Real-time cellular uptake of serotonin using fluorescence lifetime imaging with two-photon excitation

Fluorescence and phosphorescence lifetime imaging reveals a significant cell nuclear viscosity and refractive index changes upon DNA damage

Cytoplasmic viscosity is a crucial parameter in determining rates of diffusion-limited reactions. Changes in viscosity are associated with several diseases, whilst nuclear viscosity determines gene integrity, regulation and expression. Yet how drugs including DNA-damaging agents affect viscosity is unknown. We demonstrate the use of a platinum complex, Pt[L]Cl, that localizes efficiently mostly in the nucleus as a probe for nuclear viscosity. The phosphorescence lifetime of Pt[L]Cl is sensitive to viscosity and provides an excellent tool to investigate the impact of DNA damage. We show using Fluorescence Lifetime Imaging (FLIM) that the lifetime of both green and red fluorescent proteins (FP) are also sensitive to changes in cellular viscosity and refractive index. However, Pt[L]Cl proved to be a more sensitive viscosity probe, by virtue of microsecond phosphorescence lifetime versus nanosecond fluorescence lifetime of FP, hence greater sensitivity to bimolecular reactions. DNA damage was inflicted by either a two-photon excitation, one-photon excitation microbeam and X-rays. DNA damage of live cells causes significant increase in the lifetime of either Pt[L]Cl (HeLa cells, 12.5-14.1 µs) or intracellularly expressed mCherry (HEK293 cells, 1.54-1.67 ns), but a decrease in fluorescence lifetime of GFP from 2.65 to 2.29 ns (in V15B cells). These values represent a viscosity change from 8.59 to 20.56 cP as well as significant changes in the refractive index (RI), according to independent calibration. Interestingly DNA damage localized to a submicron region following a laser microbeam induction showed a whole cell viscosity change, with those in the nucleus being greater than the cytoplasm. We also found evidence of a by-stander effect, whereby adjacent un-irradiated cells also showed nuclear viscosity change. Finally, an increase in viscosity following DNA damage was also observed in bacterial cells with an over-expressed mNeonGreen FP, evidenced by the change in its lifetime from 2.8 to 2.4 ns.

Directly imaging the localisation and photosensitization properties of the pan-mTOR inhibitor, AZD2014, in living cancer cells

The range of cellular functions the mechanistic target of rapamycin (mTOR) protein performs makes it an attractive drug target for cancer therapy. However, the cellular localisation and mode of action of second generation inhibitors of mTOR is poorly understood despite the level of attention there is in targeting the mTOR protein. We have therefore studied the properties of the pan-mTOR inhibitor AZD2014, an ideal candidate to study because it is naturally fluorescent, characterising its photochemical properties in solution phase (DMSO, PBS and BSA) and within living cells, where it localises within both the nucleus and the cytoplasm but with different excited state lifetimes of 4.8 (+/- 0.5) and 3.9 (+/- 0.4) ns respectively. We measure the uptake of the inhibitor AZD2014 (7 μM) in monolayer HEK293 cells occurring with a half-life of 1 min but observe complex behaviour for 3D spheroids with the core of the spheroid showing a slower uptake and a slow biphasic behaviour at longer times. From a cellular perspective using fluorescence lifetime imaging microscopy AZD2014 was found to interact directly with GFP-tagged mTORC1 proteins including the downstream target, S6K1. We observe light sensitive behaviour of the cells containing AZD2014 which leads to cell death, in both monolayer and spheroids cells, demonstrating the potential of AZD2014 to act as a possible photodynamic drug under both single photon and multiphoton excitation and discuss its use as a photosensitizer. We also briefly characterise another pan-mTOR inhibitor, INK128.

