EGF signalling amplification induced by dynamic clustering of EGFR

Surface-Patterned DNA Origami Rulers Reveal Nanoscale Distance Dependency of the Epidermal Growth Factor Receptor Activation

The nanoscale arrangement of ligands can have a major effect on the activation of membrane receptor proteins and thus cellular communication mechanisms. Here we report on the technological development and use of tailored DNA origami-based molecular rulers to fabricate "Multiscale Origami Structures As Interface for Cells" (MOSAIC), to enable the systematic investigation of the effect of the nanoscale spacing of epidermal growth factor (EGF) ligands on the activation of the EGF receptor (EGFR). MOSAIC-based analyses revealed that EGF distances of about 30-40 nm led to the highest response in EGFR activation of adherent MCF7 and Hela cells. Our study emphasizes the significance of DNA-based platforms for the detailed investigation of the molecular mechanisms of cellular signaling cascades.

Distinct interactions stabilize EGFR dimers and higher-order oligomers in cell membranes

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase with important roles in many cellular processes as well as in cancer and other diseases. EGF binding promotes EGFR dimerization and autophosphorylation through interactions that are well understood structurally. How these dimers relate to higher-order EGFR oligomers seen in cell membranes, however, remains unclear. Here, we used single-particle tracking (SPT) and Förster resonance energy transfer imaging to examine how each domain of EGFR contributes to receptor oligomerization and the rate of receptor diffusion in the cell membrane. Although the extracellular region of EGFR is sufficient to drive receptor dimerization, we find that the EGF-induced EGFR slowdown seen by SPT requires higher-order oligomerization-mediated in part by the intracellular tyrosine kinase domain when it adopts an active conformation. Our data thus provide important insight into the interactions required for higher-order EGFR assemblies involved in EGF signaling.

Distinct interactions stabilize EGFR dimers and higher-order oligomers in cell membranes

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase (RTK) with important roles in many cellular processes as well as cancer and other diseases. EGF binding promotes EGFR dimerization and autophosphorylation through interactions that are well understood structurally. However, it is not clear how these dimers relate to higher-order EGFR oligomers detected at the cell surface. We used single-particle tracking (SPT) and Förster resonance energy transfer (FRET) imaging to examine how each domain within EGFR contributes to receptor dimerization and the rate of its diffusion in the cell membrane. We show that the EGFR extracellular region is sufficient to drive receptor dimerization, but that the EGF-induced EGFR slow-down seen by SPT requires formation of higher order oligomers, mediated in part by the intracellular tyrosine kinase domain - but only when in its active conformation. Our data thus provide important insight into higher-order EGFR interactions required for EGF signaling.

The clathrin adaptor complex-1 and Rab12 regulate post-golgi trafficking of WT epidermal growth factor receptor (EGFR)

The epidermal growth factor receptor (EGFR) plays important roles in cancer progression and is one of the major drug targets for targeted cancer therapy. Although fundamentally important, how newly synthesized EGFR is delivered to the cell surface to perform its cellular functions remains to be further investigated. In this study, we found using the approaches of gene knockout, siRNA knockdown, streptavidin pull-down, and co-immunoprecipitation assays that the clathrin adaptor complex-1 (AP-1) and Rab12 interact with EGFR and regulate the export of EGFR out of the trans-Golgi network (TGN). In addition, the tyrosine residue at the 998 position on human EGFR is critical to bind to AP-1, and this residue is important for TGN export of EGFR. We demonstrate that AP-1 and Rab12 are important for epidermal growth factor-induced phosphorylation of EGFR, cell elongation, and proliferation, suggesting that AP-1-mediated and Rab12-mediated post-Golgi trafficking is important for EGFR signaling. Moreover, TGN export of the constitutively activated mutant form of EGFR (EGFR^L858R) is independent of AP-1 and Rab12. Our results reveal insights into the molecular mechanisms that mediate the TGN-to-cell surface delivery of EGFR and indicate that TGN export of WT EGFR and EGFR^L858R depends on different cellular factors.

Critical roles for EGFR and EGFR-HER2 clusters in EGF binding of SW620 human carcinoma cells

Epidermal growth factor (EGF) signalling regulates normal epithelial and other cell growth, with EGF receptor (EGFR) overexpression reported in many cancers. However, the role of EGFR clusters in cancer and their dependence on EGF binding is unclear. We present novel single-molecule total internal reflection fluorescence microscopy of (i) EGF and EGFR in living cancer cells, (ii) the action of anti-cancer drugs that separately target EGFR and human EGFR2 (HER2) on these cells and (iii) EGFR–HER2 interactions. We selected human epithelial SW620 carcinoma cells for their low level of native EGFR expression, for stable transfection with fluorescent protein labelled EGFR, and imaged these using single-molecule localization microscopy to quantify receptor architectures and dynamics upon EGF binding. Prior to EGF binding, we observe pre-formed EGFR clusters. Unexpectedly, clusters likely contain both EGFR and HER2, consistent with co-diffusion of EGFR and HER2 observed in a different model CHO-K1 cell line, whose stoichiometry increases following EGF binding. We observe a mean EGFR : EGF stoichiometry of approximately 4 : 1 for plasma membrane-colocalized EGFR–EGF that we can explain using novel time-dependent kinetics modelling, indicating preferential ligand binding to monomers. Our results may inform future cancer drug developments.

It Takes More than Two to Tango: Complex, Hierarchal, and Membrane-Modulated Interactions in the Regulation of Receptor Tyrosine Kinases

The search for an understanding of how cell fate and motility are regulated is not a purely scientific undertaking, but it can also lead to rationally designed therapies against cancer. The discovery of tyrosine kinases about half a century ago, the subsequent characterization of certain transmembrane receptors harboring tyrosine kinase activity, and their connection to the development of human cancer ushered in a new age with the hope of finding a treatment for malignant diseases in the foreseeable future. However, painstaking efforts were required to uncover the principles of how these receptors with intrinsic tyrosine kinase activity are regulated. Developments in molecular and structural biology and biophysical approaches paved the way towards better understanding of these pathways. Discoveries in the past twenty years first resulted in the formulation of textbook dogmas, such as dimerization-driven receptor association, which were followed by fine-tuning the model. In this review, the role of molecular interactions taking place during the activation of receptor tyrosine kinases, with special attention to the epidermal growth factor receptor family, will be discussed. The fact that these receptors are anchored in the membrane provides ample opportunities for modulatory lipid-protein interactions that will be considered in detail in the second part of the manuscript. Although qualitative and quantitative alterations in lipids in cancer are not sufficient in their own right to drive the malignant transformation, they both contribute to tumor formation and also provide ways to treat cancer. The review will be concluded with a summary of these medical aspects of lipid-protein interactions.

Conserved roles for receptor tyrosine kinase extracellular regions in regulating receptor and pathway activity

Receptor Tyrosine Kinases (RTKs) comprise a diverse group of cell-surface receptors that mediate key signaling events during animal development and are frequently activated in cancer. We show here that deletion of the extracellular regions of 10 RTKs representing 7 RTK classes or their substitution with the dimeric immunoglobulin Fc region results in constitutive receptor phosphorylation but fails to result in phosphorylation of downstream signaling effectors Erk or Akt. Conversely, substitution of RTK extracellular regions with the extracellular region of the Epidermal Growth Factor Receptor (EGFR) results in increases in effector phosphorylation in response to EGF. These results indicate that the activation signal generated by the EGFR extracellular region is capable of activating at least seven different RTK classes. Failure of phosphorylated Fc-RTK chimeras or RTKs with deleted extracellular regions to stimulate phosphorylation of downstream effectors indicates that either dimerization and receptor phosphorylation per se are insufficient to activate signaling or constitutive dimerization leads to pathway inhibition.

