Multifrequency phase and modulation fluorometry

Two-photon autofluorescence lifetime assay of rabbit photoreceptors and retinal pigment epithelium during light-dark visual cycles in rabbit retina

Two-photon excited fluorescence (TPEF) is a powerful technique that enables the examination of intrinsic retinal fluorophores involved in cellular metabolism and the visual cycle. Although previous intensity-based TPEF studies in non-human primates have successfully imaged several classes of retinal cells and elucidated aspects of both rod and cone photoreceptor function, fluorescence lifetime imaging (FLIM) of the retinal cells under light-dark visual cycle has yet to be fully exploited. Here we demonstrate a FLIM assay of photoreceptors and retinal pigment epithelium (RPE) that reveals key insights into retinal physiology and adaptation. We found that photoreceptor fluorescence lifetimes increase and decrease in sync with light and dark exposure, respectively. This is likely due to changes in all-trans-retinol and all-trans-retinal levels in the outer segments, mediated by phototransduction and visual cycle activity. During light exposure, RPE fluorescence lifetime was observed to increase steadily over time, as a result of all-trans-retinol accumulation during the visual cycle and decreasing metabolism caused by the lack of normal perfusion of the sample. Our system can measure the fluorescence lifetime of intrinsic retinal fluorophores on a cellular scale, revealing differences in lifetime between retinal cell classes under different conditions of light and dark exposure.

A Recent Update on Rhodamine Dye Based Sensor Molecules: A Review

Herein we have discussed such important modified rhodamine compounds which have been used as chemosensors for the last 7-8 years. This review covered some chemosensors for the detection of metal ions like Al(III), Cu(II), Hg(II), Co(II), Fe(III), Au(III), Cr(III), and some anion like CN-. The selectivity, sensitivity, photophysical properties (i.e., UV-Vis spectral studies, fluorescence studies giving special emphasis to absorption wavelength in UV-Vis spectra and excitation and emission wavelength in fluorescence spectra), binding affinity, the limit of detection, and the application of those chemosensors are described clearly. Here we have also discussed some functionalized rhodamine-based chemosensors that emit in the near-infrared region (NIR) and can target lysosomes and detect lysosomal pH. Their versatile applicability in the medicinal ground is also delineated. We have focused on the photophysical properties of spirolactam rhodamine photoswitches and applications in single-molecule localization microscopy and volumetric 3D light photoactivable dye displays. The real-time detection of radical intermediates has also been exemplified.

Linear Combination Properties of the Phasor Space in Fluorescence Imaging

The phasor approach to fluorescence lifetime imaging, and more recently hyperspectral fluorescence imaging, has increased the use of these techniques, and improved the ease and intuitiveness of the data analysis. The fit-free nature of the phasor plots increases the speed of the analysis and reduces the dimensionality, optimization of data handling and storage. The reciprocity principle between the real and imaginary space-where the phasor and the pixel that the phasor originated from are linked and can be converted from one another-has helped the expansion of this method. The phasor coordinates calculated from a pixel, where multiple fluorescent species are present, depends on the phasor positions of those components. The relative positions are governed by the linear combination properties of the phasor space. According to this principle, the phasor position of a pixel with multiple components lies inside the polygon whose vertices are occupied by the phasor positions of these individual components and the distance between the image phasor to any of the vertices is inversely proportional to the fractional intensity contribution of that component to the total fluorescence from that image pixel. The higher the fractional intensity contribution of a vertex, the closer is the resultant phasor. The linear additivity in the phasor space can be exploited to obtain the fractional intensity contribution from multiple species and quantify their contribution. This review details the various mathematical models that can be used to obtain two/three/four components from phasor space with known phasor signatures and then how to obtain both the fractional intensities and phasor positions without any prior knowledge of either, assuming they are mono-exponential in nature. We note that other than for blind components, there are no restrictions on the type of the decay or their phasor positions for linear combinations to be valid-and they are applicable to complicated fluorescence lifetime decays from components with intensity decays described by multi-exponentials.

AKAP13 couples GPCR signaling to mTORC1 inhibition

The mammalian target of rapamycin complex 1 (mTORC1) senses multiple stimuli to regulate anabolic and catabolic processes. mTORC1 is typically hyperactivated in multiple human diseases such as cancer and type 2 diabetes. Extensive research has focused on signaling pathways that can activate mTORC1 such as growth factors and amino acids. However, less is known about signaling cues that can directly inhibit mTORC1 activity. Here, we identify A-kinase anchoring protein 13 (AKAP13) as an mTORC1 binding protein, and a crucial regulator of mTORC1 inhibition by G-protein coupled receptor (GPCR) signaling. GPCRs paired to Gα_s proteins increase cyclic adenosine 3’5’ monophosphate (cAMP) to activate protein kinase A (PKA). Mechanistically, AKAP13 acts as a scaffold for PKA and mTORC1, where PKA inhibits mTORC1 through the phosphorylation of Raptor on Ser 791. Importantly, AKAP13 mediates mTORC1-induced cell proliferation, cell size, and colony formation. AKAP13 expression correlates with mTORC1 activation and overall lung adenocarcinoma patient survival, as well as lung cancer tumor growth in vivo. Our study identifies AKAP13 as an important player in mTORC1 inhibition by GPCRs, and targeting this pathway may be beneficial for human diseases with hyperactivated mTORC1.

