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

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.

Co-clustering of EphB6 and ephrinB1 in trans restrains cancer cell invasion

EphB6 is an understudied ephrin receptor tyrosine pseudokinase that is downregulated in multiple types of metastatic cancers. Unlike its kinase-active counterparts which autophosphorylate and transmit signals upon intercellular interaction, little is known about how EphB6 functions in the absence of intrinsic kinase activity. Here, we unveil a molecular mechanism of cell-cell interaction driven by EphB6. We identify ephrinB1 as a cognate ligand of EphB6 and show that in trans interaction of EphB6 with ephrinB1 on neighboring cells leads to the formation of large co-clusters at the plasma membrane. These co-clusters exhibit a decreased propensity towards endocytosis, suggesting a unique characteristic for this type of cell-cell interaction. Using lattice light-sheet microscopy, 3D structured illumination microscopy and cryo-electron tomography techniques, we show that co-clustering of EphB6 and ephrinB1 promotes the formation of double-membrane tubular structures between cells. Importantly, we also demonstrate that these intercellular structures stabilize cell-cell adhesion, leading to a reduction in the invasive behavior of cancer cells. Our findings rationalize a role for EphB6 pseudokinase as a tumor suppressor when interacting with its ligands in trans.

Ligand bias underlies differential signaling of multiple FGFs via FGFR1

The differential signaling of multiple FGF ligands through a single fibroblast growth factor (FGF) receptor (FGFR) plays an important role in embryonic development. Here, we use quantitative biophysical tools to uncover the mechanism behind differences in FGFR1c signaling in response to FGF4, FGF8, and FGF9, a process which is relevant for limb bud outgrowth. We find that FGF8 preferentially induces FRS2 phosphorylation and extracellular matrix loss, while FGF4 and FGF9 preferentially induce FGFR1c phosphorylation and cell growth arrest. Thus, we demonstrate that FGF8 is a biased FGFR1c ligand, as compared to FGF4 and FGF9. Förster resonance energy transfer experiments reveal a correlation between biased signaling and the conformation of the FGFR1c transmembrane domain dimer. Our findings expand the mechanistic understanding of FGF signaling during development and bring the poorly understood concept of receptor tyrosine kinase ligand bias into the spotlight.

Dioxinodehydroeckol: A Potential Neuroprotective Marine Compound Identified by In Silico Screening for the Treatment and Management of Multiple Brain Disorders

Neurodegenerative disorders such as Alzheimer's disease (AD), Glioblastoma multiforme (GBM), Amyotrophic lateral sclerosis (ALS), and Parkinson's disease (PD) are some of the most prevalent neurodegenerative disorders in humans. Even after a variety of advanced therapies, prognosis of all these disorders is not favorable, with survival rates of 14-20 months only. To further improve the prognosis of these disorders, it is imperative to discover new compounds which will target effector proteins involved in these disorders. In this study, we have focused on in silico screening of marine compounds against multiple target proteins involved in AD, GBM, ALS, and PD. Fifty marine-origin compounds were selected from literature, out of which, thirty compounds passed ADMET parameters. Ligand docking was performed after ADMET analysis for AD, GBM, ALS, and PD-associated proteins in which four protein targets Keap1, Ephrin A2, JAK3 Kinase domain, and METTL3-METTL14 N6-methyladenosine methyltransferase (MTA70) were found to be binding strongly with the screened compound Dioxinodehydroeckol (DHE). Molecular dynamics simulations were performed at 100 ns with triplicate runs to validate the docking score and assess the dynamics of DHE interactions with each target protein. The results indicated Dioxinodehydroeckol, a novel marine compound, to be a putative inhibitor among all the screened molecules, which might be effective against multiple target proteins involved in neurological disorders, requiring further in vitro and in vivo validations.

Targeting Interleukin-13 Receptor α2 and EphA2 in Aggressive Breast Cancer Subtypes with Special References to Chimeric Antigen Receptor T-Cell Therapy

Breast cancer (BCA) remains the leading cause of cancer-related mortality among women worldwide. This review delves into the therapeutic challenges of BCA, emphasizing the roles of interleukin-13 receptor α2 (IL-13Rα2) and erythropoietin-producing hepatocellular receptor A2 (EphA2) in tumor progression and resistance. Highlighting their overexpression in BCA, particularly in aggressive subtypes, such as Her-2-enriched and triple-negative breast cancer (TNBC), we discuss the potential of these receptors as targets for chimeric antigen receptor T-cell (CAR-T) therapies. We examine the structural and functional roles of IL-13Rα2 and EphA2, their pathological significance in BCA, and the promising therapeutic avenues their targeting presents. With an in-depth analysis of current immunotherapeutic strategies, including the limitations of existing treatments and the potential of dual antigen-targeting CAR T-cell therapies, this review aims to summarize potential future novel, more effective therapeutic interventions for BCA. Through a thorough examination of preclinical and clinical studies, it underlines the urgent need for targeted therapies in combating the high mortality rates associated with Her-2-enriched and TNBC subtypes and discusses the potential role of IL-13Rα2 and EphA2 as promising candidates for the development of CAR T-cell therapies.

Claudin-1 interacts with EPHA2 to promote cancer stemness and chemoresistance in colorectal cancer

Therapy resistance is the primary problem in treating late-stage colorectal cancer (CRC). Claudins are frequently dysregulated in cancer, and several are being investigated as novel therapeutic targets and biomarkers. We have previously demonstrated that Claudin-1 (CLDN1) expression in CRC promotes epithelial-mesenchymal transition, metastasis, and resistance to anoikis. Here, we hypothesize that CLDN1 promotes cancer stemness and chemoresistance in CRC. We found that high CLDN1 expression in CRC is associated with cancer stemness and chemoresistance signaling pathways in patient datasets, and it promotes chemoresistance both in vitro and in vivo. Using functional stemness assays, proteomics, biophysical binding assays, and patient-derived organoids, we found that CLDN1 promotes properties of cancer stemness including CD44 expression, tumor-initiating potential, and chemoresistance through a direct interaction with ephrin type-A receptor 2 (EPHA2) tyrosine kinase. This interaction is dependent on the CLDN1 PDZ-binding motif, increases EPHA2 protein expression by inhibiting its degradation, and enhances downstream AKT signaling and CD44 expression to promote stemness and chemoresistance. These results suggest CLDN1 is a viable target for pharmacological intervention and/or biomarker development.

