EphA2 receptor activation by monomeric Ephrin-A1 on supported membranes

Targeting the EphA2 pathway: could it be the way for bone sarcomas?

Bone sarcomas are malignant tumors of mesenchymal origin. Complete surgical resection is the cornerstone of multidisciplinary treatment. However, advanced, unresectable forms remain incurable. A crucial step towards addressing this challenge involves comprehending the molecular mechanisms underpinning tumor progression and metastasis, laying the groundwork for innovative precision medicine-based interventions. We previously showed that tyrosine kinase receptor Ephrin Type-A Receptor 2 (EphA2) is overexpressed in bone sarcomas. EphA2 is a key oncofetal protein implicated in metastasis, self-renewal, and chemoresistance. Molecular, genetic, biochemical, and pharmacological approaches have been developed to target EphA2 and its signaling pathway aiming to interfere with its tumor-promoting effects or as a carrier for drug delivery. This review synthesizes the main functions of EphA2 and their relevance in bone sarcomas, providing strategies devised to leverage this receptor for diagnostic and therapeutic purposes, with a focus on its applicability in the three most common bone sarcoma histotypes: osteosarcoma, chondrosarcoma, and Ewing sarcoma.

Current Landscape of Antibody-Drug Conjugate Development in Head and Neck Cancer

Antibody-drug conjugates (ADCs) are fusions of therapeutic drugs and antibodies conjugated by a linker, designed to deliver a therapeutic payload to cells expressing the target antigen. By delivering the highly cytotoxic agent directly to cancer cells, ADCs are designed to enhance safety and broaden the therapeutic window. Recently, ADCs have demonstrated promising efficacy in various solid tumors and are rapidly expanding their indications. The prognosis of patients with advanced head and neck squamous cell carcinoma (HNSCC) remains poor, with no new therapeutics since the advent of anti-PD-1 antibodies in 2016, highlighting a critical need for innovative therapies. Recent preliminary results suggest that ADCs could be promising treatment options for HNSCC as they explore a variety of target antigens, payloads, and linkers. However, for successful adaptation of ADCs in the treatment of HNSCC, addressing key challenges such as payload toxicities, antigen heterogeneity, and adaptive resistance will be essential. Current research focused on new ADC structures, including multispecific antibodies and noncytotoxic payloads, and diverse combination approaches, show promise for future advancements.

Limits on the accuracy of contact inhibition of locomotion

Cells that collide with each other repolarize away from contact, in a process called contact inhibition of locomotion (CIL), which is necessary for correct development of the embryo. CIL can occur even when cells make a micron-scale contact with a neighbor-much smaller than their size. How precisely can a cell sense cell-cell contact and repolarize in the correct direction? What factors control whether a cell recognizes it has contacted a neighbor? We propose a theoretical model for the limits of CIL where cells recognize the presence of another cell by binding the protein ephrin with the Eph receptor. This recognition is made difficult by the presence of interfering ligands that bind nonspecifically. Both theoretical predictions and simulation results show that it becomes more difficult to sense cell-cell contact when it is difficult to distinguish ephrin from the interfering ligands, or when there are more interfering ligands, or when the contact width decreases. However, the error of estimating contact position remains almost constant when the contact width changes. This happens because the cell gains spatial information largely from the boundaries of cell-cell contact. We study using statistical decision theory the likelihood of a false-positive CIL event in the absence of cell-cell contact, and the likelihood of a false negative where CIL does not occur when another cell is present. Our results suggest that the cell is more likely to make incorrect decisions when the contact width is very small or so large that it nears the cell's perimeter. However, in general, we find that cells have the ability to make reasonably reliable CIL decisions even for very narrow (micron-scale) contacts, even if the concentration of interfering ligands is ten times that of the correct ligands.

Time-resolved live-cell spectroscopy reveals EphA2 multimeric assembly

Ephrin type-A receptor 2 (EphA2) is a receptor tyrosine kinase that initiates both ligand-dependent tumor-suppressive and ligand-independent oncogenic signaling. We used time-resolved, live-cell fluorescence spectroscopy to show that the ligand-free EphA2 assembles into multimers driven by two types of intermolecular interactions in the ectodomain. The first type entails extended symmetric interactions required for ligand-induced receptor clustering and tumor-suppressive signaling that inhibits activity of the oncogenic extracellular signal-regulated kinase (ERK) and protein kinase B (AKT) protein kinases and suppresses cell migration. The second type is an asymmetric interaction between the amino terminus and the membrane proximal domain of the neighboring receptors, which supports oncogenic signaling and promotes migration in vitro and tumor invasiveness in vivo. Our results identify the molecular interactions that drive the formation of the EphA2 multimeric signaling clusters and reveal the pivotal role of EphA2 assembly in dictating its opposing functions in oncogenesis.

Geometric Control of Cell Behavior by Biomolecule Nanodistribution

Many dynamic interactions within the cell microenvironment modulate cell behavior and cell fate. However, the pathways and mechanisms behind cell–cell or cell–extracellular matrix interactions remain understudied, as they occur at a nanoscale level. Recent progress in nanotechnology allows for mimicking of the microenvironment at nanoscale in vitro; electron-beam lithography (EBL) is currently the most promising technique. Although this nanopatterning technique can generate nanostructures of good quality and resolution, it has resulted, thus far, in the production of only simple shapes (e.g., rectangles) over a relatively small area (100 × 100 μm), leaving its potential in biological applications unfulfilled. Here, we used EBL for cell-interaction studies by coating cell-culture-relevant material with electron-conductive indium tin oxide, which formed nanopatterns of complex nanohexagonal structures over a large area (500 × 500 μm). We confirmed the potential of EBL for use in cell-interaction studies by analyzing specific cell responses toward differentially distributed nanohexagons spaced at 1000, 500, and 250 nm. We found that our optimized technique of EBL with HaloTags enabled the investigation of broad changes to a cell-culture-relevant surface and can provide an understanding of cellular signaling mechanisms at a single-molecule level.

