Dynamic recruitment of protein tyrosine phosphatase PTPD1 to EGF stimulation sites potentiates EGFR activation

Balanced activity of protein tyrosine kinases and phosphatases (PTPs) controls tyrosine phosphorylation levels and, consequently, is needed to prevent pathologies like cancer. Phosphatase activity is tightly regulated in space and time. Thus, in order to understand how phospho-tyrosine signalling is regulated, the intracellular dynamics of PTPs should be investigated. Here, we have studied the intracellular dynamics of PTPD1, a FERM (four-point-one, ezrin, radixin, moesin) domain-containing PTP that is over expressed in cancer cells and potentiates EGFR signalling. Whereas PTPD1 was excluded from E-cadherin rich cell-cell adhesions in epithelial cell monolayers, it diffused from the cytoplasm to those membranes in contact with the extracellular medium. Localisation of PTPD1 at the plasma membrane was mediated by its FERM domain and enabled the formation of EGFR/PTPD1-containing signalling complexes that pre-existed at the plasma membrane before EGF stimulation. PTPD1 and EGFR transiently co-localised at EGF stimulation sites until the formation of macropinosomes containing active species of EGFR. Interference of PTPD1 expression caused a decrease in EGFR phosphorylated species at the periphery of the cell. Presented data suggest that the transient formation of dynamic PTPD1/EGFR signalling complexes strengthens EGF signalling by promoting the spatial propagation of EGFR phosphorylated species.

Biochips for cell biology by combined dip-pen nanolithography and DNA-directed protein immobilization

A general methodology for patterning of multiple protein ligands with lateral dimensions below those of single cells is described. It employs dip pen nanolithography (DPN) patterning of DNA oligonucleotides which are then used as capture strands for DNA-directed immobilization (DDI) of oligonucleotide-tagged proteins. This study reports the development and optimization of PEG-based liquid ink, used as carrier for the immobilization of alkylamino-labeled DNA oligomers on chemically activated glass surfaces. The resulting DNA arrays have typical spot sizes of 4-5 μm with a pitch of 12 μm micrometer. It is demonstrated that the arrays can be further functionalized with covalent DNA-streptavidin (DNA-STV) conjugates bearing ligands recognized by cells. To this end, biotinylated epidermal growth factor (EGF) is coupled to the DNA-STV conjugates, the resulting constructs are hybridized with the DNA arrays and the resulting surfaces used for the culturing of MCF-7 (human breast adenocarcinoma) cells. Owing to the lateral diffusion of transmembrane proteins in the cell's plasma membrane, specific recruitment and concentration of EGF receptor can be induced specifically at the sites where the ligands are bound on the solid substrate. This is a clear demonstration that this method is suitable for precise functional manipulations of subcellular areas within living cells.

Regulation of signaling at regions of cell-cell contact by endoplasmic reticulum-bound protein-tyrosine phosphatase 1B

Protein-tyrosine phosphatase 1B (PTP1B) is a ubiquitously expressed PTP that is anchored to the endoplasmic reticulum (ER). PTP1B dephosphorylates activated receptor tyrosine kinases after endocytosis, as they transit past the ER. However, PTP1B also can access some plasma membrane (PM)-bound substrates at points of cell-cell contact. To explore how PTP1B interacts with such substrates, we utilized quantitative cellular imaging approaches and mathematical modeling of protein mobility. We find that the ER network comes in close proximity to the PM at apparently specialized regions of cell-cell contact, enabling PTP1B to engage substrate(s) at these sites. Studies using PTP1B mutants show that the ER anchor plays an important role in restricting its interactions with PM substrates mainly to regions of cell-cell contact. In addition, treatment with PTP1B inhibitor leads to increased tyrosine phosphorylation of EphA2, a PTP1B substrate, specifically at regions of cell-cell contact. Collectively, our results identify PM-proximal sub-regions of the ER as important sites of cellular signaling regulation by PTP1B.

Eph receptor function is modulated by heterooligomerization of A and B type Eph receptors

Beyond homotypic receptor interactions that are required for Eph signaling, ligand-independent association and crosstalk between members of the EphA and -B subclasses determine cell signaling outcomes.

Eph receptors interact with ephrin ligands on adjacent cells to facilitate tissue patterning during normal and oncogenic development, in which unscheduled expression and somatic mutations contribute to tumor progression. EphA and B subtypes preferentially bind A- and B-type ephrins, respectively, resulting in receptor complexes that propagate via homotypic Eph–Eph interactions. We now show that EphA and B receptors cocluster, such that specific ligation of one receptor promotes recruitment and cross-activation of the other. Remarkably, coexpression of a kinase-inactive mutant EphA3 with wild-type EphB2 can cause either cross-activation or cross-inhibition, depending on relative expression. Our findings indicate that cellular responses to ephrin contact are determined by the EphA/EphB receptor profile on a given cell rather than the individual Eph subclass. Importantly, they imply that in tumor cells coexpressing different Ephs, functional mutations in one subtype may cause phenotypes that are a result of altered signaling from heterotypic rather from homotypic Eph clusters.

PTP1B regulates Eph receptor function and trafficking

Changes in protein tyrosine phosphatase 1B expression affect duration and amplitude of EphA3 phosphorylation and cell surface concentration.

Eph receptors orchestrate cell positioning during normal and oncogenic development. Their function is spatially and temporally controlled by protein tyrosine phosphatases (PTPs), but the underlying mechanisms are unclear and the identity of most regulatory PTPs are unknown. We demonstrate here that PTP1B governs signaling and biological activity of EphA3. Changes in PTP1B expression significantly affect duration and amplitude of EphA3 phosphorylation and biological function, whereas confocal fluorescence lifetime imaging microscopy (FLIM) reveals direct interactions between PTP1B and EphA3 before ligand-stimulated receptor internalization and, subsequently, on endosomes. Moreover, overexpression of wild-type (w/t) PTP1B and the [D-A] substrate–trapping mutant decelerate ephrin-induced EphA3 trafficking in a dose-dependent manner, which reveals its role in controlling EphA3 cell surface concentration. Furthermore, we provide evidence that in areas of Eph/ephrin-mediated cell–cell contacts, the EphA3–PTP1B interaction can occur directly at the plasma membrane. Our studies for the first time provide molecular, mechanistic, and functional insights into the role of PTP1B controlling Eph/ephrin-facilitated cellular interactions.