Phosphotyrosine proteomics in cells synchronized at monopolar cytokinesis reveals EphA2 as functioning in cytokinesis

Cytokinesis is the final step of the cell division in which cellular components are separated into two daughter cells. This process is regulated through the phosphorylation of different classes of proteins by serine/threonine (Ser/Thr) kinases such as Aurora B and Polo-like kinase 1 (PLK1). Conversely, the role of phosphorylation at tyrosine residues during cytokinesis has not been studied in detail yet. In this study, we performed a phosphotyrosine proteomic analysis of cells undergoing monopolar cytokinesis synchronized by using the Eg5 inhibitor (+)-S-trityl-l-cysteine (STLC) and the CDK1 inhibitor RO-3306. Phosphotyrosine proteomics gave 362 tyrosine-phosphorylated peptides. Western blot analysis of proteins revealed tyrosine phosphorylation in mitogen-activated protein kinase 14 (MAPK14), vimentin, ephrin type-A receptor 2 (EphA2), and myelin protein zero-like protein 1 (MPZL1) during monopolar cytokinesis. Additionally, we demonstrated that EphA2, a protein with unknown function during cytokinesis, is involved in cytokinesis. EphA2 knockdown accelerated epithelial cell transforming 2 (Ect2) knockdown-induced multinucleation, suggesting that EphA2 plays a role in cytokinesis in a particular situation. The list also included many proteins previously reported to play roles during cytokinesis. These results evidence that the identified phosphopeptides facilitate the identification of novel tyrosine phosphorylation signaling involved in regulating cytokinesis.

Downregulation of EphA2 stability by RNF5 limits its tumor-suppressive function in HER2-negative breast cancers

Ephrin receptor A2 (EphA2) plays dual functions in tumorigenesis through ligand-independent tumor promotion or ligand-dependent tumor suppression. However, the regulation of EphA2 tumor-suppressive function remains unclear. Here, we showed that RNF5 interacts with EphA2 and induces its ubiquitination and degradation, decreases the stability and cell surface distribution of EphA2 and alters the balance of its phosphorylation at S897 and Y772. In turn, RNF5 inhibition decreases ERK phosphorylation and increases p53 expression through an increase in the EphA2 level in HER2-negative breast cancer cells. Consequently, RNF5 inhibition increases the adhesion and decreases the migration of HER2-negative breast cancer cells, and RNF5 silencing suppresses the growth of xenograft tumors derived from ER-positive, HER2-negative breast cancer cells with increased EphA2 expression and altered phosphorylation. RNF5 expression is inversely correlated with EphA2 expression in breast cancers, and a high EphA2 level accompanied by a low RNF5 level is related to better survival in patients with ER-positive, HER2-negative breast cancers. These studies revealed that RNF5 negatively regulates EphA2 properties and suppresses its tumor-suppressive function in HER2-negative breast cancers.

Dectin-1 multimerization and signaling depends on fungal β-glucan structure and exposure

Dectin-1A is a C-type lectin innate immunoreceptor that recognizes β-(1,3;1,6)-glucan, a structural component of Candida species cell walls. β-Glucans can adopt solution structures ranging from random coil to insoluble fiber due to tertiary (helical) and quaternary structure. Fungal β-glucans of medium and high molecular weight are highly structured, but low molecular weight glucan is much less structured. Despite similar affinity for Dectin-1, the ability of glucans to induce Dectin-1A-mediated signaling correlates with degree of structure. Glucan denaturation experiments showed that glucan structure determines agonistic potential, but not receptor binding affinity. We explored the impact of glucan structure on molecular aggregation of Dectin-1A. Stimulation with glucan signaling decreased Dectin-1A diffusion coefficient. Fluorescence measurements provided direct evidence of ligation-induced Dectin-1A aggregation, which positively correlated with increasing glucan structure content. In contrast, Dectin-1A is predominantly in a low aggregation state in resting cells. Molecular aggregates formed during interaction with highly structured, agonistic glucans did not exceed relatively small (<15 nm) clusters of a few engaged receptors. Finally, we observed increased molecular aggregation of Dectin-1A at fungal particle contact sites in a manner that positively correlated with the degree of exposed glucan on the particle surface. These results indicate that Dectin-1A senses the solution conformation of β-glucans through their varying ability to drive receptor dimer/oligomer formation and activation of membrane proximal signaling events.

Combination of protein and cell internalization SELEX identifies a potential RNA therapeutic and delivery platform to treat EphA2-expressing tumors

The EphA2 receptor tyrosine kinase is overexpressed in most solid tumors and acts as the major driver of tumorigenesis. In this study, we developed a novel approach for targeting the EphA2 receptor using a 2'-fluoro-modified pyrimidine RNA aptamer termed ATOP. We identified the ATOP EphA2 aptamer using a novel bioinformatics strategy that compared aptamers enriched during a protein SELEX using recombinant human EphA2 and a cell-internalization SELEX using EphA2-expressing MDA231 tumor cells. When applied to EphA2-expressing tumor cell lines, the ATOP EphA2 aptamer attenuated tumor cell migration and clonogenicity. In a mouse model of spontaneous metastasis, the ATOP EphA2 aptamer slowed primary tumor growth and significantly reduced the number of lung metastases. The EphA2 ATOP aptamer represents a promising candidate for the development of next-generation targeted therapies that provide safer and more effective treatment of EphA2-overexpressing tumors.

Targefrin: A Potent Agent Targeting the Ligand Binding Domain of EphA2

Overexpression of the receptor tyrosine kinase EphA2 is invariably associated with poor prognosis and development of aggressive metastatic cancers. Guided by our recently solved X-ray structure of the complex between an agonistic peptide and EphA2-LBD, we report on a novel agent, targefrin, that binds to EphA2-LBD with a 21 nM dissociation constant by isothermal titration calorimetry and presents an IC50 value of 10.8 nM in a biochemical assay. In cell-based assays, a dimeric version of the agent is as effective as the natural dimeric ligands (ephrinA1-Fc) in inducing cellular receptor internalization and degradation in several pancreatic cancer cell lines. When conjugated with chemotherapy, the agents can effectively deliver paclitaxel to pancreatic cancers in a mouse xenograft study. Given the pivotal role of EphA2 in tumor progression, we are confident that the agents reported could be further developed into innovative EphA2-targeting therapeutics.