The efficacy of receptor tyrosine kinase EphA2 autophosphorylation increases with EphA2 oligomer size

The receptor tyrosine kinase (RTK) EphA2 is expressed in epithelial and endothelial cells and controls the assembly of cell-cell junctions. EphA2 has also been implicated in many diseases, including cancer. Unlike most RTKs, which signal predominantly as dimers, EphA2 readily forms high-order oligomers upon ligand binding. Here, we investigated if a correlation exists between EphA2 signaling properties and the size of the EphA2 oligomers induced by multiple ligands, including the widely used ephrinA1-Fc ligand, the soluble monomeric m-ephrinA1, and novel engineered peptide ligands. We used fluorescence intensity fluctuation (FIF) spectrometry to characterize the EphA2 oligomer populations induced by the different ligands. Interestingly, we found that different monomeric and dimeric ligands induce EphA2 oligomers with widely different size distributions. Our comparison of FIF brightness distribution parameters and EphA2 signaling parameters reveals that the efficacy of EphA2 phosphorylation on tyrosine 588, an autophosphorylation response contributing to EphA2 activation, correlates with EphA2 mean oligomer size. However, we found that other characteristics, such as the efficacy of AKT inhibition and ligand bias coefficients, appear to be independent of EphA2 oligomer size. Taken together, this work highlights the utility of FIF in RTK signaling research and demonstrates a quantitative correlation between the architecture of EphA2 signaling complexes and signaling features.

Ligands with different dimeric configurations potently activate the EphA2 receptor and reveal its potential for biased signaling

The EphA2 receptor tyrosine kinase activates signaling pathways with different, and sometimes opposite, effects in cancer and other pathologies. Thus, highly specific and potent biased ligands that differentially control EphA2 signaling responses could be therapeutically valuable. Here, we use EphA2-specific monomeric peptides to engineer dimeric ligands with three different geometric configurations to combine a potential ability to differentially modulate EphA2 signaling responses with the high potency and prolonged receptor residence time characteristic of dimeric ligands. The different dimeric peptides readily induce EphA2 clustering, autophosphorylation and signaling, the best with sub-nanomolar potency. Yet, there are differences in two EphA2 signaling responses induced by peptides with different configurations, which exhibit distinct potency and efficacy. The peptides bias signaling when compared with the ephrinA1-Fc ligand and do so via different mechanisms. These findings provide insights into Eph receptor signaling, and proof-of-principle that different Eph signaling responses can be distinctly modulated.

Phase separation in immune signalling

Immune signalling pathways convert pathogenic stimuli into cytosolic events that lead to the resolution of infection. Upon ligand engagement, immune receptors together with their downstream adaptors and effectors undergo substantial conformational changes and spatial reorganization. During this process, nanometre-to-micrometre-sized signalling clusters have been commonly observed that are believed to be hotspots for signal transduction. Because of their large size and heterogeneous composition, it remains a challenge to fully understand the mechanisms by which these signalling clusters form and their functional consequences. Recently, phase separation has emerged as a new biophysical principle for organizing biomolecules into large clusters with fluidic properties. Although the field is still in its infancy, studies of phase separation in immunology are expected to provide new perspectives for understanding immune responses. Here, we present an up-to-date view of how liquid-liquid phase separation drives the formation of signalling condensates and regulates immune signalling pathways, including those downstream of T cell receptor, B cell receptor and the innate immune receptors cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) and retinoic acid-inducible gene I protein (RIG-I). We conclude with a summary of the current challenges the field is facing and outstanding questions for future studies.

Yeast β-glucan increases etoposide sensitivity in lung cancer cell line A549 by suppressing Nrf2 via the non-canonical NF-κB pathway

Etoposide is regarded as one of the main standard cytotoxic drugs for lung cancer. However, mutations in Keap1, the main regulator of nuclear factor erythroid 2-related factor 2 (Nrf2), are often detected in lung cancer and lead to chemoresistance. Since the aberrant activation of Nrf2 enhances drug resistance, the suppression of the Nrf2 pathway is a promising therapeutic strategy for lung cancer. We herein used the human lung adenocarcinoma cell line A549 because it harbors a Keap1 loss-of-function mutation. A treatment with β-glucan, a major component of the fungal cell wall, reduced Nrf2 protein levels, down-regulated the expression of CYP3A5, UGT1A1, and MDR1, and increased etoposide sensitivity in A549 cells. Furthermore, the ephrin type-A receptor 2 (EphA2) receptor was important for the recognition and biological activity of β-glucan in A549 cells. EphA2 signaling includes nuclear factor kappa B (NF-κB), STAT3, and p38 mitogen-activated protein kinase (MAPK). However, treatment of cells with stattic (STAT3 inhibitor) or SB203580 (p38 MAPK inhibitor) did not diminish the effects of β-glucan. In contrast, knockdown of RelB abolished the effects of β-glucan, suggesting the involvement of the non-canonical NF-κB pathway. The β-glucan effects were also attenuated by the knockdown of WDR23. The β-glucan treatment and RelB overexpression induced the expression of CUL4A, which increased WDR23 ligase activity and promoted the subsequent depletion of Nrf2. These results revealed a novel property of β-glucan as a resistance-modifying agent in addition to its widely reported immunomodulatory effects for lung cancer therapy via the EphA2-RelB-CUL4A-Nrf2 axis. Significance Statement Chemotherapeutic resistance remains a major obstacle in cancer therapy despite extensive efforts to elucidate the underlying molecular mechanisms and overcome multidrug resistance. The present study revealed a novel resistance-modifying property of β-glucan, thereby expanding our knowledge on the beneficial roles of β-glucan and providing an alternative strategy to prevent drug resistance by cancer. The present results provide evidence for the involvement of a novel mode of NF-κB and Nrf2 crosstalk in the drug resistance phenotype.

