The Eph-receptor A7 is a soluble tumor suppressor for follicular lymphoma.

Insights into cancer genetics can lead to therapeutic opportunities. By cross-referencing chromosomal changes with an unbiased genetic screen we identify the ephrin receptor A7 (EPHA7) as a tumor suppressor in follicular lymphoma (FL). EPHA7 is a target of 6q deletions and inactivated in 72% of FLs. Knockdown of EPHA7 drives lymphoma development in a murine FL model. In analogy to its physiological function in brain development, a soluble splice variant of EPHA7 (EPHA7(TR)) interferes with another Eph-receptor and blocks oncogenic signals in lymphoma cells. Consistent with this drug-like activity, administration of the purified EPHA7(TR) protein produces antitumor effects against xenografted human lymphomas. Further, by fusing EPHA7(TR) to the anti-CD20 antibody (rituximab) we can directly target this tumor suppressor to lymphomas in vivo. Our study attests to the power of combining descriptive tumor genomics with functional screens and reveals EPHA7(TR) as tumor suppressor with immediate therapeutic potential.

The Ephb2 Receptor Uses Homotypic, Head-to-Tail Interactions within Its Ectodomain as an Autoinhibitory Control Mechanism.

The Eph receptor tyrosine kinases and their ephrin ligands direct axon pathfinding and neuronal cell migration, as well as mediate many other cell-cell communication events. Their dysfunctional signaling has been shown to lead to various diseases, including cancer. The Ephs and ephrins both localize to the plasma membrane and, upon cell-cell contact, form extensive signaling assemblies at the contact sites. The Ephs and the ephrins are divided into A and B subclasses based on their sequence conservation and affinities for each other. The molecular details of Eph-ephrin recognition have been previously revealed and it has been documented that ephrin binding induces higher-order Eph assemblies, which are essential for full biological activity, via multiple, distinct Eph-Eph interfaces. One Eph-Eph interface type is characterized by a homotypic, head-to-tail interaction between the ligand-binding and the fibronectin domains of two adjacent Eph molecules. While the previous Eph ectodomain structural studies were focused on A class receptors, we now report the crystal structure of the full ectodomain of EphB2, revealing distinct and unique head-to-tail receptor-receptor interactions. The EphB2 structure and structure-based mutagenesis document that EphB2 uses the head-to-tail interactions as a novel autoinhibitory control mechanism for regulating downstream signaling and that these interactions can be modulated by posttranslational modifications.

Insights into Eph receptor tyrosine kinase activation from crystal structures of the EphA4 ectodomain and its complex with ephrin-A5.

Eph receptor tyrosine kinases and their ephrin ligands mediate cell signaling during normal and oncogenic development. Eph signaling is initiated in a multistep process leading to the assembly of higher-order Eph/ephrin clusters that set off bidirectional signaling in interacting cells. Eph and ephrins are divided in two subclasses based on their abilities to bind and activate each other and on sequence conservation. EphA4 is an exception to the general rule because it can be activated by both A- and B-class ephrin ligands. Here we present high-resolution structures of the complete EphA4 ectodomain and its complexes with ephrin-A5. The structures reveal how ligand binding promotes conformational changes in the EphA4 ligand-binding domain allowing the formation of signaling clusters at the sites of cell-cell contact. In addition, the structural data, combined with structure-based mutagenesis, reveal a previously undescribed receptor-receptor interaction between the EphA4 ligand-binding and membrane-proximal fibronectin domains, which is functionally important for efficient receptor activation.

Ectodomain structures of Eph receptors.