Processing Temporal Growth Factor Patterns by an Epidermal Growth Factor Receptor Network Dynamically Established in Space

The proto-oncogenic epidermal growth factor (EGF) receptor (EGFR) is a tyrosine kinase whose sensitivity and response to growth factor signals that vary over time and space determine cellular behavior within a developing tissue. The molecular reorganization of the receptors on the plasma membrane and the enzyme-kinetic mechanisms of phosphorylation are key determinants that couple growth factor binding to EGFR signaling. To enable signal initiation and termination while simultaneously accounting for suppression of aberrant signaling, a coordinated coupling of EGFR kinase and protein tyrosine phosphatase activity is established through space by vesicular dynamics. The dynamical operation mode of this network enables not only time-varying growth factor sensing but also adaptation of the response depending on cellular context. By connecting spatially coupled enzymatic kinase/phosphatase processes and the corresponding dynamical systems description of the EGFR network, we elaborate on the general principles necessary for processing complex growth factor signals.

Epidermal Growth Factor Stimulates Fatty Acid Synthesis Mainly via PLC-γ1/Akt Signaling Pathway in Dairy Goat Mammary Epithelial Cells

Simple Summary

Goat milk contains an abundance of fatty acids which are benefit to human health. Epidermal growth factor (EGF) is a small peptide which could positively regulate the growth, development and differentiation of the mammary gland during lactation. However, little information is available about EGF in regulating lipid metabolism in the mammary gland. This study investigated the effects of EGF on the triglyceride (TG) synthesis, lipogenic genes expression and the downstream signal protein levels in goat mammary epithelial cells (GMECs). Our findings indicated EGF might be beneficial to improve milk fat synthesis of dairy goats.

Abstract

EGF acts as a ligand of the EGF receptor (EGFR) to activate the EGFR-mediated signaling pathways and is involved in the regulation of cell physiology. However, the roles of EGFR mediated signaling pathways in the regulation of lipid metabolism in goat mammary epithelial cells (GMECs) are poorly understood. To evaluate the impact of EGF on GMECs, the triglyceride (TG) content and lipid droplet were detected, using TG assay and immunofluorescence. Further, expression of lipogenic genes, the protein kinase B (Akt), phospholipase C-γ1 (PLC-γ1) and extracellular signal-regulated kinases (ERK)1/2 signaling pathways were measured by real-time polymerase chain reaction and Western blot, respectively. The results showed that the mRNA expression of EGFR gene was significantly upregulated in lactating goat mammary gland tissues compared to non-lactation period (p < 0.05). TG contents in EGF-treated GMECs were significantly increased (p < 0.05), and an increase of lipid droplets was also detected. In vitro studies demonstrated that the mRNA levels of lipogenesis-related FASN, ACC, SCD1, LXRa, LXRb and SP1 genes were positively correlated to the mRNA level of EGFR gene shown by gene overexpression and silencing (p < 0.05). The phosphorylations of Akt, ERK1/2 and PLC-γ1 in GMECs were greatly upregulated in the presence of EGF, and specific inhibitors were capable of blocking the phosphorylation of Akt, ERK1/2 and PLC-γ1. Compared with EGF-treated GMECs, the mRNA levels of FASN, ACC and SCD1 were significantly decreased in GMECs co-treated with PLC-γ1 and Akt inhibitor and EGF (p < 0.05), and TG content was also dropped significantly. These observations implied that EGFR plays an important role in regulating de novo fatty acid synthesis in GMECs, mainly mediated by Akt and PLC-γ1 signaling pathways.

Receptor tyrosine kinase activation: From the ligand perspective

Receptor tyrosine kinases (RTKs) are single-span transmembrane receptors in which relatively conserved intracellular kinase domains are coupled to divergent extracellular modules. The extracellular domains initiate receptor signaling upon binding to either soluble or membrane-embedded ligands. The diversity of extracellular domain structures allows for coupling of many unique signaling inputs to intracellular tyrosine phosphorylation. The combinatorial power of this receptor system is further increased by the fact that multiple ligands can typically interact with the same receptor. Such ligands often act as biased agonists and initiate distinct signaling responses via activation of the same receptor. Mechanisms behind such biased agonism are largely unknown for RTKs, especially at the level of receptor-ligand complex structure. Using recent progress in understanding the structures of active RTK signaling units, we discuss selected mechanisms by which ligands couple receptor activation to distinct signaling outputs.

Clathrin switches transforming growth factor-β role to pro-tumorigenic in liver cancer

BACKGROUND & AIMS

Upon ligand binding, tyrosine kinase receptors, such as epidermal growth factor receptor (EGFR), are recruited into clathrin-coated pits for internalization by endocytosis, which is relevant for signalling and/or receptor degradation. In liver cells, transforming growth factor-β (TGF-β) induces both pro- and anti-apoptotic signals; the latter are mediated by the EGFR pathway. Since EGFR mainly traffics via clathrin-coated vesicles, we aimed to analyse the potential role of clathrin in TGF-β-induced signalling in liver cells and its relevance in liver cancer.

METHODS

Real-Time PCR and immunohistochemistry were used to analyse clathrin heavy-chain expression in human (CLTC) and mice (Cltc) liver tumours. Transient knockdown (siRNA) or overexpression of CLTC were used to analyse its role on TGF-β and EGFR signalling in vitro. Bioinformatic analysis was used to determine the effect of CLTC and TGFB1 expression on prognosis and overall survival in patients with hepatocellular carcinoma (HCC).

RESULTS

Clathrin expression increased during liver tumorigenesis in humans and mice. CLTC knockdown cells responded to TGF-β phosphorylating SMADs (canonical signalling) but showed impairment in the anti-apoptotic signals (EGFR transactivation). Experiments of loss or gain of function in HCC cells reveal an essential role for clathrin in inhibiting TGF-β-induced apoptosis and upregulation of its pro-apoptotic target NOX4. Autocrine TGF-β signalling in invasive HCC cells upregulates CLTC expression, switching its role to pro-tumorigenic. A positive correlation between TGFB1 and CLTC was found in HCC cells and patients. Patients expressing high levels of TGFB1 and CLTC had a worse prognosis and lower overall survival.

CONCLUSIONS

This work describes a novel role for clathrin in liver tumorigenesis, favouring non-canonical pro-tumorigenic TGF-β pathways. CLTC expression in human HCC samples could help select patients that would benefit from TGF-β-targeted therapy.

LAY SUMMARY

Clathrin heavy-chain expression increases during liver tumorigenesis in humans (CLTC) and mice (Cltc), altering the cellular response to TGF-β in favour of anti-apoptotic/pro-tumorigenic signals. A positive correlation between TGFB1 and CLTC was found in HCC cells and patients. Patients expressing high levels of TGFB1 and CLTC had a worse prognosis and lower overall survival. CLTC expression in HCC human samples could help select patients that would benefit from therapies targeting TGF-β.

Leader cell PLCγ1 activation during keratinocyte collective migration is induced by EGFR localization and clustering

Re-epithelialization is a critical step in wound healing and results from the collective migration of keratinocytes. Previous work demonstrated that immobilized, but not soluble, epidermal growth factor (EGF) resulted in leader cell-specific activation of phospholipase C gamma 1 (PLCγ1) in HaCaT keratinocytes, and that this PLCγ1 activation was necessary to drive persistent cell migration. To determine the mechanism responsible for wound edge-localized PLCγ1 activation, we examined differences in cell area, cell-cell interactions, and EGF receptor (EGFR) localization between wound edge and bulk cells treated with vehicle, soluble EGF, or immobilized EGF. Our results support a multistep mechanism where EGFR translocation from the lateral membrane to the basolateral/basal membrane allows clustering in response to immobilized EGF. This analysis of factors regulating PLCγ1 activation is a crucial step toward developing therapies or wound dressings capable of modulating this signal and, consequently, cell migration.