The mammalian target of rapamycin complex 1 (mTORC1) can sense multiple upstream stimuli to regulate cell growth and metabolism. Increased mTORC1 activation results in many human diseases such as cancer. Small molecules like rapamycin that target and inhibit mTORC1, are available in the clinic with limited success. Thus, decoding the mechanisms involved in mTORC1 regulation is crucial. Most of the research has focused on stimuli that activate mTORC1. Less is known about signaling pathways that can directly inhibit mTORC1 activity. G-protein coupled receptors (GPCRs) coupled to Gα_s proteins signal to and potently inhibit mTORC1. In this study, we have identified AKAP13 to play a crucial role in mTORC1 inhibition by GPCR signaling. Importantly, GPCRs are the largest family of drug targets with many approved FDA compounds. Targeting this signaling pathway may be beneficial for human diseases with hyperactivated mTORC1.

Distinguishing chemically induced NADPH- and NADH-related metabolic responses using phasor analysis of UV-excited autofluorescence

NADPH and NADH are well known for their role in antioxidant defense and energy metabolism, respectively, however distinguishing their cellular autofluorescence signals is a challenge due to their nearly identical optical properties. Recent studies applying spectral phasor analysis to autofluorescence emission during chemically induced metabolic responses showed that two-component spectral behavior, i.e., spectral change acting as a superposition of two spectra, depended on whether one or multiple metabolic pathways were affected. Here, we use this property of spectral behavior to show that metabolic responses primarily involving NADPH or NADH can be distinguished. We start by observing that the cyanide-induced response at micro- and millimolar concentrations does not follow mutual two-component spectral behavior, suggesting their response mechanisms differ. While respiratory inhibition at millimolar cyanide concentration is well known and associated with the NADH pool, we find the autofluorescence response at micromolar cyanide concentration exhibits two-component spectral behavior with NADPH-linked EGCG- and peroxide-induced responses, suggesting an association with the NADPH pool. What emerges is a spectral phasor map useful for distinguishing cellular autofluorescence responses related to oxidative stress versus cellular respiration.

A phasor approach was used to show that chemically induced cellular autofluorescence responses linked to NADPH and NADH pathways can be distinguished.

Multicomponent Analysis of Phasor Plot in a Single Pixel to Calculate Changes of Metabolic Trajectory in Biological Systems

Phasor FLIM in cells undergoing oxidative stress and in mice liver sections have shown the presence of a third autofluorescent component indicative of lipid droplets along with free and enzyme-bound NADH with similar emissions. This third component affects the position and shape of the phasor distribution, pushing it away from the metabolic trajectory. Phasor rule of addition is still valid and was exploited here to create a multicomponent analysis where the phasor distribution can be reassigned to the metabolic trajectory and changes in metabolism can be detected independently of the intensity of this third component. Calculation of multiple components from FLIM imaging data of biological systems is a difficult process, especially if different fluorescent species are present at the same pixel. This paper describes the methodology that can be used to separate these multiple components when they are present in the phasor signature acquired in a single pixel of an image.

Assessment on interactive prospectives of nanoplastics with plasma proteins and the toxicological impacts of virgin, coronated and environmentally released-nanoplastics

Recently, the concerns about micro- and nano-plastics (NPs) toxicity have been increasing constantly, however the investigations are quiet meager. The present study provides evidences on the toxicological prospectives of virgin-, coronated- and isolated-NPs on human blood cells and Allium cepa root tip, respectively. Several plasma proteins displayed strong affinity towards NPs and produced multi-layered corona of 13 nm to 600 nm size. The coronated-NPs often attracted each other via non-specific protein-protein attraction which subsequently induced protein-induced coalescence in NPs. In the protein point of view, the interaction caused conformational changes and denaturation of protein thereby turned it as bio-incompatible. The coronated-NPs with increased protein confirmation changes caused higher genotoxic and cytotoxic effect in human blood cells than the virgin-NPs. On the other hand, virgin-NPs and the NPs isolated from facial scrubs hindered the root growth and caused chromosome aberration (ring formation, C-mitotic and chromosomal breaks, etc.) in root of Allium cepa. At the outset, the present study highlights the urgent need of scrutinization and regulation of NPs use in medical applications and pre-requisition of additional studies for assessing the bio-accumulation and bio-magnification of NPs.

Widefield multifrequency fluorescence lifetime imaging using a two-tap complementary metal-oxide semiconductor camera with lateral electric field charge modulators

Widefield frequency-domain fluorescence lifetime imaging microscopy (FD-FLIM) measures the fluorescence lifetime of entire images in a fast and efficient manner. We report a widefield FD-FLIM system based on a complementary metal-oxide semiconductor camera equipped with two-tap true correlated double sampling lock-in pixels and lateral electric field charge modulators. Owing to the fast intrinsic response and modulation of the camera, our system allows parallel multifrequency FLIM in one measurement via fast Fourier transform. We demonstrate that at a fundamental frequency of 20 MHz, 31-harmonics can be measured with 64 phase images per laser repetition period. As a proof of principle, we analyzed cells transfected with Cerulean and with a construct of Cerulean-Venus that shows Förster Resonance Energy Transfer at different modulation frequencies. We also tracked the temperature change of living cells via the fluorescence lifetime of Rhodamine B at different frequencies. These results indicate that our widefield multifrequency FD-FLIM system is a valuable tool in the biomedical field.

