Arjen N. Bader

EGF induces coalescence of different lipid rafts

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.

Homo-FRET Imaging Enables Quantification of Protein Cluster Sizes with Subcellular Resolution

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.

Ligand-induced EGF Receptor Oligomerization Is Kinase-dependent and Enhances Internalization

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, ∼40% 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 pre-oligomers 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.

Spectral phasor analysis allows rapid and reliable unmixing of fluorescence microscopy spectral images

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.

Homo-FRET Imaging as a tool to quantify protein and lipid clustering

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.

Imaging of protein cluster sizes by means of confocal time-gated fluorescence anisotropy microscopy

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.

In vivo monitoring of protein-bound and free NADH during ischemia by nonlinear spectral imaging microscopy

Nonlinear spectral imaging microscopy (NSIM) allows simultaneous morphological and spectroscopic investigation of intercellular events within living animals. In this study we used NSIM for in vivo time-lapse in-depth spectral imaging and monitoring of protein-bound and free reduced nicotinamide adenine dinucleotide (NADH) in mouse keratinocytes following total acute ischemia for 3.3 h at ~3 min time intervals. The high spectral resolution of NSIM images allows discrimination between the two-photon excited fluorescence emission of protein-bound and free NAD(P)H by applying linear spectral unmixing to the spectral image data. Results reveal the difference in the dynamic response between protein-bound and free NAD(P)H to ischemia-induced hypoxia/anoxia. Our results demonstrate the capability of nonlinear spectral imaging microscopy in unraveling dynamic cellular metabolic events within living animals for long periods of time.

Phasor analysis of multiphoton spectral images distinguishes autofluorescence components of in vivo human skin

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.

Endocytosis of EGFR requires its kinase activity and N-terminal transmembrane dimerization motif.

Summary EGFR signaling is attenuated by endocytosis and degradation of receptor–ligand complexes in lysosomes. Endocytosis of EGFR is known to be regulated by multiple post-translational modifications. The observation that prevention of these modifications does not block endocytosis completely, suggests the involvement of other mechanism(s). Recently, receptor clustering has been suggested to induce internalization of multiple types of membrane receptors. However, the mechanism of clustering-induced internalization remains unknown. We have used biparatopic antibody fragments from llama (VHHs) to induce EGFR clustering without stimulating tyrosine kinase activity. Using this approach, we have found an essential role for the N-terminal GG4-like dimerization motif in the transmembrane domain (TMD) for clustering-induced internalization. Moreover, conventional EGF-induced receptor internalization depends exclusively on this TMD dimerization and kinase activity. Mutations in this dimerization motif eventually lead to reduced EGFR degradation and sustained signaling. We propose a novel role for the TMD dimerization motif in the negative-feedback control of EGFR. The widely conserved nature of GG4-like dimerization motifs in transmembrane proteins suggests a general role for these motifs in clustering-induced internalization.

Fast nonlinear spectral microscopy of in vivo human skin

An optimized system for fast, high-resolution spectral imaging of in vivo human skin is developed and evaluated. The spectrograph is composed of a dispersive prism in combination with an electron multiplying CCD camera. Spectra of autofluorescence and second harmonic generation (SHG) are acquired at a rate of 8 kHz and spectral images within seconds. Image quality is significantly enhanced by the simultaneous recording of background spectra. In vivo spectral images of 224 × 224 pixels were acquired, background corrected and previewed in real RGB color in 6.5 seconds. A clear increase in melanin content in deeper epidermal layers in in vivo human skin was observed.