Cytoplasmic relaxation of active Eph controls ephrin shedding by ADAM10

Novel imaging strategies reveal a conformational shift in a receptor tyrosine kinase domain that controls ligand shedding by an ADAM metalloprotease.

Release of cell surface-bound ligands by A-Disintegrin-And-Metalloprotease (ADAM) transmembrane metalloproteases is essential for signalling by cytokine, cell adhesion, and tyrosine kinase receptors. For Eph receptor ligands, it provides the switch between cell-cell adhesion and repulsion. Ligand shedding is tightly controlled by intrinsic tyrosine kinase activity, which for Eph receptors relies on the release of an inhibitory interaction of the cytoplasmic juxtamembrane segment with the kinase domain. However, a mechanism linking kinase and sheddase activities had remained elusive. We demonstrate that it is a membrane-proximal localisation of the latent kinase domain that prevents ephrin ligand shedding in trans. Fluorescence lifetime imaging microscopy and electron tomography reveal that activation extends the Eph receptor tyrosine kinase intracellular domain away from the cell membrane into a conformation that facilitates productive association with ADAM10. Accordingly, EphA3 mutants with constitutively-released kinase domains efficiently support shedding, even when their kinase is disabled. Our data suggest that this phosphorylation-activated conformational switch of EphA3 directly controls ADAM-mediated shedding.

The Eph transmembrane receptors are part of the receptor tyrosine kinase family and play important roles in communication between neighbouring cells. An Eph receptor binds to its ligand, membrane-tethered ephrin, on a neighbouring cell so as to form a stable complex and activate downstream signalling events. One such event is regulation of ADAM10, a transmembrane protease of the ADAM metalloprotease family, which provides a feedback mechanism to Eph signalling. ADAM10 is located on Eph-expressing cells and cleaves ephrin from its membrane tether on the opposite cell (through its so-called sheddase activity), thereby separating the cell-cell connection and allowing the signalling complex to internalise. In other biological contexts, activity of the ADAM metalloprotease family underlies signalling mechanisms such as oncogenic EGF-receptor transactivation, adhesion molecule shedding and cytokine/chemokine release. In general, ADAM function is enhanced when receptor tyrosine signalling is active and repressed when tyrosine kinase signalling is inhibited. However, the mechanism through which receptor tyrosine kinase signalling regulates ADAM10, have remained elusive. By combining fluorescence lifetime imaging microscopy (FLIM) and electron microscopic tomography of EphA3, we have demonstrated in live cells at molecular resolution that tyrosine phosphorylation of activated EphA3 triggers a measurable movement of the kinase domain away from the plasma membrane. Only this conformation of the EphA3 kinase domain away from the plasma membrane permits ADAM10 to come close enough to EphA3 so that it can reach its tightly EphA3-bound substrate, ephrin-A5. Our findings delineate a new regulatory concept in cell-cell communication, whereby control over proteolytic sheddase activity is provided by an activation-induced switch in the conformation of the cytoplasmic domain of a receptor tyrosine kinase, rather than by a cytosolic signalling pathway.

Imaging protein-protein interactions by fluorescence resonance energy transfer (FRET) microscopy

Detection of specific protein-protein interactions has long been restricted to bulk biochemical methods such as immunoprecipitation and immunoblotting. Even more sensitive methods using general immunofluorescence are limited, and it is difficult to infer protein-protein interactions from the results of these tests. Fluoresence Resonance Energy Transfer (FRET) is a photophysical process that can be exploited to obtain highly sensitive information about such interactions. It can sense the presence of acceptor fluorophores in the vicinity of a donor fluorophore within a separation distance that is the size of a single protein molecule. This unit details FRET microscopy based on release of quenched donor fluorescence after acceptor photobleaching, microinjection of reagents into the nucleus or cytosol, and labeling of antibodies for these procedures.

Imaging protein-protein interactions by fluorescence resonance energy transfer (FRET) microscopy

FRET microscopy enables the detection of different biochemical states of proteins in cells. The use of fluorescence in the detection of proteins, by chemical modification, by immunofluorescence, or by genetic encoding of a green fluorescent protein fusion protein, provides more information than just the location of the protein in the cell. The properties of the fluorophore can be exploited to extract information on protein-protein interactions. A straightforward, quantitative imaging approach is presented to measure FRET that is based on internal calibration by acceptor photobleaching.

Prolonged EGFR signaling by ERBB2-mediated sequestration at the plasma membrane

We have analyzed the spatial-temporal regulation of epidermal growth factor receptor (EGFR) phosphorylation by the orphan erbB2 receptor. It is shown that EGFR association with erbB2 is sufficient to prolong and enhance the net phosphorylation of EGFR, independent of the kinase activity of erbB2. This enhanced EGFR signaling was rather caused by erbB2-mediated retention of phosphorylated EGFR at the plasma membrane (PM), thereby preventing EGFR dephosphorylation and signal termination by endomembrane-bound protein tyrosine phosphatases (PTPs). EGF-induced EGFR internalization was indeed blocked in the presence of high levels of erbB2 or if cbl binding of EGFR was impaired. This erbB2-mediated blockage of the entry of activated EGFR into clathrin-coated vesicles could be alleviated by antibody-mediated disruption of the interaction between EGFR and erbB2. These results identify erbB2-mediated dominant trapping of phosphorylated EGFR at the PM as a mechanism that prolongs EGFR signaling, by sequestration of activated EGFR away from intracellular sites of high PTP activity.

