluorescence lifetime imaging microscopy (FD-FLIM) to visualize and quantify the dynamics of intracellular glucose within living cells using a biosensor protein based on fluorescence resonance energy transfer (FRET).

METHOD

The biosensor protein was developed by fusing a FRET pair, an AcGFP1 donor and a mCherry acceptor to N- and C- termini of a mutant glucose-binding protein (GBP), respectively. The probe was expressed and biosynthesized inside the cells, offering continuous monitoring of glucose dynamics in real time through fluorescence lifetime imaging microscopy (FLIM) measurement.

RESULTS

We transfected the deoxyribonucleic acid of the AcGFP1-GBP-mCherry sensor into murine myoblast cells, C2C12, and continuously monitored the changes in intracellular glucose concentrations in response to the variation in extracellular glucose, from which we determined glucose uptake and clearance rates. The distribution of intracellular glucose concentration was also characterized. We detected a high glucose concentration in a region close to the cell membrane and a low glucose concentration in a region close to the nucleus. The monoexponential decay of AcGFP1 was distinguished using FD-FLIM.

CONCLUSIONS

This work enables continuous glucose monitoring (CGM) within living cells using FD-FLIM and a biosensor protein. The sensor protein developed offers a new means for quantitatively analyzing glucose homeostasis at the cellular level. Data accumulated from these studies will help increase our understanding of the pathology of diabetes.

Fast fluorescence lifetime imaging of calcium in living cells

A fast fluorescence lifetime imaging (FLIM) system is developed that can acquire images at a rate of hundreds of frames per second. The FLIM system is based on a wide-field microscope equipped with a time-gated intensified CCD detector and a pulsed laser. The time-gated detector acquires the signals from two time gates simultaneously and is therefore insensitive to movements of the specimen and photo-bleaching. The system is well suited for quantitative biological FLIM experiments and its performance is evaluated in calcium imaging experiments on beating neonatal rat myocytes. Several calcium sensitive dyes are characterized and tested for their suitability for fast FLIM experiments: Oregon Green Bapta-1 (OGB1), Oregon Green Bapta-2 (OGB2), and Oregon Green Bapta-5N (OGB5N). Overall the sensitivity range of these dyes is shifted to low calcium concentrations when used as lifetime dyes. OGB1 and OGB2 behave very similarly and can be used for FLIM-based calcium imaging in the range 1 to approximately 500 nM and OGB5N can be used up to 3 microM. The fast FLIM experiments on the myocytes could be carried out at a 100-Hz frame rate. During the beating of the myocytes a lifetime change of about 20% is observed. From the lifetime images a rest calcium level of about 65 nM is found.

Characterization of two-photon excitation fluorescence lifetime imaging microscopy for protein localization

Two-photon excitation fluorescence resonance energy transfer (2P-FRET) imaging microscopy can provide details of specific protein molecule interactions inside living cells. Fluorophore molecules used for 2P-FRET imaging have characteristic absorption and emission spectra that introduce spectral cross-talk (bleed-through) in the FRET signal that should be removed in the 2P-FRET images, to establish that FRET has actually occurred and to have a basis for distance estimations. These contaminations in the FRET signal can be corrected using a mathematical algorithm to extract the true FRET signal. Another approach is 2P-FRET fluorescence lifetime imaging (FLIM). This methodology allows studying the dynamic behavior of protein-protein interactions in living cells and tissues. 2P-FRET-FLIM was used to study the dimerization of the CAATT/enhancer binding protein alpha (C/EBPalpha). Results show that the reduction in donor lifetime in the presence of acceptor reveals the dimerization of the protein molecules and also determines more precisely the distance between the donor and acceptor. We describe the development and characterization of the 2P-FRET-FLIM imaging system with the Bio-Rad Radiance2100 confocal/multiphoton microscopy system.

Discrimination of DNA and RNA in cells by a vital fluorescent probe: lifetime imaging of SYTO13 in healthy and apoptotic cells

BACKGROUND

Of the few vital DNA and RNA probes, the SYTO dyes are the most specific for nucleic acids. However, they show no spectral contrast upon DNA or RNA binding. We show that fluorescence lifetime imaging using two-photon excitation of SYTO13 allows differential and simultaneous imaging of DNA and RNA in living cells, as well as sequential and repetitive assessment of staining patterns.

METHODS

Two-photon imaging of SYTO13 is combined with lifetime contrast, using time-gated detection. We focus on distinguishing DNA and RNA in healthy and apoptotic Chinese hamster ovary cells.

RESULTS

In healthy cells, SYTO13 has a fluorescence lifetime of 3.4 +/- 0.2 ns when associated with nuclear DNA. Bound to RNA, its lifetime is 4.1 +/- 0.1 ns. After induction of apoptosis, clusters of SYTO13 with fluorescence lifetime of 3.4 +/- 0.2 ns become apparent in the cytoplasm. They are identified as mitochondrial DNA on the basis of colocalization experiments with the DNA-specific dye, DRAQ5, and the mitochondrial-specific dye, CMXRos. Upon progression of apoptosis, the lifetime of SYTO13 attached to DNA shortens significantly, which is indicative of changes in the molecular environment of the dye.