The Effects of Serotonin in Immune Cells

Serotonin [5-hydroxytryptamine (5-HT)] plays an important role in many organs as a peripheral hormone. Most of the body’s serotonin is circulating in the bloodstream, transported by blood platelets and is released upon activation. The functions of serotonin are mediated by members of the 7 known mammalian serotonin receptor subtype classes (15 known subtypes), the serotonin transporter (SERT), and by covalent binding of serotonin to different effector proteins. Almost all immune cells express at least one serotonin component. In recent years, a number of immunoregulatory functions have been ascribed to serotonin. In monocytes/macrophages, for example, serotonin modulates cytokine secretion. Serotonin can also suppress the release of tumor necrosis factor-α and interleukin-1β by activating serotonin receptors. Furthermore, neutrophil recruitment and T-cell activation can both be mediated by serotonin. These are only a few of the known immunomodulatory roles of serotonin that we will review here.

A mechanistic study of gold nanoparticle radiosensitisation using targeted microbeam irradiation

Gold nanoparticles (GNPs) have been demonstrated as effective radiosensitizing agents in a range of preclinical models using broad field sources of various energies. This study aimed to distinguish between these mechanisms by applying subcellular targeting using a soft X-ray microbeam in combination with GNPs. DNA damage and repair kinetics were determined following nuclear and cytoplasmic irradiation using a soft X-ray (carbon K-shell, 278 eV) microbeam in MDA-MB-231 breast cancer and AG01522 fibroblast cells with and without GNPs. To investigate the mechanism of the GNP induced radiosensitization, GNP-induced mitochondrial depolarisation was quantified by TMRE staining, and levels of DNA damage were compared in cells with depolarised and functional mitochondria. Differential effects were observed following radiation exposure between the two cell lines. These findings were validated 24 hours after removal of GNPs by flow cytometry analysis of mitochondrial depolarisation. This study provides further evidence that GNP radiosensitisation is mediated by mitochondrial function and it is the first report applying a soft X-ray microbeam to study the radiobiological effects of GNPs to enable the separation of physical and biological effects.

Autofluorescence Spectroscopy for Monitoring Metabolism in Animal Cells and Tissues

Excitation of biological substrates with light at a suitable wavelength can give rise to a light emission in the ultraviolet (UV)-visible, near-infrared (IR) spectral range, called autofluorescence (AF). This is a widespread phenomenon, ascribable to the general presence of biomolecules acting as endogenous fluorophores (EFs) in the organisms of the whole life kingdom. In cytochemistry and histochemistry, AF is often an unwanted signal enhancing the background and affecting in particular the detection of low signals or rare positive labeling spots of exogenous markers. Conversely, AF is increasingly considered as a powerful diagnostic tool because of its role as an intrinsic biomarker directly dependent on the nature, amount, and microenvironment of the EFs, in a strict relationship with metabolic processes and structural organization of cells and tissues. As a consequence, AF carries multiple information that can be decrypted by a proper analysis of the overall emission signal, allowing the characterization and monitoring of cell metabolism in situ, in real time and in the absence of perturbation from exogenous markers. In the animal kingdom, AF studies at the cellular level take advantage of the essential presence of NAD(P)H and flavins, primarily acting as coenzymes at multiple steps of common metabolic pathways for energy production, reductive biosynthesis and antioxidant defense. Additional EFs such as vitamin A, porphyrins, lipofuscins, proteins, and neuromediators can be detected in different kinds of cells and bulk tissues, and can be exploited as photophysical biomarkers of specific normal or altered morphofunctional properties, from the retinoid storage in the liver to aging processes, metabolic disorders or cell transformation processes. The AF phenomenon involves all living system, and literature reports numerous investigations and diagnostic applications of AF, taking advantage of continuously developing self-assembled or commercial instrumentation and measuring procedures, making almost impossible to provide their comprehensive description. Therefore a brief summary of the history of AF observations and of the development of measuring systems is provided, along with a description of the most common EFs and their metabolic significance. From our direct experience, examples of AF imaging and microspectrofluorometric procedures performed under a single excitation in the near-UV range for cell and tissue metabolism studies are then reported.

