Interaction between ephrins/Eph receptors and excitatory amino acid receptors: possible relevance in the regulation of synaptic plasticity and in the pathophysiology of neuronal degeneration

There is increasing evidence that Eph receptors and their transmembrane ligands, named ephrins, interact with glutamate receptors in both developing and adult neurons. EphB receptors interact with proteins that regulate the membrane trafficking of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunits, and both ephrins and EphB receptors have been found to co-localize with N-methyl-d-aspartate (NMDA) receptors and to positively modulate NMDA receptor function. Moreover, pharmacologic activation of ephrin-Bs amplifies group-I metabotropic glutamate receptor signaling through mechanisms that involve NMDA receptors. The interaction with ionotropic or metabotropic glutamate receptors provides a substrate for the emerging role of ephrins and Eph receptors in the regulation of activity-dependent forms of synaptic plasticity, such as long-term potentiation and long-term depression, which are established electrophysiologic models of associative learning. In addition, these interactions explain the involvement of ephrins/Eph receptors in the regulation of pain threshold and epileptogenesis, as well as their potential implication in processes of neuronal degeneration. This may stimulate the search for new drugs that might modulate excitatory synaptic transmission by interacting with the ephrin/Eph receptor system.     

Axon guidance processes in the retinotectal and vomeronasal projection are controlled by Eph receptor tyrosine kinases and ephrins

The Eph family of receptor tyrosine kinases and their ‘ligands’, the ephrins, have been shown to play key roles in a number of different developmental processes such as cell migration, boundary formation, axon guidance, synapse formation and vasculogenesis. Here, we summarize recent findings derived from investigating the role of the EphA family during development of the retinotectal and vomeronasal projection uncovering a role of ephrin-A molecules as axon guidance receptors.     

Irradiation affects cellular properties and Eph receptor expression in human melanoma cells

X-ray irradiation influences metastatic properties of tumor cells and, moreover, metastasis and cellular motility can be modified by members of the Eph receptor/ephrin family of receptor tyrosine kinases. We hypothesized that irradiation-induced changes in cellular properties relevant for metastasis in melanoma cells could be mediated by Eph receptor/ephrin signaling. In this pilot study, we analyzed one pre-metastatic (Mel-Juso) and three metastatic human melanoma (Mel-Juso-L3, A375, and A2058) cells lines and predominantly found anti-metastatic effects of X-ray irradiation with impaired cell growth, clonal growth and motility. Additionally, we observed an irradiation-induced increase in adhesion paralleled by a decrease in migration in Mel-Juso and Mel-Juso-L3 cells and, in part, also in A375 cells. We further demonstrate a decrease of EphA2 both in expression and activity at 7 d after irradiation paralleled by an upregulation of EphA3. Analyzing downstream signaling after irradiation, we detected decreased Src kinase phosphorylation, but unchanged focal adhesion kinase (FAK) phosphorylation, indicating, in part, irradiation-induced downregulation of signaling via the EphA2-Src-FAK axis in melanoma cells. However, to which extent this finding contributes to the modification of metastasis-relevant cellular properties remains to be elucidated.     

Irradiation affects cellular properties and Eph receptor expression in human melanoma cells

X-ray irradiation influences metastatic properties of tumor cells and, moreover, metastasis and cellular motility can be modified by members of the Eph receptor/ephrin family of receptor tyrosine kinases. We hypothesized that irradiation-induced changes in cellular properties relevant for metastasis in melanoma cells could be mediated by Eph receptor/ephrin signaling. In this pilot study, we analyzed one pre-metastatic (Mel-Juso) and three metastatic human melanoma (Mel-Juso-L3, A375, and A2058) cells lines and predominantly found anti-metastatic effects of X-ray irradiation with impaired cell growth, clonal growth and motility. Additionally, we observed an irradiation-induced increase in adhesion paralleled by a decrease in migration in Mel-Juso and Mel-Juso-L3 cells and, in part, also in A375 cells. We further demonstrate a decrease of EphA2 both in expression and activity at 7 d after irradiation paralleled by an upregulation of EphA3. Analyzing downstream signaling after irradiation, we detected decreased Src kinase phosphorylation, but unchanged focal adhesion kinase (FAK) phosphorylation, indicating, in part, irradiation-induced downregulation of signaling via the EphA2-Src-FAK axis in melanoma cells. However, to which extent this finding contributes to the modification of metastasis-relevant cellular properties remains to be elucidated.     