Improved spatial discrimination of protein reaction states in cells by global analysis and deconvolution of fluorescence lifetime imaging microscopy data

The deconvolution of fluorescence lifetime imaging microscopy (FLIM) data that were processed with global analysis techniques is described. Global analysis of FLIM data enables the determination of relative numbers of molecules in different protein reaction states on a pixel-by-pixel basis in cells. The three-dimensional fluorescence distributions of each protein state can then be calculated and deconvolved. High-resolution maps of the relative concentrations of each state are then obtained from the deconvolved images. We applied these techniques to quantitatively image the phosphorylation state of ErbB1 receptors tagged with green fluorescent protein in MCF7 cells.

Imaging biochemistry inside cells

Proteins provide the building blocks for multicomponent molecular units, or pathways, from which higher cellular functions emerge. These units consist of either assemblies of physically interacting proteins or dispersed biochemical activities connected by rapidly diffusing second messengers, metabolic intermediates, ions or other proteins. It will probably remain within the realm of genetics to identify the ensemble of proteins that constitute these functional units and to establish the first-order connectivity. The dynamics of interactions within these protein machines can be assessed in living cells by the application of fluorescence spectroscopy on a microscopic level, using fluorescent proteins that are introduced within these functional units. Fluorescence is sensitive, specific and non-invasive, and the spectroscopic properties of a fluorescent probe can be analysed to obtain information on its molecular environment. The development and use of sensors based on the genetically encoded variants of green-fluorescent proteins has facilitated the observation of 'live' biochemistry on a microscopic level, with the advantage of preserving the cellular context of biochemical connectivity, compartmentalization and spatial organization. Protein activities and interactions can be imaged and localized within a single cell, allowing correlation with phenomena such as the cell cycle, migration and morphogenesis.

Imaging FRET between spectrally similar GFP molecules in single cells

Fluorescence resonance energy transfer (FRET) detection in fusion constructs consisting of green fluorescent protein (GFP) variants linked by a sequence that changes conformation upon modification by enzymes or binding of ligands has enabled detection of physiological processes such as Ca(2+) ion release, and protease and kinase activity. Current FRET microscopy techniques are limited to the use of spectrally distinct GFPs such as blue or cyan donors in combination with green or yellow acceptors. The blue or cyan GFPs have the disadvantages of less brightness and of autofluorescence. Here a FRET imaging method is presented that circumvents the need for spectral separation of the GFPs by determination of the fluorescence lifetime of the combined donor/acceptor emission by fluorescence lifetime imaging microscopy (FLIM). This technique gives a sensitive, reproducible, and intrinsically calibrated FRET measurement that can be used with the spectrally similar and bright yellow and green fluorescent proteins (EYFP/EGFP), a pair previously unusable for FRET applications. We demonstrate the benefits of this approach in the analysis of single-cell signaling by monitoring caspase activity in individual cells during apoptosis.

Observing proteins in their natural habitat: the living cell

Fluorescence microscopy has played a tremendous role in uncovering the morphological features of cells and the expression pattern of proteins by immunofluorescence. Since the discovery of green-fluorescent proteins (GFPs), this technique has undergone a revival in the life sciences as the spatial distribution of ectopically expressed fusion proteins inside living cells can now be followed more easily. By further exploiting the photophysical properties of the emitted fluorescence with microspectroscopic methods, spatial information on the biochemical parameters of intracellular processes and reactions can be obtained. This possibility will not only play an important role in the understanding of biochemical reactions in signal processing and fidelity but also help to uncover the molecular mechanisms of organelle and cell morphogenesis.

Quantitative imaging of lateral ErbB1 receptor signal propagation in the plasma membrane

Evidence for a new signaling mechanism consisting of ligand-independent lateral propagation of receptor activation in the plasma membrane is presented. We visualized the phosphorylation of green fluorescent protein (GFP)-tagged ErbB1 (ErbB1-GFP) receptors in cells focally stimulated with epidermal growth factor (EGF) covalently attached to beads. This was achieved by quantitative imaging of protein reaction states in cells by fluorescence resonance energy transfer (FRET) with global analysis of fluorescence lifetime imaging microscopy (FLIM) data. The rapid and extensive propagation of receptor phosphorylation over the entire cell after focal stimulation demonstrates a signaling wave at the plasma membrane resulting in full activation of all receptors.

Global analysis of fluorescence lifetime imaging microscopy data

Global analysis techniques are described for frequency domain fluorescence lifetime imaging microscopy (FLIM) data. These algorithms exploit the prior knowledge that only a limited number of fluorescent molecule species whose lifetimes do not vary spatially are present in the sample. Two approaches to implementing the lifetime invariance constraint are described. In the lifetime invariant fit method, each image in the lifetime image sequence is spatially averaged to obtain an improved signal-to-noise ratio. The lifetime estimations from these averaged data are used to recover the fractional contribution to the steady-state fluorescence on a pixel-by-pixel basis for each species. The second, superior, approach uses a global analysis technique that simultaneously fits the fractional contributions in all pixels and the spatially invariant lifetimes. In frequency domain FLIM the maximum number of lifetimes that can be fit with the global analysis method is twice the number of lifetimes that can be fit with conventional approaches. As a result, it is possible to discern two lifetimes with a single-frequency FLIM setup. The algorithms were tested on simulated data and then applied to separate the cellular distributions of coexpressed green fluorescent proteins in living cells.