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.

Systematic Analysis of Genetic and Pathway Determinants of Eribulin Sensitivity across 100 Human Cancer Cell Lines from the Cancer Cell Line Encyclopedia (CCLE)

Eribulin, a natural product-based microtubule targeting agent with cytotoxic and noncytotoxic mechanisms, is FDA approved for certain patients with advanced breast cancer and liposarcoma. To investigate the feasibility of developing drug-specific predictive biomarkers, we quantified antiproliferative activities of eribulin versus paclitaxel and vinorelbine against 100 human cancer cell lines from the Cancer Cell Line Encyclopedia, and correlated results with publicly available databases to identify genes and pathways associated with eribulin response, either uniquely or shared with paclitaxel or vinorelbine. Mean expression ratios of 11,985 genes between the most and least sensitive cell line quartiles were sorted by p-values and drug overlaps, yielding 52, 29 and 80 genes uniquely associated with eribulin, paclitaxel and vinorelbine, respectively. Further restriction to minimum 2-fold ratios followed by reintroducing data from the middle two quartiles identified 9 and 13 drug-specific unique fingerprint genes for eribulin and vinorelbine, respectively; surprisingly, no gene met all criteria for paclitaxel. Interactome and Reactome pathway analyses showed that unique fingerprint genes of both drugs were primarily associated with cellular signaling, not microtubule-related pathways, although considerable differences existed in individual pathways identified. Finally, four-gene (C5ORF38, DAAM1, IRX2, CD70) and five-gene (EPHA2, NGEF, SEPTIN10, TRIP10, VSIG10) multivariate regression models for eribulin and vinorelbine showed high statistical correlation with drug-specific responses across the 100 cell lines and accurately calculated predicted mean IC50s for the most and least sensitive cell line quartiles as surrogates for responders and nonresponders, respectively. Collectively, these results provide a foundation for developing drug-specific predictive biomarkers for eribulin and vinorelbine.

Direct quantification of ligand-induced lipid and protein microdomains with distinctive signaling properties

Lipid rafts are ordered lipid domains that are enriched in saturated lipids, such as the ganglioside GM1. While lipid rafts are believed to exist in cells and to serve as signaling platforms through their enrichment in signaling components, they have not been directly observed in the plasma membrane without treatments that artificially cluster GM1 into large lattices. Here, we report that microscopic GM1-enriched domains can form, in the plasma membrane of live mammalian cells expressing the EphA2 receptor tyrosine kinase in response to its ligand ephrinA1-Fc. The GM1-enriched microdomains form concomitantly with EphA2-enriched microdomains. To gain insight into how plasma membrane heterogeneity controls signaling, we quantify the degree of EphA2 segregation and study initial EphA2 signaling steps in both EphA2-enriched and EphA2-depleted domains. By measuring dissociation constants, we demonstrate that the propensity of EphA2 to oligomerize is similar in EphA2-enriched and -depleted domains. However, surprisingly, EphA2 interacts preferentially with its downstream effector SRC in EphA2-depleted domains. The ability to induce microscopic GM1-enriched domains in live cells using a ligand for a transmembrane receptor will give us unprecedented opportunities to study the biophysical chemistry of lipid rafts.

Near-Infrared-Emissive AIE Bioconjugates: Recent Advances and Perspectives

Near-infrared (NIR) fluorescence materials have exhibited formidable power in the field of biomedicine, benefiting from their merits of low autofluorescence background, reduced photon scattering, and deeper penetration depth. Fluorophores possessing planar conformation may confront the shortcomings of aggregation-caused quenching effects at the aggregate level. Fortunately, the concept of aggregation-induced emission (AIE) thoroughly reverses this dilemma. AIE bioconjugates referring to the combination of luminogens showing an AIE nature with biomolecules possessing specific functionalities are generated via the covalent conjugation between AIEgens and functional biological species, covering carbohydrates, peptides, proteins, DNA, and so on. This perfect integration breeds unique superiorities containing high brightness, good water solubility, versatile functionalities, and prominent biosafety. In this review, we summarize the recent progresses of NIR-emissive AIE bioconjugates focusing on their design principles and biomedical applications. Furthermore, a brief prospect of the challenges and opportunities of AIE bioconjugates for a wide range of biomedical applications is presented.

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.

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.

EphA2 Expression in Bone Sarcomas: Bioinformatic Analyses and Preclinical Characterization in Patient-Derived Models of Osteosarcoma, Ewing's Sarcoma and Chondrosarcoma

Bone sarcomas are a group of heterogeneous malignant mesenchymal tumors. Complete surgical resection is still the cornerstone of treatment, but, in the advanced/unresectable setting, their management remains challenging and not significantly improved by target- and immuno-therapies. We focused on the tyrosine kinase Eph type-A receptor-2 (EphA2), a key oncoprotein implicated in self-renewal, angiogenesis, and metastasis, in several solid tumors and thus representing a novel potential therapeutic target. Aiming at better characterizing its expression throughout the main bone sarcoma histotypes, we investigated EPHA2 expression in the Cancer Cell Lines Encyclopedia and in public datasets with clinical annotations. looking for correlations with molecular, histopathological and patients’ features and clinical outcomes in a total of 232 osteosarcomas, 197 Ewing’s sarcomas, and 102 chondrosarcomas. We observed EPHA2 expression in bone sarcoma cell lines. We demonstrated higher EPHA2 expression in tumor tissues when compared to normal counterparts. A significant correlation was found between EPHA2 expression and Huvos grade (osteosarcoma) and with worse overall survival (dedifferentiated chondrosarcoma). Next, we characterized EPHA2 expression and activation in bone sarcoma primary tissues and in patient-derived xenografts generated in our laboratory to verify their reliability as in vivo models of osteosarcoma, Ewing’s sarcoma and chondrosarcoma. Furthermore, for the first time, we demonstrated EPHA2 expression in chondrosarcoma, suggesting its potential key role in this histotype. Indeed, we observed a significant dose-dependent antitumor effect of the EphA2-inhibitor ALW-II-41-27 in patient-derived in vitro models. In conclusion, EphA2 targeting represents a promising novel therapeutic strategy against bone sarcomas.