The use of DAPI fluorescence lifetime imaging for investigating chromatin condensation in human chromosomes

Chromatin undergoes dramatic condensation and decondensation as cells transition between the different phases of the cell cycle. The organization of chromatin in chromosomes is still one of the key challenges in structural biology. Fluorescence lifetime imaging (FLIM), a technique which utilizes a fluorophore's fluorescence lifetime to probe changes in its environment, was used to investigate variations in chromatin compaction in fixed human chromosomes. Fixed human metaphase and interphase chromosomes were labeled with the DNA minor groove binder, DAPI, followed by measurement and imaging of the fluorescence lifetime using multiphoton excitation. DAPI lifetime variations in metaphase chromosome spreads allowed mapping of the differentially compacted regions of chromatin along the length of the chromosomes. The heteromorphic regions of chromosomes 1, 9, 15, 16, and Y, which consist of highly condensed constitutive heterochromatin, showed statistically significant shorter DAPI lifetime values than the rest of the chromosomes. Differences in the DAPI lifetimes for the heteromorphic regions suggest differences in the structures of these regions. DAPI lifetime variations across interphase nuclei showed variation in chromatin compaction in interphase and the formation of chromosome territories. The successful probing of differences in chromatin compaction suggests that FLIM has enormous potential for application in structural and diagnostic studies.

Imaging and radiation effects of gold nanoparticles in tumour cells

Gold nanoparticle radiosensitization represents a novel technique in enhancement of ionising radiation dose and its effect on biological systems. Variation between theoretical predictions and experimental measurement is significant enough that the mechanism leading to an increase in cell killing and DNA damage is still not clear. We present the first experimental results that take into account both the measured biodistribution of gold nanoparticles at the cellular level and the range of the product electrons responsible for energy deposition. Combining synchrotron-generated monoenergetic X-rays, intracellular gold particle imaging and DNA damage assays, has enabled a DNA damage model to be generated that includes the production of intermediate electrons. We can therefore show for the first time good agreement between the prediction of biological outcomes from both the Local Effect Model and a DNA damage model with experimentally observed cell killing and DNA damage induction via the combination of X-rays and GNPs. However, the requirement of two distinct models as indicated by this mechanistic study, one for short-term DNA damage and another for cell survival, indicates that, at least for nanoparticle enhancement, it is not safe to equate the lethal lesions invoked in the local effect model with DNA damage events.

Exploration of the two-photon excitation spectrum of fluorescent dyes at wavelengths below the range of the Ti:Sapphire laser

We have studied the wavelength dependence of the two-photon excitation efficiency for a number of common UV excitable fluorescent dyes; the nuclear stains DAPI, Hoechst and SYTOX Green, chitin- and cellulose-staining dye Calcofluor White and Alexa Fluor 350, in the visible and near-infrared wavelength range (540-800 nm). For several of the dyes, we observe a substantial increase in the fluorescence emission intensity for shorter excitation wavelengths than the 680 nm which is the shortest wavelength usually available for two-photon microscopy. We also find that although the rate of photo-bleaching increases at shorter wavelengths, it is still possible to acquire many images with higher fluorescence intensity. This is particularly useful for applications where the aim is to image the structure, rather than monitoring changes in emission intensity over extended periods of time. We measure the excitation spectrum when the dyes are used to stain biological specimens to get a more accurate representation of the spectrum of the dye in a cell environment as compared to solution-based measurements.

Environmental Pollutant Ozone Causes Damage to Lung Surfactant Protein B (SP-B)

Lung surfactant protein B (SP-B) is an essential protein found in the surfactant fluid at the air-water interface of the lung. Exposure to the air pollutant ozone could potentially damage SP-B and lead to respiratory distress. We have studied two peptides, one consisting of the N-terminus of SP-B [SP-B(1-25)] and the other a construct of the N- and C-termini of SP-B [SP-B(1-25,63-78)], called SMB. Exposure to dilute levels of ozone (~2 ppm) of monolayers of each peptide at the air-water interface leads to a rapid reaction, which is evident from an increase in the surface tension. Fluorescence experiments revealed that this increase in surface tension is accompanied by a loss of fluorescence from the tryptophan residue at the interface. Neutron and X-ray reflectivity experiments show that, in contrast to suggestions in the literature, the peptides are not solubilized upon oxidation but rather remain at the interface with little change in their hydration. Analysis of the product material reveals that no cleavage of the peptides occurs, but a more hydrophobic product is slowly formed together with an increased level of oligomerization. We attributed this to partial unfolding of the peptides. Experiments conducted in the presence of phospholipids reveal that the presence of the lipids does not prevent oxidation of the peptides. Our results strongly suggest that exposure to low levels of ozone gas will damage SP-B, leading to a change in its structure. The implication is that the oxidized protein will be impaired in its ability to interact at the air-water interface with negatively charged phosphoglycerol lipids, thus compromising what is thought to be its main biological function.

