Therapeutic potential of targeting the Eph/ephrin signaling complex

Abstract

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.

Keywords: ADAM proteinase; Antibody drug; Eph receptor; Eph-specific peptide; Small molecule inhibitor.

Figure 1:

Schematic representation of the Eph receptors and their ephrin (efn) ligands. Ephrin-A is shown in red, ephrin-B in light brown, EphA receptor in cyan, and EphB receptor in blue. The A class ligands are attached to the cell membrane by a glycosylphosphatidylinositol (GPI) linkage and bind nearly exclusively the A class receptors. On the other hand, the B ligands that bind almost exclusively the B class receptors, contain a transmembrane stretch and a small cytoplasmic domain. The overall structures of the EphA and the EphB receptors are very similar. The ectodomain forms a fairly rigid, rod-like structure that undergoes minimal conformational change upon activation. Ligand binding causes a clustering of the Eph receptors, leading to the opening of an activation loop, and a subsequent tyrosine phosphorylation (shown in green) of the kinase domain. SAM domain offers a docking surface for downstream signaling and is reported to be involved in receptor oligomerization (Shi et al., 2017). RBD, Receptor-Binding Domain; LBD, Ligand-Binding Domain; CRD, Cysteine-Rich Domain; FN, Fibronectin III domain; Y-P, Phospho-Tyrosine; SAM, Sterile Alpha Motif.

Figure 2.

Ligand-Binding Domain (LBD) of the EphB4 receptor (in blue) in complex with an antagonistic peptide, TNYL-RAW (in cyan) (Chrencik et al., 2006). LBD has a jellyroll folding topology with 13 antiparallel B-sheets connected by several loops of varying lengths. Binding of TNYL-RAW makes the fairly flexible D-E and J-K loops more structured and they can be visualized in the electron density map.

Figure 3.

Crystal structure of an anti-EphA2 scFv Ab in complex with the ligand-binding domain of EphA2. The scFv variable light chain is pink, the variable heavy chain is red, and the EphA2 receptor is cyan. A long loop of the D2 heavy chain penetrates a hydrophobic surface cavity of the EphA2 in a way very similar to Ephrin binding to Eph.

Figure 4.

Crystal structure of the Nipah virus G protein in complex with ephrin-B3. G protein is shown in yellow and Ephrin in red. The main binding interfaces appear around a long G-H loop of Ephrin and the central cavity of the G protein beta-propeller. The same surface elements of Ephrin are involved in both Eph and Nipah G binding.

Figure 5.

Domain organization of transmembrane ADAM metalloproteases. ADAMs contain a prodomain that is cleaved upon maturation. The metalloprotease (MP) harbors the active site. The disintegrin (D) and cysteine–rich domains are primarily responsible for substrate-recognition. The epidermal growth factor (EGF)-like domain is absent in ADAM10 and ADAM17. The EGF domain is followed by the transmembrane domain and a cytoplasmic tail.

Figure 6.

Crystal structure of mAb/8C7-bound ADAM10. The heavy chain of the 8C7 mAb is in green and the light in wheat. The D+C domains of ADAM10 are represented in red.