New Insights in Rap-mediated Control of Cell Adhesion

The small G-protein Rap1 is a key regulator of cell adhesion. In this thesis, we have further investigated Rap signaling, particularly in the control endothelial cell-cell adhesion and cell-matrix adhesion. In chapter 2, we have investigated the Rap1 effector RIAM, which is activated by Rap1 in platelets to induce the high affinity conformation of αIIbβ3 integrins. We offer evidence that RIAM is not a direct effector of Rap1 in the control of β1 integrins and C. elegans touch neuron development, but in these settings rather regulates Rap1 function by controlling a pathway parallel to Rap1 signaling. In chapter 3, we review the literature on Rap1 signaling in the control of cell-cell adhesion. This analysis emphasizes that multiple GEFs are reported to activate Rap1 to control cell-cell adhesion, which is further elaborated on in chapter 4, where we show that PDZ-GEF and Epac1 cooperate to control endothelial cell-cell adhesion. Specifically, we show that Epac1 controls endothelial barrier upon cAMP stimulation, whereas PDZ-GEF functions to maintain a basal level of barrier, especially the latter being hitherto unappreciated. In chapter 5 we proceed to investigate Rap isoform contribution to this basal barrier function. This reveals an intriguing function of Rap2, which suppresses endothelial barrier. Thus, basal endothelial barrier is maintained by the antagonistic functions of Rap1 and Rap2. Finally, we return to the notion that multiple GEFs regulate cell-cell adhesion in chapter 6, which shows that Rap1 is not the only target of GEF C3G regulating junctional levels of E-cadherin, thereby stressing that Rap1-independent functions of RapGEFs have to be appreciated. These findings identify novel layers of complexity to our understanding of Rap G-proteins and will prove pivotal in the ongoing unraveling of Rap signaling