Differences between FLIM phasor analyses for data collected with the Becker and Hickl SPC830 card and with the FLIMbox card

The phasor approach to FLIM (Fluorescence Lifetime Imaging Microscopy) is becoming popular due to the powerful fit free analysis and the visualization of the decay at each point in images of cells and tissues. However, although several implementation of the method are offered by manufactures of FLIM accessories for microscopes, the details of the conversion of the decay to phasors at each point in an image requires some consideration. Here, we show that if the decay is not properly acquired, the apparently simple phasor transformation can provide incorrect phasor plots and the results may be misinterpreted. In particular, we show the disagreement in experimental data acquired on the same samples using the two cards (FLIMbox, frequency domain and Becker & Hickl BH 830, time domain) and the effect produced by using the BH 830 card with different settings. This difference in data acquisition translates to the assignment of phasor components calculated using different acquisition parameters. This effect is already present in the original data that are not acquired with the proper parameters for the phasor conversion. We also show that the difference in the resolution of components already exists in the data acquired in the time domain when used with settings that do not allow acquisition of the fluorescence decay on a sufficient large time scale. RESEARCH HIGHLIGHTS: This paper is intended to made researchers aware of some simple requirements for the conversion of time-domain data (typically TCSPC) to phasors. The use of phasors for FLIM analysis has seen a surge of popularity. Since the phasor approach is a fit free method and has a powerful visualization of the data, it appears very simple to use. This paper shows that when the original data in the time domain is not acquired with the proper time range to cover the lifetimes in a sample, the conversion to phasors can produce very erroneous results. These results are appearing more frequently in the literature since many of the manufacturers of FLIM accessories for microscopes are now offering the phasor analysis in their software. Here, we show that the phasor transformation per se cannot correct for the problems with data acquisition and that one is misled to think that the "phasor approach" is a universal fix for the lack of the proper time range for data acquisition.

A metabolic interpretation for the response of cellular autofluorescence to chemical perturbations assessed using spectral phasor analysis

Analytical approaches for sensing cellular NADH conformation from autofluorescence signals have significance because NADH is a metabolic indicator and endogenous biomarker. Recently, spectral detection of multiple cellular NADH forms during chemically-induced metabolic response was reported, however because NADH is solvatochromic and the spectral change is small, the possibility of a non-metabolic interpretation needs to be considered. Here we investigate the response of UV-excited autofluorescence to a range of well-known chemicals affecting fermentation, respiration, and oxidative-stress pathways in Saccharomyces cerevisiae. The two-component nature of the spectral response is assessed using phasor analysis. By considering a series of physically similar and dissimilar chemicals acting on multiple pathways, we show how the two-component nature of a spectral response is of metabolic origin, indicative of whether a single or several pathways have been affected.

The two-component nature of the autofluorescence response is indicative of whether a single or several pathways are affected.

Regulation of K-Ras4B Membrane Binding by Calmodulin

K-Ras4B is a membrane-bound small GTPase with a prominent role in cancer development. It contains a polybasic farnesylated C-terminus that is required for the correct localization and clustering of K-Ras4B in distinct membrane domains. PDEδ and the Ca(2+)-binding protein calmodulin (CaM) are known to function as potential binding partners for farnesylated Ras proteins. However, they differ in the number of interaction sites with K-Ras4B, leading to different modes of interaction, and thus affect the subcellular distribution of K-Ras4B in different ways. Although it is clear that Ca(2+)-bound CaM can play a role in the dynamic spatial cycle of K-Ras4B in the cell, the exact molecular mechanism is only partially understood. In this biophysical study, we investigated the effect of Ca(2+)/CaM on the interaction of GDP- and GTP-loaded K-Ras4B with heterogeneous model biomembranes by using a combination of different spectroscopic and imaging techniques. The results show that Ca(2+)/CaM is able to extract K-Ras4B from negatively charged membranes in a nucleotide-independent manner. Moreover, the data demonstrate that the complex of Ca(2+)/CaM and K-Ras4B is stable in the presence of anionic membranes and shows no membrane binding. Finally, the influence of Ca(2+)/CaM on the interaction of K-Ras4B with membranes is compared with that of PDEδ, which was investigated in a previous study. Although both CaM and PDEδ exhibit a hydrophobic binding pocket for farnesyl, they have different effects on membrane binding of K-Ras4B and hence should be capable of regulating K-Ras4B plasma membrane localization in the cell.

Investigation of signal-to-noise ratio in frequency-domain multiphoton fluorescence lifetime imaging microscopy

Multiphoton microscopy (MPM) combined with fluorescence lifetime imaging microscopy (FLIM) has enabled three-dimensional quantitative molecular microscopy in vivo. The signal-to-noise ratio (SNR), and thus the imaging rate of MPM-FLIM, which is fundamentally limited by the shot noise and fluorescence saturation, has not been quantitatively studied yet. In this paper, we investigate the SNR performance of the frequency-domain (FD) MPM-FLIM with two figures of merit: the photon economy in the limit of shot noise, and the normalized SNR in the limit of saturation. The theoretical results and Monte Carlo simulations find that two-photon FD-FLIM requires 50% fewer photons to achieve the same SNR as conventional one-photon FLIM. We also analytically show that the MPM-FD-FLIM can exploit the DC and higher harmonic components generated by nonlinear optical mixing of the excitation light to improve SNR, reducing the required number of photons by an additional 50%. Finally, the effect of fluorophore saturation on the experimental SNR performance is discussed.

Encoding and decoding spatio-temporal information for super-resolution microscopy

The challenge of increasing the spatial resolution of an optical microscope beyond the diffraction limit can be reduced to a spectroscopy task by proper manipulation of the molecular states. The nanoscale spatial distribution of the molecules inside the detection volume of a scanning microscope can be encoded within the fluorescence dynamics and decoded by resolving the signal into its dynamics components. Here we present a robust and general method to decode this information using phasor analysis. As an example of the application of this method, we optically generate spatially controlled gradients in the fluorescence lifetime by stimulated emission. Spatial resolution can be increased indefinitely by increasing the number of resolved dynamics components up to a maximum determined by the amount of noise. We demonstrate that the proposed method provides nanoscale imaging of subcellular structures, opening new routes in super-resolution microscopy based on the encoding/decoding of spatial information through manipulation of molecular dynamics.

