A new global analysis algorithm to analyse (hyper-) spectral images is presented. It is based on the phasor representation that has been demonstrated to be very powerful for the analysis of lifetime imaging data. In spectral phasor analysis the fluorescence spectrum of each pixel in the image is Fourier transformed. Next, the real and imaginary components of the first harmonic of the transform are employed as X and Y coordinates in a scatter (spectral phasor) plot. Importantly, the spectral phasor representation allows for rapid (real time) semi-blind spectral unmixing of up to three components in the image. This is demonstrated on slides with fixed cells containing three fluorescent labels. In addition the method is used to analyse autofluorescence of cells in a fresh grass blade. It is shown that the spectral phasor approach is compatible with spectral imaging data recorded with a low number of spectral channels.

Fluorescence-anisotropy-based homo-FRET detection methods can be employed to study clustering of identical proteins in cells. Here, the potential of fluorescence anisotropy microscopy for the quantitative imaging of protein clusters with subcellular resolution is investigated. Steady-state and time-resolved anisotropy detection and both one- and two-photon excitation methods are compared. The methods are evaluated on cells expressing green fluorescent protein (GFP) constructs that contain one or two FK506-binding proteins. This makes it possible to control dimerization and oligomerization of the constructs and yields the experimental relation between anisotropy and cluster size. The results show that, independent of the experimental method, the commonly made assumption of complete depolarization after a single energy transfer step is not valid here. This is due to a nonrandom relative orientation of the fluorescent proteins. Our experiments show that this relative orientation is restricted by interactions between the GFP barrels. We describe how the experimental relation between anisotropy and cluster size can be employed in quantitative cluster size imaging experiments of other GFP fusions. Experiments on glycosylphosphatidylinisotol (GPI)-anchored proteins reveal that GPI forms clusters with an average size of more than two subunits. For epidermal growth factor receptor (EGFR), we observe that approximately 40% of the unstimulated receptors are present in the plasma membrane as preexisting dimers. Both examples reveal subcellular heterogeneities in cluster size and distribution.

The suggestion that microdomains may function as signaling platforms arose from the presence of growth factor receptors, such as the EGFR, in biochemically isolated lipid raft fractions. To investigate the role of EGFR activation in the organization of lipid rafts we have performed FLIM analyses using putative lipid raft markers such as ganglioside GM1 and glycosylphosphatidylinositol (GPI)-anchored GFP (GPI-GFP). The EGFR was labeled using single domain antibodies from Llama glama that specifically bind the EGFR without stimulating its kinase activity. Our FLIM analyses demonstrate a cholesterol-independent colocalization of GM1 with EGFR, which was not observed for the transferrin receptor. By contrast, a cholesterol-dependent colocalization was observed for GM1 with GPI-GFP. In the resting state no colocalization was observed between EGFR and GPI-GFP, but stimulation of the cell with EGF resulted in the colocalization at the nanoscale level of EGFR and GPI-GFP. Moreover, EGF induced the enrichment of GPI-GFP in a detergent-free lipid raft fraction. Our results suggest that EGF induces the coalescence of the two types of GM1-containing microdomains that might lead to the formation of signaling platforms.

The current activation model of the EGF receptor (EGFR) predicts that binding of EGF results in dimerization and oligomerization of the EGFR, leading to the allosteric activation of the intracellular tyrosine kinase. Little is known about the regulatory mechanism of receptor oligomerization. In this study, we have employed FRET between identical fluorophores (homo-FRET) to monitor the dimerization and oligomerization state of the EGFR before and after receptor activation. Our data show that, in the absence of ligand, ϳ40% of the EGFR molecules were present as inactive dimers or predimers. The monomer/predimer ratio was not affected by deletion of the intracellular domain. Ligand binding induced the formation of receptor oligomers, which were found in both the plasma membrane and intracellular structures. Ligand-induced oligomerization required tyrosine kinase activity and nine different tyrosine kinase substrate residues. This indicates that the binding of signaling molecules to activated EGFRs results in EGFR oligomerization. Induction of EGFR predimers or preoligomers using the EGFR fused to the FK506-binding protein did not affect signaling but was found to enhance EGF-induced receptor internalization. Our data show that EGFR oligomerization is the result of EGFR signaling and enhances EGFR internalization.The EGF receptor (EGFR 2 ; ErbB1) has an essential role in the regulation of growth and differentiation of a large range of cell types. The EGFR belongs to the ErbB family, all four members of which have been implicated in the development of different cancers (1). The first step in the signal transduction cascade is the binding of its ligand such as EGF or TGF-␣ to the ectodomain, which provokes receptor dimerization and oligomerization. Deletion of the dimerization domain, which is present in domain II of the EGFR ectodomain, blocks receptor activation completely, demonstrating that receptor dimerization is critical for the allosteric activation of the tyrosine kinase (2, 3). Activation of the receptor tyrosine kinase results in cross-phosphorylation of the receptors, and the phosphotyrosines in the intracellular domain serve subsequently as docking sites for adaptor proteins such as Grb2 and Shc and enzymes such as phospholipase C␥, which contain phosphotyrosine-specific SH2 (Src homology 2) or phosphotyrosine-binding domains. Eventually, the active ligand-receptor complex becomes internalized via both clathrin-dependent and clathrin-independent pathways, followed by the intracellular transport to lysosomes, where the ligand-receptor complexes are degraded (4).Although EGF binding and dimerization seem to be strictly connected, both microscopic and biochemical studies have demonstrated that, in resting cells, the receptor is already found on the cell surface as non-active dimers, the so-called predimers. This phenomenon was initially discovered using electron microscopy and immunogold labeling of the EGFR: in the resting cell, ϳ35% of the total receptor population was present as receptor predimers (5). Th...