Eph receptors, the largest subfamily of receptor tyrosine kinases (RTKs), and their ephrin ligands are important mediators of cell-cell communication that regulate axon guidance, long-term potentiation, and stem cell development, among others. By now, many Eph receptors and ephrins have also been found to play important roles in the progression of cancer. Since both the receptor and the ligand are membrane-bound, their interaction leads to the multimerization of both molecules to distinct clusters within their respective plasma membranes, resulting in the formation of discrete signaling centers. In addition, and unique to Eph receptors and ephrins, their interaction initiates bi-directional signaling cascades where information is transduced in the direction of both the receptor- and the ligand-bearing cells. The Ephs and the ephrins are divided into two subclasses, A and B, based on their affinities for each other and on sequence conservation. Crystal structures and other biophysical studies have indicated that isolated extracellular Eph and ephrin domains initially form high-affinity heterodimers around a hydrophobic loop of the ligand that is buried in a hydrophobic pocket on the surface of the receptor. The dimers can then further arrange by weaker interactions into higher-order Eph/ephrin clusters observed in vivo at the sites of cell-cell contact. Although the hetero-dimerization is a universal way to initiate signaling, other extracellular domains of Ephs are involved in the formation of higher-order clusters. The structures also show important differences defining the unique partner preferences of the two ligand and receptor subclasses, namely, how subclass specificity is determined both by individual interacting residues and by the precise architectural arrangement of ligands and receptors within the complexes.

Biological and structural characterization of glycosylation on ephrin-A1, a preferred ligand for EphA2 receptor tyrosine kinase.

The EphA2 receptor tyrosine kinase is overexpressed in a number of malignancies and is activated by ephrin ligands, most commonly by ephrin-A1. The crystal structure of the ligand-receptor complex revealed a glycosylation on the Asn-26 of ephrin-A1. Here we report for the first time the significance of the glycosylation in the biology of EphA2 and ephrin-A1. Ephrin-A1 was enzymatically deglycosylated, and its activity was evaluated in several assays using glioblastoma (GBM) cells and recombinant EphA2. We found that deglycosylated ephrin-A1 does not efficiently induce EphA2 receptor internalization and degradation, and does not activate the downstream signaling pathways involved in cell migration and proliferation. Data obtained by surface plasmon resonance confirms that deglycosylated ephrin-A1 does not bind EphA2 with high affinity. Mutations in the glycosylation site on ephrin-A1 result in protein aggregation and mislocalization. Analysis of Eph/ephrin crystal structures reveals an interaction between the ligand's carbohydrates and two residues of EphA2: Asp-78 and Lys-136. These findings suggest that the glycosylation on ephrin-A1 plays a critical role in the binding and activation of the EphA2 receptor.

Eph/ephrin recognition and the role of Eph/ephrin clusters in signaling initiation.

The Eph receptors and their ephrin ligands play crucial roles in a large number of cell-cell interaction events, including those associated with axon pathfinding, neuronal cell migration and vasculogenesis. They are also involved in the patterning of most tissues and overall cell positioning in the development of the vertebrate body plan. The Eph/ephrin signaling system manifests several unique features that differentiate it from other receptor tyrosine kinases, including initiation of bi-directional signaling cascades and the existence of ligand and receptor subclasses displaying promiscuous intra-subclass interactions, but very rare inter-subclass interactions. In this review we briefly discuss these features and focus on recent studies of the unique and expansive high-affinity Eph/ephrin assemblies that form at the sites of cell-cell contact and are required for Eph signaling initiation. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Generation and characterization of a single-chain anti-EphA2 antibody.

Recombinant antibody phage library technology provides multiple advantages, including that human antibodies can be generated against proteins that are highly conserved between species. We used this technology to isolate and characterize an anti-EphA2 single-chain antibody. We show that the antibody binds the antigen with 1:1 stoichiometry and has high specificity for EphA2. The crystal structure of the complex reveals that the antibody targets the same receptor surface cavity as the ephrin ligand. Specifically, a lengthy CDR-H3 loop protrudes deep into the ligand-binding cavity, with several hydrophobic residues at its tip forming an anchor-like structure buried within the hydrophobic Eph pocket, in a way similar to the ephrin receptor-binding loop in the Eph/ephrin structures. Consequently, the antibody blocks ephrin binding to EphA2. Furthermore, it induces apoptosis and reduces cell proliferation in lymphoma cells lines. Since Ephs are important mediators of tumorigenesis, such antibodies could have applications both in research and therapy.