Quantitative characterization of tetraspanin 8 homointeractions in the plasma membrane

The spatial distribution of proteins in cell membranes is crucial for signal transduction, cell communication and membrane trafficking. Members of the Tetraspanin family organize functional protein clusters within the plasma membrane into so-called Tetraspanin-enriched microdomains (TEMs). Direct interactions between Tetraspanins are believed to be important for this organization. However, studies thus far have utilized mainly co-immunoprecipitation methods that cannot distinguish between direct and indirect, through common partners, interactions. Here we study Tetraspanin 8 homointeractions in living cells via quantitative fluorescence microscopy. We demonstrate that Tetraspanin 8 exists in a monomer-dimer equilibrium in the plasma membrane. Tetraspanin 8 dimerization is described by a high dissociation constant (Kd = 14 700 ± 1100 Tspan8/µm2), one of the highest dissociation constants measured for membrane proteins in live cells. We propose that this high dissociation constant, and thus the short lifetime of the Tetraspanin 8 dimer, is critical for Tetraspanin 8 functioning as a master regulator of cell signaling.

Human herpesvirus 8 molecular mimicry of ephrin ligands facilitates cell entry and triggers EphA2 signaling

Human herpesvirus 8 (HHV-8) is an oncogenic virus that enters cells by fusion of the viral and endosomal cellular membranes in a process mediated by viral surface glycoproteins. One of the cellular receptors hijacked by HHV-8 to gain access to cells is the EphA2 tyrosine kinase receptor, and the mechanistic basis of EphA2-mediated viral entry remains unclear. Using X-ray structure analysis, targeted mutagenesis, and binding studies, we here show that the HHV-8 envelope glycoprotein complex H and L (gH/gL) binds with subnanomolar affinity to EphA2 via molecular mimicry of the receptor’s cellular ligands, ephrins (Eph family receptor interacting proteins), revealing a pivotal role for the conserved gH residue E52 and the amino-terminal peptide of gL. Using FSI-FRET and cell contraction assays, we further demonstrate that the gH/gL complex also functionally mimics ephrin ligand by inducing EphA2 receptor association via its dimerization interface, thus triggering receptor signaling for cytoskeleton remodeling. These results now provide novel insight into the entry mechanism of HHV-8, opening avenues for the search of therapeutic agents that could interfere with HHV-8–related diseases.

Herpesviruses are known to hijack cellular receptors to enter cells, but this study shows that human herpesvirus 8 takes this to another level by using its envelope glycoprotein complex gH/gL to mimic the EphA2 receptor’s natural ligands, ephrins.

Pondering the mechanism of receptor tyrosine kinase activation: The case for ligand-specific dimer microstate ensembles

Receptor tyrosine kinases (RTKs) are single-pass membrane proteins that regulate cell growth, differentiation, motility, and metabolism. Here, we review recent advancements in RTK structure determination and in the understanding of RTK activation. We argue that further progress in the field will necessitate new ways of thinking, and we introduce the concept that RTK dimers explore ensembles of microstates, each characterized by different kinase domain dimer conformations, but the same extracellular domain dimer structure. Many microstates are phosphorylation-competent and ensure the phosphorylation of one specific tyrosine. The prevalence of each microstate correlates with its stability. A switch in ligand will lead to a switch in the extracellular domain configuration and to a subsequent switch in the ensemble of microstates. This model can explain how different ligands produce specific phosphorylation patterns, how receptor overexpression leads to enhanced signaling even in the absence of activating ligands, and why RTK kinase domain structures have remained unresolved in cryogenic electron microscopy studies.

A cancer mutation promotes EphA4 oligomerization and signaling by altering the conformation of the SAM domain

The Eph receptor tyrosine kinases and their ephrin ligands regulate many physiological and pathological processes. EphA4 plays important roles in nervous system development and adult homeostasis, while aberrant EphA4 signaling has been implicated in neurodegeneration. EphA4 may also affect cancer malignancy, but the regulation and effects of EphA4 signaling in cancer are poorly understood. A correlation between decreased patient survival and high EphA4 mRNA expression in melanoma tumors that also highly express ephrinA ligands suggests that enhanced EphA4 signaling may contribute to melanoma progression. A search for EphA4 gain-of-function mutations in melanoma uncovered a mutation of the highly conserved leucine 920 in the EphA4 sterile alpha motif (SAM) domain. We found that mutation of L920 to phenylalanine (L920F) potentiates EphA4 autophosphorylation and signaling, making it the first documented EphA4 cancer mutation that increases kinase activity. Quantitative Föster resonance energy transfer and fluorescence intensity fluctuation (FIF) analyses revealed that the L920F mutation induces a switch in EphA4 oligomer size, from a dimer to a trimer. We propose this switch in oligomer size as a novel mechanism underlying EphA4-linked tumorigenesis. Molecular dynamics simulations suggest that the L920F mutation alters EphA4 SAM domain conformation, leading to the formation of EphA4 trimers that assemble through two aberrant SAM domain interfaces. Accordingly, EphA4 wild-type and the L920F mutant are affected differently by the SAM domain and are differentially regulated by ephrin ligand stimulation. The increased EphA4 activation induced by the L920F mutation, through the novel mechanism we uncovered, supports a functional role for EphA4 in promoting pathogenesis.

Effective Tumor Targeting by EphA2-Agonist-Biotin-Streptavidin Conjugates

We recently reported on a potent synthetic agent, 135H11, that selectively targets the receptor tyrosine kinase, EphA2. While 135H11 possesses a relatively high binding affinity for the ligand-binding domain of EphA2 (Kd~130 nM), receptor activation in the cell required the synthesis of dimeric versions of such agent (namely 135H12). This was expected given that the natural ephrin ligands also need to be dimerized or clustered to elicit agonistic activity in cell. In the present report we investigated whether the agonistic activity of 135H11 could be enhanced by biotin conjugation followed by complex formation with streptavidin. Therefore, we measured the agonistic EphA2 activity of 135H11-biotin (147B5) at various agent/streptavidin ratios, side by side with 135H12, and a scrambled version of 147B5 in pancreatic- and breast-cancer cell lines. The (147B5)n-streptavidin complexes (when n = 2, 3, 4, but not when n = 1) induced a strong receptor degradation effect in both cell lines compared to 135H12 or the (scrambled-147B5)4-streptavidin complex as a control, indicating that multimerization of the targeting agent resulted in an increased ability to cause receptor clustering and internalization. Subsequently, we prepared an Alexa-Fluor-streptavidin conjugate to demonstrate that (147B5)4-AF-streptavidin, but not the scrambled equivalent complex, concentrates in pancreatic and breast cancers in orthotopic nude-mouse models. Hence, we conclude that these novel targeting agents, with proper derivatization with imaging reagents or chemotherapy, can be used as diagnostics, and/or to deliver chemotherapy selectively to EphA2-expressing tumors.

