Endothelial cell signaling at the plasma membrane and at endosomes
Group leader: Mar Fernandez-Borja PhD
Role of the prion protein in cell adhesion and migration
The cellular prion protein (PrP) is a highly conserved glycoprotein expressed primarily in the nervous and immune systems. PrP is clearly involved in the pathogenicity of prion diseases such as Creutzfeldt-Jakob disease. However, the physiological role of PrP remains elusive. The fact that PrP associates to several molecules involved in cell adhesion suggests that PrP may participate in the modulation of cell-cell and cell-substrate adhesion. We have studied the role of PrP in cell adhesion and migration in the context of our research theme on the molecular mechanisms of leukocyte transendothelial migration and endothelial permeability regulation.
Antibody-mediated surface PrP ligation blocks cell chemotaxis towards the chemokine CXCL-12 in the pro-monocytic cell line U937. In contrast, the silencing of PrP results in increased CXCL-12-driven migration. This suggests that PrP negatively regulates U937 cell migration. To understand the molecular mechanisms underlying PrPC action, we followed a proteomic approach based on the identification by mass spectrometry of proteins binding to PrP. This analysis resulted in the identification of the integrin Mac-1 as a possible binding partner for PrPC as well as several signaling molecules that could be involved in PrP-dependent signaling during cell migration. In human endothelial cells, PrP accumulates at the intercellular junctions suggesting that it could be involved in the regulation of cell-cell adhesion.
Furthermore, we have observed that PrP can be transferred from cell to cell via tunneling nanotubes (see figure). These thin membrane tubes have been observed in a variety of cell types and represent a novel form of long distance cell-to-cell communication.
In summary, our data support the hypothesis that PrPC regulates cell adhesion to the substratum and to other cells. Studies to identify the mode of action of PrP are currently being performed in our group.
|Nanotube-mediated transfer of PrP between human endothelial cells. Endothelial cells transfected with GFP-tagged PrP were fixed and imaged by confocal microscopy (green: PrP; blue: nuclei). GFP-PrP was observed in long membrane tubes connecting two cells. In the image, GFP-PrP appears to be transferred from a transfected to a non-transfected cell.|
Signaling of the TNF receptor from endosomes
TNF is a pro-inflammatory cytokine that activates endothelial cells to express other inflammatory cytokines and receptors for leukocyte integrins facilitating the onset of inflammation. Binding of TNF to its receptor triggers intracellular signaling leading to gene expression. Recently, it was recognized that the TNF receptor signals not only from the plasma membrane but also from endosomes after being internalized. We found that TNF induces the synthesis of the endosomal small GTPase RhoB in endothelial cells and that RhoB regulates TNF receptor signaling. RhoB regulates intracellular TNF receptor traffic and MAP-kinase activation by TNF, however, RhoB has no effect on TNF receptor signaling from the plasma membrane. We propose that RhoB regulates TNF receptor inflammatory signaling from endosomes. Therefore, RhoB inhibition could be used as a strategy to selectively block TNF signaling from endosomes.
- Fernandez-Borja M, Kroon J, Tol S, Elias JA, Hordijk PL. The small GTPase RhoB regulates TNFα signaling in endothelial cells. Submitted to PLoS ONE.
- Fernandez-Borja M, van Buul JD, Hordijk PL. The regulation of leucocyte transendothelial migration by endothelial signaling events. Cardiovasc Res 2010; 86:202-10.
- Fernandez-Borja M, Janssen L, Verwoerd D, Hordijk P, Neefjes J. RhoB regulates endosome transport by promoting actin assembly on endosomal membranes through Dia1. J Cell Sci. 2005; 118:2661-70.