Electromagnetic fields alter the motility of metastatic breast cancer cells

Interactions between cells and their environment influence key physiologic processes such as their propensity to migrate. However, directed migration controlled by extrinsically applied electrical signals is poorly understood. Using a novel microfluidic platform, we found that metastatic breast cancer cells sense and respond to the net direction of weak (∼100 µV cm-1), asymmetric, non-contact induced Electric Fields (iEFs). iEFs inhibited EGFR (Epidermal Growth Factor Receptor) activation, prevented formation of actin-rich filopodia, and hindered the motility of EGF-treated breast cancer cells. The directional effects of iEFs were nullified by inhibition of Akt phosphorylation. Moreover, iEFs in combination with Akt inhibitor reduced EGF-promoted motility below the level of untreated controls. These results represent a step towards isolating the coupling mechanism between cell motility and iEFs, provide valuable insights into how iEFs target multiple diverging cancer cell signaling mechanisms, and demonstrate that electrical signals are a fundamental regulator of cancer cell migration.

Characterizing Large-Scale Receptor Clustering on the Single Cell Level: A Comparative Plasmon Coupling and Fluorescence Superresolution Microscopy Study

Spatial clustering of cell membrane receptors has been indicated to play a regulatory role in signal initiation, and the distribution of receptors on the cell surface may represent a potential biomarker. To realize its potential for diagnostic purposes, scalable assays capable of mapping spatial receptor heterogeneity with high throughput are needed. In this work, we use gold nanoparticle (NP) labels with an average diameter of 72.17 ± 2.16 nm as bright markers for large-scale epidermal growth factor receptor (EGFR) clustering in hyperspectral plasmon coupling microscopy and compare the obtained clustering maps with those obtained through fluorescence superresolution microscopy (direct stochastic optical reconstruction microscopy, dSTORM). Our dSTORM experiments reveal average EGFR cluster sizes of 172 ± 99 and 150 ± 90 nm for MDA-MB-468 and HeLa, respectively. The cluster sizes decrease after EGFR activation. Hyperspectral imaging of the NP labels shows that differences in the EGFR cluster sizes are accompanied by differences in the average separations between electromagnetically coupled NPs. Because of the distance dependence of plasmon coupling, changes in the average interparticle separation result in significant spectral shifts. For the experimental conditions investigated in this work, hyperspectral plasmon coupling microscopy of NP labels identified the same trends in large-scale EGFR clustering as dSTORM, but the NP imaging approach provided the information in a fraction of the time. Both dSTORM and hyperspectral plasmon coupling microscopy confirm the cortical actin network as one structural component that determines the average size of EGFR clusters.

Raftophilic rhodopsin-clusters offer stochastic platforms for G protein signalling in retinal discs

Rhodopsin is a G protein-coupled receptor (GPCR) that initiates the phototransduction cascade in retinal disc membrane. Recent studies have suggested that rhodopsin forms highly ordered rows of dimers responsible for single-photon detection by rod photoreceptors. Dimerization is also known to confer to rhodopsin a high affinity for ordered lipids (raftophilicity). However, the role of rhodopsin organization and its raftophilicity in phototransduction remains obscure, owing to the lack of direct observation of rhodopsin dynamics and distribution in native discs. Here, we explore the single-molecule and semi-multimolecule behaviour of rhodopsin in native discs. Rhodopsin forms transient meso-scale clusters, even in darkness, which are loosely confined to the disc centre. Cognate G protein transducin co-distributes with rhodopsin, and exhibits lateral translocation to the disc periphery upon activation. We demonstrate that rhodopsin offers inherently distributed and stochastic platforms for G protein signalling by self-organizing raftophilic clusters, which continually repeat generation/extinction in the disc membrane.

A Brief History of Single-Particle Tracking of the Epidermal Growth Factor Receptor

Single-particle tracking (SPT) has been used and developed over the last 25 years as a method to investigate molecular dynamics, structure, interactions, and function in the cellular context. SPT is able to show how fast and how far individual molecules move, identify different dynamic populations, measure the duration and strength of intermolecular interactions, and map out structures on the nanoscale in cells. In combination with other techniques such as macromolecular crystallography and molecular dynamics simulation, it allows us to build models of complex structures, and develop and test hypotheses of how these complexes perform their biological roles in health as well as in disease states. Here, we use the example of the epidermal growth factor receptor (EGFR), which has been studied extensively by SPT, demonstrating how the method has been used to increase our understanding of the receptor’s organization and function, including its interaction with the plasma membrane, its activation, clustering, and oligomerization, and the role of other receptors and endocytosis. The examples shown demonstrate how SPT might be employed in the investigation of other biomolecules and systems.

Ligand Density and Nanoparticle Clustering Cooperate in the Multivalent Amplification of Epidermal Growth Factor Receptor Activation

Multivalent presentation of ligands on nanoparticles (NPs) is considered a general strategy for enhancing receptor binding and activation through amplification of ligand-receptor interactions within the footprint of the individual NPs. The spatial clustering of ligand-functionalized NPs represents an additional, less well understood mechanism for increasing local ligand-receptor interactions, especially for receptors that form higher-order assemblies, such as the epidermal growth factor (EGF) receptor (EGFR). To shed light on the interplay between ligand density ( i.e., multivalency) and NP clustering in signal amplification, we apply EGF-functionalized 72 ± 1 nm gold nanoparticles (NP-EGF) with known ligand loading (10-200 EGF/NP) as quantifiable and experimentally tractable units of EGFR activation and characterize the NP-mediated amplification of EGFR phosphorylation as a function of both EGF surface density and NP-EGF clustering for two cancer cell lines (HeLa and MDA-MB-468). The measurements confirm a strong multivalent amplification of EGFR phosphorylation through NP-EGF on the cellular level that results in EGF-loading-dependent maximum EGFR phosphorylation levels. A microscopic analysis of NP-EGF-induced EGFR phosphorylation reveals a heterogeneous spatial distribution of EGFR activation across the cell surface. Clustering of multivalent NP-EGF on sub-diffraction-limited length scales is found to result in a local enhancement of EGFR phosphorylation in signaling "hot spots" from where the signal can spread laterally in an EGF-independent fashion. Increasing EGF loadings of the NP enhances NP-EGF clustering and intensifies EGFR phosphorylation. These observations suggest that NP-EGF clustering and the associated local enhancement of ligand-receptor interactions are intrinsic components of the multivalent amplification of phosphorylation for the heterogeneously distributed EGFR through NP-EGF.

The extracellular matrix as a key regulator of intracellular signalling networks

The extracellular matrix (ECM) is a salient feature of all solid tissues within the body. This complex, acellular entity is composed of hundreds of individual molecules whose assembly, architecture and biomechanical properties are critical to controlling the behaviour and phenotype of the different cell types residing within tissues. Cells are the basic unit of life and the core building block of tissues and organs. At their simplest, they follow a set of rules, governed by their genetic code and effected through the complex protein signalling networks that these genes encode. These signalling networks assimilate and process the information received by the cell to control cellular decisions that govern cell fate. The ECM is the biggest provider of external stimuli to cells and as such is responsible for influencing intracellular signalling dynamics. In this review, we discuss the inclusion of ECM as a central regulatory signalling sub-network in computational models of cellular decision making, with a focus on its role in diseases such as cancer. LINKED ARTICLES: This article is part of a themed section on Translating the Matrix. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.1/issuetoc.

Extracellular matrix: The driving force of mammalian diseases

Like the major theme of a Mozart concerto, the immense and pervasive extracellular matrix drives each movement and ultimately closes the symphony, embracing a unique role as the fundamental mediator for most, if not all, ensuing intracellular events. As such, it comes as no surprise that the mechanism of just about every known disease can be traced back to some part of the matrix, typically in the form of an abnormal amount or activity level of a particular matrix component. These defects considerably affect downstream signaling axes leading to overt cellular dysfunction, organ failure, and death. From skin to bone, from vessels to brain, from eyes to all the internal organs, the matrix plays an incredible role as both a cause and potential means to reverse diseases. Human malaises including connective tissue disorders, muscular dystrophy, fibrosis, and cancer are all extracellular matrix-driven diseases. The ability to understand and modulate these matrix-related mechanisms may lead to the future discovery of novel therapeutic options for these patients.