Probing the effect of clustering on EphA2 receptor signaling efficiency by subcellular control of ligand-receptor mobility

Clustering of ligand:receptor complexes on the cell membrane is widely presumed to have functional consequences for subsequent signal transduction. However, it is experimentally challenging to selectively manipulate receptor clustering without altering other biochemical aspects of the cellular system. Here, we develop a microfabrication strategy to produce substrates displaying mobile and immobile ligands that are separated by roughly 1 µm, and thus experience an identical cytoplasmic signaling state, enabling precision comparison of downstream signaling reactions. Applying this approach to characterize the ephrinA1:EphA2 signaling system reveals that EphA2 clustering enhances both receptor phosphorylation and downstream signaling activity. Single-molecule imaging clearly resolves increased molecular binding dwell times at EphA2 clusters for both Grb2:SOS and NCK:N-WASP signaling modules. This type of intracellular comparison enables a substantially higher degree of quantitative analysis than is possible when comparisons must be made between different cells and essentially eliminates the effects of cellular response to ligand manipulation.

Targeting EphA2 in cancer

Eph receptors and the corresponding Eph receptor-interacting (ephrin) ligands jointly constitute a critical cell signaling network that has multiple functions. The tyrosine kinase EphA2, which belongs to the family of Eph receptors, is highly produced in tumor tissues, while found at relatively low levels in most normal adult tissues, indicating its potential application in cancer treatment. After 30 years of investigation, a large amount of data regarding EphA2 functions have been compiled. Meanwhile, several compounds targeting EphA2 have been evaluated and tested in clinical studies, albeit with limited clinical success. The present review briefly describes the contribution of EphA2-ephrin A1 signaling axis to carcinogenesis. In addition, the roles of EphA2 in resistance to molecular-targeted agents were examined. In particular, we focused on EphA2's potential as a target for cancer treatment to provide insights into the application of EphA2 targeting in anticancer strategies. Overall, EphA2 represents a potential target for treating malignant tumors.

Single-cell analysis of EphA clustering phenotypes to probe cancer cell heterogeneity

The Eph family of receptor tyrosine kinases is crucial for assembly and maintenance of healthy tissues. Dysfunction in Eph signaling is causally associated with cancer progression. In breast cancer cells, dysregulated Eph signaling has been linked to alterations in receptor clustering abilities. Here, we implemented a single-cell assay and a scoring scheme to systematically probe the spatial organization of activated EphA receptors in multiple carcinoma cells. We show that cancer cells retain EphA clustering phenotype over several generations, and the degree of clustering reported for migration potential both at population and single-cell levels. Finally, using patient-derived cancer lines, we probed the evolution of EphA signalling in cell populations that underwent metastatic transformation and acquisition of drug resistance. Taken together, our scalable approach provides a reliable scoring scheme for EphA clustering that is consistent over multiple carcinomas and can assay heterogeneity of cancer cell populations in a cost- and time-effective manner.

Ravasio et al. develop an assay to quantify the clustering of Eph receptor EphA2 upon ligand binding, based on the scoring of single cells. They discover that clustering phenotype predicts cell and population migration potential in cancer cells and reflects Eph associated gene expression profiles in cancer cell lines.

Approaches to Manipulate Ephrin-A:EphA Forward Signaling Pathway

Erythropoietin-producing hepatocellular carcinoma A (EphA) receptors and their ephrin-A ligands are key players of developmental events shaping the mature organism. Their expression is mostly restricted to stem cell niches in adults but is reactivated in pathological conditions including lesions in the heart, lung, or nervous system. They are also often misregulated in tumors. A wide range of molecular tools enabling the manipulation of the ephrin-A:EphA system are available, ranging from small molecules to peptides and genetically-encoded strategies. Their mechanism is either direct, targeting EphA receptors, or indirect through the modification of intracellular downstream pathways. Approaches enabling manipulation of ephrin-A:EphA forward signaling for the dissection of its signaling cascade, the investigation of its physiological roles or the development of therapeutic strategies are summarized here.

Helicobacter Pylori Targets the EPHA2 Receptor Tyrosine Kinase in Gastric Cells Modulating Key Cellular Functions

Helicobacter pylori, a stomach-colonizing Gram-negative bacterium, is the main etiological factor of various gastroduodenal diseases, including gastric adenocarcinoma. By establishing a life-long infection of the gastric mucosa, H. pylori continuously activates host-signaling pathways, in particular those associated with receptor tyrosine kinases. Using two different gastric epithelial cell lines, we show that H. pylori targets the receptor tyrosine kinase EPHA2. For long periods of time post-infection, H. pylori induces EPHA2 protein downregulation without affecting its mRNA levels, an effect preceded by receptor activation via phosphorylation. EPHA2 receptor downregulation occurs via the lysosomal degradation pathway and is independent of the H.pylori virulence factors CagA, VacA, and T4SS. Using small interfering RNA, we show that EPHA2 knockdown affects cell–cell and cell–matrix adhesion, invasion, and angiogenesis, which are critical cellular processes in early gastric lesions and carcinogenesis mediated by the bacteria. This work contributes to the unraveling of the underlying mechanisms of H. pylori–host interactions and associated diseases. Additionally, it raises awareness for potential interference between H. pylori infection and the efficacy of gastric cancer therapies targeting receptors tyrosine kinases, given that infection affects the steady-state levels and dynamics of some receptor tyrosine kinases (RTKs) and their signaling pathways.

Hypoxia-Induced Cleavage Of Soluble ephrinA1 From Cancer Cells Is Mediated By MMP-2 And Associates With Angiogenesis In Oral Squamous Cell Carcinoma

INTRODUCTION

ephrinA1 plays important roles in tumor angiogenesis. Matrix metalloproteases (MMPs) can cleave ephrinA1 from the cell membrane into extracellular environment. However, how soluble ephrinA1 is modulated by hypoxia and whether MMPs participate in this hypoxic process remains to be investigated in detail.

METHODS

Thirty-seven patients with oral squamous cell carcinoma (OSCC) were included in the present study for HIF-1α, MMP-2, MMP-9 and ephrinA1 detection by immunohistochemistry. Serum samples from 35 patients were collected both preoperatively and postoperatively to confirm the existence of soluble ephrinA1 by ELISA. Block assay and Western blot analysis were further carried out to elucidate the proteolysis mechanism of ephrinA1 under hypoxic condition in vitro.