Multiple frequency fluorescence lifetime imaging microscopy

The experimental configuration and the computational algorithms for performing multiple frequency fluorescence lifetime imaging microscopy (mfFLIM) are described. The mfFLIM experimental set-up enables the simultaneous homodyne detection of fluorescence emission modulated at a set of harmonic frequencies. This was achieved in practice by using monochromatic laser light as an excitation source modulated at a harmonic set of frequencies. A minimum of four frequencies were obtained by the use of two standing wave acousto-optic modulators placed in series. Homodyne detection at each of these frequencies was performed simultaneously by mixing with matching harmonics present in the gain characteristics of a microchannel plate (MCP) image intensifier. These harmonics arise as a natural consequence of applying a high frequency sinusoidal voltage to the photocathode of the device, which switches the flow of photoelectrons 'on' and 'off' as the sinus voltage swings from negative to positive. By changing the bias of the sinus it was possible to control the duration of the 'on' state of the intensifier relative to its 'off' state, enabling the amplitude of the higher harmonic content in the gain to be controlled. Relative modulation depths of 400% are theoretically possible from this form of square-pulse modulation. A phase-dependent integrated image is formed by the sum of the mixed frequencies on the phosphor of the MCP. Sampling this signal over a full period of the fundamental harmonic enables each harmonic to be resolved, provided that the Nyquist sampling criterion is satisfied for the highest harmonic component in the signal. At each frequency both the phase and modulation parameters can be estimated from a Fourier analysis of the data. These parameters enable the fractional populations and fluorescence lifetimes of individual components of a complex fluorescence decay to be resolved on a pixel-by-pixel basis using a non-linear fit to the dispersion relationships. The fitting algorithms were tested on a simulated data set and were successful in disentangling two populations having 1 ns and 4 ns fluorescence lifetimes. Spatial invariance of the lifetimes was exploited to improve the accuracy significantly. Multiple frequency fluorescence lifetime imaging microscopy was then successfully applied to resolve the fluorescence lifetimes and fluorescence intensity contributions in a rhodamine dye mixture in solution, and green fluorescent protein variants co-expressed in live cells.

Wide-field optically sectioning fluorescence microscopy with laser illumination

We describe an extremely simple method by which optically sectioned fluorescence images may be obtained with conventional microscopes using laser illumination. A one-dimensional grid pattern is introduced into the illumination system, together with a rotating ground glass diffuser. This causes an image of the grid pattern to be projected into the specimen. Images taken at three spatial positions of the grid are processed in a simple manner to provide optically sectioned images of fluorescent specimens.

Fluorescence lifetime imaging of receptor tyrosine kinase activity in cells

We report a highly specific fluorescence lifetime imaging microscopy (FLIM) method for monitoring epidermal growth factor receptor (EGFR) phosphorylation in cells based on fluorescence resonance energy transfer (FRET). EGFR phosphorylation was monitored using a green fluorescent protein (GFP)-tagged EGFR and Cy3-conjugated anti-phosphotyrosine antibodies. In this FRET-based imaging method, the information about phosphorylation is contained only in the (donor) GFP fluorescence lifetime and is independent of the antibody-derived (acceptor) fluorescence signal. A pixel-by-pixel reference lifetime of the donor GFP in the absence of FRET was acquired from the same cell after photobleaching of the acceptor. We show that this calibration, by acceptor photobleaching, works for the GFP-Cy3 donor-acceptor pair and allows the full quantitation of FRET efficiencies, and therefore the degree of exposed phosphotyrosines, at each pixel. The hallmark of EGFR stimulation is receptor dimerisation [1] [2] [3] [4] and concomitant activation of its intracellular tyrosine kinase domain [5] [6] [7]. Trans-autophosphorylation of the receptor [8] [9] on specific tyrosine residues couples the activated dimer to the intracellular signal transduction machinery as these phosphorylated residues serve as docking sites for adaptor and effector molecules containing Src homology 2 (SH2; reviewed in [10]) and phosphotyrosine-binding (PTB) [11] domains. The time-course and extent of EGFR phosphorylation are therefore important determinants of the underlying pathway and resulting cellular response. Our results strongly suggest that secondary proteins are recruited by activated receptors in endosomes, indicating that these are active compartments in signal transduction.

PKCalpha regulates beta1 integrin-dependent cell motility through association and control of integrin traffic

Protein kinase C (PKC) has been implicated in integrin-mediated spreading and migration. In mammary epithelial cells there is a partial co-localization between beta1 integrin and PKCalpha. This reflects complexes between these proteins as demonstrated by fluorescense resonance energy transfer (FRET) monitored by fluorescence lifetime imaging microscopy and also by coprecipitation. Constitutive complexes are observed for the intact PKCalpha and also form with the regulatory domain in an activation-dependent manner. Expression of PKCalpha causes upregulation of beta1 integrin on the cell surface, whereas stimulation of PKC induces internalization of beta1 integrin. The integrin initially traffics to an endosomal compartment in a Ca(2+)/PI 3-kinase/dynamin I-dependent manner and subsequently enters an endocytic recycling pathway. This induction of endocytosis by PKCalpha is a function of activity and is not observed for the regulatory domain. PKCalpha, but not PKCalpha regulatory domain expression stimulates migration on beta1 integrin substrates. This PKCalpha-enhanced migratory response is inhibited by blockade of endocytosis.

Imaging protein kinase Calpha activation in cells

Spatially resolved fluorescence resonance energy transfer (FRET) measured by fluorescence lifetime imaging microscopy (FLIM), provides a method for tracing the catalytic activity of fluorescently tagged proteins inside live cell cultures and enables determination of the functional state of proteins in fixed cells and tissues. Here, a dynamic marker of protein kinase Calpha (PKCalpha) activation is identified and exploited. Activation of PKCalpha is detected through the binding of fluorescently tagged phosphorylation site-specific antibodies; the consequent FRET is measured through the donor fluorophore on PKCalpha by FLIM. This approach enabled the imaging of PKCalpha activation in live and fixed cultured cells and was also applied to pathological samples.