Neural network strategies for plasma membrane selection in fluorescence microscopy images

In recent years, there has been an explosion of fluorescence microscopy studies of live cells in the literature. The analysis of the images obtained in these studies often requires labor-intensive manual annotation to extract meaningful information. In this study, we explore the utility of a neural network approach to recognize, classify, and select plasma membranes in high-resolution images, thus greatly speeding up data analysis and reducing the need for personnel training for highly repetitive tasks. Two different strategies are tested: 1) a semantic segmentation strategy, and 2) a sequential application of an object detector followed by a semantic segmentation network. Multiple network architectures are evaluated for each strategy, and the best performing solutions are combined and implemented in the Recognition Of Cellular Membranes software. We show that images annotated manually and with the Recognition Of Cellular Membranes software yield identical results by comparing Förster resonance energy transfer binding curves for the membrane protein fibroblast growth factor receptor 3. The approach that we describe in this work can be applied to other image selection tasks in cell biology.

Supramolecular Self-Assembly-Facilitated Aggregation of Tumor-Specific Transmembrane Receptors for Signaling Activation and Converting Immunologically Cold to Hot Tumors

Supramolecular self-assembling peptide systems are attracting increasing interest in the field of cancer theranostics. Additionally, transformation of the immunologically cold tumor microenvironment into hot is of great importance for obtaining high antitumor responses for most immunotherapies. However, as far as it is known, there are nearly no studies on self-assembling peptides reported to be able to convert cold to hot tumors. Herein, a self-assembling peptide-based cancer theranostic agent (named DBT-2FFGYSA) is designed and synthesized, which can target tumor-specific transmembrane Eph receptor A2 (EphA2) receptors selectively and make the receptors form large aggregates. Such aggregate formation promotes the cross-phosphorylations among EphA2 receptors, leading to signal transduction of antitumor pathway. As a consequence, DBT-2FFGYSA can not only visualize EphA2 receptors in a fluorescence turn-on manner, but also specifically suppress the EphA2 receptor-overexpressed cancer cell proliferation and tumor growth. What is more, DBT-2FFGYSA also serves as an effective agent to convert immunologically cold tumors to hot by inducing the immunogenic cell death of EphA2 receptor-overexpressed cancer cells and recruiting massive tumor-infiltrating T cells. This study, thus, introduces a new category of agents capable of converting cold to hot tumors by pure supramolecular self-assembly without any aid of known anticancer drugs.

Nanotechnology to the Rescue: Treatment Perspective for the Immune Dysregulation Observed in COVID-19

The study of the use of nanotechnology for drug delivery has been extensive. Nanomedical approaches for therapeutics; drug delivery in particular is superior to conventional methods in that it allows for controlled targeted delivery and release, higher stability, extended circulation time, minimal side-effects, and improved pharmacokinetic clearance (of the drug) form the body, to name a few. The magnitude of COVID-19, the current ongoing pandemic has been severe; it has caused widespread the loss of human life. In individuals with severe COVID-19, immune dysregulation and a rampant state of hyperinflammation is observed. This kind of an immunopathological response is detrimental and results in rapid disease progression, development of secondary infections, sepsis and can be fatal. Several studies have pin-pointed the reason for this immune dysregulation; deviations in the signaling pathways involved in the mediation and control of immune responses. In severe COVID-19 patients, many signaling cascades including JAK/STAT, NF-κB, MAPK/ERK, TGF beta, VEGF, and Notch signaling were found to be either upregulated or inactivated. Targeting these aberrant signaling pathways in conjunction with antiviral therapy will effectuate mitigation of the hyperinflammation, hypercytokinemia, and promote faster recovery. The science of the use of nanocarriers as delivery agents to modulate these signaling pathways is not new; it has already been explored for other inflammatory diseases and in particular, cancer therapy. Numerous studies have evaluated the efficacy and potential of nanomedical approaches to modulate these signaling pathways and have been met with positive results. A treatment regime, that includes nanotherapeutics and antiviral therapies will prove effective and holds great promise for the successful treatment of COVID-19. In this article, we review different nanomedical approaches already studied for targeting aberrant signaling pathways, the host immune response to SARS-CoV-2, immunopathology and the dysregulated signaling pathways observed in severe COVID-19 and the current treatment methods in use for targeting signaling cascades in COVID-19. We then conclude by suggesting that the use of nanomedical drug delivery systems for targeting signaling pathways can be extended to effectively target the aberrant signaling pathways in COVID-19 for best treatment results.

The biophysical basis of receptor tyrosine kinase ligand functional selectivity: Trk-B case study

Tropomyosin receptor kinase B (Trk-B) belongs to the second largest family of membrane receptors, Receptor Tyrosine Kinases (RTKs). Trk-B is known to interact with three different neurotrophins: Brain-Derived Neurotrophic Factor (BDNF), Neurotrophin-4 (NT-4), and Neurotrophin-3 (NT-3). All three neurotrophins are involved in survival and proliferation of neuronal cells, but each induces distinct signaling through Trk-B. We hypothesize that the different biological effects correlate with differences in the interactions between the Trk-B receptors, when bound to different ligands, in the plasma membrane. To test this hypothesis, we use quantitative FRET to characterize Trk-B dimerization in response to NT-3 and NT-4 in live cells, and compare it to the previously published data for Trk-B in the absence and presence of BDNF. Our study reveals that the distinct Trk-B signaling outcomes are underpinned by both different configurations and different stabilities of the three ligand-bound Trk-B dimers in the plasma membrane.

EphA2 and EGFR: Friends in Life, Partners in Crime. Can EphA2 Be a Predictive Biomarker of Response to Anti-EGFR Agents?