Imaging intracellular and systemic in vivo gold nanoparticles to enhance radiotherapy

Nanoparticles offer alternative options in cancer therapy both as drug delivery carriers and as direct therapeutic agents for cancer cell inactivation. More recently, gold nanoparticles (AuNPs) have emerged as promising radiosensitizers achieving significantly elevated radiation dose enhancement factors when irradiated with both kilo-electron-volt and mega-electron-volt X-rays. Use of AuNPs in radiobiology is now being intensely driven by the desire to achieve precise energy deposition in tumours. As a consequence, there is a growing demand for efficient and simple techniques for detection, imaging and characterization of AuNPs in both biological and tumour samples. Spatially accurate imaging on the nanoscale poses a serious challenge requiring high- or super-resolution imaging techniques. In this mini review, we discuss the challenges in using AuNPs as radiosensitizers as well as various current and novel imaging techniques designed to validate the uptake, distribution and localization in mammalian cells. In our own work, we have used multiphoton excited plasmon resonance imaging to map the AuNP intracellular distribution. The benefits and limitations of this approach will also be discussed in some detail. In some cases, the same “excitation” mechanism as is used in an imaging modality can be harnessed to make it also a part of therapy modality (e.g. phototherapy)—such examples are discussed in passing as extensions to the imaging modality concerned.

A series of flexible design adaptations to the Nikon E-C1 and E-C2 confocal microscope systems for UV, multiphoton and FLIM imaging

Multiphoton microscopy is widely employed in the life sciences using extrinsic fluorescence of low- and high-molecular weight labels with excitation and emission spectra in the visible and near infrared regions. For imaging of intrinsic and extrinsic fluorophores with excitation spectra in the ultraviolet region, multiphoton excitation with one- or two-colour lasers avoids the need for ultraviolet-transmitting excitation optics and has advantages in terms of optical penetration in the sample and reduced phototoxicity. Excitation and detection of ultraviolet emission around 300 nm and below in a typical inverted confocal microscope is more difficult and requires the use of expensive quartz optics including the objective. In this technical note we describe the adaptation of a commercial confocal microscope (Nikon, Japan E-C1 or E-C2) for versatile use with Ti-sapphire and OPO laser sources and the addition of a second detection channel that enables detection of ultraviolet fluorescence and increases detection sensitivity in a typical fluorescence lifetime imaging microscopy experiment. Results from some experiments with this setup illustrate the resulting capabilities.

Time-resolved confocal microscopy using lanthanide centred near-IR emission

Time-resolved NIR imaging of lanthanide coated silica particles using Photon Arrival Time Imaging allows fast acquisition of high contrast images based on the probe luminescence lifetime.

Hydrothermal conversion of one-photon-fluorescent poly(4-vinylpyridine) into two-photon-fluorescent carbon nanodots

A novel two-photon-fluorescent N,O-heteroatom-rich carbon nanomaterial has been synthesized and characterized. The new carbon nanoparticles were produced by hydrothermal conversion from a one-photon-fluorescent poly(4-vinylpyridine) precursor (P4VP). The carbonized particles (cP4VP dots) with nonuniform particle diameter (ranging from sub-6 to 20 nm with some aggregates up to 200 nm) exhibit strong fluorescence properties in different solvents and have also been investigated for applications in cell culture media. The cP4VP dots retain their intrinsic fluorescence in a cellular environment and exhibit an average excited-state lifetime of 2.0 ± 0.9 ns in the cell. The cP4VP dots enter HeLa cells and do not cause significant damage to outer cell membranes. They provide one-photon or two-photon fluorescent synthetic scaffolds for imaging applications and/or drug delivery.