Increasing the resolution of fluorescence microscopy is a fundamental need for modern cell biology. Lanzanò et al. demonstrate that arbitrary spatial resolution is, in principle, possible by encoding the fluorophore's spatial distribution information in the temporal dynamics of the fluorophore's transition.

Creating diversified response profiles from a single quenchometric sensor element by using phase-resolved luminescence

We report a new strategy for generating a continuum of response profiles from a single luminescence-based sensor element by using phase-resolved detection. This strategy yields reliable responses that depend in a predictable manner on changes in the luminescent reporter lifetime in the presence of the target analyte, the excitation modulation frequency, and the detector (lock-in amplifier) phase angle. In the traditional steady-state mode, the sensor that we evaluate exhibits a linear, positive going response to changes in the target analyte concentration. Under phase-resolved conditions the analyte-dependent response profiles: (i) can become highly non-linear; (ii) yield negative going responses; (iii) can be biphasic; and (iv) can exhibit super sensitivity (e.g., sensitivities up to 300 fold greater in comparison to steady-state conditions).

A comparative study of Ir(iii) complexes with pyrazino[2,3-f][1,10]phenanthroline and pyrazino[2,3-f][4,7]phenanthroline ligands in light-emitting electrochemical cells (LECs)

Two new cationic Ir(iii) complexes are compared with respect to their performance in light electrochemical cells (LECs).

Polar plot representation of time-resolved fluorescence

Measuring changes in a molecule's fluorescence emission is a common technique to study complex biological systems such as cells and tissues. Although the steady-state fluorescence intensity is frequently used, measuring the average amount of time that a molecule spends in the excited state (the fluorescence lifetime) reveals more detailed information about its local environment. The lifetime is measured in the time domain by detecting directly the decay of fluorescence following excitation by short pulse of light. The lifetime can also be measured in the frequency domain by recording the phase and amplitude of oscillation in the emitted fluorescence of the sample in response to repetitively modulated excitation light. In either the time or frequency domain, the analysis of data to extract lifetimes can be computationally intensive. For example, a variety of iterative fitting algorithms already exist to determine lifetimes from samples that contain multiple fluorescing species. However, recently a method of analysis referred to as the polar plot (or phasor plot) is a graphical tool that projects the time-dependent features of the sample's fluorescence in either the time or frequency domain into the Cartesian plane to characterize the sample's lifetime. The coordinate transformations of the polar plot require only the raw data, and hence, there are no uncertainties from extensive corrections or time-consuming fitting in this analysis. In this chapter, the history and mathematical background of the polar plot will be presented along with examples that highlight how it can be used in both cuvette-based and imaging applications.

Frequency domain fluorometry: theory and application

Frequency domain fluorometry is a widely utilized tool in the physical, chemical, and biological sciences. This chapter focuses on the theory of the method and the practical aspects required to carry out intensity decay, i.e., lifetime measurements on a modern frequency domain fluorometer. Several chemical/biological systems are utilized to illustrate data acquisition protocols. Data analysis procedures and methodologies are also discussed.

Spectral properties of the prototropic forms of fluorescein in aqueous solution

The commonly used fluorescent probe, fluorescein, can exist in seven prototropic forms. We have used global analysis procedures to reanalyze the absorption data of Diehl and Horchak-Morris (Talanta 34, 739-741, 1987) in terms of five alternative ionization models. We identify the forms of fluorescein present in aqueous solution and the pK a of each ionisation transition. The pKa values of the neutral xanthene, carboxylic acid, and cationic xanthene groups are 6.3, 3.1-3.4, and 3.1-3.4, respectively, and the pKa value of lactonization is 2.4. As a consequence, the neutral form of fluorescein is a mixture of the lactone (70%), zwitterionic (15%), and quinoid (15%) forms. A knowledge of the forms present in solution permits the characterization of their spectral properties. It is shown that the quinoid and monoanion forms have similar absorption spectra, while the zwitterion spectrum is similar to that of the cation but blue-shifted by 3 nm. The emission spectra of the monoanion and quinoid forms are also identified and shown to be similar but not identical. A model for the excited-state reactions of fluorescein is presented.

Rotational and translational dynamics of ras proteins upon binding to model membrane systems

Plasma-membrane-associated Ras proteins typically control signal transduction processes. As nanoclustering and membrane viscosity sensing provide plausible signaling mechanisms, determination of the rotational and translational dynamics of membrane-bound Ras isoforms can help to link their dynamic mobility to their function. Herein, by using time-resolved fluorescence anisotropy and correlation spectroscopic measurements, we obtain the rotational-correlation time and the translational diffusion coefficient of lipidated boron-dipyrromethene-labeled Ras, both in bulk Ras and upon membrane binding. The results show that the second lipidation motif of N-Ras triggers dimer formation in bulk solution, whereas K-Ras4B is monomeric. Upon membrane binding, an essentially free rotation of the G-domain is observed, along with a high lateral mobility; the latter is essentially limited by the viscosity of the membrane and by lipid-mediated electrostatic interactions. This high diffusional mobility warrants rapid recognition-binding sequences in the membrane-bound state, thereby facilitating efficient interactions between the Ras proteins and scaffolding or effector proteins. The lipid-like rapid lateral diffusion observed here complies with in vivo data.

