Eph family receptors as therapeutic targets

Anti-angiogenic therapy is currently a commonly accepted and rapidly developing approach in oncology and other pathologies linked to aberrant neovascularization. Discovery and validation of additional molecular targets in angiogenesis is needed due to the limitations of the existing clinical therapeutics inhibiting activity of vascular endothelial growth factor (VEGF) and its receptors. A brief review of normal and pathological biological functions of the Eph family of receptor tyrosine kinases and their ephrin ligands is presented, and the approaches to developing therapeutics with anti- and pro-angiogenic and anti-tumor activity based on selective molecular modulation of Eph-ephrin signaling pairs are discussed. Functional roles of Eph-kinases and ephrins in such mechanisms of carcinogenesis as cell proliferation and invasion are also addressed.     

The BCL-2 protein family: opposing activities that mediate cell death

BCL-2 family proteins, which have either pro- or anti-apoptotic activities, have been studied intensively for the past decade owing to their importance in the regulation of apoptosis, tumorigenesis and cellular responses to anti-cancer therapy. They control the point of no return for clonogenic cell survival and thereby affect tumorigenesis and host-pathogen interactions and regulate animal development. Recent structural, phylogenetic and biological analyses, however, suggest the need for some reconsideration of the accepted organizational principles of the family and how the family members interact with one another during programmed cell death. Although these insights into interactions among BCL-2 family proteins reveal how these proteins are regulated, a unifying hypothesis for the mechanisms they use to activate caspases remains elusive.     

Promiscuous and specific recognition among ephrins and Eph receptors

Eph–ephrin interactions control the signal transduction between cells and play an important role in carcinogenesis and other diseases. The interactions between Eph receptors and ephrins of the same subclass are promiscuous; there are cross-interactions between some subclasses, but not all. To understand how Eph–ephrin interactions can be both promiscuous and specific, we investigated sixteen energy landscapes of four Eph receptors (A2, A4, B2, and B4) interacting with four ephrin ligands (A1, A2, A5, and B2). We generated conformational ensembles and recognition energy landscapes starting from separated Eph and ephrin molecules and proceeding up to the formation of Eph–ephrin complexes. Analysis of the Eph–ephrin recognition trajectories and the co-evolution entropy of 400 ligand binding domains of Eph receptor and 241 ephrin ligands identified conserved residues during the recognition process. Our study correctly predicted the promiscuity and specificity of the interactions and provided insights into their recognition. The dynamic conformational changes during Eph–ephrin recognition can be described by progressive conformational selection and population shift events, with two dynamic salt bridges between EphB4 and ephrin-B2 contributing to the specific recognition. EphA3 cancer-related mutations lowered the binding energies. The specificity is not only controlled by the final stage of the interaction across the protein–protein interface, but also has large contributions from binding kinetics with the help of dynamic intermediates along the pathway from the separated Eph and ephrin to the Eph–ephrin complex.     

Eph receptor-ephrin bidirectional signals that target Ras and Rho proteins

The ability of cells to respond to their surrounding environment and relay signals to the cell interior is essential for numerous processes during the development and maintenance of tissues. Eph receptors and their membrane-bound ligands, the ephrins, are unique in the receptor tyrosine kinase family in that their signaling is bidirectional, through both the receptor and the ligand. Eph receptors and ephrins are essential for a variety of biological processes, and play a particularly important role in regulating cell shape and cell movement. Recent data have linked Eph receptor-ephrin signaling complexes to the Ras and Rho families of small molecular weight GTPases and also to heterotrimeric G proteins. Understanding the signaling networks involved is an important step to understand the molecular basis for normal and defective cell-cell communication through Eph receptors and ephrins.     

Peroxisome proliferator-activated receptors: three isotypes for a multitude of functions

The peroxisome proliferator-activated receptors (PPARs) are fatty acid and eicosanoid inducible nuclear receptors, which occur in three different isotypes. Upon activator binding, they modulate the expression of various target genes implicated in several important physiological pathways. During the past few years, the identification of both PPAR ligands, natural and synthetic, and PPAR targets and their associated functions has been one of the most important achievements in the field. It underscores the potential therapeutic application of PPAR-specific compounds on the one side, and the crucial biological roles of endogenous PPAR ligands on the other.     

The microbial and danger signals that activate Nod-like receptors

Nod-like receptors (NLRs) are a family of intracellular sensors that play key roles in innate immunity and inflammation. While some NLRs, including Nod1, Nod2, NAIP and IPAF, detect conserved bacterial molecular signatures from within the host cytosol, other members of this family seem to have evolved the capacity to sense danger signals perhaps independently of a microbial trigger. This is illustrated by the discovery that Nalp3 and Nalp1 are specifically activated by low concentrations of intracellular potassium. The fact that several stimuli, including bacterial toxins and some viruses, but also sterile crystals made of uric acid, asbestos or aluminium hydroxide, can trigger the Nalp3 inflammasome illustrate the fascinating prospect that microbial infections and certain danger signals may be perceived similarly by host recognition systems. Gaining insight into the function of NLR proteins in general will impact considerably on our understanding of the mechanisms underlying immunity to infection, adjuvanticity and auto-inflammatory disorders. In this review, we summarize the current knowledge on the microbial- and danger-derived signals that activate NLRs.     

