ReviewNon-SH2/PDZ reverse signaling by ephrins
Introduction
Members of the Eph/ephrin family have been implicated in regulating numerous morphogenetic processes such as axon outgrowth, neural crest and retinal progenitor cell migration, hindbrain segmentation, skeletal patterning, and angiogenesis [1], [2], [3]. Interactions between the Eph receptor tyrosine kinases residing on one cell with their membrane bound ligands (ephrins) on another cell results in bi-directional signaling, in which both molecules transmit intracellular signals upon cell–cell contact. Although there is evidence supporting an ultimate role for both Eph receptors and ligands in affecting small GTPases such as Rho, numerous signaling molecules and pathways intersect with Eph receptor or ligand signaling. Additional studies are needed to define the Eph/ephrin signal transduction systems in various cellular contexts. Cell–cell contact events during development can initiate Eph/ephrin signaling that leads to cell sorting and boundary formation between receptor and ligand bearing cells [4]. When motile ligand or receptor-bearing cells come in contact with cells expressing the cognate receptor or ligand, the response is often adhesion or repulsion. Alternative growth factors and signaling pathways can mediate or regulate Eph/ephrin signaling to cooperatively regulate the movement and positioning of the cognate receptor or ligand-bearing cells [2]. These ligands and receptors play important roles in several morphogenetic events during development, but when de-regulated can lead to cancer invasion and metastasis [5], [6]. Recent data also show that members of the Eph/ephrin family mediate cell–cell interactions both in tumor cells and in the tumor microenvironment (i.e. stroma and vasculature) [7], [8].
Section snippets
Ephrin-A and -B ligands
Eph receptors are transmembrane receptor tyrosine kinases possessing an extracellular domain that includes an N-terminal ligand-binding domain, a cysteine-rich EGF-like domain, and two fibronectin type III motifs. These receptors are divided into two subclasses (A and B) by sequence similarities and binding specificity toward two subclasses of ligands (A and B) known as ephrins. The ephrins are all membrane-bound proteins with the A subclass consisting of glycosylphosphatidylinositol
Ephrin-Bs and reverse signaling in tissue boundaries
Ephrin-Bs and reverse signaling have been shown to play a role in tissue boundary formation that is dependent upon cell–cell adhesive interactions [4], [55]. One example of this process is hindbrain segmentation (Fig. 4), where Eph receptors and ephrins are expressed in alternating rhombomeres and are required for the proper sorting of cells at rhombomere boundaries [56], [57]. Using ectodermal explants from zebrafish, it was shown that bidirectional (but not unidirectional) signaling between
Reverse signaling affecting cell–cell contact and movement
Recent work indicates that ephrin-B2 over-expression in endothelial cells, increases motility and triggers repeated cycles of actomyosin-dependent cell contraction as well as cell spreading in the absence of receptor (101). In response to soluble recombinant EphB4, cell shape changes were observed, but in a non-repetitive fashion that ceased with ligand internalization [73]. The C-terminal PDZ binding motif of ephrin-B is required for the morphological alterations within the cell, but the
Outstanding issues
Significant gains have been made in our understanding of the mechanisms of action and regulation of ephrin-A and -B reverse signaling in cell adhesive events, but further studies are required. For A-type reverse signaling, it is still unclear how a GPI-linked ligand transduces a signal, and what mechanism and molecules are engaged proximally that lead to the reported downstream effects. Although ephrin-Bs are more thoroughly studied in this regard, many interesting questions remain. How does
Acknowledgement
I wish to apologize to all of my colleagues whose work was not cited in this review. Many have contributed greatly to our understanding of the role of the Eph–ephrin system in biology, but space considerations prevented the inclusion of their work. Research in the Daar laboratory on the described topics was supported by the Intramural Research Program of the NIH, National Cancer Institute.