Interaction of carboxylated single-walled carbon nanotubes with bovine serum albumin

Carboxylated single-walled carbon nanotubes (c-SWNTs) were synthesized prosperously in order to improve dispersion of raw carbon nanotubes. Then, bovine serum albumin (BSA) was used as the template protein to study the biocompatibility of c-SWNTs by UV-Vis, fluorescence and circular dichroism (CD) spectroscopic methods at the molecular level. Results from fluorescence spectrum showed obvious decreases in fluorescence intensity of BSA induced by c-SWNTs, indicating the occurrence of interaction between BSA and c-SWNTs. Static quenching effect of c-SWNTs was verified by linear Stern-Volmer plots and K(SV) values. Thermodynamic parameters at different temperatures demonstrated that the interaction between c-SWNTs and BSA was mainly favored by hydrophobic force. In addition, Na(+) interfered with the quenching effect of c-SWNTs, which revealed that electrostatic force played a role in binding roles of BSA to c-SWNTs simultaneously. The results of UV and synchronous fluorescence spectrum validated that hydrophobicity of amino acid residues expressly increased with the addition of c-SWNTs. The content of α-helix structure in BSA decreased by 14.06% with c-SWNTs viewed from CD spectrum. Effect of SWNTs on the conformation of BSA could be controlled by the surface chemistry of SWNTs.

The effect of translational motion on FLIM measurements-single particle phasor-FLIM

Fluorescence lifetime imaging microscopy or FLIM provides a versatile tool for spatially-mapping macromolecular interactions and environments through pixel-by-pixel resolution of the excited-state lifetime. In conventional frequency-domain FLIM the phase and modulation of the detected fluorescence are determined by the photophysics of the fluorophore only. However, translational motion on the timescale of FLIM acquisition can significantly perturb apparent phase and modulation values owing to intensity fluctuations and phase decoherence. Using the phasor plot we outline a simple analytic theory, numerical simulations and measurements on fluorescent beads (ex 470 nm, em 520 nm). Fluctuations due to particle motions result in an increase in the number and spread of phasors, an effect we refer to as phasor broadening. The approach paves the way for the measurement of lifetimes and translational motion from one experiment.

Fluorescence anisotropy measurements in solution: Methods and reference materials (IUPAC Technical Report)

After recalling the basic relations relevant to both steady-state and time-resolved fluorescence polarization, it is shown how the values of steady-state polarized intensities recorded experimentally usually need to be corrected for systematic effects and errors, caused by instrumentation and sample properties. A list of selected reference values of steady-state fluorescence anisotropy and polarization is given. Attention is also paid to analysis of time-resolved fluorescence anisotropy data obtained by pulse fluorometry or phase and modulation fluorometry techniques. Recommendations for checking the accuracy of measurements are provided together with a list of selected time-resolved fluorescence anisotropy data as reported in the literature.

A practical implementation of multifrequency widefield frequency-domain fluorescence lifetime imaging microscopy

Widefield frequency-domain fluorescence lifetime imaging microscopy (FD-FLIM) is a fast and accurate method to measure the fluorescence lifetime, especially in kinetic studies in biomedical researches. However, the small range of modulation frequencies available in commercial instruments makes this technique limited in its applications. Herein, we describe a practical implementation of multifrequency widefield FD-FLIM using a pulsed supercontinuum laser and a direct digital synthesizer. In this instrument we use a pulse to modulate the image intensifier rather than the more conventional sine-wave modulation. This allows parallel multifrequency FLIM measurement using the Fast Fourier Transform and the cross-correlation technique, which permits precise and simultaneous isolation of individual frequencies. In addition, the pulse modulation at the cathode of image intensifier restores the loss of optical resolution caused by the defocusing effect when the cathode is sinusoidally modulated. Furthermore, in our implementation of this technique, data can be graphically analyzed by the phasor method while data are acquired, which allows easy fit-free lifetime analysis of FLIM images. Here, our measurements of standard fluorescent samples and a Föster resonance energy transfer pair demonstrate that the widefield multifrequency FLIM system is a valuable and simple tool in fluorescence imaging studies.

Red-shifted fluorescent proteins monitor enzymatic activity in live HT-1080 cells with fluorescence lifetime imaging microscopy (FLIM)

Membrane type 1 matrix metalloproteinase (MT1-MMP) is a membrane-tethered collagenase primarily involved in the mechanical destruction of extracellular matrix proteins. MT1-MMP has also been shown to be upregulated in several types of cancers. Many coordinated functions of MT1-MMP during migration and invasion remain to be determined. In this paper, live cells from the invasive cell line HT-1080 were imaged using an intracellular Förster resonance energy transfer-based biosensor specific for MT1-MMP; a substrate specific for MT1-MMP was hybridized with the mOrange2 and mCherry fluorescent proteins to form the Förster resonance energy transfer-based sensor. The configuration of the biosensor was determined with fluorescence lifetime-resolved imaging microscopy using both a polar plot-based analysis and a rapid data acquisition modality of fluorescence lifetime-resolved imaging microscopy known as phase suppression. Both configurations of the biosensor (with or without cleavage by MT1-MMP) were clearly resolvable in the same cell. Changes in the configuration of the MT1-MMP biosensor were observed primarily along the edge of the cell following the removal of the MMP inhibitor GM6001. The intensities highlighted by phase suppression correlated well with the fractional intensities derived from the polar plot.