Phosphorylated EGFR Dimers Are Not Sufficient to Activate Ras

Growth factor binding to EGFR drives conformational changes that promote homodimerization and transphosphorylation, followed by adaptor recruitment, oligomerization, and signaling through Ras. Whether specific receptor conformations and oligomerization states are necessary for efficient activation of Ras is unclear. We therefore evaluated the sufficiency of a phosphorylated EGFR dimer to activate Ras without growth factor by developing a chemical-genetic strategy to crosslink and "trap" full-length EGFR homodimers on cells. Trapped dimers become phosphorylated and recruit adaptor proteins at stoichiometry equivalent to that of EGF-stimulated receptors. Surprisingly, these phosphorylated dimers do not activate Ras, Erk, or Akt. In the absence of EGF, phosphorylated dimers do not further oligomerize or reorganize on cell membranes. These results suggest that a phosphorylated EGFR dimer loaded with core signaling adapters is not sufficient to activate Ras and that EGFR ligands contribute to conformational changes or receptor dynamics necessary for oligomerization and efficient signal propagation through the SOS-Ras-MAPK pathway.

Transient Acceleration of Epidermal Growth Factor Receptor Dynamics Produces Higher-Order Signaling Clusters

Cell signaling depends on spatiotemporally regulated molecular interactions. Although the movements of signaling proteins have been analyzed with various technologies, how spatial dynamics influence the molecular interactions that transduce signals is unclear. Here, we developed a single-molecule method to analyze the spatiotemporal coupling between motility, clustering, and signaling. The analysis was performed with the epidermal growth factor receptor (EGFR), which triggers signaling through its dimerization and phosphorylation after association with EGF. Our results show that the few EGFRs isolated in membrane subdomains were released by an EGF-dependent increase in their diffusion area, facilitating molecular associations and producing immobile clusters. Using a two-color single-molecule analysis, we found that the EGF-induced state transition alters the properties of the immobile clusters, allowing them to interact for extended periods with the cytoplasmic protein, GRB2. Our study reveals a novel correlation between this molecular interaction and its mesoscale dynamics, providing the initial signaling node.

The Effect of Nanoparticles on the Cluster Size Distributions of Activated EGFR Measured with Photobleaching Image Correlation Spectroscopy

The epidermal growth factor receptor (EGFR) is an important cell surface receptor in normal physiology and disease. Recent work has shown that EGF-gold nanoparticle conjugates can influence cell behaviour, but the underlying mechanism at the receptor quaternary structural level remains poorly understood.In the present work, the cluster density and cluster size of activated (phosphorylated) EGFR clusters in HeLa cells were determined with photobleaching image correlation spectroscopy. EGFR activation was probed via immunofluorescence-detected phosphorylation of tyrosines (pY-mAb) located in the kinase domain of EGFR (Y845) and at the EGFR cytoplasmic tail (Y1173). Cell activation was probed via nuclear extracellular-regulated kinase (ERK) phosphorylation. The cluster size of activated EGFR was 1.3-2.4 pY-mAb/cluster in unstimulated HeLa cells. EGF or nanorod treatment led to an increase in EGFR oligomers containing multiple phosphotyrosines (>2 phosphotyrosines per EGFR oligomer, average cluster size range = 3-5 pY-mAb/cluster) which paralleled increases in nuclear p-ERK. In contrast, EGF-nanorods decreased the contribution from higher-order phospho-clusters and decreased nuclear p-ERK relative to the nanorod control. These studies provide direct evidence that targeted nanotechnology can manipulate receptor organization and lead to changes in receptor activation and subsequent signalling processes.

Single-molecule fluorescence-based analysis of protein conformation, interaction, and oligomerization in cellular systems

Single-molecule imaging (SMI) of proteins in operation has a history of intensive investigations over 20 years and is now widely used in various fields of biology and biotechnology. We review the recent advances in SMI of fluorescently-tagged proteins in structural biology, focusing on technical applicability of SMI to the measurements in living cells. Basic technologies and recent applications of SMI in structural biology are introduced. Distinct from other methods in structural biology, SMI directly observes single molecules and single-molecule events one-by-one, thus, explicitly analyzing the distribution of protein structures and the history of protein dynamics. It also allows one to detect single events of protein interaction. One unique feature of SMI is that it is applicable in complicated and heterogeneous environments, including living cells. The numbers, location, movements, interaction, oligomerization, and conformation of single-protein molecules have been determined using SMI in cellular systems.

Multi-physics interactions drive VEGFR2 relocation on endothelial cells

Vascular Endothelial Growth Factor Receptor-2 (VEGFR2) is a pro-angiogenic receptor, expressed on endothelial cells (ECs). Although biochemical pathways that follow the VEGFR2 activation are well established, knowledge about the dynamics of receptors on the plasma membrane remains limited. Ligand stimulation induces the polarization of ECs and the relocation of VEGFR2, either in cell protrusions or in the basal aspect in cells plated on ligand-enriched extracellular matrix (ECM). We develop a mathematical model in order to simulate the relocation of VEGFR2 on the cell membrane during the mechanical adhesion of cells onto a ligand-enriched substrate. Co-designing the in vitro experiments with the simulations allows identifying three phases of the receptor dynamics, which are controlled respectively by the high chemical reaction rate, by the mechanical deformation rate, and by the diffusion of free receptors on the membrane. The identification of the laws that regulate receptor polarization opens new perspectives toward developing innovative anti-angiogenic strategies through the modulation of EC activation.

Enzymatic biomineralization of biocompatible CuInS2, (CuInZn)S2 and CuInS2/ZnS core/shell nanocrystals for bioimaging

This work demonstrates a bioenabled fully aqueous phase and room temperature route to the synthesis of CuInS₂/ZnS core/shell quantum confined nanocrystals conjugated to IgG antibodies and used for fluorescent tagging of THP-1 leukemia cells. This elegant, straightforward and green approach avoids the use of solvents, high temperatures and the necessity to phase transfer the nanocrystals prior to application. Non-toxic CuInS₂, (CuInZn)S₂, and CuInS₂/ZnS core/shell quantum confined nanocrystals are synthesized via a biomineralization process based on a single recombinant cystathionine γ-lyase (CSE) enzyme. First, soluble In-S complexes are formed from indium acetate and H₂S generated by CSE, which are then stabilized by l-cysteine in solution. The subsequent addition of copper, or both copper and zinc, precursors then results in the immediate formation of CuInS₂ or (CuInZn)S₂ quantum dots. Shell growth is realized through subsequent introduction of Zn acetate to the preformed core nanocrystals. The size and optical properties of the nanocrystals are tuned by adjusting the indium precursor concentration and initial incubation period. CuInS₂/ZnS core/shell particles are conjugated to IgG antibodies using EDC/NHS cross-linkers and then applied in the bioimaging of THP-1 cells. Cytotoxicity tests confirm that CuInS₂/ZnS core/shell quantum dots do not cause cell death during bioimaging. Thus, this biomineralization enabled approach provides a facile, low temperature route for the fully aqueous synthesis of non-toxic CuInS₂/ZnS quantum dots, which are ideal for use in bioimaging applications.