Architecture of Eph receptor clusters.

Eph receptor tyrosine kinases and their ephrin ligands regulate cell navigation during normal and oncogenic development. Signaling of Ephs is initiated in a multistep process leading to the assembly of higher-order signaling clusters that set off bidirectional signaling in interacting cells. However, the structural and mechanistic details of this assembly remained undefined. Here we present high-resolution structures of the complete EphA2 ectodomain and complexes with ephrin-A1 and A5 as the base unit of an Eph cluster. The structures reveal an elongated architecture with novel Eph/Eph interactions, both within and outside of the Eph ligand-binding domain, that suggest the molecular mechanism underlying Eph/ephrin clustering. Structure-function analysis, by using site-directed mutagenesis and cell-based signaling assays, confirms the importance of the identified oligomerization interfaces for Eph clustering.

Integration of cancer-related genetic landscape of Eph receptors and ephrins with proteomics identifies a crosstalk between EPHB6 and EGFR.

Eph receptors and their ephrin ligands are viewed as promising targets for cancer treatment; however, targeting them is hindered by their context-dependent functionalities. To circumvent this, we explore molecular landscapes underlying their pro- and anti-malignant activities. Using unbiased bioinformatics approaches, we construct a cancer-related network of genetic interactions (GIs) of all Ephs and ephrins to assist in their therapeutic manipulation. We also apply genetic screening and BioID proteomics and integrate them with machine learning approaches to select the most relevant GIs of one Eph receptor, EPHB6. This identifies a crosstalk between EPHB6 and EGFR, and further experiments confirm the ability of EPHB6 to modulate EGFR signaling, enhancing the proliferation of cancer cells and tumor development. Taken together, our observations show EPHB6 involvement in EGFR action, suggesting its targeting might be beneficial in EGFR-dependent tumors, and confirm that the Eph family genetic interactome presented here can be effectively exploited in developing cancer treatment approaches.

A Multipronged Unbiased Strategy Guides the Development of an Anti-EGFR/EPHA2-Bispecific Antibody for Combination Cancer Therapy.

PURPOSE: Accumulating analyses of pro-oncogenic molecular mechanisms triggered a rapid development of targeted cancer therapies. Although many of these treatments produce impressive initial responses, eventual resistance onset is practically unavoidable. One of the main approaches for preventing this refractory condition relies on the implementation of combination therapies. This includes dual-specificity reagents that affect both of their targets with a high level of selectivity. Unfortunately, selection of target combinations for these treatments is often confounded by limitations in our understanding of tumor biology. Here, we describe and validate a multipronged unbiased strategy for predicting optimal co-targets for bispecific therapeutics. EXPERIMENTAL DESIGN: Our strategy integrates ex vivo genome-wide loss-of-function screening, BioID interactome profiling, and gene expression analysis of patient data to identify the best fit co-targets. Final validation of selected target combinations is done in tumorsphere cultures and xenograft models. RESULTS: Integration of our experimental approaches unambiguously pointed toward EGFR and EPHA2 tyrosine kinase receptors as molecules of choice for co-targeting in multiple tumor types. Following this lead, we generated a human bispecific anti-EGFR/EPHA2 antibody that, as predicted, very effectively suppresses tumor growth compared with its prototype anti-EGFR therapeutic antibody, cetuximab. CONCLUSIONS: Our work not only presents a new bispecific antibody with a high potential for being developed into clinically relevant biologics, but more importantly, successfully validates a novel unbiased strategy for selecting biologically optimal target combinations. This is of a significant translational relevance, as such multifaceted unbiased approaches are likely to augment the development of effective combination therapies for cancer treatment. See related commentary by Kumar, p. 2570.

Ligand recognition by A-class Eph receptors: crystal structures of the EphA2 ligand-binding domain and the EphA2/ephrin-A1 complex.