Simultaneous detection of multiple green fluorescent proteins in live cells by fluorescence lifetime imaging microscopy

The green fluorescent protein (GFP) has proven to be an excellent fluorescent marker for protein expression and localisation in living cells [1] [2] [3] [4] [5]. Several mutant GFPs with distinct fluorescence excitation and emission spectra have been engineered for intended use in multi-labelling experiments [6] [7] [8] [9]. Discrimination of these co-expressed GFP variants by wavelength is hampered, however, by a high degree of spectral overlap, low quantum efficiencies and extinction coefficients [10], or rapid photobleaching [6]. Using fluorescence lifetime imaging microscopy (FLIM) [11] [12] [13] [14] [15] [16], four GFP variants were shown to have distinguishable fluorescence lifetimes. Among these was a new variant (YFP5) with spectral characteristics reminiscent of yellow fluorescent protein [8] and a comparatively long fluorescence lifetime. The fluorescence intensities of co-expressed spectrally similar GFP variants (either alone or as fusion proteins) were separated using lifetime images obtained with FLIM at a single excitation wavelength and using a single broad band emission filter. Fluorescence lifetime imaging opens up an additional spectroscopic dimension to wavelength through which novel GFP variants can be selected to extend the number of protein processes that can be imaged simultaneously in cells.

Fluorescence lifetime imaging microscopy: spatial resolution of biochemical processes in the cell

Fluorescence lifetime imaging microscopy (FLIM) is a technique in which the mean fluorescence lifetime of a chromophore is measured at each spatially resolvable element of a microscope image. The nanosecond excited-state lifetime is independent of probe concentration or light path length but dependent upon excited-state reactions such as fluorescence resonance energy transfer (FRET). These properties of fluorescence lifetimes allow exploration of the molecular environment of labelled macromolecules in the interior of cells. Imaging of fluorescence lifetimes enables biochemical reactions to be followed at each microscopically resolvable location within the cell.

Three dimensional image restoration in fluorescence lifetime imaging microscopy

A microscope set-up and numerical methods are described which enable the measurement and reconstruction of three-dimensional nanosecond fluorescence lifetime images in every voxel. The frequency domain fluorescence lifetime imaging microscope (FLIM) utilizes phase detection of high-frequency modulated light by homodyne mixing on a microchannel plate image intensifier. The output signal at the image intensifier's phosphor screen is integrated on a charge coupled device camera. A scanning stage is employed to obtain a series of phase-dependent intensity images at equally separated depths in a specimen. The Fourier transform of phase-dependent data gives three-dimensional (3D) images of the Fourier coefficients. These images are deblurred using an Iterative Constrained Tikhonov-Miller (ICTM) algorithm in conjunction with a measured point spread function. The 3D reconstruction of fluorescence lifetimes are calculated from the deblurred images of the Fourier coefficients. An improved spatial and temporal resolution of fluorescence lifetimes was obtained using this approach to the reconstruction of simulated 3D FLIM data. The technique was applied to restore 3D FLIM data of a live cell specimen expressing two green fluorescent protein fusion constructs having distinct fluorescence lifetimes which localized to separate cellular compartments.

Calcium-dependent oligomerization of synaptotagmins I and II. Synaptotagmins I and II are localized on the same synaptic vesicle and heterodimerize in the presence of calcium

Synaptotagmins constitute a large family of membrane proteins characterized by their distinct distributions and different biochemical features. Genetic evidence suggests that members of this protein family are likely to function as calcium sensors in calcium-regulated events in neurons, although the precise molecular mechanism remains ill defined. Here we demonstrate that different synaptotagmin isoforms (Syt I, II, and IV) are present in the same synaptic vesicle population from rat brain cortex. In addition, Syt I and II co-localize on the same small synaptic vesicle (SSV), and they heterodimerize in the presence of calcium with a concentration dependence resembling that of the starting phase of SSV exocytosis (EC50 = 6 +/- 4 microM). The association between Syt I and Syt II was demonstrated by immunoprecipitation of the native proteins and the recombinant cytoplasmic domains and by using fluorescence resonance energy transfer (FRET). Although a subpopulation of SSV containing Syt I and IV can be isolated, these two isoforms do not show a calcium-dependent interaction. These results suggest that the self-association of synaptotagmins with different calcium binding features may create a variety of calcium sensors characterized by distinct calcium sensitivities. This combinatorial hypothesis predicts that the probability of a single SSV exocytic event is determined, in addition to the gating properties of the presynaptic calcium channels, by the repertoire and relative abundance of distinct synaptotagmin isoforms present on the SSV surface.

FRET microscopy demonstrates molecular association of non-specific lipid transfer protein (nsL-TP) with fatty acid oxidation enzymes in peroxisomes

The fate of fluorescently labeled pre-nsL-TP (Cy3-pre-nsL-TP) microinjected into BALB/c 3T3 fibroblasts was investigated by confocal laser scanning microscopy. The protein exhibited a distinct punctate fluorescence pattern and colocalized to a high degree with the immunofluorescence pattern for the peroxisomal enzyme acyl-CoA oxidase. Proteolytic removal of the C-terminal leucine of the putative peroxisomal targeting sequence (AKL) resulted in a diffuse cytosolic fluorescence. These results indicate that microinjected Cy3-pre-nsL-TP is targeted to peroxisomes. The association of nsL-TP with peroxisomal enzymes was investigated in cells by measuring fluorescence resonance energy transfer (FRET) between the microinjected Cy3-pre-nsL-TP and Cy5-labeled antibodies against the peroxisomal enzymes acyl-CoA oxidase, 3-ketoacyl-CoA thiolase, bifunctional enzyme, PMP70 and catalase. The technique of photobleaching digital imaging microscopy (pbDIM), used to quantitate the FRET efficiency on a pixel-by-pixel basis, revealed a specific association of nsL-TP with acyl-CoA oxidase, 3-ketoacyl-CoA thiolase and bifunctional enzyme in the peroxisomes. These observations were corroborated by subjecting a peroxisomal matrix protein fraction to affinity chromatography on Sepharose-immobilized pre-nsL-TP. Acyl-CoA oxidase was retained. These studies provide strong evidence for a role of nsL-TP in the regulation of peroxisomal fatty acid beta-oxidation, e.g. by facilitating the presentation of substrates and/or stabilization of the enzymes.