EGF and NRG induce phosphorylation of HER3/ERBB3 by EGFR using distinct oligomeric mechanisms

Heteromeric interactions between the catalytically impaired human epidermal growth factor receptor (HER3/ERBB3) and its catalytically active homologs EGFR and HER2 are essential for their signaling. Different ligands can activate these receptor pairs but lead to divergent signaling outcomes through mechanisms that remain largely unknown. We used stochastic optical reconstruction microscopy (STORM) with pair-correlation analysis to show that EGF and neuregulin (NRG) can induce different extents of HER3 clustering that are dependent on the nature of the coexpressed HER receptor. We found that the presence of these clusters correlated with distinct patterns and mechanisms of receptor phosphorylation. NRG induction of HER3 phosphorylation depended on the formation of the asymmetric kinase dimer with EGFR in the absence of detectable higher-order oligomers. Upon EGF stimulation, HER3 paralleled previously observed EGFR behavior and formed large clusters within which HER3 was phosphorylated via a noncanonical mechanism. HER3 phosphorylation by HER2 in the presence of NRG proceeded through still another mechanism and involved the formation of clusters within which receptor phosphorylation depended on asymmetric kinase dimerization. Our results demonstrate that the higher-order organization of HER receptors is an essential feature of their ligand-induced behavior and plays an essential role in lateral cross-activation of the receptors. We also show that HER receptor ligands exert unique effects on signaling by modulating this behavior.

Activation-Dependent Rapid Postsynaptic Clustering of Glycine Receptors in Mature Spinal Cord Neurons

Inhibitory synapses are established during development but continue to be generated and modulated in strength in the mature nervous system. In the spinal cord and brainstem, presynaptically released inhibitory neurotransmitter dominantly switches from GABA to glycine during normal development in vivo. While presynaptic mechanisms of the shift of inhibitory neurotransmission are well investigated, the contribution of postsynaptic neurotransmitter receptors to this shift is not fully elucidated. Synaptic clustering of glycine receptors (GlyRs) is regulated by activation-dependent depolarization in early development. However, GlyR activation induces hyperpolarization after the first postnatal week, and little is known whether and how presynaptically released glycine regulates postsynaptic receptors in a depolarization-independent manner in mature developmental stage. Here we developed spinal cord neuronal culture of rodents using chronic strychnine application to investigate whether initial activation of GlyRs in mature stage could change postsynaptic localization of GlyRs. Immunocytochemical analyses demonstrate that chronic blockade of GlyR activation until mature developmental stage resulted in smaller clusters of postsynaptic GlyRs that could be enlarged upon receptor activation for 1 h in the mature stage. Furthermore, live cell-imaging techniques show that GlyR activation decreases its lateral diffusion at synapses, and this phenomenon is dependent on PKC, but neither Ca2+ nor CaMKII activity. These results suggest that the GlyR activation can regulate receptor diffusion and cluster size at inhibitory synapses in mature stage, providing not only new insights into the postsynaptic mechanism of shifting inhibitory neurotransmission but also the inhibitory synaptic plasticity in mature nervous system.

EGFR oligomerization organizes kinase-active dimers into competent signalling platforms

Epidermal growth factor receptor (EGFR) signalling is activated by ligand-induced receptor dimerization. Notably, ligand binding also induces EGFR oligomerization, but the structures and functions of the oligomers are poorly understood. Here, we use fluorophore localization imaging with photobleaching to probe the structure of EGFR oligomers. We find that at physiological epidermal growth factor (EGF) concentrations, EGFR assembles into oligomers, as indicated by pairwise distances of receptor-bound fluorophore-conjugated EGF ligands. The pairwise ligand distances correspond well with the predictions of our structural model of the oligomers constructed from molecular dynamics simulations. The model suggests that oligomerization is mediated extracellularly by unoccupied ligand-binding sites and that oligomerization organizes kinase-active dimers in ways optimal for auto-phosphorylation in trans between neighbouring dimers. We argue that ligand-induced oligomerization is essential to the regulation of EGFR signalling.

Epidermal growth factor receptors have been shown to oligomerise upon binding to their cognate ligands. Here, the authors use biochemical, biophysical and cell biology techniques to analyse the structures of these oligomers, and argue that these formations are required for signalling.

Breast Cancer Cell Invasion into a Three Dimensional Tumor-Stroma Microenvironment

In this study, to model 3D chemotactic tumor-stroma invasion in vitro, we developed an innovative microfluidic chip allowing side-by-side positioning of 3D hydrogel-based matrices. We were able to (1) create a dual matrix architecture that extended in a continuous manner, thus allowing invasion from one 3D matrix to another, and (2) establish distinct regions of tumor and stroma cell/ECM compositions, with a clearly demarcated tumor invasion front, thus allowing us to quantitatively analyze progression of cancer cells into the stroma at a tissue or single-cell level. We showed significantly enhanced cancer cell invasion in response to a transient gradient of epidermal growth factor (EGF). 3D tracking at the single-cell level displayed increased migration speed and persistence. Subsequently, we analyzed changes in expression of EGF receptors, cell aspect ratio, and protrusive activity. These findings show the unique ability of our model to quantitatively analyze 3D chemotactic invasion, both globally by tracking the progression of the invasion front, and at the single-cell level by examining changes in cellular behavior and morphology using high-resolution imaging. Taken together, we have shown a novel model recapitulating 3D tumor-stroma interactions for studies of real-time cell invasion and morphological changes within a single platform.

Plasma Membrane Organization of Epidermal Growth Factor Receptor in Resting and Ligand-Bound States

The spatial arrangement of the epidermal growth factor receptor (EGFR) on the cellular plasma membrane is one of the prime factors that control its downstream signaling pathways and related functions. However, the molecular organization, which spans the scale from nanometers to micrometer-size clusters, has not been resolved in detail, mainly due to a lack of techniques with the required spatiotemporal resolution. Therefore, we used imaging total internal reflection-fluorescence correlation spectroscopy to investigate EGFR dynamics on live CHO-K1 plasma membranes in resting and ligand-bound states. In combination with the fluorescence correlation spectroscopy diffusion law, this provides information on the subresolution organization of EGFR on cell membranes. We found that overall EGFR organization is sensitive to both cholesterol and the actin cytoskeleton. EGFR in the resting state is partly trapped in cholesterol-containing domains, whereas another fraction exhibits cholesterol independent trapping on the membrane. Disruption of the cytoskeleton leads to a broader range of EGFR diffusion coefficients and a reduction of hop diffusion. In the ligand-bound state we found a dose-dependent behavior. At 10 ng/mL EGF the EGFR is endocytosed and recycled to the membrane, whereas diffusion and organization do not change significantly. At 100 ng/mL EGF the EGFR forms clusters, which are subsequently internalized, whereas outside the clusters diffusivity increases and the organization of the receptor remains unchanged. After disruption of cholesterol-containing domains or actin cytoskeleton, EGF induces microscopic EGFR clusters on the membrane and endocytosis is inhibited.

Reversible cryo-arrest for imaging molecules in living cells at high spatial resolution

The dynamics of molecules in living cells hampers precise imaging of molecular patterns by functional and super-resolution microscopy. We developed a method that circumvents lethal chemical fixation and allows on-stage cryo-arrest for consecutive imaging of molecular patterns within the same living, but arrested, cells. The reversibility of consecutive cryo-arrests was demonstrated by the high survival rate of different cell lines and by intact growth factor signaling that was not perturbed by stress response. Reversible cryo-arrest was applied to study the evolution of ligand-induced receptor tyrosine kinase activation at different scales. The nanoscale clustering of epidermal growth factor receptor (EGFR) in the plasma membrane was assessed by single-molecule localization microscopy, and endosomal microscale activity patterns of ephrin receptor A2 (EphA2) were assessed by fluorescence lifetime imaging microscopy. Reversible cryo-arrest allows the precise determination of molecular patterns while conserving the dynamic capabilities of living cells.

Time-resolved multimodal analysis of Src Homology 2 (SH2) domain binding in signaling by receptor tyrosine kinases

While the affinities and specificities of SH2 domain-phosphotyrosine interactions have been well characterized, spatio-temporal changes in phosphosite availability in response to signals, and their impact on recruitment of SH2-containing proteins in vivo, are not well understood. To address this issue, we used three complementary experimental approaches to monitor phosphorylation and SH2 binding in human A431 cells stimulated with epidermal growth factor (EGF): 1) phospho-specific mass spectrometry; 2) far-Western blotting; and 3) live cell single-molecule imaging of SH2 membrane recruitment. Far-Western and MS analyses identified both well-established and previously undocumented EGF-dependent tyrosine phosphorylation and binding events, as well as dynamic changes in binding patterns over time. In comparing SH2 binding site phosphorylation with SH2 domain membrane recruitment in living cells, we found in vivo binding to be much slower. Delayed SH2 domain recruitment correlated with clustering of SH2 domain binding sites on the membrane, consistent with membrane retention via SH2 rebinding.

