Pathway Map Details

Cell adhesion_Ephrins signaling

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p120GAP, RHO6, PAK1, Ephexin, Ephrin-A5, Grb4, PI3K cat class IB, CDC42, NCK1, Ephrin-A receptors, Intersectin, VAV-2, RAP-1A, Ephrin-A receptor 8, Paxillin, ADAM10, Ephrin-A, Ephrin-B receptor 1, TAK1, c-Src, Fyn, Tiam 1, G-protein alpha-i, HGK, H-Ras, glycosyl- phosphatidyl- inositol, GRB2, Fyn, GRB7, Ephrin-B, PtdIns(3,4,5)P3, PtdIns(4,5)P2, Ephrin-A2, Ephrin-B receptors, c-Raf-1,, FAK1, RGS3, Ephrin-A receptor 2, FAK1, c-Src, RhoA, Rac1, GRB10, SLAP, Kalirin, FAP-1, JNK1



The Ephrin receptor tyrosine kinases and their Ephrin ligands play a pivotal role during axon guidance, synaptogenesis, neuronal circuitry formation, angiogenesis and proliferation of neuronalstem cells [1]. Ephrin receptors and Ephrin ligands transduce intracellular responses only upon binding and clustering in the membrane [2].

Ephrin-A receptors (Ephrin-A receptors 1-8) bind glycosylphosphatidyl-anchored Ephrin-A ligands (Ephrin-A1-5), whereas Ephrin-B receptors (Ephrin-B receptors 1-6) bind transmembrane Ephrin-B ligands (Ephrin-B1-3) [3].

Ephrin receptors signaling occurs through specific Guanine nucleotide exchange factors (GEFs) and therby can activate multiple Rho family GTPases including RhoA, Rac1 and CDC42.

Ephrin-A stimulation of Ephrin-A receptors activates three exchange factors: Ephexin [4], VAV-2 [4] and Tiam 1 [5]. VAV-2 can also bind to Ephrin-B receptors.

In the absence of Ephrin-A stimulation, Ephrin-A receptors alternatively engage Ephexin at the plasma membrane. This interaction induces Ephexin phosphorylation by c-Src tyrosine kinase [6] and this phosphorylation enhances Ephexin activity toward the GTPase RhoA and not Rac1 or CDC42 [7].

VAV-2 is rapidly phosphorylated by c-Src upon Ephrin stimulation of both Ephrin-A receptors and Ephrin-B receptors [4] leading to RhoA activation [8].

RhoA -dependent signaling in both cases leads to the growth cone retraction and collapse.

The growth cone collapse may be due to Rac1 -dependent endocytosis events. Following Ephrin-A activation, VAV-2 induces activation of Rac1 which leads to actin cytoskeleton reorganization and endocytosis [3].

Ephrin-A receptors also signal through the Rac1 exchange factor Tiam 1 to promote neurite outgrowth [5].

Ephrin-A receptor 8 localizes p110gamma isoform of phosphatidylinositol 3-kinase ( PI3K cat class IB (p110-gamma) ) to the plasma membrane, thereby allowing access to lipid substrates that facilitate integrin-mediated cell adhesion [9].

Src-like adapter protein SLAP binds to activated Ephrin-A receptor 2 [10] and this interaction leads to the inhibiting c-Src signaling [11].

Guanine exchange factors Kalirin and Intersectin are downstream effectors of Ephrin-B receptors. Kalirin and Intersectin promote dendritic spine morphogenesis by modulating Rac1 and CDC42 activity, respectively [12]. Intersectin binds to Ephrin-B receptors independently of activation by Ephrins, while Kalirin appear to require Ephrin stimulation. Kalirin is also phosphorylated on tyrosine residues following Ephrin-B receptors activation [13].

The majority of Ephrin receptors negatively regulate the Ras/ MAP-kinase pathways in most cell types [14]. For instance, Ephrin-B receptor 2 via GTPase activated protein (GAP), p120GAP, down-regulates H-Ras activity and MAP kinase phosphorylation and induces neurite retraction in the some neuronal cell lines [15] However the phosphorylation of p120GAP by c-Src inhibites its GAP activity [16]. Ephrin-A1 stimulation leads to Ras-related protein Rap-1A activation [17] and inhibits MAPK signaling cascade by decreasing c-Raf-1 kinase activation [18]. Alternatively c-Raf-1 can also be phosphorylated and activated by PAK1 [19]. Recruitment of the adaptor proteins GRB2 and GRB10 to the activated Ephrin-B receptor 1 also promotes MAP-kinase activation [20], [21].

Ephrin-B receptor 1 also associates with GRB7 [22], that specifically activates RHO6, a member of Rho family GTPases, and promotes axon growth repulsion [23].

Ephrin-B receptor 1 and Ephrin-B receptor 2 bind adaptor protein NCK1, thereby increasing the activity of specifically Nck-interacting kinase HGK [24]. HGK -induced JNK (stress-activated protein kinases) activation leads to the phosphorylation of Paxillin by JNK, which is essential for maintaining the dynamic cytoskeletal remodeling required for rapid cell migration [25].

Ephrin receptors also maintain feedback mechanisms that reverse signaling through their Ephrin ligands [14].

Src family kinases are responsible for Ephrin-B phosphorylation upon Ephrin receptor engagement [26]. The adaptor protein GRB4 links Ephrin-B to a vast signaling network that modifies cell morphology through reorganization of the actin cytoskeleton. Phosphorylated Ephrin-B recruits the phosphotyrosine phosphatase FAP-1, that dephosphorylates the cytoplasmic domain of Ephrin-B [26].

The GTPase-activating protein RGS3, can also transduce Ephrin-B signaling by catalyzing the hydrolysis of GTP to GDP in the alpha-i-subunits of G-proteins ( G-protein alpha-i family ). This signaling mechanism has broad implications for cell migratory behavior in different systems [14].

Ephrin-A ligands can also induce signals that modify cell behavior. Clustering of Ephrin-A molecules with Ephrin-A receptors recruits the Src family kinase Fyn to lipid rafts. This is accompanied by activation of MAP kinases and leading to an increase in cellular adhesion [27], [14].

Inhibition of Ephrin-A signaling may be modulated at the cell surface by induction of ligand-receptor dissociation by the metalloprotease ADAM10. Upon binding of Ephrin-A receptors, ADAM10 cleaves Ephrin-A2 ligands from the cell surface [28], serving two functions: 1) Ephrin-A cleavage allows Ephrin-A-receptor -bearing structures such as growth cones to revert from cellular adhesion to repulsion, and 2) ligand cleavage leads to direct inhibition of receptor activation [14].


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