CONCLUSIONS

We have characterized SYTO13 as a vital lifetime probe, allowing repetitive and differential imaging of DNA and RNA.

Nanosecond fluorescence resonance energy transfer-fluorescence lifetime imaging microscopy to localize the protein interactions in a single living cell

Visualizing and quantifying protein-protein interactions is a recent trend in biomedical imaging. The current advances in fluorescence microscopy, coupled with the development of new fluorescent probes such as green fluorescent proteins, allow fluorescence resonance energy transfer (FRET) to be used to study protein interactions in living specimens. Intensity-based FRET microscopy is limited by spectral bleed-through and fluorophore concentration. Fluorescence lifetime imaging (FLIM) microscopy and lifetime measurements are independent of change in fluorophore concentration or excitation intensity, and the combination of FRET and FLIM provides high spatial (nanometre) and temporal (nanoseconds) resolution. Because only the donor fluorophore lifetime is measured, spectral bleed-through is not an issue in FRET-FLIM imaging. In this paper we describe the development of a nanosecond FRET-FLIM microscopy instrumentation to acquire the time-resolved images of donor in the presence and the absence of the acceptor. Software was developed to process the acquired images for single and double exponential decays. Measurement of donor lifetime in two different conditions allowed us to calculate accurately the distance between the interacting proteins. We used this approach to quantify the dimerization of the transcription factor CAATT/enhancer binding protein alpha in living pituitary cells. The one- and two-component analysis of the donor molecule lifetime in the presence of acceptor demonstrates the distance distribution between interacting proteins.

Chondroitin sulphate inhibits connective tissue mast cells

1. Mast cells derive from the bone marrow and are responsible for the development of allergic and possibly inflammatory reactions. Mast cells are stimulated by immunoglobulin E (IgE) and specific antigen, but also by a number of neuropeptides such as neurotensin (NT), somatostatin or substance P (SP), to secrete numerous pro-inflammatory molecules that include histamine, cytokines and proteolytic enzymes. 2. Chondroitin sulphate, a major constituent of connective tissues and of mast cell secretory granules, had a dose-dependent inhibitory effect on rat peritoneal mast cell release of histamine induced by the mast cell secretagogue compound 48/80 (48/80). This inhibition was stronger than that of the clinically available mast cell 'stabilizer' disodium cromoglycate (cromolyn). Inhibition by chondroitin sulphate increased with the length of preincubation and persisted after the drug was washed off, while the effect of cromolyn was limited by rapid tachyphylaxis. 3. Immunologic stimulation of histamine secretion from rat connective tissue mast cells (CTMC) was also inhibited, but this effect was weaker in umbilical cord-derived human mast cells and was absent in rat basophilic leukemia (RBL) cells which are considered homologous to mucosal mast cells (MMC). Oligo- and monosaccharides were not as effective as the polysaccharides. 4. Inhibition, documented by light and electron microscopy, involved a decrease of intracellular calcium ion levels shown by confocal microscopy and image analysis. Autoradiography at the ultrastructural level showed that chondroitin sulphate was mostly associated with plasma and perigranular membranes. 5. Chondroitin sulphate appears to be a potent mast cell inhibitor of allergic and nonimmune stimulation with potential clinical implications.

Measurement of intracellular calcium

To a certain extent, all cellular, physiological, and pathological phenomena that occur in cells are accompanied by ionic changes. The development of techniques allowing the measurement of such ion activities has contributed substantially to our understanding of normal and abnormal cellular function. Digital video microscopy, confocal laser scanning microscopy, and more recently multiphoton microscopy have allowed the precise spatial analysis of intracellular ion activity at the subcellular level in addition to measurement of its concentration. It is well known that Ca2+ regulates numerous physiological cellular phenomena as a second messenger as well as triggering pathological events such as cell injury and death. A number of methods have been developed to measure intracellular Ca2+. In this review, we summarize the advantages and pitfalls of a variety of Ca2+ indicators used in both optical and nonoptical techniques employed for measuring intracellular Ca2+ concentration.

Lifetime-based pH sensors: indicators for acidic environments

We characterized the pH-dependent intensity decays of three fluorophores, Oregon green 514 carboxylic acid, Cl-NERF, and DM-NERF, using frequency-domain fluorometry, with the objective of identifying lifetime-based sensors for low pH values. These three probes were originally designed as dual excitation wavelength-ratiometric probes, with high photostability and high quantum yields in aqueous solutions. We found that their fluorescence intensity decays were strongly dependent on pH. Moreover, global intensity decays analysis reveals that these probes have double exponential intensity decays at intermediate pH values and that the decay time amplitudes are greatly dependent on pH. The longer lifetime components originated from the unprotonated forms and the shorter components from the protonated forms. Both forms can emit fluorescence at intermediate pH values. The apparent pKa values were also determined from the titration curves of phase angles and modulations versus pH for the purpose of pH sensing. The apparent pKa values range from pH 3 to 5, a range where lifetime-based sensors are not presently reported. Since these probes show low pKa values and display substantial phase and modulation changes with pH, they are suitable as lifetime-based pH sensors to monitor the pH changes in acidic environments. One potential application of these probes is to trace the pH in different cellular compartments.