Rho GTPase signaling in cell adhesion and migration

Group leader: Prof PL Hordijk PhD
As of September 2015 part of the Hordijk group has moved to the Free University Medical Center (VUMC). For questions regarding the lab please contact dr. Hordijk at p.hordijk@vumc.nl.

Leukocyte transendothelial migration comprises a properly organized interplay between leukocytes and the endothelium. β1- and β2 integrins on the leukocyte bind to the integrin-ligands VCAM-1 and ICAM-1 on the endothelial cell surface, which allows firm leukocyte adhesion, even under conditions of (blood) flow. These molecular interactions induce adhesion-induced signaling in both cell types. This results in the leukocyte to spreading and migration, and in the endothelium to remodeling of the cortical cytoskeleton. This is paralleled by the induction of kinase-mediated signaling that induces ultimately the transient opening of homotypic cell-cell contacts, a prerequisite for efficient transendothelial migration.

Our department focuses on endothelial signaling towards cytoskeletal dynamics. These are regulated by the family of Rho-like small GTPases. We have studied primarily the Rac1 GTPase, a key member of this family, which is known for its induction of actin polymerization and regulation of integrin- and cadherin-based cell adhesion. Over the past years we identified a series of novel Rac1-interacting proteins and have investigated the biology that accompanies these interactions.

One of these novel Rac1-binding proteins is the BAR-domain protein PACSIN2. We found PACSIN2 to be a negative regulator of Rac1 signaling, because PACSIN2 mediates the internalization of activated Rac1 from the plasma membrane to endosomes. This internalization leads to Rac1 inactivation, most likely through GAP-stimulated GTP hydrolysis.

In addition, we have also studied the role of protein ubiquitylation in Rac1 function. We found that the Rac1 binding protein Caveolin1 regulates the poly-ubiquitylation and degradation of activated Rac1. This pathway functions as a alternative means of Rac1 inactivation, next to the classical inactivation through stimulation of GTP-hydrolysis.

Finally, we found that Rac1 binds to the ubiquitin E3 ligase Nedd4. Rac1 itself is not a Nedd4 substrate, but promotes Nedd4-dependent ubiquitylation of the adapter protein disheveled. This in turn induces a stabilization of homotypic cell-cell contacts probably because disheveled regulates microtubule dynamics at the intercellular junctions.
We aim to pursue this line of research further, focusing on the role of mono- and poly ubiquitylation in the control of Rac1 signaling towards cytoskeletal remodeling and regulation of cell-cell contact in primary human endothelial cells.

Model of Rac1 regulation by PACSIN2 and Caveolin1.w

Model of Rac1 regulation by PACSIN2 and Caveolin1. (1) Following its activation, Rac1GTP can either be targeted for ubiquitylation and degradation via a Caveolin1-dependent pathway, or (2) enter a PACSIN2-dependent pathway associated with GTP hydrolysis.

Key publications

Last edited on: 13 April 2016