RESULTS

Our data demonstrated that HIF-1α, MMP-2, MMP-9 and ephrinA1 expressed positively, and correlated with microvessel density in OSCCs, except for MMP-9. The serum expression level of ephrinA1 in OSCC patients decreased significantly after surgical removal of the solid tumors. In vitro experiments indicated that GM6001, a MMP-specific inhibitor, could reduce hypoxia-induced soluble ephrinA1 secretion from SCC cells. Further Western blot analysis confirmed that both HIF-1α and MMP-2 were up-regulated by hypoxia in a similar time-dependent manner, with the MMP-9 expression unchanged during this course.

CONCLUSION

These results suggested a possible novel mechanism that ephrinA1 secretion is mediated by HIF-1α/MMP-2 signaling cascade which may play pivotal roles in OSCC neovascularization in a paracrine manner.

Discoidin domain receptors: Micro insights into macro assemblies

Assembly of cell-surface receptors into specific oligomeric states and/or clusters before and after ligand binding is an important feature governing their biological function. Receptor oligomerization can be mediated by specific domains of the receptor, ligand binding, configurational changes or other interacting molecules. In this review we summarize our understanding of the oligomeric state of discoidin domain receptors (DDR1 and DDR2), which belong to the receptor tyrosine kinase family (RTK). DDRs form an interesting system from an oligomerization perspective as their ligand collagen(s) can also undergo supramolecular assembly to form fibrils. Even though DDR1 and DDR2 differ in the domains responsible to form ligand-free dimers they share similarities in binding to soluble, monomeric collagen. However, only DDR1b forms globular clusters in response to monomeric collagen and not DDR2. Interestingly, both DDR1 and DDR2 are assembled into linear clusters by the collagen fibril. Formation of these clusters is important for receptor phosphorylation and is mediated in part by other membrane components. We summarize how the oligomeric status of DDRs shares similarities with other members of the RTK family and with collagen receptors. Unraveling the multiple macro-molecular configurations adopted by this receptor-ligand pair can provide novel insights into the intricacies of cell-matrix interactions.

Hybrid Live Cell–Supported Membrane Interfaces for Signaling Studies

A wide range of cell–microenvironmental interactions are mediated by membrane-localized receptors that bind ligands present on another cell or the extracellular matrix. This situation introduces a number of physical effects including spatial organization of receptor–ligand complexes and development of mechanical forces in cells. Unlike traditional experimental approaches, hybrid live cell–supported lipid bilayer (SLB) systems, wherein a live cell interacts with a synthetic substrate supported membrane, allow interrogation of these aspects of receptor signaling. The SLB system directly offers facile control over the identity, density, and mobility of ligands used for engaging cellular receptors. Further, application of various nano- and micropatterning techniques allows for spatial patterning of ligands. In this review, we describe the hybrid live cell–SLB system and its application in uncovering a range of spatial and mechanical aspects of receptor signaling. We highlight the T cell immunological synapse, junctions formed between EphA2- and ephrinA1-expressing cells, and adhesions formed by cadherin and integrin receptors.

Pinpointed Stimulation of EphA2 Receptors via DNA-Templated Oligovalence

DNA nanostructures enable the attachment of functional molecules to nearly any unique location on their underlying structure. Due to their single-base-pair structural resolution, several ligands can be spatially arranged and closely controlled according to the geometry of their desired target, resulting in optimized binding and/or signaling interactions. Here, the efficacy of SWL, an ephrin-mimicking peptide that binds specifically to EphrinA2 (EphA2) receptors, increased by presenting up to three of these peptides on small DNA nanostructures in an oligovalent manner. Ephrin signaling pathways play crucial roles in tumor development and progression. Moreover, Eph receptors are potential targets in cancer diagnosis and treatment. Here, the quantitative impact of SWL valency on binding, phosphorylation (key player for activation) and phenotype regulation in EphA2-expressing prostate cancer cells was demonstrated. EphA2 phosphorylation was significantly increased by DNA trimers carrying three SWL peptides compared to monovalent SWL. In comparison to one of EphA2’s natural ligands ephrin-A1, which is known to bind promiscuously to multiple receptors, pinpointed targeting of EphA2 by oligovalent DNA-SWL constructs showed enhanced cell retraction. Overall, we show that DNA scaffolds can increase the potency of weak signaling peptides through oligovalent presentation and serve as potential tools for examination of complex signaling pathways.

Spatiomechanical Modulation of EphB4-Ephrin-B2 Signaling in Neural Stem Cell Differentiation

Interactions between EphB4 receptor tyrosine kinases and their membrane-bound ephrin-B2 ligands on apposed cells play a regulatory role in neural stem cell differentiation. With both receptor and ligand constrained to move within the membranes of their respective cells, this signaling system inevitably experiences spatial confinement and mechanical forces in conjunction with receptor-ligand binding. In this study, we reconstitute the EphB4-ephrin-B2 juxtacrine signaling geometry using a supported-lipid-bilayer system presenting laterally mobile and monomeric ephrin-B2 ligands to live neural stem cells. This experimental platform successfully reconstitutes EphB4-ephrin-B2 binding, lateral clustering, downstream signaling activation, and neuronal differentiation, all in a configuration that preserves the spatiomechanical aspects of the natural juxtacrine signaling geometry. Additionally, the supported bilayer system allows control of lateral movement and clustering of the receptor-ligand complexes through patterns of physical barriers to lateral diffusion fabricated onto the underlying substrate. The results from this study reveal a distinct spatiomechanical effect on the ability of EphB4-ephrin-B2 signaling to induce neuronal differentiation. These observations parallel similar studies of the EphA2-ephrin-A1 system in a very different biological context, suggesting that such spatiomechanical regulation may be a common feature of Eph-ephrin signaling.

The EphA2 receptor is activated through induction of distinct, ligand-dependent oligomeric structures

The EphA2 receptor tyrosine kinase is capable of activating multiple diverse signaling pathways with roles in processes such as tissue homeostasis and cancer. EphA2 is known to form activated oligomers in the presence of ephrin-A ligands. Here, we characterize the lateral interactions between full-length EphA2 molecules in the plasma membrane in the presence of three types of ligands (dimeric ephrinA1-Fc, monomeric ephrinA1, and an engineered peptide ligand) as well as in the absence of ligand, using a quantitative FRET technique. The data show that EphA2 forms higher-order oligomers and two different types of dimers that all lead to increased EphA2 tyrosine phosphorylation, which is indicative of increased kinase-dependent signaling. We find that different ligands stabilize conformationally distinct oligomers that are assembled through two different interfaces. Our results suggest that these different oligomeric assemblies could have distinct signaling properties, contributing to the diverse activities of the EphA2 receptor.