DOI:http://dx.doi.org/10.7554/eLife.11835.001

Individual cells in a multicellular organism must receive signals from the environment and from other cells, and adjust their behavior accordingly. Such signals may cause a cell to grow and multiply, move, or even die. Often these signals are received by receptor proteins, which span the cell membrane and thus provide a way for signals from outside the cell to cause changes inside the cell.

The tyrosine kinases are one such group of membrane receptors. When a signal binds to a tyrosine kinase, the receptor is activated and it can add chemical tags called phosphates to the part of itself, or a neighboring protein, that is inside the cell. These phosphates provide binding sites for other types of proteins, many of which contain a section called a SH2 domain. This transmits the signal and leads to further changes in the cell. However, there are over a hundred different SH2 domain-containing proteins in human cells and we do not have a clear picture of what exactly happens when receptor tyrosine kinases are activated.

Jadwin, Oh et al. have now looked at how the number of SH2 domain binding sites changes over time after a signal is received. The experiments used three different experimental approaches to study a tyrosine kinase called the Epidermal Growth Factor (EGF) receptor, which is often over-active in human cancers. Jadwin, Oh et al. found that the timing of the changes in the number of SH2 domain binding sites on EGF varied widely. The different methods provided different perspectives on exactly when the changes happen, for example, directly observing the binding of SH2 domains to the membrane of living cells under the microscope showed that binding was much slower than expected from other methods that used purified proteins in solutions. This might be due to the receptors taking a relatively long time to form clusters at the membrane after they receive a signal.

Further experiments suggested that what happens when EGF is activated may depend not only on the number of SH2 domain binding sites made, but also the timing and the physical arrangement of those sites. A long-term goal for further studies is to understand how various types of signals can lead to different outcomes in the cell.

DOI:http://dx.doi.org/10.7554/eLife.11835.002

Immobilized epidermal growth factor stimulates persistent, directed keratinocyte migration via activation of PLCγ1

Epidermal growth factor (EGF) is a critical element in dermal repair, but EGF-containing wound dressings have not been successful clinically. However, these dressings have delivered only soluble EGF, and the native environment provides both soluble and matrix-bound EGF. To address our hypothesis that tethered EGF can stimulate cell behaviors not achievable with soluble EGF, we examined single-cell movement and signaling in human immortalized HaCaT keratinocytes treated with soluble or immobilized EGF. Although both EGF treatments increased collective sheet displacement and individual cell speed, only cells treated with immobilized EGF exhibited directed migration, as well as 2-fold greater persistence compared with soluble EGF. Immunofluorescence showed altered EGF receptor (EGFR) trafficking, where EGFR remained membrane-localized in the immobilized EGF condition. Cells treated with soluble EGF demonstrated higher phosphorylated ERK1/2, and cells on immobilized EGF exhibited higher pPLCγ1, which was localized at the leading edge. Treatment with U0126 inhibited migration in both conditions, demonstrating that ERK1/2 activity was necessary but not responsible for the observed differences. In contrast, PLCγ1 inhibition with U73122 significantly decreased persistence on immobilized EGF. Combined, these results suggest that immobilized EGF increases collective keratinocyte displacement via an increase in single-cell migration persistence resulting from altered EGFR trafficking and PLCγ1 activation.-Kim, C. S., Mitchell, I. P., Desotell, A. W., Kreeger, P. K., Masters, K. S. Immobilized epidermal growth factor stimulates persistent, directed keratinocyte migration via activation of PLCγ1.

Evaluating the Raftophilicity of Rhodopsin Photoreceptor in a Patterned Model Membrane

Lipid rafts in the cell membrane are believed to affect various membrane functions, including the signaling by G-protein coupled receptors (GPCRs). However, the regulatory roles of lipid rafts on GPCRs' functions are still poorly understood, partially owing to the lack of the methods to quantitatively evaluate the affinity of membrane proteins to lipid raft (raftophilicity). Here, we describe a methodology to gauge the raftophilicity of a representative GPCR in vertebrate photoreceptor, i.e., rhodopsin (Rh), and its cognate G protein transducin (Gt) by using a patterned model membrane. We generated a substrate-supported planar lipid bilayer that has patterned regions of liquid-ordered (Lo) and liquid-disordered (Ld) membrane domains. We reconstituted Rh and Gt into the patterned membrane and observed their lateral distribution and diffusion. Mobile and functional Rh molecules could be reconstituted through the rapid dilution of solubilized Rh, by optimizing the reconstitution conditions including the chamber design, protein/detergent concentrations, and solution mixing. We determined the partition and diffusion coefficients of Rh and Gt in the Lo-rich and Ld-rich regions. Both Rh and Gt were predominantly localized in the Ld phase, suggesting their low affinity to lipid rafts. Patterned model membrane offers a robust and scalable platform for systematically and quantitatively studying the functional roles of lipid rafts in biological membranes including retinal disk membranes.

Single Particle Tracking Reveals that EGFR Signaling Activity Is Amplified in Clathrin-Coated Pits

Signaling from the epidermal growth factor receptor (EGFR) via phosphorylation on its C-terminal tyrosine residues requires self-association, which depends on the diffusional properties of the receptor and its density in the plasma membrane. Dimerization is a key event for EGFR activation, but the role of higher order clustering is unknown. We employed single particle tracking to relate the mobility and aggregation of EGFR to its signaling activity. EGFR mobility alternates between short-lived free, confined and immobile states. In the immobile state, EGFR tends to aggregate in clathrin-coated pits, which is further enhanced in a phosphorylation-dependent manner and does not require ligand binding. EGFR phosphorylation is further amplified by cross-phosphorylation in clathrin-coated pits. Because phosphorylated receptors can escape from the pits, local gradients of signaling active EGFR are formed. These results show that amplification of EGFR phosphorylation by receptor clustering in clathrin-coated pits supports signal activation at the plasma membrane.

Endosomal escape efficiency of fusogenic B18 and B55 peptides fused with anti-EGFR single chain Fv as estimated by nuclear translocation

Although monoclonal antibodies have been used not only as analytical tools but also as biologic therapeutics, they cannot target intracellular proteins due to their large molecular size and low membrane permeability, which limit their applications. During previous attempts to delivery antibodies intracellularly, the low efficiency of escape from endosomes to the cytosol reduced the bioavailability of antibodies or antibody-conjugated effectors. Recently, we found that the fusogenic peptides (FPs) B18 and B55 from bindin, a sea urchin gamete recognition protein, facilitated the endosomal escape of FP-fused enhanced green fluorescent protein (eGFP) and/or of co-administered cargos such as dextrans [Niikura et al. A fusogenic peptide from a sea urchin fertilization protein promotes intracellular delivery of biomacromolecules by facilitating endosomal escape. J.

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Release 2015;212:85-93]. In this study, we constructed FP-fused anti-epidermal growth factor receptor (EGFR) single-chain Fv (αEGFR[scFv]) proteins and evaluated their endosomal escape efficiency by utilizing a nuclear localization signal). When the FP-fused αEGFR[scFv] proteins were incubated with A431 cells, the estimated endosomal escape efficiency of αEGFR[scFv]-B18 was significantly higher than that of αEGFR[scFv] alone, suggesting that the B18 peptide facilitates endosomal escape of the conjugated scFv in cis. Moreover, αEGFR[scFv]-B55 promoted the intracellular uptake of co-administered eGFP and dextrans in trans. These results imply that B18- and B55-fused antibodies may be useful for the cell-specific intracellular delivery of biomacromolecules.