The Eph receptors represent the largest group among Receptor Tyrosine kinase (RTK) families. The Eph/ephrin signaling axis plays center stage during development, and the deep perturbation of signaling consequent to its dysregulation in cancer reveals the multiplicity and complexity underlying its function. In the last decades, they have emerged as key players in solid tumors, including colorectal cancer (CRC); however, what causes EphA2 to switch between tumor-suppressive and tumor-promoting function is still an active theater of investigation. This review summarizes the recent advances in understanding EphA2 function in cancer, with detail on the molecular determinants of the oncogene-tumor suppressor switch function of EphA2. We describe tumor context-specific examples of EphA2 signaling and the emerging role EphA2 plays in supporting cancer-stem-cell-like populations and overcoming therapy-induced stress. In such a frame, we detail the interaction of the EphA2 and EGFR pathway in solid tumors, including colorectal cancer. We discuss the contribution of the EphA2 oncogenic signaling to the resistance to EGFR blocking agents, including cetuximab and TKIs.

PIP2 promotes conformation-specific dimerization of the EphA2 membrane region

The impact of the EphA2 receptor on cancer malignancy hinges on the two different ways it can be activated. EphA2 induces antioncogenic signaling after ligand binding, but ligand-independent activation of EphA2 is pro-oncogenic. It is believed that the transmembrane (TM) domain of EphA2 adopts two alternate conformations in the ligand-dependent and the ligand-independent states. However, it is poorly understood how the difference in TM helical crossing angles found in the two conformations impacts the activity and regulation of EphA2. We devised a method that uses hydrophobic matching to stabilize two conformations of a peptide comprising the EphA2 TM domain and a portion of the intracellular juxtamembrane (JM) segment. The two conformations exhibit different TM crossing angles, resembling the ligand-dependent and ligand-independent states. We developed a single-molecule technique using styrene maleic acid lipid particles to measure dimerization in membranes. We observed that the signaling lipid PIP2 promotes TM dimerization, but only in the small crossing angle state, which we propose corresponds to the ligand-independent conformation. In this state the two TMs are almost parallel, and the positively charged JM segments are expected to be close to each other, causing electrostatic repulsion. The mechanism PIP2 uses to promote dimerization might involve alleviating this repulsion due to its high density of negative charges. Our data reveal a conformational coupling between the TM and JM regions and suggest that PIP2 might directly exert a regulatory effect on EphA2 activation in cells that is specific to the ligand-independent conformation of the receptor.

Ligand bias in receptor tyrosine kinase signaling

Ligand bias is the ability of ligands to differentially activate certain receptor signaling responses compared with others. It reflects differences in the responses of a receptor to specific ligands and has implications for the development of highly specific therapeutics. Whereas ligand bias has been studied primarily for G protein-coupled receptors (GPCRs), there are also reports of ligand bias for receptor tyrosine kinases (RTKs). However, the understanding of RTK ligand bias is lagging behind the knowledge of GPCR ligand bias. In this review, we highlight how protocols that were developed to study GPCR signaling can be used to identify and quantify RTK ligand bias. We also introduce an operational model that can provide insights into the biophysical basis of RTK activation and ligand bias. Finally, we discuss possible mechanisms underpinning RTK ligand bias. Thus, this review serves as a primer for researchers interested in investigating ligand bias in RTK signaling.

The Biased Ligands NGF and NT-3 Differentially Stabilize Trk-A Dimers

Trk-A is a receptor tyrosine kinase (RTK) that plays an essential role in the development and functioning of the nervous system. Trk-A is expressed in neurons and signals in response to two ligands, NGF and neurotrophin-3 (NT-3), with very different functional consequences. Thus, NGF and NT-3 are "biased" ligands for Trk-A. Because it has been hypothesized that biased RTK ligands induce differential stabilization of RTK dimers, here, we seek to test this hypothesis for NGF and NT-3. In particular, we use Förster resonance energy transfer (FRET) and fluorescence intensity fluctuation spectroscopy to assess the strength of Trk-A interactions and Trk-A oligomer size in the presence of the two ligands. Although the difference in Trk-A behavior in response to the two ligands has been previously attributed to differences in their binding to Trk-A in the endosomes at low pH, here, we further show differences in the stabilities of the NGF- and NT-3-bound Trk-A dimers in the plasma membrane and at neutral pH. We discuss the biological significance of these new findings and their implications for the design of Trk-A ligands with novel functionalities.

Activation of EPHA2-ROBO1 Heterodimer by SLIT2 Attenuates Non-canonical Signaling and Proliferation in Squamous Cell Carcinomas

The tyrosine kinase receptor ephrin receptor A2 (EPHA2) is overexpressed in lung (LSCC) and head and neck (HNSCC) squamous cell carcinomas. Although EPHA2 can inhibit tumorigenesis in a ligand-dependent fashion via phosphorylation of Y588 and Y772, it can promote tumorigenesis in a ligand-independent manner via phosphorylation of S897. Here, we show that EPHA2 and Roundabout Guidance Receptor 1 (ROBO1) interact to form a functional heterodimer. Furthermore, we show that the ROBO1 ligand Slit Guidance Ligand 2 (SLIT2) and ensartinib, an inhibitor of EPHA2, can attenuate growth of HNSCC cells and act synergistically in LSCC cells. Our results suggest that patients with LSCC and HNSCC may be stratified and treated based on their EPHA2 and ROBO1 expression patterns. Although ~73% of patients with LSCC could benefit from SLIT2+ensartinib treatment, ~41% of patients with HNSCC could be treated with either SLIT2 or ensartinib. Thus, EPHA2 and ROBO1 represent potential LSCC and HNSCC theranostics.

Prostate Cancer Metastases Are Strongly Inhibited by Agonistic Epha2 Ligands in an Orthotopic Mouse Model

Simple Summary

We demonstrate that pro-oncogenic EphA2 (ephrin type-A receptor 2) expression is activated in aggressive prostate cancers, and in mouse models of prostate cancers that are treated with enzalutamide. We also demonstrate in mouse models, that agonistic EphA2 targeting agents are very effective in suppressing cell migration and tumor metastases, hence anticipating the possible use of such agents in innovative anti-metastatic therapeutic modalities.