Deo Singh et al. use Fully Quantified Spectral Imaging-FRET to show that the EphA2 receptor forms dimers or higher order oligomers depending on the type of ligand, and that different ligands stabilize EphA2 dimers through distinct interfaces. These findings may explain how EphA2 activates diverse signaling pathways.

Nanopatterns of Surface-Bound EphrinB1 Produce Multivalent Ligand-Receptor Interactions That Tune EphB2 Receptor Clustering

Here we present a nanostructured surface able to produce multivalent interactions between surface-bound ephrinB1 ligands and membrane EphB2 receptors. We created ephrinB1 nanopatterns of regular size (<30 nm in diameter) by using self-assembled diblock copolymers. Next, we used a statistically enhanced version of the Number and Brightness technique, which can discriminate-with molecular sensitivity-the oligomeric states of diffusive species to quantitatively track the EphB2 receptor oligomerization process in real time. The results indicate that a stimulation using randomly distributed surface-bound ligands was not sufficient to fully induce receptor aggregation. Conversely, when nanopatterned onto our substrates, the ligands effectively induced a strong receptor oligomerization. This presentation of ligands improved the clustering efficiency of conventional ligand delivery systems, as it required a 9-fold lower ligand surface coverage and included faster receptor clustering kinetics compared to traditional cross-linked ligands. In conclusion, nanostructured diblock copolymers constitute a novel strategy to induce multivalent ligand-receptor interactions leading to a stronger, faster, and more efficient receptor activation, thus providing a useful strategy to precisely tune and potentiate receptor responses. The efficiency of these materials at inducing cell responses can benefit applications such as the design of new bioactive materials and drug-delivery systems.

N-Cadherin modified lipid bilayers promote neural network formation and circuitry

Neural adhesion, maturation, and the correct wiring of the brain to establish each neuron's intended connectivity are controlled by complex interactions of bioactive molecules such as ligands, growth factors, or enzymes. The correct pairing of adjacent neurons is thought to be highly regulated by ligand-mediated cell-cell adhesion proteins, which are known to induce signaling activities. We developed a new platform consisting of supported lipid bilayers incorporated with Fc-chimera synaptic proteins like ephrinA5 or N-cadherin. We extensively characterized their function employing a quartz crystal microbalance with dissipation (QCM-D), calcium imaging, and immunofluorescence analysis. Our biomimetic platform has been shown to promote neural cell adhesion and to improve neural maturation at day in vitro 7 (DIV7) as indicated by an elevated expression of synaptophysin.

A role of the SAM domain in EphA2 receptor activation

Among the 20 subfamilies of protein receptor tyrosine kinases (RTKs), Eph receptors are unique in possessing a sterile alpha motif (SAM domain) at their C-terminal ends. However, the functions of SAM domains in Eph receptors remain elusive. Here we report on a combined cell biology and quantitative fluorescence study to investigate the role of the SAM domain in EphA2 function. We observed elevated tyrosine autophosphorylation levels upon deletion of the EphA2 SAM domain (EphA2ΔS) in DU145 and PC3 prostate cancer cells and a skin tumor cell line derived from EphA1/A2 knockout mice. These results suggest that SAM domain deletion induced constitutive activation of EphA2 kinase activity. In order to explain these effects, we applied fluorescence correlation spectroscopy to investigate the lateral molecular organization of EphA2. Our results indicate that SAM domain deletion (EphA2ΔS-GFP) increases oligomerization compared to the full length receptor (EphA2FL-GFP). Stimulation with ephrinA1, a ligand for EphA2, induced further oligomerization and activation of EphA2FL-GFP. The SAM domain deletion mutant, EphA2ΔS-GFP, also underwent further oligomerization upon ephrinA1 stimulation, but the oligomers were larger than those observed for EphA2FL-GFP. Based on these results, we conclude that the EphA2 SAM domain inhibits kinase activity by reducing receptor oligomerization.

Membrane Compartmentalization Reducing the Mobility of Lipids and Proteins within a Model Plasma Membrane

The cytoskeleton underlying cell membranes may influence the dynamic organization of proteins and lipids within the bilayer by immobilizing certain transmembrane (TM) proteins and forming corrals within the membrane. Here, we present coarse-grained resolution simulations of a biologically realistic membrane model of asymmetrically organized lipids and TM proteins. We determine the effects of a model of cytoskeletal immobilization of selected membrane proteins using long time scale coarse-grained molecular dynamics simulations. By introducing compartments with varying degrees of restraints within the membrane models, we are able to reveal how compartmentalization caused by cytoskeletal immobilization leads to reduced and anomalous diffusional mobility of both proteins and lipids. This in turn results in a reduced rate of protein dimerization within the membrane and of hopping of membrane proteins between compartments. These simulations provide a molecular realization of hierarchical models often invoked to explain single-molecule imaging studies of membrane proteins.

A Microbead Supported Membrane-Based Fluorescence Imaging Assay Reveals Intermembrane Receptor-Ligand Complex Dimension with Nanometer Precision

Receptor-ligand complexes spanning a cell-cell interface inevitably establish a preferred intermembrane spacing based on the molecular dimensions and orientation of the complexes. This couples molecular binding events to membrane mechanics and large-scale spatial organization of receptors on the cell surface. Here, we describe a straightforward, epi-fluorescence-based method to precisely determine intermembrane receptor-ligand dimension at adhesions established by receptor-ligand binding between apposed membranes in vitro. Adhesions were reconstituted between planar and silica microbead supported membranes via specific interaction between cognate receptor/ligand pairs (EphA2/EphrinA1 and E-cadherin/anti-E-cadherin antibody). Epi-fluorescence imaging of the ligand enrichment zone in the supported membrane beneath the adhering microbead, combined with a simple geometrical interpretation, proves sufficient to estimate intermembrane receptor-ligand dimension with better than 1 nm precision. An advantage of this assay is that no specialized equipment or imaging methods are required.