Rapid prototyping of electrochromatography chips for improved two-photon excited fluorescence detection

In the present study, we introduce two-photon excitation at 532 nm for label-free fluorescence detection in chip electrochromatography. Two-photon excitation at 532 nm offers a promising alternative to one-photon excitation at 266 nm, as it enables the use of economic chip materials instead of fused silica. In order to demonstrate these benefits, one-photon and two-photon induced fluorescence detection are compared in different chip layouts and materials with respect to the achievable sensitivity in the detection of polycyclic aromatic hydrocarbons (PAHs). Customized chromatography chips with cover or bottom slides of different material and thickness are produced by means of a rapid prototyping method based on liquid-phase lithography. The design of thin bottom chips (180 μm) enables the use of high-performance immersion objectives with low working distances, which allows one to exploit the full potential of two-photon excitation for a sensitive detection. The developed method is applied for label-free analysis of PAHs separated on a polymer monolith inside polymer glass sandwich chips made from fused silica or soda-lime glass. The obtained limits of detection range from 40 nM to 1.95 μM, with similar sensitivities in fused silica thin bottom chips for one-photon and two-photon excitation. In deep-UV non- or less-transparent devices two-photon excitation is mandatory for label-free detection of aromatics with high sensitivity.

Label-free fluorescence detection of aromatic compounds in chip electrophoresis applying two-photon excitation and time-correlated single-photon counting

In this study, we introduce time-resolved fluorescence detection with two-photon excitation at 532 nm for label-free analyte determination in microchip electrophoresis. In the developed method, information about analyte fluorescence lifetimes is collected by time-correlated single-photon counting, improving reliable peak assignment in electrophoretic separations. The determined limits of detection for serotonin, propranolol, and tryptophan were 51, 37, and 280 nM, respectively, using microfluidic chips made of fused silica. Applying two-photon excitation microchip separations and label-free detection could also be performed in borosilicate glass chips demonstrating the potential for label-free fluorescence detection in non-UV-transparent devices. Microchip electrophoresis with two-photon excited fluorescence detection was then applied for analyses of active compounds in plant extracts. Harmala alkaloids present in methanolic plant extracts from Peganum harmala could be separated within seconds and detected with on-the-fly determination of fluorescence lifetimes.

Laser-induced radiation microbeam technology and simultaneous real-time fluorescence imaging in live cells

The use of nano- and microbeam techniques to induce and identify subcellular localized energy deposition within a region of a living cell provides a means to investigate the effects of low radiation doses. Particularly within the nucleus where the propagation and processing of deoxyribonucleic acid (DNA) damage (and repair) in both targeted and nontargeted cells, the latter being able to study cell-cell (bystander) effects. We have pioneered a near infrared (NIR) femtosecond laser microbeam to mimic ionizing radiation through multiphoton absorption within a 3D femtoliter volume of a highly focused Gaussian laser beam. The novel optical microbeam mimics both complex ionizing and UV-radiation-type cell damage including double strand breaks (DSBs). Using the microbeam technology, we have been able to investigate the formation of DNA DSB and subsequent recruitment of repair proteins to the submicrometer size site of damage introduced in viable cells. The use of a phosphorylated H2AX (γ-H2AX a marker for DSBs, visualized by immunofluorescent staining) and real-time imaging of fluorescently labeling proteins, the dynamics of recruitment of repair proteins in viable mammalian cells can be observed. Here we show the recruitment of ATM, p53 binding protein 1 (53BP1), and RAD51, an integral protein of the homologous recombination process in the DNA repair pathway and Ku-80-GFP involved in the nonhomologous end joining (NHEJ) pathway as exemplar repair process to show differences in the repair kinetics of DNA DSBs. The laser NIR multiphoton microbeam technology shows persistent DSBs at later times post laser irradiation which are indicative of DSBs arising at replication presumably from UV photoproducts or clustered damage containing single strand breaks (SSBs) that are also observed. Effects of the cell cycle may also be investigated in real time. Postirradiation and fixed cells studies show that in G1 cells a fraction of multiphoton laser-induced DSBs is persistent for >6h in addition to those induced at replication demonstrating the broad range of timescales taken to repair DNA damage.