Ligand binding induces Cbl-dependent EphB1 receptor degradation through the lysosomal pathway

Eph receptor tyrosine kinases play a critical role in embryonic patterning and angiogenesis. In the adult, they are involved in carcinogenesis and pathological neovascularization. However, the mechanisms underlying their role in tumor formation and metastasis remain to be defined. Here, we demonstrated that stimulation of EphB1 with ephrinB1/Fc led to a marked downregulation of EphB1 protein, a process blocked by the lysosomal inhibitor bafilomycin. Following ephrinB1 stimulation, the ubiquitin ligase Cbl was recruited by EphB1 and then phosphorylated. Both Cbl phosphorylation and EphB1 ubiquitination were blocked by the Src inhibitor PP2. Overexpression of wild-type Cbl, but not of 70Z mutant lacking ligase activity, enhanced EphB1 ubiquitination and degradation. This negative regulation required the tyrosine kinase activity of EphB1 as kinase-dead EphB1-K652R was resistant to Cbl. Glutathione S-transferase binding experiments showed that Cbl bound to EphB1 through its tyrosine kinase-binding domain. In aggregate, we demonstrated that Cbl induces the ubiquitination and lysosomal degradation of activated EphB1, a process requiring EphB1 and Src kinase activity. To our knowledge, this is the first study dissecting the molecular mechanisms leading to EphB1 downregulation, thus paving the way to new means of modulating their angiogenic and tumorigenic properties.     

EphrinB3 blocks EphB3 dependence receptor functions to prevent cell death following traumatic brain injury

Eph receptor tyrosine kinases and their membrane-bound ligands, ephrins, have a variety of roles in the developing and adult central nervous system that require direct cell–cell interactions; including regulating axon path finding, cell proliferation, migration and synaptic plasticity. Recently, we identified a novel pro-survival role for ephrins in the adult subventricular zone, where ephrinB3 blocks Eph-mediated cell death during adult neurogenesis. Here, we examined whether EphB3 mediates cell death in the adult forebrain following traumatic brain injury and whether ephrinB3 infusion could limit this effect. We show that EphB3 co-labels with microtubule-associated protein 2-positive neurons in the adult cortex and is closely associated with ephrinB3 ligand, which is reduced following controlled cortical impact (CCI) injury. In the complete absence of EphB3 (EphB3^−/−), we observed reduced terminal deoxynucleotidyl transferase-dUTP nick end labeling (TUNEL), and functional improvements in motor deficits after CCI injury as compared with wild-type and ephrinB3^−/− mice. We also demonstrated that EphB3 exhibits dependence receptor characteristics as it is cleaved by caspases and induces cell death, which is not observed in the presence of ephrinB3. Following trauma, infusion of pre-clustered ephrinB3-Fc molecules (eB3-Fc) into the contralateral ventricle reduced cortical infarct volume and TUNEL staining in the cortex, dentate gyrus and CA3 hippocampus of wild-type and ephrinB3^−/− mice, but not EphB3^−/− mice. Similarly, application of eB3-Fc improved motor functions after CCI injury. We conclude that EphB3 mediates cell death in the adult cortex through a novel dependence receptor-mediated cell death mechanism in the injured adult cortex and is attenuated following ephrinB3 stimulation.     

Toward the semisynthesis of multidomain transmembrane receptors: modification of Eph tyrosine kinases

Expressed protein ligation (EPL) is a protein engineering approach that allows the modification or assembly of a target protein from multiple recombinant and synthetic polypeptides. EPL has been previously used to modify intracellular proteins and small integral membrane proteins for structural and functional studies. Here we describe the semisynthetic site-specific modification of the complete, multidomain extracellular regions of both A and B classes of Eph receptor tyrosine kinases. We show that the ectodomains of these receptors can be ligated to different peptides under carefully established experimental conditions, while their biological activity is retained. This work extends the boundaries of the EPL technique for semisynthesis of multidomain, extracellular, disulfide-bonded, and glycosylated proteins and highlights its potential application for reconstituting entire single-pass transmembrane proteins.