Robo4 maintains vessel integrity and inhibits angiogenesis by interacting with UNC5B

Robo4 is an endothelial cell-specific member of the Roundabout axon guidance receptor family. To identify Robo4 binding partners, we performed a protein-protein interaction screen with the Robo4 extracellular domain. We find that Robo4 specifically binds to UNC5B, a vascular Netrin receptor, revealing unexpected interactions between two endothelial guidance receptors. We show that Robo4 maintains vessel integrity by activating UNC5B, which inhibits signaling downstream of vascular endothelial growth factor (VEGF). Function-blocking monoclonal antibodies against Robo4 and UNC5B increase angiogenesis and disrupt vessel integrity. Soluble Robo4 protein inhibits VEGF-induced vessel permeability and rescues barrier defects in Robo4(-/-) mice, but not in mice treated with anti-UNC5B. Thus, Robo4-UNC5B signaling maintains vascular integrity by counteracting VEGF signaling in endothelial cells, identifying a novel function of guidance receptor interactions in the vasculature.     

Regulatory signals for endothelial podosome formation

Podosomes are punctate actin-rich adhesion structures which spontaneously form in cells of the myelomonocytic lineage. Their formation is dependent on Src and RhoGTPases. Recently, podosomes have also been described in vascular cells. These podosomes differ from the former by the fact that they are inducible. In endothelial cells, such a signal can be provided by either constitutively active Cdc42, the PKC activator PMA or TGFbeta, depending on the model. Consequently, other regulatory pathways have been reported to contribute to podosome formation. To get more insight into the mechanisms by which podosomes form in endothelial cells, we have explored the respective contribution of signal transducers such as Cdc42-related GTPases, Smads and PKCs in three endothelial cell models. Results presented demonstrate that, in addition to Cdc42, TC10 and TCL GTPases can also promote podosome formation in endothelial cells. We also show that PKCalpha can be either necessary or entirely dispensable, depending on the cell model. In contrast, PKCdelta is essential for podosome formation in endothelial cells but not smooth muscle cells. Finally, although podosomes vary very little in their molecular composition, the signalling pathways involved in their assembly appear very diverse.     

Oligodendrocyte development in the embryonic brain: the contribution of the plp lineage

Oligodendrocytes are the myelin forming cells of the central nervous system. Over the last decade, their development in the embryonic brain and spinal cord has been documented following the discovery of early oligodendroglial markers. This review highlights the fundamental results obtained on the specification and migration of oligodendroglial cells and illustrates our advances in the knowledge of the cell lineage expressing plp (proteolipid protein), one of the early oligodendroglial genes.     

Defective neuroepithelial cell cohesion affects tangential branchiomotor neuron migration in the zebrafish neural tube

Facial branchiomotor neurons (FBMNs) in zebrafish and mouse embryonic hindbrain undergo a characteristic tangential migration from rhombomere (r) 4, where they are born, to r6/7. Cohesion among neuroepithelial cells (NCs) has been suggested to function in FBMN migration by inhibiting FBMNs positioned in the basal neuroepithelium such that they move apically between NCs towards the midline of the neuroepithelium instead of tangentially along the basal side of the neuroepithelium towards r6/7. However, direct experimental evaluation of this hypothesis is still lacking. Here, we have used a combination of biophysical cell adhesion measurements and high-resolution time-lapse microscopy to determine the role of NC cohesion in FBMN migration. We show that reducing NC cohesion by interfering with Cadherin 2 (Cdh2) activity results in FBMNs positioned at the basal side of the neuroepithelium moving apically towards the neural tube midline instead of tangentially towards r6/7. In embryos with strongly reduced NC cohesion, ectopic apical FBMN movement frequently results in fusion of the bilateral FBMN clusters over the apical midline of the neural tube. By contrast, reducing cohesion among FBMNs by interfering with Contactin 2 (Cntn2) expression in these cells has little effect on apical FBMN movement, but reduces the fusion of the bilateral FBMN clusters in embryos with strongly diminished NC cohesion. These data provide direct experimental evidence that NC cohesion functions in tangential FBMN migration by restricting their apical movement.     

The role of adhesion energy in controlling cell-cell contacts

Recent advances in microscopy techniques and biophysical measurements have provided novel insight into the molecular, cellular and biophysical basis of cell adhesion. However, comparably little is known about a core element of cell-cell adhesion--the energy of adhesion at the cell-cell contact. In this review, we discuss approaches to understand the nature and regulation of adhesion energy, and propose strategies to determine adhesion energy between cells in vitro and in vivo.     

A novel role for anosmin-1 in the adhesion and migration of oligodendrocyte precursors

At embryonic stages of development, oligodendrocyte precursors (OPCs) generated in the preoptic area colonize the entire optic nerve (ON). Different factors controlling migration of ON OPCs have been identified, including secreted growth factors, morphogens and guidance cues, as well as cell adhesion molecules. We have shown previously that the soluble form of the extracellular matrix (ECM) protein anosmin-1, impairs OPC migration induced by FGF-2. In the present work, we show that anosmin-1 is expressed by both migrating OPCs and axons of the retinal ganglion cells in the embryonic ON. In vitro, we observe that OPC migration is strongly impaired by contact with anosmin-1 when used as a substrate and, in contrast to previous results, this effect is independent of FGF-2/FGFR1 signaling. We also show that OPCs preferentially adhere to anosmin-1 when compared with other ECM molecules used as substrates, and that when the endogenous anosmin-1 expressed by OPCs is blocked, OPC adhesion to all the different substrates (including anosmin-1), is significantly reduced. This novel effect of anosmin-1 on cell adhesion is also independent of FGF-2/FGFR1. We finally demonstrate that the blockade of the endogenous anosmin-1 expressed by OPCs impairs their migration. Our data suggest that the endogenous anosmin-1 expressed by OPCs is necessary for the correct adhesion of these cells to the different components of the ECM (including anosmin-1 itself), contributing to the migration of these cells.     

Control of axonophilic migration of oligodendrocyte precursor cells by Eph-ephrin interaction

The migration of oligodendrocyte precursor cells (OPCs) is modulated by secreted molecules in their environment and by cell-cell and matrix-cell interactions. Here, we ask whether membrane-anchored guidance cues, such as the ephrin ligands and their Eph receptors, participate in the control of OPC migration in the optic nerve. We postulate that EphA and EphB receptors, which are expressed on axons of retinal ganglion cells, interact with ephrins on the surface of OPCs. We show the expression of ephrinA5, ephrinB2 and ephrinB3 in the migrating OPCs of the optic nerve as well as in the diencephalic sites from where they originate. In addition, we demonstrate that coated EphB2-Fc receptors, which are specific for ephrinB2/B3 ligands, induce dramatic changes in the contact and migratory properties of OPCs, indicating that axonal EphB receptors activate ephrinB signaling in OPCs.Based on these findings, we propose that OPCs are characterized by an ephrin code, and that Eph-ephrin interactions between axons and OPCs control the distribution of OPCs in the optic axonal tracts, and the progress and arrest of their migration.