Fluorescence lifetime imaging of E-combretastatin uptake and distribution in live mammalian cells

To investigate within live mammalian cells the uptake and disposition of combretastatins, fluorescence lifetime imaging was used with two-photon excitation (2PE). Combretastatin A4 (CA4) and analogues are potential anticancer drugs due to their ability to inhibit angiogenesis. E(trans)-combretastatins are considerably less active than the Z(cis)-combretastatins proposed for clinical use. However the E-combretastatins exhibit stronger intrinsic fluorescence with quantum yields and lifetimes that depend markedly on solvent polarity and viscosity. It is proposed that 2PE in the red and near-infrared tissue window may allow in situ isomerization of E-combretastatins to the more active Z-isomer, offering spatial and temporal control of drug activation and constitute a novel form of photodynamic therapy. In the present work we have characterised 2PE of E-CA4 and have used fluorescence lifetime imaging with 2PE to study uptake and intracellular disposition of E-CA4 and an analogue. The results show that these molecules accumulate rapidly in cells and are located mainly in lipidic environments such as lipid droplets. Within the droplets the local concentrations may be up to two orders of magnitude higher than that of the drug in the surrounding medium.

Direct interaction of baculovirus capsid proteins VP39 and EXON0 with kinesin-1 in insect cells determined by fluorescence resonance energy transfer-fluorescence lifetime imaging microscopy

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) replicates in the nucleus of insect cells to produce nucleocapsids, which are transported from the nucleus to the plasma membrane for budding through GP64-enriched areas to form budded viruses. However, little is known about the anterograde trafficking of baculovirus nucleocapsids in insect cells. Preliminary confocal scanning laser microscopy studies showed that enhanced green fluorescent protein (EGFP)-tagged nucleocapsids and capsid proteins aligned and colocalized with the peripheral microtubules of virus-infected insect cells. A colchicine inhibition assay of virus-infected insect cells showed a significant reduction in budded virus production, providing further evidence for the involvement of microtubules and suggesting a possible role of kinesin in baculovirus anterograde trafficking. We investigated the interaction between AcMNPV nucleocapsids and kinesin-1 with fluorescence resonance energy transfer-fluorescence lifetime imaging microscopy (FRET-FLIM) and show for the first time that AcMNPV capsid proteins VP39 and EXON0, but not Orf1629, interact with the tetratricopeptide repeat (TPR) domain of kinesin. The excited-state fluorescence lifetime of EGFP fused to VP39 or EXON0 was quenched from 2.4 ± 1 ns to 2.1 ± 1 ns by monomeric fluorescent protein (mDsRed) fused to TPR (mDsRed-TPR). However, the excited-state fluorescence lifetime of an EGFP fusion of Orf1629 remained unquenched by mDsRed-TPR. These data indicate that kinesin-1 plays an important role in the anterograde trafficking of baculovirus in insect cells.

Fluorescent gallium and indium bis(thiosemicarbazonates) and their radiolabelled analogues: synthesis, structures and cellular confocal fluorescence imaging investigations

New fluorescent and biocompatible aromatic Ga(III)- and In(III)-bis(thiosemicarbazonato) complexes for dual mode optical and PET or SPECT molecular imaging have been synthesised via a synthetic method based on transmetallation reactions from Zn(II) precursors. Complexes have been fully characterised in the solid state by single crystal X-ray diffraction and in solution by spectroscopic methods (UV/Vis, fluorescence, (1)H and (13)C{(1)H} NMR). The bis(thiosemicarbazones) radiolabelled rapidly in high yields under mild conditions with (111)In (a gamma and Auger emitter for SPECT imaging and radiotherapy with t(1/2) = 2.8 d) and (68)Ga (a generator-available positron emitter for PET imaging with t(1/2) = 68 min). Cytotoxicity and biolocalisation studies using confocal fluorescence imaging and fluorescence lifetime imaging (FLIM) techniques have been used to study their in vitro activities and stabilities in HeLa and PC-3 cells to ascertain their suitability as synthetic scaffolds for future multimodality molecular imaging in cancer diagnosis and therapy. The observation that the indium complexes show certain nuclear uptake could be of relevance towards developing (111)In therapeutic agents based on Auger electron emission to induce DNA damage.