Ephrin (Eph) receptor tyrosine kinases fall into two subclasses (A and B) according to preferences for their ephrin ligands. All published structural studies of Eph receptor/ephrin complexes involve B-class receptors. Here, we present the crystal structures of an A-class complex between EphA2 and ephrin-A1 and of unbound EphA2. Although these structures are similar overall to their B-class counterparts, they reveal important differences that define subclass specificity. The structures suggest that the A-class Eph receptor/ephrin interactions involve smaller rearrangements in the interacting partners, better described by a 'lock-and-key'-type binding mechanism, in contrast to the 'induced fit' mechanism defining the B-class molecules. This model is supported by structure-based mutagenesis and by differential requirements for ligand oligomerization by the two subclasses in cell-based Eph receptor activation assays. Finally, the structure of the unligated receptor reveals a homodimer assembly that might represent EphA2-specific homotypic cell adhesion interactions.

Crystal structures of the Tie2 receptor ectodomain and the angiopoietin-2-Tie2 complex.

The Tie receptor tyrosine kinases and their angiopoietin (Ang) ligands play central roles in developmental and tumor-induced angiogenesis. Here we present the crystal structures of the Tie2 ligand-binding region alone and in complex with Ang2. In contrast to prediction, Tie2 contains not two but three immunoglobulin (Ig) domains, which fold together with the three epidermal growth factor domains into a compact, arrowhead-shaped structure. Ang2 binds at the tip of the arrowhead utilizing a lock-and-key mode of ligand recognition-unique for a receptor kinase-where two complementary surfaces interact with each other with no domain rearrangements and little conformational change in either molecule. Ang2-Tie2 recognition is similar to antibody-protein antigen recognition, including the location of the ligand-binding site within the Ig fold. Analysis of the structures and structure-based mutagenesis provide insight into the mechanism of receptor activation and support the hypothesis that all angiopoietins interact with Tie2 in a structurally similar manner.

Host cell recognition by the henipaviruses: crystal structures of the Nipah G attachment glycoprotein and its complex with ephrin-B3.

Nipah virus (NiV) and Hendra virus are the type species of the highly pathogenic paramyxovirus genus Henipavirus, which can cause severe respiratory disease and fatal encephalitis infections in humans, with case fatality rates approaching 75%. NiV contains two envelope glycoproteins, the receptor-binding G glycoprotein (NiV-G) that facilitates attachment to host cells and the fusion (F) glycoprotein that mediates membrane merger. The henipavirus G glycoproteins lack both hemagglutinating and neuraminidase activities and, instead, engage the highly conserved ephrin-B2 and ephrin-B3 cell surface proteins as their entry receptors. Here, we report the crystal structures of the NiV-G both in its receptor-unbound state and in complex with ephrin-B3, providing, to our knowledge, the first view of a paramyxovirus attachment complex in which a cellular protein is used as the virus receptor. Complex formation generates an extensive protein-protein interface around a protruding ephrin loop, which is inserted in the central cavity of the NiV-G beta-propeller. Analysis of the structural data reveals the molecular basis for the highly specific interactions of the henipavirus G glycoproteins with only two members (ephrin-B2 and ephrin-B3) of the very large ephrin family and suggests how they mediate in a unique fashion both cell attachment and the initiation of membrane fusion during the virus infection processes. The structures further suggest that the NiV-G/ephrin interactions can be effectively targeted to disrupt viral entry and provide the foundation for structure-based antiviral drug design.

Structure of the EphB6 receptor ectodomain.