Reconstitution of Fusion Proteins in Supported Lipid Bilayers for the Study of Cell Surface Receptor-Ligand Interactions in Cell-Cell Contact

Bioactive molecules such as adhesion ligands, growth factors, or enzymes play an important role in modulating cell behavior such as cell adhesion, spreading, and differentiation. Deciphering the mechanism of ligand-mediated cell adhesion and associated signaling is of great interest not only for fundamental biophysical investigations but also for applications in medicine and biotechnology. In the presented work, we developed a new biomimetic platform that enables culturing primary neurons and testing cell surface-receptor ligand interactions in cell-cell contacts as, e.g., in neuronal synapses. This platform consists of a supported lipid bilayer modified with incorporated neuronal adhesion proteins conjugated with the Fc-domain of IgG (ephrin A5 Fc-chimera). We extensively characterized properties of these protein containing bilayers using fluorescence recovery after photobleaching (FRAP), quartz crystal microbalance with dissipation (QCM-D), and immunostaining. We conclude that the Fc-domain is the part responsible for the incorporation of the protein into the bilayer. The biomimetic platform prepared by this new approach was able to promote neuronal cell adhesion and maintain growth as well as facilitate neuronal maturation as shown by electrophysiological measurements. We believe that our approach can be extended to insert other proteins to create a general culture platform for neurons and other cell types.

Supported Lipid Bilayers for the Generation of Dynamic Cell-Material Interfaces

Supported lipid bilayers (SLB) offer unique possibilities for studying cellular membranes and have been used as a synthetic architecture to interact with cells. Here, the state-of-the-art in SLB-based technology is presented. The fabrication, analysis, characteristics and modification of SLBs are described in great detail. Numerous strategies to form SLBs on different substrates, and the means to patteren them, are described. The use of SLBs as model membranes for the study of membrane organization and membrane processes in vitro is highlighted. In addition, the use of SLBs as a substratum for cell analysis is presented, with discrimination between cell-cell and cell-extracellular matrix (ECM) mimicry. The study is concluded with a discussion of the potential for in vivo applications of SLBs.

Spatiotemporally Controlled Reorganization of Signaling Complexes in the Plasma Membrane of Living Cells

Triggered immobilization of proteins in the plasma membrane of living cells into functional micropatterns is established by using an adaptor protein, which is comprised of an antiGFP nanobody fused to the HaloTag protein. Efficient in situ reorganization of the type I interferon receptor subunits as well as intact, fully functional signaling complexes in living cells are achieved by this method.

E-cadherin junction formation involves an active kinetic nucleation process

Epithelial (E)-cadherin-mediated cell-cell junctions play important roles in the development and maintenance of tissue structure in multicellular organisms. E-cadherin adhesion is thus a key element of the cellular microenvironment that provides both mechanical and biochemical signaling inputs. Here, we report in vitro reconstitution of junction-like structures between native E-cadherin in living cells and the extracellular domain of E-cadherin (E-cad-ECD) in a supported membrane. Junction formation in this hybrid live cell-supported membrane configuration requires both active processes within the living cell and a supported membrane with low E-cad-ECD mobility. The hybrid junctions recruit α-catenin and exhibit remodeled cortical actin. Observations suggest that the initial stages of junction formation in this hybrid system depend on the trans but not the cis interactions between E-cadherin molecules, and proceed via a nucleation process in which protrusion and retraction of filopodia play a key role.

Proteolysis of EphA2 Converts It from a Tumor Suppressor to an Oncoprotein

Eph receptor tyrosine kinases are considered candidate therapeutic targets in cancer, but they can exert opposing effects on cell growth. In the presence of its ligands, Eph receptor EphA2 suppresses signaling by other growth factor receptors, including ErbB, whereas ligand-independent activation of EphA2 augments ErbB signaling. To deploy EphA2-targeting drugs effectively in tumors, the anti-oncogenic ligand-dependent activation state of EphA2 must be discriminated from its oncogenic ligand-independent state. Because the molecular basis for the latter is little understood, we investigated how the activation state of EphA2 can be switched in tumor tissue. We found that ligand-binding domain of EphA2 is cleaved frequently by the membrane metalloproteinase MT1-MMP, a powerful modulator of the pericellular environment in tumor cells. EphA2 immunostaining revealed a significant loss of the N-terminal portion of EphA2 in areas of tumor tissue that expressed MT1-MMP. Moreover, EphA2 phosphorylation patterns that signify ligand-independent activation were observed specifically in these areas of tumor tissue. Mechanistic experiments revealed that processing of EphA2 by MT1-MMP promoted ErbB signaling, anchorage-independent growth, and cell migration. Conversely, expression of a proteolysis-resistant mutant of EphA2 prevented tumorigenesis and metastasis of human tumor xenografts in mice. Overall, our results showed how the proteolytic state of EphA2 in tumors determines its effector function and influences its status as a candidate biomarker for targeted therapy.

Attractive and repulsive factors act through multi-subunit receptor complexes to regulate nerve fiber growth

In the nervous system, attractive and repulsive factors guide neuronal growth, pathfinding and target innervation during development, learning and regeneration after injury. Repulsive and growth-inhibitory factors, such as some ephrins, semaphorins, netrins and myelin-associated growth inhibitors, restrict nerve fiber growth, whereas neurotrophins, and other ephrins, semaphorins and netrins attract fibers and promote neurite growth. Several of these guidance molecules also play crucial roles in vasculogenesis, and regulate cell migration and tissue formation in different organs. Precise and highly specific signal transduction in space and time is required in all these cases, which primarily depends on the presence and function of specific receptors. Interestingly, many of these ligands act through multi-subunit receptor complexes. In this Commentary, we review the current knowledge of how complexes of the receptors for attractive and repulsive neurite growth regulatory factors are reorganized in a spatial and temporal manner, and reveal the implications that such dynamics have on the signaling events that coordinate neurite fiber growth.