Abstract

The EphA2 tyrosine kinase receptor is highly expressed in several types of solid tumors. In our recent studies, we targeted EphA2 in pancreatic cancer with agonistic agents and demonstrated that suppression of EphA2 significantly reduced cancer-cell migration in cell-based assays. In the present study, we focused on targeting EphA2 in prostate cancer. While not all prostate cancers express EphA2, we showed that enzalutamide induced EphA2 expression in prostate cancer cells and in a patient-derived xenograft (PDX) animal model, which provides further impetus to target EphA2 in prostate cancer. Western blot studies showed that agonistic dimeric synthetic (135H12) and natural (ephrinA1-Fc) ligands effectively degraded EphA2 receptor in the prostate cancer cell line PC-3. The agents also delayed cell migration of prostate cancer (PC-3) cells, while an in vivo PC-3 orthotopic metastatic nude-mouse model also revealed that administration of ephrinA1-Fc or 135H12 strongly reduced metastases. The present study further validates EphA2 as an important target in metastatic prostate cancer treatment. Our results should incentivize further efforts aimed at developing potent and effective EphA2 synthetic agonistic agents for the treatment of EphA2-driven aggressive metastatic tumors including prostate, pancreatic, and breast cancer.

Identification of Targets to Redirect CAR T Cells in Glioblastoma and Colorectal Cancer: An Arduous Venture

The chimeric antigen receptor (CAR) is an artificial molecule engineered to induce cytolytic T cell reactions in tumors. Generally, this molecule combines an extracellular single-chain variable fragment (scFv) able to recognize tumor-associated epitopes together with the intracellular signaling domains that are required for T cell activation. When expressed by T cells, the CAR enables the recognition and subsequent destruction of cancer cells expressing the complementary antigen on their surface. Although the clinical application for CAR T cells is currently limited to some hematological malignancies, researchers are trying to develop CAR T cell-based therapies for the treatment of solid tumors. However, while in the case of CD19, or other targets restricted to the hematopoietic compartment, the toxicity is limited and manageable, the scarcity of specific antigens expressed by solid tumors and not by healthy cells from vital organs makes the clinical development of CAR T cells in this context particularly challenging. Here we summarize relevant research and clinical trials conducted to redirect CAR T cells to surface antigens in solid tumors and cancer stem cells with a focus on colorectal cancer and glioblastoma. Finally, we will discuss current knowledge of altered glycosylation of CSCs and cancer cells and how these novel epitopes may help to target CAR T cell-based immunotherapy in the future.

Spatial organization-dependent EphA2 transcriptional responses revealed by ligand nanocalipers

Ligand binding induces extensive spatial reorganization and clustering of the EphA2 receptor at the cell membrane. It has previously been shown that the nanoscale spatial distribution of ligands modulates EphA2 receptor reorganization, activation and the invasive properties of cancer cells. However, intracellular signaling downstream of EphA2 receptor activation by nanoscale spatially distributed ligands has not been elucidated. Here, we used DNA origami nanostructures to control the positions of ephrin-A5 ligands at the nanoscale and investigated EphA2 activation and transcriptional responses following ligand binding. Using RNA-seq, we determined the transcriptional profiles of human glioblastoma cells treated with DNA nanocalipers presenting a single ephrin-A5 dimer or two dimers spaced 14, 40 or 100 nm apart. These cells displayed divergent transcriptional responses to the differing ephrin-A5 nano-organization. Specifically, ephrin-A5 dimers spaced 40 or 100 nm apart showed the highest levels of differential expressed genes compared to treatment with nanocalipers that do not present ephrin-A5. These findings show that the nanoscale organization of ephrin-A5 modulates transcriptional responses to EphA2 activation.

Role of EFNA1 in tumorigenesis and prospects for cancer therapy

Cancer is one of the major threats to human health. It is of vital importance to reveal the mechanisms of tumorigenesis, identify effective biomarkers and develop novel treatments to improve patient outcome. EFNA1 (ephrinA1) is a member of the EFN family, and it has been studied extensively since its discovery in 1990. Increasing evidence indicates that EFNA1 plays a pivotal role in the pathogenesis of tumors. We provide a detailed overview of the expression and prognostic value of EFNA1 in different types of human malignancies. We briefly discuss the mechanisms of EFNA1 induction in hypoxic environments and its pro-angiogenic function in different cancer cells. We describe the effects of EFNA1 on tumor growth, invasiveness and metastasis. We summarize recent advances in EFNA1-associated cancer therapeutics with emphasis on the prospect of novel anti-tumor methods based on EFNA1.

Quantifying the strength of heterointeractions among receptor tyrosine kinases from different subfamilies: Implications for cell signaling

Receptor tyrosine kinases (RTKs) are single-pass membrane proteins that control vital cell processes such as cell growth, survival, and differentiation. There is a growing body of evidence that RTKs from different subfamilies can interact and that these diverse interactions can have important biological consequences. However, these heterointeractions are often ignored, and their strengths are unknown. In this work, we studied the heterointeractions of nine RTK pairs, epidermal growth factor receptor (EGFR)-EPH receptor A2 (EPHA2), EGFR-vascular endothelial growth factor receptor 2 (VEGFR2), EPHA2-VEGFR2, EPHA2-fibroblast growth factor receptor 1 (FGFR1), EPHA2-FGFR2, EPHA2-FGFR3, VEGFR2-FGFR1, VEGFR2-FGFR2, and VEGFR2-FGFR3, using a FRET-based method. Surprisingly, we found that RTK heterodimerization and homodimerization strengths can be similar, underscoring the significance of RTK heterointeractions in signaling. We discuss how these heterointeractions can contribute to the complexity of RTK signal transduction, and we highlight the utility of quantitative FRET for probing multiple interactions in the plasma membrane.

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.

Therapeutic Targeting of Pancreatic Cancer via EphA2 Dimeric Agonistic Agents

Recently, we reported on potent EphA2 targeting compounds and demonstrated that dimeric versions of such agents can exhibit remarkably increased agonistic activity in cellular assays compared to the monomers. Here we further characterize the activity of dimeric compounds at the structural, biochemical, and cellular level. In particular, we propose a structural model for the mechanism of receptor activation by dimeric agents and characterize the effect of most potent compounds in inducing EphA2 activation and degradation in a pancreatic cancer cell line. These cellular studies indicate that the pro-migratory effects induced by the receptor can be reversed in EphA2 knockout cells, by treatment with either a dimeric natural ligand (ephrinA1-Fc), or by our synthetic agonistic dimers. Based on these data we conclude that the proposed agents hold great potential as possible therapeutics in combination with standard of care, where these could help suppressing a major driver for cell migration and tumor metastases. Finally, we also found that, similar to ephrinA1-Fc, dimeric agents cause a sustained internalization of the EphA2 receptor, hence, with proper derivatizations, these could also be used to deliver chemotherapy selectively to pancreatic tumors.