Eph receptors are the largest group amongst the receptor tyrosine kinases and are divided into two subgroups, A and B, based on ligand binding specificities and sequence conservation. Through ligand-induced and ligand-independent activities, Ephs play central roles in diverse biological processes, including embryo development, regulation of neuronal signaling, immune responses, vasculogenesis, as well as tumor initiation, progression, and metastasis. The Eph extracellular regions (ECDs) are constituted of multiple domains, and previous structural studies of the A class receptors revealed how they interact with ephrin ligands and simultaneously mediate Eph-Eph clustering necessary for biological activity. Specifically, EphA structures highlighted a model, where clustering of ligand-bound receptors relies on two distinct receptor/receptor interfaces. Interestingly, most unliganded A class receptors also form an additional, third interface, between the ligand binding domain (LBD) and the fibronectin III domain (FN3) of neighboring molecules. Structures of B-class Eph ECDs, on the other hand, have never been reported. To further our understanding of Eph receptor function, we crystallized the EphB6-ECD and determined its three-dimensional structure using X-ray crystallography. EphB6 has important functions in both normal physiology and human malignancies and is especially interesting because this atypical receptor innately lacks kinase activity and our understanding of the mechanism of action is still incomplete. Our structural data reveals the overall EphB6-ECD architecture and shows EphB6-LBD/FN3 interactions similar to those observed for the unliganded A class receptors, suggesting that these unusual interactions are of general importance to the Eph group. We also observe unique structural features, which likely reflect the atypical signaling properties of EphB6, namely the need of co-receptor(s) for this kinase-inactive Eph. These findings provide new valuable information on the structural organization and mechanism of action of the B-class Ephs, and specifically EphB6, which in the future will assist in identifying clinically relevant targets for cancer therapy.

Ephrin-A1 and the sheddase ADAM12 are upregulated in COVID-19.

More than 3.5 million people have died globally from COVID-19, yet an effective therapy is not available. It is, therefore, important to understand the signaling pathways that mediate disease progression in order to identify new molecular targets for therapeutic development. Here, we report that the blood serum levels of ephrin-A1 and the sheddase ADAM12 were significantly elevated in COVID-19 patients treated at SUNY Downstate Hospital of Brooklyn, New York. Both ephrin-A1 and ADAM12 are known to be involved in inflammation and regulate endothelial cell permeability, thus providing a gateway to lung injury. The clinical outcome correlated with the ephrin-A1 and ADAM12 serum levels during the first week of hospitalization. In contrast, the serum levels of TNFα were elevated in only a small subset of the patients, and these same patients also had highly elevated levels of the sheddase ADAM17. These data indicate that ephrin-A1-mediated inflammatory signaling may contribute to COVID-19 disease progression more so than TNFα-mediated inflammatory signaling. They also support the notion that, in COVID-19 inflammation, ADAM12 sheds ephrin-A1, while ADAM17 sheds TNFα. Furthermore, the results suggest that elevated serum levels and activity of cytokines, such as TNFα, and other secreted inflammatory molecules, such as ephrin-A1, are not simply due to overexpression, but also to upregulation of sheddases that release them into the blood circulation. Our results identify ephrin-A1, ADAM12, and other molecules in the ephrin-A1 signaling pathway as potential pharmacological targets for treating COVID-19 inflammation.

Crystal structure of the ligand-binding domain of the promiscuous EphA4 receptor reveals two distinct conformations.

Eph receptors and their ephrin ligands are important mediators of cell-cell communication. They are divided in two subclasses based on their affinities for each other and on sequence conservation. Receptor-ligand binding within each subclass is fairly promiscuous, while binding cross the subclasses happens rarely. EphA4 is an exception to this general rule, since it has long been known to bind both A- and B-class ephrin ligands but the reason for this exceptional behavior has not been worked out at molecular level. Recent structural and biochemical studies on EphA4 ligand-binding domain alone and in complex with its ligands have addressed this question. However, the published structures of EphA4/ephrin complexes differ considerably from each other and strikingly different explanations for the exceptional promiscuity of EphA4 were proposed. To address these contradictory findings, we have determined a crystal structure of the EphA4 ligand-binding domain at 2.3A resolution and show that the receptor has an unprecedented ability to exist in two very different, well-ordered conformations even in the unbound state. Our results suggest that the ligand promiscuity of the Ephs is directly correlated with the structural flexibility of the ligand-binding surface of the receptor.