Binding of a diphosphotyrosine-containing peptide that mimics activated platelet-derived growth factor receptor beta induces oligomerization of phosphatidylinositol 3-kinase

Phosphatidylinositol 3-kinase (PI3K) is a heterodimeric enzyme comprising a p110 catalytic subunit and a p85 regulatory subunit. We have recently shown that the isolated p85 subunit exists as a dimer; therefore, we examined whether the heterodimeric enzyme was capable of further self-association. Size-exclusion chromatography demonstrated that PI3K was a 1:1 complex of p85 and p110 under native conditions. However, binding of a diphosphotyrosine-containing peptide that mimics an activated platelet-derived growth factor receptor beta induced an increase in the apparent molecular mass of PI3K. This increase was due to dimerization of PI3K and was dependent on PI3K concentration but not diphosphopeptide concentration. Dimer formation was also observed directly using fluorescence resonance energy transfer. Diphosphopeptide-induced activation of PI3K (Carpenter, C. L., Auger, K. R., Chanudhuri, M., Yoakim, M., Schaffhausen, B., Shoelson, S., and Cantley, L. C. (1993) J. Biol. Chem. 268, 9478-9483; Rordorf-Nikolic, T., Van Horn, D. J., Chen, D., White, M. F., and Backer, J. M. (1995) J. Biol. Chem. 270, 3662-3666) was not a direct result of dimerization and occurred only when phosphatidylinositol, and not phosphatidylinositol-4,5-diphosphate, was the phosphorylation substrate. Binding of the tandem SH2 domains of the p85 regulatory subunit to activated receptor tyrosine kinases therefore induces dimerization of PI3K, which may be an early step in inositol lipid-mediated signal transduction.

Fluorescently labeled phosphatidylinositol transfer protein isoforms (alpha and beta), microinjected into fetal bovine heart endothelial cells, are targeted to distinct intracellular sites

Upon permeabilization of Swiss mouse 3T3 fibroblasts, an isoform of phosphatidylinositol transfer protein (PI-TP) was preferentially retained, a major part of which was associated with the perinuclear Golgi system (K. J. de Vries, A. Momchilova-Pankova, G. T. Snoek, and K. W. A. Wirtz, Exp. Cell Res. 215, 109-113, 1994). In the present study, the intracellular localization of this isoform (PI-TP beta) and the regular form (PI-TP alpha) was investigated in fetal bovine heart endothelial cells by microinjection of fluorescently labeled analogs followed by confocal laser scanning microscopy. The PI-TP alpha and PI-TP beta used were purified from bovine brain cytosol and covalently labeled with sulfoindocyanine dyes. By this novel method it was found that PI-TP beta was preferentially associated with perinuclear membrane structures whereas PI-TP alpha was predominantly present in the nucleus and in the cytoplasm. This intracellular localization was confirmed by indirect immunofluorescence indicating that the fluorescently labeled PI-TP alpha and PI-TP beta were targeted to the same sites as their endogeneous counterparts.

Imaging the intracellular trafficking and state of the AB5 quaternary structure of cholera toxin

The subcellular localization and corresponding quaternary state of fluorescent labelled cholera toxin were determined at different time points after exposure to living cells by a novel form of fluorescence confocal microscopy. The compartmentalization and locus of separation of the pentameric B subunits (CTB) from the A subunit (CTA) of the toxin were evaluated on a pixel-by-pixel (voxel-by-voxel) basis by measuring the fluorescence resonance energy transfer (FRET) between CTB labelled with the sulfoindocyanine dye Cy3 and an antibody against CTA labelled with Cy5. The FRET efficiency was determined by a new technique based on the release of quenching of the Cy3 donor after photodestruction of the Cy5 acceptor in a region of interest within the cell. The results demonstrate vesicular transport of the holotoxin from the plasma membrane to the Golgi compartment with subsequent separation of the CTA and CTB subunits. The CTA subunit is redirected to the plasma membrane by retrograde transport via the endoplasmic reticulum whereas the CTB subunit persists in the Golgi compartment.

Microspectroscopic imaging tracks the intracellular processing of a signal transduction protein: fluorescent-labeled protein kinase C beta I

We have devised a microspectroscopic strategy for assessing the intracellular (re)distribution and the integrity of the primary structure of proteins involved in signal transduction. The purified proteins are fluorescent-labeled in vitro and reintroduced into the living cell. The localization and molecular state of fluorescent-labeled protein kinase C beta I isozyme were assessed by a combination of quantitative confocal laser scanning microscopy, fluorescence lifetime imaging microscopy, and novel determinations of fluorescence resonance energy transfer based on photobleaching digital imaging microscopy. The intensity and fluorescence resonance energy transfer efficiency images demonstrate the rapid nuclear translocation and ensuing fragmentation of protein kinase C beta I in BALB/c3T3 fibroblasts upon phorbol ester stimulation, and suggest distinct, compartmentalized roles for the regulatory and catalytic fragments.

Transport of an external Lys-Asp-Glu-Leu (KDEL) protein from the plasma membrane to the endoplasmic reticulum: studies with cholera toxin in Vero cells

The A2 chain of cholera toxin (CTX) contains a COOH-terminal Lys-Asp-Glu-Leu (KDEL) sequence. We have, therefore, analyzed by immunofluorescence and by subcellular fractionation in Vero cells whether CTX can used to demonstrate a retrograde transport of KDEL proteins from the Golgi to the ER. Immunofluorescence studies reveal that after a pulse treatment with CTX, the CTX-A and B subunits (CTX-A and CTX-B) reach Golgi-like structures after 15-20 min (maximum after 30 min). Between 30 and 90 min, CTX-A (but not CTX-B) appear in the intermediate compartment and in the ER, whereas the CTX-B are translocated to the lysosomes. Subcellular fractionation studies confirm these results: after CTX uptake for 15 min, CTX-A is associated only with endosomal and Golgi compartments. After 30 min, a small amount of CTX-A appears in the ER in a trypsin-resistant form, and after 60 min, a significant amount appears. CTX-A seems to be transported mainly in its oxidized form (CTX-A1-S-S-CTX-A2) from the Golgi to the ER, where it becomes slowly reduced to form free CTX A1 and CTX-A2, as indicated by experiments in which cells were homogenized 30 and 90 min after the onset of CTX uptake in the presence of N-ethylmaleimide. Nocodazol applied after accumulation of CTX in Golgi inhibits the appearance of CTX-A in the ER and delays the increase of 3',5'cAMP, indicating the participation of microtubules in the retrograde Golgi-ER transport.