Epidermal growth factor-stimulated Akt phosphorylation requires clathrin or ErbB2 but not receptor endocytosis

Upon ligand binding, the epidermal growth factor receptor (EGFR) activates signaling and undergoes endocytosis. EGFR signaling leading to Akt activation is impaired by perturbation of clathrin but not by inhibition of internalization through perturbation of dynamin. Clathrin may thus directly regulate receptor signaling at the cell surface.

Epidermal growth factor (EGF) binding to its receptor (EGFR) activates several signaling intermediates, including Akt, leading to control of cell survival and metabolism. Concomitantly, ligand-bound EGFR is incorporated into clathrin-coated pits—membrane structures containing clathrin and other proteins—eventually leading to receptor internalization. Whether clathrin might regulate EGFR signaling at the plasma membrane before vesicle scission is poorly understood. We compared the effect of clathrin perturbation (preventing formation of, or receptor recruitment to, clathrin structures) to that of dynamin2 (allowing formation of clathrin structures but preventing EGFR internalization) under conditions in which EGFR endocytosis is clathrin dependent. Clathrin perturbation by siRNA gene silencing, with the clathrin inhibitor pitstop2, or knocksideways silencing inhibited EGF-simulated Gab1 and Akt phosphorylation in ARPE-19 cells. In contrast, perturbation of dynamin2 with inhibitors or by siRNA gene silencing did not affect EGF-stimulated Gab1 or Akt phosphorylation. EGF stimulation enriched Gab1 and phospho-Gab1 within clathrin structures. ARPE-19 cells have low ErbB2 expression, and overexpression and knockdown experiments revealed that robust ErbB2 expression bypassed the requirement for clathrin for EGF-stimulated Akt phosphorylation. Thus clathrin scaffolds may represent unique plasma membrane signaling microdomains required for signaling by certain receptors, a function that can be separated from vesicle formation.

Inhibiting EGFR clustering and cell proliferation with gold nanoparticles

Gold nanoparticles are functionalized with epidermal growth factor (EGF) molecules and incubated with HeLa cells. These new complexes mechanically interfere with the activation of EGF receptors in a length-dependent manner. Protein-functionalized gold nanoparticles hold great potential for unveiling the fundamental characteristics of cell receptors and for future pharmacological studies on receptor targeting.

Effect of phosphorylation on EGFR dimer stability probed by single-molecule dynamics and FRET/FLIM

Deregulation of epidermal growth factor receptor (EGFR) signaling has been correlated with the development of a variety of human carcinomas. EGF-induced receptor dimerization and consequent trans- auto-phosphorylation are among the earliest events in signal transduction. Binding of EGF is thought to induce a conformational change that consequently unfolds an ectodomain loop required for dimerization indirectly. It may also induce important allosteric changes in the cytoplasmic domain. Despite extensive knowledge on the physiological activation of EGFR, the effect of targeted therapies on receptor conformation is not known and this particular aspect of receptor function, which can potentially be influenced by drug treatment, may in part explain the heterogeneous clinical response among cancer patients. Here, we used Förster resonance energy transfer/fluorescence lifetime imaging microscopy (FRET/FLIM) combined with two-color single-molecule tracking to study the effect of ATP-competitive small molecule tyrosine kinase inhibitors (TKIs) and phosphatase-based manipulation of EGFR phosphorylation on live cells. The distribution of dimer on-times was fitted to a monoexponential to extract dimer off-rates (koff). Our data show that pretreatment with gefitinib (active conformation binder) stabilizes the EGFR ligand-bound homodimer. Overexpression of EGFR-specific DEP-1 phosphatase was also found to have a stabilizing effect on the homodimer. No significant difference in the koff of the dimer could be detected when an anti-EGFR antibody (425 Snap single-chain variable fragment) that allows for dimerization of ligand-bound receptors, but not phosphorylation, was used. These results suggest that both the conformation of the extracellular domain and phosphorylation status of the receptor are involved in modulating the stability of the dimer. The relative fractions of these two EGFR subpopulations (interacting versus free) were obtained by a fractional-intensity analysis of ensemble FRET/FLIM images. Our combined imaging approach showed that both the fraction and affinity (surrogate of conformation at a single-molecule level) increased after gefitinib pretreatment or DEP-1 phosphatase overexpression. Using an EGFR mutation (I706Q, V948R) that perturbs the ability of EGFR to dimerize intracellularly, we showed that a modest drug-induced increase in the fraction/stability of the EGFR homodimer may have a significant biological impact on the tumor cell's proliferation potential.

In situ single molecule imaging of cell membranes: linking basic nanotechniques to cell biology, immunology and medicine

The cell membrane, which consists of a viscous phospholipid bilayer, different kinds of proteins and various nano/micrometer-sized domains, plays a very important role in ensuring the stability of the intracellular environment and the order of cellular signal transductions. Exploring the precise cell membrane structure and detailed functions of the biomolecules in a cell membrane would be helpful to understand the underlying mechanisms involved in cell membrane signal transductions, which could further benefit research into cell biology, immunology and medicine. The detection of membrane biomolecules at the single molecule level can provide some subtle information about the molecular structure and the functions of the cell membrane. In particular, information obtained about the molecular mechanisms and other information at the single molecule level are significantly different from that detected from a large amount of biomolecules at the large-scale through traditional techniques, and can thus provide a novel perspective for the study of cell membrane structures and functions. However, the precise investigations of membrane biomolecules prompts researchers to explore cell membranes at the single molecule level by the use of in situ imaging methods, as the exact conformation and functions of biomolecules are highly controlled by the native cellular environment. Recently, the in situ single molecule imaging of cell membranes has attracted increasing attention from cell biologists and immunologists. The size of biomolecules and their clusters on the cell surface are set at the nanoscale, which makes it mandatory to use high- and super-resolution imaging techniques to realize the in situ single molecule imaging of cell membranes. In the past few decades, some amazing imaging techniques and instruments with super resolution have been widely developed for molecule imaging, which can also be further employed for the in situ single molecule imaging of cell membranes. In this review, we attempt to summarize the characteristics of these advanced techniques for use in the in situ single molecule imaging of cell membranes. We believe that this work will help to promote the technological and methodological developments of super-resolution techniques for the single molecule imaging of cell membranes and help researchers better understand which technique is most suitable for their future exploring of membrane biomolecules; ultimately promoting further developments in cell biology, immunology and medicine.

Ligand-induced homotypic and heterotypic clustering of apolipoprotein E receptor 2

ApoE Receptor 2 (ApoER2) and the very low density lipoprotein receptor (VLDLR) are type I transmembrane proteins belonging to the LDLR family of receptors. They are neuronal proteins found in synaptic compartments that play an important role in neuronal migration during development. ApoER2 and VLDLR bind to extracellular glycoproteins, such as Reelin and F-spondin, which leads to phosphorylation of adaptor proteins and subsequent activation of downstream signaling pathways. It is thought that ApoER2 and VLDLR undergo clustering upon binding to their ligands, but no direct evidence of clustering has been shown. Here we show strong clustering of ApoER2 induced by the dimeric ligands Fc-RAP, F-spondin, and Reelin but relatively weak clustering with the ligand apoE in the absence of lipoproteins. This clustering involves numerous proteins besides ApoER2, including amyloid precursor protein and the synaptic adaptor protein PSD-95. Interestingly, we did not observe strong clustering of ApoER2 with VLDLR. Clustering was modulated by both extracellular and intracellular domains of ApoER2. Together, our data demonstrate that several multivalent ligands for ApoER2 induce clustering in transfected cells and primary neurons and that these complexes included other synaptic molecules, such as APP and PSD-95.

