Ira O. Daar, Ph.D.
Cancer and Developmental Biology Laboratory
Building 560, Room 12-88
Frederick, MD 21702-1201
Developmental Signal Transduction
Our current research interests are focused on the mechanism by which Eph receptor tyrosine kinases and their ephrin ligands signal events affecting cell-cell adhesion and morphogenetic movements. From the elucidation of these signal transduction pathways we may improve our understanding of oncogenesis. The cell-cell adhesion system plays a major role in normal development and morphogenesis. Inactivation of this adhesion system is thought to play a critical role in cancer invasion and metastasis. The Xenopus embryo is well suited for investigations of these processes because the frog has a well characterized and invariant cell fate map and cell lineage can be easily traced during experiments. Mutant receptors, ligands, and other proteins can be ectopically expressed in embryos. Thus, their effects on signal transduction, motility, and differentiation can be assessed morphologically and histologically as well as biochemically in a developing vertebrate.
Our laboratory is currently investigating the role of the Eph receptor tyrosine kinases and ephrinB transmembrane ligands in cell signaling and function using the Xenopus embryo. We have placed some emphasis upon the mechanism by which these Eph family members send signals affecting morphogenetic movements and exhibit crosstalk with the fibroblast growth factor (FGF) signaling pathway. Members of the Eph family have been implicated in regulating numerous developmental processes, and their de-regulation is found in several metastatic cancers.
Using the Xenopus embryonic system, we have demonstrated that signaling mediated by the intracellular domain of ephrinB affects cell-cell adhesion, and that this activity can be modulated by tyrosine phosphorylation initiated by binding to the extracellular domain of a cognate Eph receptor or by an interaction with an activated FGF receptor. Using the epithelial cells of early stage Xenopus embryos, we previously showed that loss- or gain-of function of ephrinB1 can disrupt cell-cell contacts and tight junctions (Lee HS, et al., Nature Cell Biol. 2008). This study reveals a mechanism where ephrinB1 competes with active Cdc42 for binding to Par-6, a scaffold protein central to the Par polarity complex (Par-3/Par-6/Cdc42/aPKC) and disrupts the localization of tight junction-associated proteins (ZO-1, Cingulin). This competition affects formation of tight junctions, and is regulated by tyrosine phosphorylation of ephrinB1.
In addition to our interest in cell-cell adhesion, our laboratory has focused on the role of ephrinB1 in cell movement. We along with our collaborators in Sally Moody’s laboratory reported that Dishevelled (a scaffold central to the Wnt signaling pathway) mediates ephrinB signaling that controls retinal progenitor cell movement into the eye field (Lee HS, et al., Nature Cell Biol. 2006), and we previously showed that morphogenetic movements underlying eye field formation require interactions between the FGF and ephrinB1 signaling pathways. We determined the mechanism by which FGFR modulates signaling from the ephrinB1 molecule; that FGFR or Eph-induced phosphorylation of ephrinB1 disrupts the ephrinB1/Dishevelled interaction, leading to a loss of ephrinB1-induced planar cell polarity (PCP) signaling (Moore KB, et al., Dev. Cell 2004; Lee HS, et al., Mol. Biol. Cell 2009). Thus, cross-talk between FGF signaling and the ephrinB1/Dsh/PCP pathway can regulate the movements and positioning of specific progenitor cells during embryogenesis.
Although these studies allowed us to gain mechanistic insight into how ephrinB1 regulates cell movement and cell-cell boundaries, we understood little of how ephrinBs are regulated. Since we believe that the regulation of ephrinB proteins will be a critical process in controlling morphogenetic events, we have recently turned our attention to the post-translational regulation of ephrinBs. One of our studies has revealed a system of differential interactions between ephrinB1 and the E3 ubiquitin ligases, Smurf1 and Smurf2. These interactions regulate the maintenance of tissue boundaries through the control of ephrinB protein levels (Hwang YS, et al., Genes Dev. 2013). In another study that examines regulation of ephrinB2, we identified a new ephrinB2 interacting protein, the flotillin-1 scaffold. The presence of this protein is critical to maintain ephrinB2 protein levels. Loss of flotillin-1 renders eprhinB2 susceptible to cleavage by the ADAM10 metalloprotease, and leads to the failure of neural tube closure, an important morphogenetic event (Ji YJ, et al., Nature Comm. 2014). We continued studies examining ephrinB2, and found that ephrinB2 regulates contact inhibition of locomotion (CIL) in migrating neural crest cells through its association with TBC1d24, a Rab-GAP protein that negatively regulates Ecadherin recycling via Rab35 (Yoon et al., Nature Comm. 2018). These findings provide significant mechanistic insight into the signaling pathways that regulate CIL and cell movement during embryogenesis and in cancer progression.
Our future goals are to discover how the interactions between Eph/ephrin and other signaling pathways (such as Wnt, FGF, and TGF-β coordinate morphogenesis. Interactions between Eph/ephrin and other pathways may serve as an apparatus to organize the extracellular inputs received by cells at critical determination points to direct a morphogenetic outcome. When this signaling system goes awry, a disease state (such as birth defects and cancer progression) may be the outcome.
Gaur S, Mandelbaum M, Herold M, Majumdar HD, Neilson KM, Maynard TM, Mood K, Daar IO, Moody SA. Neural transcription factors bias cleavage stage blastomeres to give rise to neural ectoderm. Genesis. 2016;54(6):334-49.
Ji YJ, Hwang YS, Mood K, Cho HJ, Lee HS, Winterbottom E, Cousin H, Daar IO. EphrinB2 affects apical constriction in Xenopus embryos and is regulated by ADAM10 and flotillin-1. Nat Commun. 2014;5:3516.
Cho HJ, Hwang YS, Mood K, Ji YJ, Lim J, Morrison DK, Daar IO. EphrinB1 interacts with CNK1 and promotes cell migration through c-Jun N-terminal kinase (JNK) activation. J Biol Chem. 2014;289(26):18556-68.
Lu Q, Insinna C, Ott C, Stauffer J, Pintado PA, Rahajeng J, Baxa U, Walia V, Cuenca A, Hwang YS, Daar IO, Lopes S, Lippincott-Schwartz J, Jackson PK, Caplan S, Westlake CJ. Early steps in primary cilium assembly require EHD1/EHD3-dependent ciliary vesicle formation. Nat Cell Biol. 2015;17(3):228-240.
Hwang YS, Lee HS, Kamata T, Mood K, Cho HJ, Winterbottom E, Ji YJ, Singh A, Daar IO. The Smurf ubiquitin ligases regulate tissue separation via antagonistic interactions with ephrinB1. Genes Dev. 2013;27(5):491-503.
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This page was last updated on August 30th, 2018