ADAM proteases: Emerging role and targeting of the non-catalytic domains.

ADAM proteases are multi domain transmembrane metalloproteases that cleave a range of cell surface proteins and activate signaling pathways implicated in tumor progression, including those mediated by Notch, EFGR, and the Eph receptors. Consequently, they have emerged as key therapeutic targets in the efforts to inhibit tumor initiation and progression. To that end, two main approaches have been taken to develop ADAM antagonists: (i) small molecule inhibitors, and (ii) monoclonal antibodies. In this mini-review we describe the distinct features of ADAM proteases, particularly of ADAM10 and ADAM17, their domain organization, conformational rearrangements, regulation, as well as their emerging importance as therapeutic targets in cancer. Further, we highlight an anti-ADAM10 monoclonal antibody that we have recently developed, which has shown significant promise in inhibiting Notch signaling and deterring growth of solid tumors in pre-clinical settings.

Therapeutic potential of targeting the Eph/ephrin signaling complex.

The Eph-ephrin signaling pathway mediates developmental processes and the proper functioning of the adult human body. This distinctive bidirectional signaling pathway includes a canonical downstream signal cascade inside the Eph-bearing cells, as well as a reverse signaling in the ephrin-bearing cells. The signaling is terminated by ADAM metalloproteinase cleavage, internalization, and degradation of the Eph/ephrin complexes. Consequently, the Eph-ephrin-ADAM signaling cascade has emerged as a key target with immense therapeutic potential particularly in the context of cancer. An interesting twist was brought forth by the emergence of ephrins as the entry receptors for the pathological Henipaviruses, which has spurred new studies to target the viral entry. The availability of high-resolution structures of the multi-modular Eph receptors in complexes with ephrins and other binding partners, such as peptides, small molecule inhibitors and antibodies, offers a wealth of information for the structure-guided development of therapeutic intervention. Furthermore, genomic data mining of Eph mutants involved in cancer provides information for targeted drug development. In this review we summarize the distinct avenues for targeting the Eph-ephrin signaling pathway, including its termination by ADAM proteinases. We highlight the latest developments in Eph-related pharmacology in the context of Eph-ephrin-ADAM-based antibodies and small molecules. Finally, the future prospects of genomics- and proteomics-based medicine are discussed.

Binding of EphrinA5 to RET receptor tyrosine kinase: An in vitro study.

Eph/Ephrin signaling pathways are crucial in regulating a large variety of physiological processes during development, such as cell morphology, proliferation, migration and axonal guidance. EphrinA (efn-A) ligands, in particular, can be activated by EphA receptors at cell-cell interfaces and have been proposed to cause reverse signaling via RET receptor tyrosine kinase. Such association has been reported to mediate spinal motor axon navigation, but conservation of the interactive signaling pathway and the molecular mechanism of the interaction are unclear. Here, we found Danio rerio efn-A5b bound to Mus musculus EphA4 with high affinity, revealing structurally and functionally conserved EphA/efn-A signaling. Interestingly, we observed no interaction between efn-A5b and RET from zebrafish, unlike earlier cell-based assays. Their lack of association indicates how complex efn-A signaling is and suggests that there may be other molecules involved in efn-A5-induced RET signaling.

Homotypic receptor-receptor interactions regulating Eph signaling.

The Eph receptor tyrosine kinases and their ephrin ligands direct axon pathfinding and neuronal cell migration, and mediate many other cell-cell communication events. The Ephs and ephrins both localize to the plasma membrane and, upon cell-cell contact, form extensive signaling assemblies at the contact sites. Recent structural, biochemical and cell-biological studies revealed that these assemblies are generated not only via Eph-ephrin interactions, but also via homotypic interactions between neighboring receptor molecules. In addition, Eph-Eph interactions mediate receptor pre-clustering, which ensures fast and efficient activation once ligands come into contact range. Here we summarize the current knowledge about the homotypic Eph-Eph interactions and discuss how they could modulate the initiation of Eph/ephrin signaling.