Phasor-FLIM analysis of NADH distribution and localization in the nucleus of live progenitor myoblast cells

Analysis of the cellular distributions of coenzymes including NADH may aid in understanding a cells metabolic status. We altered serum concentration (0, 2, and 10%) to induce living myoblast cells to undergo the early stages of differentiation. Through microscopy and phasor-FLIM, we spatially mapped and identified variations in the distribution of free and bound NADH. Undifferentiated cells displayed abundant free NADH within the nucleus along with specific regions of more bound NADH. Complete serum starvation dramatically increased the fraction of bound NADH in the nucleus, indicating heightened requirement for transcriptional processes. In comparison, cells exposed to 2% serum exhibited intermediate free nuclear NADH fraction. Overall our results suggest an order of events in which a cell metabolic status alters significantly during the early stages of serum induced differentiation.

Differential dynamic and structural behavior of lipid-cholesterol domains in model membranes

Changes in the cholesterol (Chol) content of biological membranes are known to alter the physicochemical properties of the lipid lamella and consequently the function of membrane-associated enzymes. To characterize these changes, we used steady-state and time resolved fluorescence spectroscopy and two photon-excitation microscopy techniques. The membrane systems were chosen according to the techniques that were used: large unilamellar vesicles (LUVs) for cuvette and giant unilamellar vesicles (GUVs) for microscopy measurements; they were prepared from dipalmitoyl phosphatidylcholine (DPPC) and dioctadecyl phosphatidylcholine (DOPC) in mixtures that are well known to form lipid domains. Two fluorescent probes, which insert into different regions of the bilayer, were selected: 1,6-diphenyl-1,3,5-hexatriene (DPH) was located at the deep hydrophobic core of the acyl chain regions and 2-dimethylamino-6-lauroylnaphthalene (Laurdan) at the hydrophilic-hydrophobic membrane interface. Our spectroscopy results show that (i) the changes induced by cholesterol in the deep hydrophobic phospholipid acyl chain domain are different from the ones observed in the superficial region of the hydrophilic-hydrophobic interface, and these changes depend on the state of the lamella and (ii) the incorporation of cholesterol into the lamella induces an increase in the orientation dynamics in the deep region of the phospholipid acyl chains with a corresponding decrease in the orientation at the region close to the polar lipid headgroups. The microscopy data from DOPC/DPPC/Chol GUVs using Laurdan generalized polarization (Laurdan GP) suggest that a high cholesterol content in the bilayer weakens the stability of the water hydrogen bond network and hence the stability of the liquid-ordered phase (Lo).

Human erythrocytes and neuroblastoma cells are in vitro affected by sodium orthovanadate

Research on biological influence of vanadium has gained major importance because it exerts potent toxic, mutagenic, and genotoxic effects on a wide variety of biological systems. However, hematological toxicity is one of the less studied effects. The lack of information on this issue prompted us to study the structural effects induced on the human erythrocyte membrane by vanadium (V). Sodium orthovanadate was incubated with intact erythrocytes, isolated unsealed human erythrocyte membranes (IUM) and molecular models of the erythrocyte membrane. The latter consisted of bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE), phospholipid classes located in the outer and inner monolayers of the human erythrocyte membrane, respectively. This report presents evidence in order that orthovanadate interacted with red cell membranes as follows: a) in scanning electron microscopy (SEM) studies it was observed that morphological changes on human erythrocytes were induced; b) fluorescence spectroscopy experiments in isolated unsealed human erythrocyte membranes (IUM) showed that an increase in the molecular dynamics and/or water content at the shallow depth of the lipids glycerol backbone at concentrations as low as 50μM was produced; c) X-ray diffraction studies showed that orthovanadate 0.25-1mM range induced increasing structural perturbation to DMPE; d) somewhat similar effects were observed by differential scanning calorimetry (DSC) with the exception of the fact that DMPC pretransition was shown to be affected; and e) fluorescence spectroscopy experiments performed in DMPC large unilamellar vesicles (LUV) showed that at very low concentrations induced changes in DPH fluorescence anisotropy at 18°C. Additional experiments were performed in mice cholinergic neuroblastoma SN56 cells; a statistically significant decrease of cell viability was observed on orthovanadate in low or moderate concentrations.

Calculation of rate spectra from noisy time series data

As the resolution of experiments to measure folding kinetics continues to improve, it has become imperative to avoid bias that may come with fitting data to a predetermined mechanistic model. Toward this end, we present a rate spectrum approach to analyze timescales present in kinetic data. Computing rate spectra of noisy time series data via numerical discrete inverse Laplace transform is an ill-conditioned inverse problem, so a regularization procedure must be used to perform the calculation. Here, we show the results of different regularization procedures applied to noisy multiexponential and stretched exponential time series, as well as data from time-resolved folding kinetics experiments. In each case, the rate spectrum method recapitulates the relevant distribution of timescales present in the data, with different priors on the rate amplitudes naturally corresponding to common biases toward simple phenomenological models. These results suggest an attractive alternative to the "Occam's razor" philosophy of simply choosing models with the fewest number of relaxation rates.