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Cited by (50)
Cellular and molecular mechanisms of EPH/EPHRIN signaling in evolution and development
2022, Current Topics in Developmental BiologyCitation Excerpt :The EPHRIN-B C-terminal PDZ binding motif has also been shown to bind to a handful of PDZ-proteins independent of EPH-receptor binding to transduce a reverse signal (Lin, Gish, Songyang, & Pawson, 1999; Torres et al., 1998). Both phosphorylation-dependent and PDZ-dependent modes of reverse signaling have been well-studied, but other less-studied mechanisms, including serine phosphorylation of EPHRIN-B2, also exist (Bush & Soriano, 2012; Daar, 2012; Davy & Soriano, 2005; Niethamer & Bush, 2019). We recently published an extensive review of genetic interrogation of forward and reverse signaling function of B-type EPHRINs in mice (Niethamer & Bush, 2019), which led us to speculate that some of the in vivo functions currently attributed to B-type reverse signaling may instead be due to impacts of EPHRIN-B intracellular mutations on forward signaling; here, I will not dwell on this challenging question beyond noting whether forward or reverse signaling has been implicated.
EFNA4 promotes cell proliferation and tumor metastasis in hepatocellular carcinoma through a PIK3R2/GSK3β/β-catenin positive feedback loop
2021, Molecular Therapy Nucleic AcidsCitation Excerpt :EFNA ligands bind to the corresponding EPH receptor, activate the tyrosine kinase in the cytoplasm of the receptor by changing the conformation of EPH, and result in phosphorylation of the corresponding receptor and activation of downstream signaling.3 In addition, EFNA ligands activate the relevant surface receptors of their host cells, such as the p75NT receptor (p75NTR).3,4 EFNA4 is mainly expressed in the spleen, lymph nodes, ovary, small intestine, and colon of adults, as well as in the heart, lungs, liver, and kidneys of the fetus.
Getting direction(s): The Eph/ephrin signaling system in cell positioning
2019, Developmental BiologyCitation Excerpt :However, Eph/ephrin signaling via cellular protrusions may be capable of mediating signaling between nonadjacent cells (Cayuso et al., 2016), and release of Ephs and ephrins by exosomes also allows for the possibility of signaling at greater distances (Gong et al., 2016). Whereas Eph and ephrin ectodomains can be proteolytically cleaved (Georgakopoulos et al., 2006; Hattori et al., 2000), the ectodomain alone is incapable of initiating oligomerization and is therefore unlikely to activate forward signaling; indeed, the unclustered ectodomain is often used as a competitive antagonist (Daar, 2012; Pegg et al., 2017), suggesting that antagonistic modulation of signaling at a distance by soluble Eph/ephrin ectodomains may be possible. Adding an additional layer of complexity to the regulation of Eph/ephrin signaling, there is evidence that expression of Eph receptors and ephrins in the same cell (in cis) can negatively regulate Eph receptor signaling through ephrins in an adjacent cell (in trans) (Hornberger et al., 1999).
Autonomous and non-autonomous roles for ephrin-B in interneuron migration
2017, Developmental BiologyCitation Excerpt :Unfortunately, we cannot assess the interneuron populations in the EB26YFΔV/6YFΔV adult brain because the ‘weak-ligand’ activity in the homozygotes provides too little forward signaling for lymphatic/blood vessels and animal viability. Nevertheless, the results observed in the embryo indicate that SH2/PDZ binding domains of ephrin-B2 reverse signaling are not absolutely required for interneuron migration and raises the possibility that other phosphorylation sites such as serine residues in the intracellular domain, or perhaps other binding and signaling avenues (e.g. Par6, Connexin43, and Claudins), are involved in the migration of interneurons (Daar, 2012). Furthermore, it is entirely possible that additional modes or components of EphB/ephrin-B cell-cell signaling are at play during interneuron migration, including involvement of trans-endocytosis, downstream Rho/Rac/Cdc42 signaling, or perhaps even Eph-independent mechanisms (Lisabeth et al., 2013).
Taspine derivative 12k suppressed A549 cell migration through the Wnt/β-catenin and EphrinB2 signaling pathway
2017, Biomedicine and PharmacotherapyCitation Excerpt :Meanwhile, MMP3 and MMP7 are targeted protein of canonical Wnt signal pathway, which is accordance with the inhibition of Wnt signal molecules. EphrinB2 is well known for its vital role in angiogenesis during development and disease [17]. We detected the expression of EphrinB2 in non-small lung cancer cell lines including A549, NCI-H1299 and NCI-H460 cells.