Eph receptors and ephrins: effectors of morphogenesis

Eph receptor tyrosine kinases and their ligands, the ephrins, appear to lie functionally at the interface between pattern formation and morphogenesis. We review the role of Eph and ephrin signalling in the formation of segmented structures, in the control of axon guidance and cell migration and in the development of the vasculature. We address the question of how the specificity of response is achieved and discuss the specificity of ephrin-Eph interactions and the significance of structural domains in Eph receptors.     

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.     

Control of cell behaviour by signalling through Eph receptors and ephrins

Eph receptor tyrosine kinases and ephrins mediate contact-dependent cell interactions that regulate the repulsion and adhesion mechanisms involved in the guidance and assembly of cells. Recent work has revealed a role of overlapping Eph receptor and ephrin expression in modulating neuronal growth cone repulsion, and has shown that bidirectional activation restricts intermingling and communication between cell populations. In addition, progress has been made in understanding how Eph receptors and ephrins control cell adhesion.     

The ephrins and Eph receptors in angiogenesis.

Eph receptors are a unique family of receptor tyrosine kinases that play critical roles in embryonic patterning, neuronal targeting, vascular development and adult neovascularization. Engagement of Eph receptors by ephrin ligands mediates critical steps of angiogenesis, including juxtacrine cell-cell contacts, cell adhesion to extracellular matrix, and cell migration. Recent evidence from in vitro angiogenesis assays and analysis of mice deficient for one or more members of the Eph family establishes the role of Eph signaling in sprouting angiogenesis and blood vessel remodeling during vascular development. Furthermore, elevated expression of Eph receptors and ephrin ligands is associated with tumors and associated tumor vasculature, suggesting that Eph receptors and their ephrin ligands also play critical roles in tumor angiogenesis and tumor growth. This review will focus on the relevance of Eph receptor signaling in embryonic and adult neovascularization, and possible contributions to tumor growth and metastasis.     

Eph-ephrin与突触可塑性

Eph受体酪氨酸激酶及其配体ephrin广泛参与神经系统的发育,如轴突导向、细胞迁移、体节形成和血管生成。最近研究显示的Ephephrin在突触的定位提示其与突触可塑性有关。Ephephrin对成年神经系统的可塑性、学习和记忆,以及神经损伤后的再生可能具有重要的调节作用。     

Eph receptors tangled up in two: Independent control of cell positioning and proliferation

Comment on: Genander M, et al. Cell 2009; 139:679-92.     

The role of ephrins and Eph receptors in cancer

Eph receptors are the largest receptor tyrosine kinase family of transmembrane proteins with an extracellular domain capable of recognizing signals from the cells’ environment and influencing cell–cell interaction and cell migration. Ephrins are the ligands to Eph receptors and stimulate bi-directional signaling of the Eph/ephrin axis. Eph receptor and ephrin overexpression can result in tumorigenesis as related to tumor growth and survival and is associated with angiogenesis and metastasis in many types of human cancer. Recent data suggest that Eph/ephrin signaling could play an important role in the development of novel inhibition strategies and cancer treatments to potentially target this receptor tyrosine kinase and/or its ligand. A deeper understanding of the molecular basis for normal versus defective cell–cell interaction through the Eph/ephrin axis will enable the potential development of novel cancer treatments. This review emphasizes the biology of Eph/ephrin as well as the potential for novel targeted therapy through this pathway.     

The Eph family in the patterning of neural development

The Eph family represents the largest subfamily of receptor tyrosine kinases. Its members are predominantly expressed in the developing and adult nervous system. Besides playing an important role in the contact-mediated repulsion of axons, they have recently also been implicated in the control of cell migration. Characteristics of the Eph family are extended promiscuity in the interaction between receptors and ligands, the necessity of membrane attachment of the ligands to exert their function, the lack of induction of mitogenic responses, and the bi-directional signalling of receptors and ligands.     

Eph receptors: two ways to sharpen boundaries

Eph receptors and ephrins can sharpen domains within developing tissues by mediating repulsion at interfaces. An Eph receptor has now been shown also to regulate cell adhesion within tissue subdivisions.     

Additional signals from VPAC/PAC family receptors

The receptors for the neuropeptides vasoactive intestinal polypeptide and pituitary adenylate cyclase-activating polypeptide are strong activators of adenylate cyclase, but recent evidence suggests that they can elicit a number of additional intracellular signals. Some of these are likely to be downstream of the conventional adenylate cyclase pathway, but it is now clear that others reflect novel primary coupling events of the receptors.     

Protein surface recognition by synthetic agents: design and structural requirements of a family of artificial receptors that bind to cytochrome c

The design, synthesis, and evaluation of a novel series of receptors for protein surface recognition are described. The design of these agents is based around the attachment of four constrained peptide loops onto a central calix[4]arene scaffold. This arrangement mimics the role of the hypervariable loops in antibody combining regions and defines a large surface area for binding to a complementary region of the exterior of a target protein. Using affinity and gel filtration chromatographies we show that one particular receptor binds strongly to the surface of cytochrome c. The site of binding is presumably close to the heme edge region, which contains several charged lysine residues. This is supported by the observation that the receptor inhibits the reduction of Fe(III) cytochrome c to its Fe(II) form. We also show that binding is strongly dependent on the nature of the substituents on the lower rim of the calixarene. The nmr and computational studies suggest that this effect may be due to conformational differences among the differently substituted receptors.