High-resolution 10 K Shpol'skii spectra of 3-hydroxyflavone (3HF) and its deuterated analogue (3DF) in n-octane and n-octane/octanol mixtures are presented for the first time. In pure n-octane for both 3HF and 3DF, well-resolved excitation and emission spectra were observed, showing fluorescence shifted from 380−460 to 513−550 nm because of excited-state intramolecular proton/deuteron transfer (ESIPT/ESIDT). Compared to those of 3DF, the 3HF excitation and emission bands are much wider because of lifetime-limited homogeneous broadening. Proton transfer is at least a factor of 4 faster than deuteron transfer. From the homogeneous contribution to the total bandwidth of 3HF, the rate constants of ESIPT and ground-state back proton transfer were estimated to be 39 ± 10 and 210 ± 30 fs, respectively. The effect of four octanol additives was investigated. Only for 2-octanol andthough less favorable3-octanol, a new site in the emission spectrum was observed, blue-shifted over 7 and 10 nm, respectively, versus the 3HF spectrum in n-octane. The new site is attributed to a 1:1 3HF/octanol complex. Its ground-state vibrational pattern differs from that of free 3HF. For 3DF, no Shpol'skii spectrum of a complex could be obtained. It is suggested that in the complex the proton/deuteron transfer mechanisms differ from those of the free molecules; furthermore, a molecular structure for the tautomeric form of the complex is proposed.

Homo‐FRET, Förster resonance energy transfer between identical fluorophores, can be conveniently measured by observing its effect on the fluorescence anisotropy. This review aims to summarize the possibilities of fluorescence anisotropy imaging techniques to investigate clustering of identical proteins and lipids. Homo‐FRET imaging has the ability to determine distances between fluorophores. In addition it can be employed to quantify cluster sizes as well as cluster size distributions. The interpretation of homo‐FRET signals is complicated by the fact that both the mutual orientations of the fluorophores and the number of fluorophores per cluster affect the fluorescence anisotropy in a similar way. The properties of the fluorescence probes are very important. Taking these properties into account is critical for the correct interpretation of homo‐FRET signals in protein‐ and lipid‐clustering studies. This is be exemplified by studies on the clustering of the lipid raft markers GPI and K‐ras, as well as for EGF receptor clustering in the plasma membrane.

A time-resolved fluorescence anisotropy imaging method for studying nanoscale clustering of proteins or lipids was developed and evaluated. It is based on FRET between the identical fluorophores (homo-FRET), which results in a rapid depolarization of the fluorescence. The method employs the time-resolved fluorescence anisotropy decays recorded in a confocal microscope equipped with pulsed excitation and time-gated detection. From the decay the limiting anisotropy r(inf) was derived, which is a direct measure for the number of fluorophores per cluster. The method was evaluated by imaging GPI-GFP, a lipid raft marker. Small clusters were observed in the plasma membrane while the cytoplasm and the Golgi contained predominantly monomers.

Skin contains many autofluorescent components that can be studied using spectral imaging. We employed a spectral phasor method to analyse two photon excited autofluorescence and second harmonic generation images of in vivo human skin. This method allows segmentation of images based on spectral features. Various structures in the skin could be distinguished, including Stratum Corneum, epidermal cells and dermis. The spectral phasor analysis allowed investigation of their fluorescence composition and identification of signals from NADH, keratin, FAD, melanin, collagen and elastin. Interestingly, two populations of epidermal cells could be distinguished with different melanin content.