Effects of an antimalarial quinazoline derivative on human erythrocytes and on cell membrane molecular models

Plasmodium, the parasite which causes malaria in humans multiplies in the liver and then infects circulating erythrocytes. Thus, the role of the erythrocyte cell membrane in antimalarial drug activity and resistance has key importance. The effects of the antiplasmodial N(6)-(4-methoxybenzyl)quinazoline-2,4,6-triamine (M4), and its inclusion complex (M4/HPβCD) with 2-hydroxypropyl-β-cyclodextrin (HPβCD) on human erythrocytes and on cell membrane molecular models are herein reported. This work evidences that M4/HPβCD interacts with red cells as follows: a) in scanning electron microscopy (SEM) studies on human erythrocytes induced shape changes at a 10μM concentration; b) in isolated unsealed human erythrocyte membranes (IUM) a concentration as low as 1μM induced sharp DPH fluorescence anisotropy decrease whereas increasing concentrations produced a monotonically decrease of DPH fluorescence lifetime at 37°C; c) X-ray diffraction studies showed that 200μM induced a complete structural perturbation of dimyristoylphosphatidylcholine (DMPC) bilayers whereas no significant effects were detected in dimyristoylphosphatidylethanolamine (DMPE) bilayers, classes of lipids present in the outer and inner monolayers of the human erythrocyte membrane, respectively; d) fluorescence spectroscopy data showed that increasing concentrations of the complex interacted with the deep hydrophobic core of DMPC large unilamellar vesicles (LUV) at 18°C. All these experiments are consistent with the insertion of M4/HPβCD in the outer monolayer of the human erythrocyte membrane; thus, it can be considered a promising and novel antimalarial agent.

Phase differential enhancement of FLIM to distinguish FRET components of a biosensor for monitoring molecular activity of Membrane Type 1 Matrix Metalloproteinase in live cells

Fluorescence lifetime-resolved imaging microscopy (FLIM) has been used to monitor the enzymatic activity of a proteolytic enzyme, Membrane Type 1 Matrix Metalloproteinase (MT1-MMP), with a recently developed FRET-based biosensor in vitro and in live HeLa and HT1080 cells. MT1-MMP is a collagenaise that is involved in the destruction of extra-cellular matrix (ECM) proteins, as well as in various cellular functions including migration. The increased expression of MT1-MMP has been positively correlated with the invasive potential of tumor cells. However, the precise spatiotemporal activation patterns of MT1-MMP in live cells are still not well-established. The activity of MT1-MMP was examined with our biosensor in live cells. Imaging of live cells was performed with full-field frequency-domain FLIM. Image analysis was carried out both with polar plots and phase differential enhancement. Phase differential enhancement, which is similar to phase suppression, is shown to facilitate the differentiation between different conformations of the MT1-MMP biosensor in live cells when the lifetime differences are small. FLIM carried out in differential enhancement or phase suppression modes, requires only two acquired phase images, and permits rapid imaging of the activity of MT1-MMP in live cells.

Role of PDZK1 protein in apical membrane expression of renal sodium-coupled phosphate transporters

The sodium-dependent phosphate (Na/P(i)) transporters NaPi-2a and NaPi-2c play a major role in the renal reabsorption of P(i). The functional need for several transporters accomplishing the same role is still not clear. However, the fact that these transporters show differential regulation under dietary and hormonal stimuli suggests different roles in P(i) reabsorption. The pathways controlling this differential regulation are still unknown, but one of the candidates involved is the NHERF family of scaffolding PDZ proteins. We propose that differences in the molecular interaction with PDZ proteins are related with the differential adaptation of Na/P(i) transporters. Pdzk1(-/-) mice adapted to chronic low P(i) diets showed an increased expression of NaPi-2a protein in the apical membrane of proximal tubules but impaired up-regulation of NaPi-2c. These results suggest an important role for PDZK1 in the stabilization of NaPi-2c in the apical membrane. We studied the specific protein-protein interactions of Na/P(i) transporters with NHERF-1 and PDZK1 by FRET. FRET measurements showed a much stronger interaction of NHERF-1 with NaPi-2a than with NaPi-2c. However, both Na/P(i) transporters showed similar FRET efficiencies with PDZK1. Interestingly, in cells adapted to low P(i) concentrations, there were increases in NaPi-2c/PDZK1 and NaPi-2a/NHERF-1 interactions. The differential affinity of the Na/P(i) transporters for NHERF-1 and PDZK1 proteins could partially explain their differential regulation and/or stability in the apical membrane. In this regard, direct interaction between NaPi-2c and PDZK1 seems to play an important role in the physiological regulation of NaPi-2c.

Fluorescence spectroscopy as a probe of the effect of phosphorylation at serine 40 of tyrosine hydroxylase on the conformation of its regulatory domain

Phosphorylation of Ser40 in the regulatory domain of tyrosine hydroxylase activates the enzyme by increasing the rate constant for dissociation of inhibitory catecholamines from the active site by 3 orders of magnitude. To probe the changes in the structure of the N-terminal domain upon phosphorylation, individual phenylalanine residues at positions 14, 34, and 74 were replaced with tryptophan in a form of the protein in which the endogenous tryptophans had all been mutated to phenylalanine (W(3)F TyrH). The steady-state fluorescence anisotropy of F74W W(3)F TyrH was unaffected by phosphorylation, but the anisotropies of both F14W and F34W W(3)F TyrH increased significantly upon phosphorylation. The fluorescence of the single tryptophan residue at position 74 was less readily quenched by acrylamide than those at the other two positions; fluorescence increased the rate constant for quenching of the residues at positions 14 and 34 but did not affect that for the residue at position 74. Frequency domain analyses were consistent with phosphorylation having no effect on the amplitude of the rotational motion of the indole ring at position 74, resulting in a small increase in the rotational motion of the residue at position 14 and resulting in a larger increase in the rotational motion of the residue at position 34. These results are consistent with the local environment at position 74 being unaffected by phosphorylation, that at position 34 becoming much more flexible upon phosphorylation, and that at position 14 becoming slightly more flexible upon phosphorylation. The results support a model in which phosphorylation at Ser40 at the N-terminus of the regulatory domain causes a conformational change to a more open conformation in which the N-terminus of the protein no longer inhibits dissociation of a bound catecholamine from the active site.