Excited state intramolecular proton transfer and photophysics of a new fluorenyl two-photon fluorescent probe

The steady-state photophysical, NMR, and two-photon absorption (2PA) properties of a new fluorene derivative (1) containing the 2-(2'-hydroxyphenyl)benzothiazole (HBT) terminal construct is investigated for use as a fluorescence probe in bioimaging. A comprehensive analysis of the linear spectral properties reveals inter- and intramolecular hydrogen bonding and excited state intramolecular proton transfer (ESIPT) processes in the HBT substituent. A specific electronic model with a double minimum potential energy surface is consistent with the observed spectral properties. The 2PA spectra are obtained using a standard two-photon induced fluorescence method with a femtosecond kHz laser system, affording a maximum 2PA cross section of approximately 600 GM, a sufficiently high value for two-photon fluorescence imaging. No dependence of two-photon absorption efficiency on solvent properties and hydrogen bonding in the HBT substituent is observed. The potential use of this fluorenyl probe in bioimaging is demonstrated via one- and two-photon fluorescence imaging of COS-7 cells.

Fluorescence lifetime imaging of interactions between Golgi tethering factors and small GTPases in plants

Peripheral tethering factors bind to small GTPases in order to obtain their correct location within the Golgi apparatus. Using fluorescence resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM) we visualized interactions between Arabidopsis homologues of tethering factors and small GTPases at the Golgi stacks in planta. Co-expression of the coiled-coil proteins AtGRIP and golgin candidate 5 (GC5) [TATA element modulatory factor (TMF)] and the putative post-Golgi tethering factor AtVPS52 fused to green fluorescent protein (GFP) with mRFP (monomeric red fluorescent protein) fusions to the small GTPases AtRab-H1(b), AtRab-H1(c) and AtARL1 resulted in reduced GFP lifetimes compared to the control proteins. Interestingly, we observed differences in GFP quenching between the different protein combinations as well as selective quenching of GFP-AtVPS52-labelled structures. The data presented here indicate that the FRET-FLIM technique should prove invaluable in assessing protein interactions in living plant cells at the organelle level.

Time-resolved and two-photon emission imaging microscopy of live cells with inert platinum complexes

This work explores time-resolved emission imaging microscopy (TREM) for noninvasive imaging and mapping of live cells on a hitherto uncharted microsecond time scale. Simple robust molecules for this purpose have long been sought. We have developed highly emissive, synthetically versatile, and photostable platinum(II) complexes that make TREM a practicable reality. [PtLCl], {HL = 1,3-di(2-pyridyl)benzene and derivatives}, are charge-neutral, small molecules that have low cytotoxicity and accumulate intracellularly within a remarkably short incubation time of 5 min, apparently under diffusion control. Their microsecond lifetimes and emission quantum yields of up to 70% are exceptionally high for transition metal complexes and permit the application of TREM to be demonstrated in a range of live cell types-normal human dermal fibroblast, neoplastic C8161 and CHO cells. [PtLCl] are thus likely to be suitable emission labels for any eukaryotic cell types. The high photostability of [PtLCl] under intense prolonged irradiation has allowed the development of tissue-friendly NIR two-photon excitation (TPE) in conjunction with transition metal complexes in live cells. A combination of confocal one-photon excitation, nonlinear TPE, and microsecond time-resolved imaging has revealed (i) preferential localization of the complexes to intracellular nucleic acid structures, in particular the nucleoli and (ii) the possibility of measuring intracellular emission lifetimes in the microsecond range. The combination of TREM, TPE, and Pt(II) complexes will be a powerful tool for investigating intracellular processes in vivo, because the long lifetimes allow discrimination from autofluorescence and open up the use of commonplace technology.