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.

Ephrin or not? Six tough questions on Eph targeting

Introduction: The Eph-ephrin is a cell-cell communication system generating a forward signal in cell expressing Eph receptors and a reverse signal in ephrin-ligand expressing cells. While clearly involved in the insurgence and progression of cancer, the understanding of the molecular mechanisms regulated by this system needs development; this is a hurdle to the development of therapeutic strategies that can target the Eph receptors and/or their ephrin ligands.Areas covered: We have taken the opportunity to share some key questions on the most effective strategies to target the Eph-ephrin system. This article is based on our experience of the field and therefore is a Perspective and not comprehensive examination of the literature.Expert opinion: Targeting of the Eph-ephrin system has emerged as a potentially valuable approach for cancer therapy. Pharmacological tools have been reported in the last 15 years and these include forward signaling blockers such as kinases inhibitors and antagonists of forward and reverse signaling. Also, biologics including antibodies and recombinant proteins have been developed and some have reached early clinical stages. Data deem the Eph-ephrin system as a signaling axis that is an elusive target. A better understanding of the basic pharmacology behind the activity of available agents and a comprehensive knowledge of the ephrin biology are necessary. We are looking forward to knowing the opinion of the readers.

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.

Nongenetic engineering strategies for regulating receptor oligomerization in living cells

Nongenetic strategies for regulating receptor oligomerization in living cells based on DNA, protein, small molecules and physical stimuli.

Membrane receptor activation mechanisms and transmembrane peptide tools to elucidate them

Single-pass membrane receptors contain extracellular domains that respond to external stimuli and transmit information to intracellular domains through a single transmembrane (TM) α-helix. Because membrane receptors have various roles in homeostasis, signaling malfunctions of these receptors can cause disease. Despite their importance, there is still much to be understood mechanistically about how single-pass receptors are activated. In general, single-pass receptors respond to extracellular stimuli via alterations in their oligomeric state. The details of this process are still the focus of intense study, and several lines of evidence indicate that the TM domain (TMD) of the receptor plays a central role. We discuss three major mechanistic hypotheses for receptor activation: ligand-induced dimerization, ligand-induced rotation, and receptor clustering. Recent observations suggest that receptors can use a combination of these activation mechanisms and that technical limitations can bias interpretation. Short peptides derived from receptor TMDs, which can be identified by screening or rationally developed on the basis of the structure or sequence of their targets, have provided critical insights into receptor function. Here, we explore recent evidence that, depending on the target receptor, TMD peptides cannot only inhibit but also activate target receptors and can accommodate novel, bifunctional designs. Furthermore, we call for more sharing of negative results to inform the TMD peptide field, which is rapidly transforming into a suite of unique tools with the potential for future therapeutics.

The transition model of RTK activation: A quantitative framework for understanding RTK signaling and RTK modulator activity

Here, we discuss the transition model of receptor tyrosine kinase (RTK) activation, which is derived from biophysical investigations of RTK interactions and signaling. The model postulates that (1) RTKs can interact laterally to form dimers even in the absence of ligand, (2) different unliganded RTK dimers have different stabilities, (3) ligand binding stabilizes the RTK dimers, and (4) ligand binding causes structural changes in the RTK dimer. The model is grounded in the principles of physical chemistry and provides a framework to understand RTK activity and to make predictions in quantitative terms. It can guide basic research aimed at uncovering the mechanism of RTK activation and, in the long run, can empower the search for modulators of RTK function.

Newly Discovered Micropeptide Regulators of SERCA Form Oligomers but Bind to the Pump as Monomers

The recently-discovered single-span transmembrane proteins endoregulin (ELN), dwarf open reading frame (DWORF), myoregulin (MLN), and another-regulin (ALN) are reported to bind to the SERCA calcium pump in a manner similar to that of known regulators of SERCA activity, phospholamban (PLB) and sarcolipin (SLN). To determine how micropeptide assembly into oligomers affects the availability of the micropeptide to bind to SERCA in a regulatory complex, we used co-immunoprecipitation and fluorescence resonance energy transfer (FRET) to quantify micropeptide oligomerization and SERCA-binding. Micropeptides formed avid homo-oligomers with high-order stoichiometry (n > 2 protomers per homo-oligomer), but it was the monomeric form of all micropeptides that interacted with SERCA. In view of these two alternative binding interactions, we evaluated the possibility that oligomerization occurs at the expense of SERCA-binding. However, even the most avidly oligomeric micropeptide species still showed robust FRET with SERCA, and there was a surprising positive correlation between oligomerization affinity and SERCA-binding. This comparison of micropeptide family members suggests that the same structural determinants that support oligomerization are also important for binding to SERCA. Moreover, the unique oligomerization/SERCA-binding profile of DWORF is in harmony with its distinct role as a PLB-competing SERCA activator, in contrast to the inhibitory function of the other SERCA-binding micropeptides.

Eph receptor signalling: from catalytic to non-catalytic functions

Eph receptors, the largest subfamily of receptor tyrosine kinases, are linked with proliferative disease, such as cancer, as a result of their deregulated expression or mutation. Unlike other tyrosine kinases that have been clinically targeted, the development of therapeutics against Eph receptors remains at a relatively early stage. The major reason is the limited understanding on the Eph receptor regulatory mechanisms at a molecular level. The complexity in understanding Eph signalling in cells arises due to following reasons: (1) Eph receptors comprise 14 members, two of which are pseudokinases, EphA10 and EphB6, with relatively uncharacterised function; (2) activation of Eph receptors results in dimerisation, oligomerisation and formation of clustered signalling centres at the plasma membrane, which can comprise different combinations of Eph receptors, leading to diverse downstream signalling outputs; (3) the non-catalytic functions of Eph receptors have been overlooked. This review provides a structural perspective of the intricate molecular mechanisms that drive Eph receptor signalling, and investigates the contribution of intra- and inter-molecular interactions between Eph receptors intracellular domains and their major binding partners. We focus on the non-catalytic functions of Eph receptors with relevance to cancer, which are further substantiated by exploring the role of the two pseudokinase Eph receptors, EphA10 and EphB6. Throughout this review, we carefully analyse and reconcile the existing/conflicting data in the field, to allow researchers to further the current understanding of Eph receptor signalling.