Importin-β modulates the permeability of the nuclear pore complex in a Ran-dependent manner

Soluble karyopherins of the importin-β (impβ) family use RanGTP to transport cargos directionally through the nuclear pore complex (NPC). Whether impβ or RanGTP regulate the permeability of the NPC itself has been unknown. In this study, we identify a stable pool of impβ at the NPC. A subpopulation of this pool is rapidly turned-over by RanGTP, likely at Nup153. Impβ, but not transportin-1 (TRN1), alters the pore's permeability in a Ran-dependent manner, suggesting that impβ is a functional component of the NPC. Upon reduction of Nup153 levels, inert cargos more readily equilibrate across the NPC yet active transport is impaired. When purified impβ or TRN1 are mixed with Nup153 in vitro, higher-order, multivalent complexes form. RanGTP dissolves the impβ•Nup153 complexes but not those of TRN1•Nup153. We propose that impβ and Nup153 interact at the NPC's nuclear face to form a Ran-regulated mesh that modulates NPC permeability.

DOI:http://dx.doi.org/10.7554/eLife.04052.001

In our cells, genetic material is contained within the nucleus, which is separated from the rest of the cell by a double-layered membrane called the nuclear envelope. Within this membrane there are pores that allow proteins and other molecules to enter and exit the nucleus.

Small molecules can pass through these pores unaided, which is known as ‘passive’ transport. However, larger cargos need help from transport receptor proteins in a process called ‘active’ transport. Large cargos bind to transport receptors, such as importin-β, in the cytoplasm and are then guided through the pore. Once the cargo and importin-β are inside the nucleus, a protein called RanGTP binds to importin-β to release the cargo.

It is thought that importin-β and RanGTP are only important for the active transport of cargo. Here, Lowe et al. studied how importin-β interacts with the pore. The experiments show that in the absence of RanGTP, importin-β accumulates inside the pore and binds to a protein called Nup153, which is part of the complex of proteins that makes up the pore. However, when RanGTP is present, some of the importin-β is displaced from Nup153 and leaves the pore, which makes it easier for cargo to pass through.

Further experiments show that when Nup153 and importin-β are mixed, they associate into a gel-like material that can be ‘melted’ by RanGTP. Lowe et al. propose a model for how RanGTP may control the flow of cargo through the nuclear pore by affecting the binding of importin-β to Nup153. Lowe et al.'s findings suggest that passive and active transport of cargo across the nuclear pore are fundamentally connected and suggest that RanGTP provides the cell with an additional layer of control over nucleocytoplasmic transport.

DOI:http://dx.doi.org/10.7554/eLife.04052.002

Live cell micropatterning reveals the dynamics of signaling complexes at the plasma membrane

The use of micropatterned surfaces that bind HaloTag fusion proteins allows spatial organization of plasma membrane proteins for efficient visualization and quantification of protein–protein interactions in live cells.

Interactions of proteins in the plasma membrane are notoriously challenging to study under physiological conditions. We report in this paper a generic approach for spatial organization of plasma membrane proteins into micropatterns as a tool for visualizing and quantifying interactions with extracellular, intracellular, and transmembrane proteins in live cells. Based on a protein-repellent poly(ethylene glycol) polymer brush, micropatterned surface functionalization with the HaloTag ligand for capturing HaloTag fusion proteins and RGD peptides promoting cell adhesion was devised. Efficient micropatterning of the type I interferon (IFN) receptor subunit IFNAR2 fused to the HaloTag was achieved, and highly specific IFN binding to the receptor was detected. The dynamics of this interaction could be quantified on the single molecule level, and IFN-induced receptor dimerization in micropatterns could be monitored. Assembly of active signaling complexes was confirmed by immunostaining of phosphorylated Janus family kinases, and the interaction dynamics of cytosolic effector proteins recruited to the receptor complex were unambiguously quantified by fluorescence recovery after photobleaching.

Spatial organization of EphA2 at the cell-cell interface modulates trans-endocytosis of ephrinA1

EphA2 is a receptor tyrosine kinase (RTK) that is sensitive to spatial and mechanical aspects of the cell's microenvironment. Misregulation of EphA2 occurs in many aggressive cancers. Although its juxtacrine signaling geometry (EphA2's cognate ligand ephrinA1 is expressed on the surface of an apposing cell) provides a mechanism by which the receptor may experience extracellular forces, this also renders the system challenging to decode. By depositing living cells on synthetic supported lipid membranes displaying ephrinA1, we have reconstituted key features of the juxtacrine EphA2-ephrinA1 signaling system while maintaining the ability to perturb the spatial and mechanical properties of the membrane-cell interface with precision. In addition, we developed a trans-endocytosis assay to monitor internalization of ephrinA1 from a supported membrane into the apposing cell using a quantitative three-dimensional fluorescence microscopy assay. Using this experimental platform to mimic a cell-cell junction, we found that the signaling complex is not efficiently internalized when lateral reorganization at the membrane-cell contact sites is physically hindered. This suggests that EphA2-ephrinA1 trans-endocytosis is sensitive to the mechanical properties of a cell's microenvironment and may have implications in physical aspects of tumor biology.

Size-based chromatography of signaling clusters in a living cell membrane

Here we introduce a form of chromatography that can be imposed on the membrane of a living cell. A cell-cell signaling interaction is reconstituted in a hybrid live cell-supported membrane junction. The chromatographic material consists of a hexagonally ordered array of gold nanoparticles (nanodot array), which is fabricated onto the underlying substrate. While individual membrane components move freely throughout the array, the movement of larger assemblies is impeded if they exceed the physical dimensions of the array. This tactile approach to probing membrane structures in living cells reveals organizational aspects of the membrane environment unobservable by other techniques.

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.