Acyl-chain mismatch driven superlattice arrangements in DPPC/DLPC/cholesterol bilayers

Fluorescence and infrared spectroscopy and cholesterol oxidase activity were employed to investigate the effect of phosphatidylcholine (PC) acyl chain length mismatch on the lateral organizations of lipids in liquid-ordered dipalmitoyl-PC/dilauroyl-PC/cholesterol (DPPC/DLPC/CHOL) bilayers. Plots of steady-state fluorescence emission anisotropy of diphenylhexatriene (DPH) labeled PC (DPH-PC) embedded in the DPPC/DLPC/CHOL bilayers revealed significant peaks at several DPPC mole fractions (Y(DPPC)) when the cholesterol mole fraction (X(CHOL)) was fixed to particular values. Analogously, the DPH-PC anisotropy peaked at several critical X(CHOL)'s when Y(DPPC) was fixed. Acyl chain C-H and C horizontal lineO vibrational peak frequencies of native PC as well as the activity of cholesterol oxidase also revealed dips and peaks at similar Y(DPPC)'s. Importantly, most of the observed peaks/dips coincide with the critical mole fractions predicted by the Superlattice (SL) model. A three-dimensional map of DPH-PC anisotropy versus composition in the range 0.32 <or= X(CHOL) <or= 0.50; 0.54 <or= Y(DPPC) <or= 0.72 revealed a prominent peak at (X(CHOL), Y(DPPC)) approximately (0.42, 0.64). This suggests a simultaneous presence of two different types of superlattices, one where cholesterol is the quest molecule in a PC host lattice and another where DPPC is the guest in the DLPC host lattice. Time-resolved measurements of DPH-PC fluorescence indicated the existence of an ordered, rotationally hindered environment of acyl chains at that "critical" composition consistent with the existence of SL arrangements. We propose that beside CHOL/PC superlattices, DPPC, and DLPC as well tend to adopt regular SL-like lateral distributions relative to each other, presumably because the less hydrophobic DLPC molecule is slightly displaced toward the aqueous phase, thus allowing more room and mobility for the head groups of both DPPC and DLPC as well as for the acyl chain tails of DPPC. The parallel presence of two kinds of superlattices, that is, CHOL/PC-SL and DPPC/DLPC-SL as demonstrated here, has intriguing implications regarding lipid homeostasis of eukaryote membranes.

Low-frequency wide-field fluorescence lifetime imaging using a high-power near-infrared light-emitting diode light source

Fluorescence lifetime imaging (FLi) could potentially improve exogenous near-infrared (NIR) fluorescence imaging, because it offers the capability of discriminating a signal of interest from background, provides real-time monitoring of a chemical environment, and permits the use of several different fluorescent dyes having the same emission wavelength. We present a high-power, LED-based, NIR light source for the clinical translation of wide-field (larger than 5 cm in diameter) FLi at frequencies up to 35 MHz. Lifetime imaging of indocyanine green (ICG), IRDye 800-CW, and 3,3(')-diethylthiatricarbocyanine iodide (DTTCI) was performed over a large field of view (10 cm by 7.5 cm) using the LED light source. For comparison, a laser diode light source was employed as a gold standard. Experiments were performed both on the bench by diluting the fluorescent dyes in various chemical environments in Eppendorf tubes, and in vivo by injecting the fluorescent dyes mixed in Matrigel subcutaneously into CD-1 mice. Last, measured fluorescence lifetimes obtained using the LED and the laser diode sources were compared with those obtained using a state-of-the-art time-domain imaging system and with those previously described in the literature. On average, lifetime values obtained using the LED and the laser diode light sources were consistent, exhibiting a mean difference of 3% from the expected values and a coefficient of variation of 12%. Taken together, our study offers an alternative to laser diodes for clinical translation of FLi and explores the use of relatively low frequency modulation for in vivo imaging.

Cholesterol modulates the interaction of beta-amyloid peptide with lipid bilayers

The interaction of an amphiphilic, 40-amino acid beta-amyloid (Abeta) peptide with liposomal membranes as a function of sterol mole fraction (X(sterol)) was studied based on the fluorescence anisotropy of a site-specific membrane sterol probe, dehydroergosterol (DHE), and fluorescence resonance energy transfer (FRET) from the native Tyr-10 residue of Abeta to DHE. Without Abeta, peaks or kinks in the DHE anisotropy versus X(sterol) plot were detected at X(sterol) approximately 0.25, 0.33, and 0.53. Monomeric Abeta preserved these peaks/kinks, but oligomeric Abeta suppressed them and created a new DHE anisotropy peak at X(sterol) approximately 0.38. The above critical X(sterol) values coincide favorably with the superlattice compositions predicted by the cholesterol superlattice model, suggesting that membrane cholesterol tends to adopt a regular lateral arrangement, or domain formation, in the lipid bilayers. For FRET, a peak was also detected at X(sterol) approximately 0.38 for both monomeric and oligomeric Abeta, implying increased penetration of Abeta into the lipid bilayer at this sterol mole fraction. We conclude that the interaction of Abeta with membranes is affected by the lateral organization of cholesterol, and hypothesize that the formation of an oligomeric Abeta/cholesterol domain complex may be linked to the toxicity of Abeta in neuronal membranes.