Quantitation of the interaction of protein kinase C with diacylglycerol and phosphoinositides by time-resolved detection of resonance energy transfer

Quantitative studies of the binding of protein kinase C (PKC) to lipid cofactors were performed by monitoring resonance energy transfer with time-resolved fluorescence techniques. For that purpose, diacylglycerol (DG), phosphatidylinositol 4,5-biphosphate (PIP2), phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol (PI), phosphatidylcholine (PC), and phosphatidylserine (PS) were labeled with a pyrenyl decanoyl moiety at the sn-2 position of the lipid glycerol. These labeled lipids proved excellent energy acceptors of light-excited tryptophan residues in PKC. The quenching efficiency of the tryptophan fluorescence was determined as function of lipid probe concentration in mixed micelles consisting of poly(oxyethylene)-9-lauryl ether, PS, and various mole fractions of probe lipid. The experimental conditions and method of data analysis allowed the estimation of binding constants of single or multiple pyrene lipids to PKC. The affinity of PKC for inositide lipids increases in the order PI < PIP < PIP2. The affinity of PKC for PIP and PIP2 is higher than that for DG. Determination of PKC activity in the presence of labeled lipids and PS showed that only PIP2 and DG activate PKC. Double-labeling experiments suggest that PIP2 and DG are not able to bind simultaneously to PKC, indicating a reciprocal binding relationship of both cofactors. The results support the notion that, besides DG, PIP2 can be a primary activator of PKC.

The interaction of pyrene labeled diacylglycerol with protein kinase C in mixed micelles

The binding of protein kinase C (PKC) to pyrene-labeled diacylglycerol (pDG) has been studied in a mixed micellar system by monitoring resonance energy transfer from excited tryptophans to pyrene with time-correlated single photon counting. The average lifetime of the excited state of the tryptophans in PKC showed a clear dependence on the mole percentage pDG in micelles in contrast with pyrene-labeled phosphatidylcholine (pPC). The binding data has been analyzed to a simple model which encompasses the size of the micelles and the binding constant of the pDG-PKC complex. From our data, though, these quantities cannot be determined independently. If we have no size information on the micelles we can determine a lower boundary of this quantity compatible with the data. When the micellar size is known, a binding constant for the DG-PKC complex can be extracted. The presented analytical approach can be applied to other systems in which lipid-protein interactions must be quantified.

Conformational dynamics and intersubunit energy transfer in wild-type and mutant lipoamide dehydrogenase from Azotobacter vinelandii. A multidimensional time-resolved polarized fluorescence study

Time-resolved fluorescence and fluorescence anisotropy data surfaces of flavin adenine dinucleotide bound to lipoamide dehydrogenase from Azotobacter vinelandii in 80% glycerol have been obtained by variation of excitation energy and temperature between 203 and 303 K. The fluorescence kinetics of a deletion mutant lacking 14 COOH-terminal amino acids were compared with the wild-type enzyme to study a possible interaction of the COOH-terminal tail with the active site of the enzyme. The flavin adenine dinucleotide fluorescence in both proteins exhibits a bimodal lifetime distribution as recovered by the maximum entropy method of data analysis. The difference in standard enthalpy and entropy of associated conformational substates was retrieved from the fractional contributions of the two lifetime classes. Activation energies of thermal quenching were obtained that confirm that the isoalloxazines in the deletion mutant are solvent accessible in contrast to the wild-type enzyme. Red-edge spectroscopy in conjunction with variation of temperature provides the necessary experimental axes to interpret the fluorescence depolarization in terms of intersubunit energy transfer rather than reorientational dynamics of the flavins. The results can be explained by a compartmental model that describes the anisotropy decay of a binary, inhomogeneously broadened, homoenergy transfer system. By using this model in a global analysis of the fluorescence anisotropy decay surface, the distance between and relative orientation of the two isoalloxazine rings are elucidated. For the wild-type enzyme, this geometrical information is in agreement with crystallographic data of the A. vinelandii enzyme, whereas the mutual orientation of the subunits in the deletion mutant is slightly altered. In addition, the ambiguity in the direction of the emission transition moment in the isoalloxazine ring is solved. The anisotropy decay parameters also provide information on electronic and dipolar relaxational properties of the flavin active site. The local environment of the prosthetic groups in the deletion mutant of the A. vinelandii enzyme is highly inhomogeneous, and a transition from slow to rapid dipolar relaxation is observed over the measured temperature range. In the highly homogeneous active site of the wild-type enzyme, dipolar relaxation is slowed down beyond the time scale of fluorescence emission at any temperature studied. Our results are in favor of a COOH-terminal polypeptide interacting with the active site, thereby shielding the isoalloxazines from the solvent. This biological system forms a very appropriate tool to test the validity of photophysical models describing homoenergy transfer.