EphA2 and Src regulate equatorial cell morphogenesis during lens development

High refractive index and transparency of the eye lens require uniformly shaped and precisely aligned lens fiber cells. During lens development, equatorial epithelial cells undergo cell-to-cell alignment to form meridional rows of hexagonal cells. The mechanism that controls this morphogenesis from randomly packed cuboidal epithelial cells to highly organized hexagonal fiber cells remains unknown. In Epha2(-/-) mouse lenses, equatorial epithelial cells fail to form precisely aligned meridional rows; moreover, the lens fulcrum, where the apical tips of elongating epithelial cells constrict to form an anchor point before fiber cell differentiation and elongation at the equator, is disrupted. Phosphorylated Src-Y424 and cortactin-Y466, actin and EphA2 cluster at the vertices of wild-type hexagonal epithelial cells in organized meridional rows. However, phosphorylated Src and phosphorylated cortactin are not detected in disorganized Epha2(-/-) cells with altered F-actin distribution. E-cadherin junctions, which are normally located at the basal-lateral ends of equatorial epithelial cells and are diminished in newly differentiating fiber cells, become widely distributed in the apical, lateral and basal sides of epithelial cells and persist in differentiating fiber cells in Epha2(-/-) lenses. Src(-/-) equatorial epithelial cells also fail to form precisely aligned meridional rows and lens fulcrum. These results indicate that EphA2/Src signaling is essential for the formation of the lens fulcrum. EphA2 also regulates Src/cortactin/F-actin complexes at the vertices of hexagonal equatorial cells for cell-to-cell alignment. This mechanistic information explains how EphA2 mutations lead to disorganized lens cells that subsequently contribute to altered refractive index and cataracts in humans and mice.

Nanoscale obstacle arrays frustrate transport of EphA2-Ephrin-A1 clusters in cancer cell lines

Juxtacrine signaling interactions between the EphA2 receptor tyrosine kinase and its ephrin-A1 ligand contribute to healthy tissue maintenance and misregulation of this system is observed in at least 40% of human breast cancer. Hybrid live cell-supported membrane experiments in which membrane-linked ephrin-A1 displayed in supported membranes interacts with EphA2 in living cells have revealed large scale clustering of EphA2/ephrin-A1 complexes as well as their lateral transport across the cell surface during triggering. Here, we utilize 100 nm spaced hexagonally ordered arrays of gold nanodots embedded within supported membranes to present defined obstacles to the movement and assembly of EphA2 clusters. By functionalizing both the supported membrane and the nanodots with ephrin-A1, we perform a type of affinity chromatography on EphA2 signaling clusters in live cell membranes. Analysis of 10 different breast cancer cell lines reveals that EphA2 transport is most frustrated by nanodot arrays in the most diseased cell lines. These observations suggest that strong physical association among EphA2 receptors, as well as their assembly into larger clusters, correlates with and may contribute to the pathological misregulation of the EphA2/ephrin-A1 pathway in breast cancer.

Direct single molecule measurement of TCR triggering by agonist pMHC in living primary T cells

T cells discriminate between self and foreign antigenic peptides, displayed on antigen presenting cell surfaces, via the TCR. While the molecular interactions between TCR and its ligands are well characterized in vitro, quantitative measurements of these interactions in living cells are required to accurately resolve the physical mechanisms of TCR signaling. We report direct single molecule measurements of TCR triggering by agonist pMHC in hybrid junctions between live primary T cells and supported lipid membranes. Every pMHC:TCR complex over the entire cell is tracked while simultaneously monitoring the local membrane recruitment of ZAP70, as a readout of TCR triggering. Mean dwell times for pMHC:TCR molecular binding of 5 and 54 s were measured for two different pMHC:TCR systems. Single molecule measurements of the pMHC:TCR:ZAP70 complex indicate that TCR triggering is stoichiometric with agonist pMHC in a 1:1 ratio. Thus any signal amplification must occur downstream of TCR triggering.

DOI:http://dx.doi.org/10.7554/eLife.00778.001

The immune system identifies and combats foreign objects, including pathogens, in the body. T cells are key components of the immune system, and each has a unique variant of a signalling complex known as the T cell receptor on its surface. T cells scan the surfaces of other cells in search of antigens, which are peptides (fragments of proteins) that derive from foreign pathogens such as viruses. Successful recognition of a foreign peptide leads to an immune response that, in most cases, ultimately rids the body of the pathogen. Most importantly, however, the immune system must be able to discriminate between peptides that are produced naturally in the body (‘self’ peptides) and foreign or ‘non-self’ peptides. This is challenging because self peptides may have similar structures to non-self peptides and are often much more abundant.

Many models have been proposed to explain how T cells are able to detect just a few molecules of foreign peptide. According to some hypotheses the T cell receptors get together in clusters to function cooperatively; alternatively, it has been suggested that rapid binding of a foreign peptide to multiple T cell receptors sequentially can build up a strong signal. However, none of these phenomena have been directly observed.

O'Donoghue et al. now image the interactions between T cell receptors and peptides bound to molecules called major histocompatibility complexes (MHCs), and show that T cell activation can occur when a single foreign peptide binds to a single receptor. These interactions are long-lived and ultimately result in the recruitment of ZAP70, which has an important role in the activation of T cells, to the complex formed by the T cell, the peptide and the MHC molecule. Therefore, any amplification of the activating signal transmitted by non-self peptides occurs following receptor binding, in contrast to previous models.

DOI:http://dx.doi.org/10.7554/eLife.00778.002

DNA-mediated assembly of protein heterodimers on membrane surfaces

We present a method based on self-assembling oligonucleotides to anchor proteins to a supported membrane surface. This anchoring method allows control of the surface density of multiple proteins. By incorporating additional recognition sequences into the DNA linkers, defined heterodimers can be produced upon the addition of a heterospecific DNA cross-linking strand. Characterization by fluorescence cross-correlation spectroscopy (FCCS) confirmed lateral mobility and the formation of specific heterodimers. We further demonstrate that proteins linked in this manner as either monomers or dimers can form functional interfaces with living cells.

Receptor signaling clusters in the immune synapse

Signaling processes between various immune cells involve large-scale spatial reorganization of receptors and signaling molecules within the cell-cell junction. These structures, now collectively referred to as immune synapses, interleave physical and mechanical processes with the cascades of chemical reactions that constitute signal transduction systems. Molecular level clustering, spatial exclusion, and long-range directed transport are all emerging as key regulatory mechanisms. The study of these processes is drawing researchers from physical sciences to join the effort and represents a rapidly growing branch of biophysical chemistry. Recent advances in physical and quantitative analyses of signaling within the immune synapses are reviewed here.