Membrane microdomains: from seeing to understanding

The plasma membrane is a composite material, which forms a semi-permeable barrier and an interface for communication between the intracellular and extracellular environments. While the existence of membrane microdomains with nanoscale organization has been proved by the application of numerous biochemical and physical methods, direct observation of these heterogeneities using optical microscopy has remained challenging for decades, partly due to the optical diffraction limit, which restricts the resolution to ~200 nm. During the past years, new optical methods which circumvent this fundamental limit have emerged. Not only do these techniques allow direct visualization, but also quantitative characterization of nanoscopic structures. We discuss how these emerging optical methods have refined our knowledge of membrane microdomains and how they may shed light on the basic principles of the mesoscopic membrane organization.

Propranolol restricts the mobility of single EGF-receptors on the cell surface before their internalization

The epidermal growth factor receptor is involved in morphogenesis, proliferation and cell migration. Its up-regulation during tumorigenesis makes this receptor an interesting therapeutic target. In the absence of the ligand, the inhibition of phosphatidic acid phosphohydrolase activity by propranolol treatment leads to internalization of empty/inactive receptors. The molecular events involved in this endocytosis remain unknown. Here, we quantified the effects of propranolol on the mobility of single quantum-dot labelled receptors before the actual internalization took place. The single receptors showed a clear stop-and-go motion; their diffusive tracks were continuously interrupted by sub-second stalling events, presumably caused by transient clustering. In the presence of propranolol we found that: i) the diffusion rate reduced by 22 %, which indicates an increase in drag of the receptor. Atomic force microscopy measurements did not show an increase of the effective membrane tension, such that clustering of the receptor remains the likely mechanism for its reduced mobility. ii) The receptor got frequently stalled for longer periods of multiple seconds, which may signal the first step of the internalization process.

Manipulating the lateral diffusion of surface-anchored EGF demonstrates that receptor clustering modulates phosphorylation levels

Upon activation, the epidermal growth factor (EGF) receptor becomes phosphorylated and triggers a vast signaling network that has profound effects on cell growth. The EGF receptor is observed to assemble into clusters after ligand binding and tyrosine kinase autophosphorylation, but the role of these assemblies in the receptor signaling pathway remains unclear. To address this question, we measured the phosphorylation of EGFR when the EGF ligand was anchored onto laterally mobile and immobile surfaces. We found that cells generated clusters of ligand-receptor complex on mobile EGF surfaces, and displayed a lower ratio of phosphorylated EGFR to EGF when compared to immobilized EGF that is unable to cluster. This result was verified by tuning the lateral assembly of ligand-receptor complexes on the surface of living cells using patterned supported lipid bilayers. Nanoscale metal lines fabricated into the supported membrane constrained lipid diffusion and EGF receptor assembly into micron and sub-micron scale corrals. Single cell analysis indicated that clustering impacts EGF receptor activation, and larger clusters (>1 μm(2)) of ligand-receptor complex generated lower EGF receptor phosphorylation per ligand than smaller assemblies (<1 μm(2)) in HCC1143 cells that were engaged to ligand-functionalized surfaces. We investigated the mechanism of EGFR clustering by treating cells with compounds that disrupt the cytoskeleton (Latrunculin B), clathrin-mediated endocytosis (Pitstop2), and inhibit EGFR activation (Gefitinib). These results help elucidate the nature of large-scale EGFR clustering, thus underscoring the general significance of receptor spatial organization in tuning biochemical function.

Exploring higher-order EGFR oligomerisation and phosphorylation--a combined experimental and theoretical approach

The epidermal growth factor receptor (EGFR) kinase is generally considered to be activated by either ligand-induced dimerisation or a ligand-induced conformational change within pre-formed dimers. Ligand-induced higher-order EGFR oligomerisation or clustering has been reported but it is not clear how EGFR oligomers, as distinct from EGFR dimers, influence signaling outputs. To address this question, we combined measures of receptor clustering (microscopy; image correlation spectroscopy) and phosphorylation (Western blots) with modelling of mass-action chemical kinetics. A stable BaF/3 cell-line that contains a high proportion (>90%) of inactive dimers of EGFR-eGFP but no secreted ligand and no other detectable ErbB receptors was used as the model cell system. EGF at concentrations of greater than 1 nM was found to cluster EGFR-eGFP dimers into higher-order complexes and cause parallel increases in EGFR phosphorylation. The kinetics of EGFR clustering and phosphorylation were both rapid, plateauing within 2 minutes after stimulation with 30 nM EGF. A rule-based model was formulated to interpret the data. This model took into account ligand binding, ligand-induced conformational changes in the cytosolic tail, monomer-dimer-trimer-tetramer transitions via ectodomain- and kinase-mediated interactions, and phosphorylation. The model predicts that cyclic EGFR tetramers are the predominant phosphorylated species, in which activated receptor dimers adopt a cyclic side-by-side orientation, and that receptor kinase activation is stabilised by the intramolecular interactions responsible for cyclic tetramerization.

Measuring EGFR separations on cells with ~10 nm resolution via fluorophore localization imaging with photobleaching

Detecting receptor dimerisation and other forms of clustering on the cell surface depends on methods capable of determining protein-protein separations with high resolution in the ~10-50 nm range. However, this distance range poses a significant challenge because it is too large for fluorescence resonance energy transfer and contains distances too small for all other techniques capable of high-resolution in cells. Here we have adapted the technique of fluorophore localisation imaging with photobleaching to measure inter-receptor separations in the cellular environment. Using the epidermal growth factor receptor, a key cancer target molecule, we demonstrate ~10 nm resolution while continuously covering the range of ~10-80 nm. By labelling the receptor on cells expressing low receptor numbers with a fluorescent antagonist we have found inter-receptor separations all the way up from 8 nm to 59 nm. Our data are consistent with epidermal growth factor receptors being able to form homo-polymers of at least 10 receptors in the absence of activating ligands.

Ligand binding induces a conformational change in epidermal growth factor receptor dimers

The activation of the epidermal growth factor receptor (EGFR) kinase requires ligand binding to the extracellular domain (ECD). Previous reports demonstrate that the EGFR-ECD can be crystallized in two conformations - a tethered monomer or, in the presence of ligand, an untethered back-to-back dimer. We use Biosensor analysis to demonstrate that even in the monomeric state different C-terminal extensions of both truncated (EGFR(1-501))-ECD and full-length EGFR(1-621)-ECD can change the conformation of the ligand-binding site. The binding of a monoclonal antibody mAb806, which recognizes the dimer interface, to the truncated EGFR(1-501)-Fc fusion protein is reduced in the presence of ligand, consistent with a change in conformation. On the cell surface, the presence of erythroblastosis B2 (erbB2) increases the binding of mAb806 to the EGFR. The conformation of the erbB2: EGFR heterodimer interface changes when the cells are treated with epidermal growth factor (EGF). We propose that ligand induces kinase-inactive, pre-formed EGFR dimers and heterodimers to change conformation leading to kinase-active tetramers, where kinase activation occurs via an asymmetric interaction between EGFR dimers.

Characterization of the diffusion of epidermal growth factor receptor clusters by single particle tracking

A number of studies have shown that receptors of the epidermal growth factor receptor family (ErbBs) exist as higher-order oligomers (clusters) in cell membranes in addition to their monomeric and dimeric forms. Characterizing the lateral diffusion of such clusters may provide insights into their dynamics and help elucidate their functional relevance. To that end, we used single particle tracking to study the diffusion of clusters of the epidermal growth factor (EGF) receptor (EGFR; ErbB1) containing bound fluorescently-labeled ligand, EGF. EGFR clusters had a median diffusivity of 6.8×10(-11)cm(2)/s and were found to exhibit different modes of transport (immobile, simple, confined, and directed) similar to that previously reported for single EGFR molecules. Disruption of actin filaments increased the median diffusivity of EGFR clusters to 10.3×10(-11)cm(2)/s, while preserving the different modes of diffusion. Interestingly, disruption of microtubules rendered EGFR clusters nearly immobile. Our data suggests that microtubules may play an important role in the diffusion of EGFR clusters either directly or perhaps indirectly via other mechanisms. To our knowledge, this is the first report probing the effect of the cytoskeleton on the diffusion of EGFR clusters in the membranes of live cells.