Molecular relaxation spectroscopy of flavin adenine dinucleotide in wild type and mutant lipoamide dehydrogenase from Azotobacter vinelandii

The temperature dependence of the fluorescence emission spectra of flavin adenine dinucleotide bound to lipoamide dehydrogenase from Azotobacter vinelandii shows that the protein matrix in the vicinity of the prosthetic group is rigid on a nanosecond time scale in a medium of high viscosity (80% glycerol). The active site of a deletion mutant of this enzyme, which lacks 14 C-terminal amino acids, is converted from a solid-state environment (on the nanosecond time scale of fluorescence) into a state where efficient dipolar relaxation takes place at temperatures between 203 and 303 K. In aqueous solution, fast dipolar fluctuations are observed in both proteins. It is shown from fluorescence quenching of the flavin by iodide ions that the prosthetic groups of the mutant protein are partially iodide accessible in contrast to the wild type enzyme. A detailed analysis of the temperature dependence of spectral energies according to continuous relaxation models reveals two distinct relaxation processes in the deletion mutant, which were assigned to solvent and protein dipoles, respectively. From the long-wavelength shifts of the emission spectra upon red-edge excitation, it is demonstrated that the active site of the wild type enzyme has high structural homogeneity in comparison to the deletion mutant. In combination with results obtained by X-ray diffraction studies on crystals of the wild type enzyme, it can be concluded that the C-terminal polypeptide of the A. vinelandii enzyme interacts with the dehydrolipoamide binding site, thereby shielding the flavins from the solvent.

Comparison of the dynamical structures of lipoamide dehydrogenase and glutathione reductase by time-resolved polarized flavin fluorescence

Time-resolved polarized fluorescence spectroscopy has been applied to the bound FAD in the structurally related flavoproteins lipoamide dehydrogenase from Azotobacter vinelandii (LipDH-AV) and glutathione reductase (GR) from human erythrocytes. The fluorescence parameters as obtained from the maximum entropy analysis differ considerably in both enzymes, reflecting the unique properties of the flavin microenvironment. Three conformational substates are revealed in LipDH-AV and five in GR. Almost 90% of the population of GR molecules has a fluorescence lifetime in the order of 30 ps which originates from efficient exciplex formation with Tyr197. Equilibrium fluctuations between conformational substates are observed for LipDH-AV on a nanosecond time scale in the temperature range 277-313 K. Interconversion between conformational substates in GR is slow, indicating that large activation barriers exist between the states. In agreement with these results, a model is postulated which ascribes a role in catalysis to equilibrium fluctuations between conformational substates in GR and LipDH-AV. From time-resolved fluorescence anisotropy as a function of temperature, distinction can be made between flavin reorientational motion and interflavin energy transfer. In both proteins intersubunit energy transfer between the prosthetic groups is observed. Furthermore, it is revealed that only the flavin in glutathione reductase exhibits rapid restricted reorientational motion. Geometric information concerning the relative orientation and distance of the flavins can be extracted from the parameters describing the energy-transfer process. The obtained spatial arrangement of the flavins is in excellent agreement with crystallographic data.

Shape and lipid-binding site of the nonspecific lipid-transfer protein (sterol carrier protein 2): a steady-state and time-resolved fluorescence study

The nonspecific lipid-transfer protein (nsL-TP) from bovine liver was studied with time-resolved and steady-state fluorescence techniques. From the decay of the intrinsic tryptophanyl fluorescence, it was estimated that the rotational correlation time of nsL-TP is 15 ns. This parameter increased only slightly upon addition of an excess of negatively charged vesicles, indicating that the basic nsL-TP is not immobilized at the membrane surface under these conditions. Binding studies using fluorescent lipid analogues revealed that nsL-TP is able to extract sn-2-(pyrenehexanoyl) phosphatidylcholine and 1-palmitoyl-2-[3-(diphenylhexatrienyl) propionyl]-sn-3-phosphocholine (DPHp-PC) from a quenched donor vesicle. The fluorescence increase resulting from this binding was poorly quenched by either acrylamide or iodide. This indicates that nsL-TP shields the bound PC molecules from the aqueous environment. Time-resolved analysis of DPH fluorescence originating from DPHp-PC bound to nsL-TP yielded a rotational correlation time of 7.4 ns. This correlation time strongly suggests that the DPH moiety of the bound molecule is immobilized and that the nsL-TP/DPHp-PC complex is not attached to the donor vesicle. In view of the longer rotational correlation time obtained for the intrinsic tryptophanyl fluorescence, we conclude that nsL-TP is highly asymmetric. The data are consistent with a model in which the shape of nsL-TP is ellipsoidal with an axis ratio of 2.8. The implications for the mode of action of nsL-TP are discussed.

Time-resolved fluorescence spectroscopy of NADPH-cytochrome P-450 reductase: demonstration of energy transfer between the two prosthetic groups

Fluorescence as well as fluorescence anisotropy decay parameters have been obtained from NADPH-cytochrome P-450 reductase by time-resolved fluorescence spectroscopy. The two flavins in the enzyme, FMN and FAD, are slightly fluorescent and exhibit heterogeneous fluorescence lifetimes, as observed with other flavoproteins. The time-dependent anisotropy is also multiexponential and is wavelength-dependent. The anisotropy decay is biexponential with two correlation times when the enzyme is excited at the red edge of the first absorption band (514 nm). When the enzyme is excited in the light absorption maximum (458 nm), an additional shorter correlation time is found, which contains information about the rate of energy transfer between the two flavins present in the enzyme. FMN-depleted NADPH-cytochrome P-450 reductase shows also only two correlation times, as does the enzyme in the "air-stable" semiquinone state when excited at 458 nm. Wavelength-dependent steady-state anisotropy measurements of native and FMN-depleted protein show that the former exhibits lower values than the latter in the region of the first absorption band, but when the red edge of the absorption band is reached, the anisotropy becomes equal in both preparations. A similar situation is encountered in model compounds, monomeric and dimeric flavins, immobilized in poly(methyl methacrylate). Both in the models and in the flavoprotein this can be attributed to failure of energy transfer at the red edge of the absorption band. From the results we were able to derive both geometric parameters and dynamic properties of both flavins in the NADPH-cytochrome P-450 reductase.(ABSTRACT TRUNCATED AT 250 WORDS)