Loss of the Rap1 effector RIAM results in leukocyte adhesion deficiency due to impaired b2 integrin function in mice

Integrins are a/b heterodimeric transmembrane receptors that are expressed on all cells. They provide stable adhesion of cells to the extracellular matrix or to other cell counter receptors by forming complex adhesion sites, which serve as signaling platforms (outside-in signaling) that induce cell polarity, migration, survival, and proliferation. Because integrins reside in an inactive state, integrin-ligand binding requires an integrin activation step (inside-out signaling), which switches the heterodimer from a lowto a high-affinity state. The majority of studies that defined integrin affinity regulation were performed on aIIbb3 and b2-class integrins expressed by platelets and leukocytes, respectively. These cells circulate in the blood and keep their integrins in an inactive state until they encounter soluble or membrane-bound agonists, which activate talin-1 and the hematopoietic cell-specific kindlin-3, which in turn cooperate to induce integrin activation and clustering. Consequently, loss of kindlin-3 or talin-1 expression in platelets and leukocytes abolishes activation of aIIbb3 and b2 integrins leading to severe bleeding and leukocyte adhesion defects. Talin, which consists of an N-terminal band 4.1, ezrin, radixin, moesin (FERM)andaC-terminal roddomain, exists inan autoinhibited globular and active extended conformation that binds and activates integrins. In the autoinhibited state, the rod domain interacts with the FERMdomain, whichmasks the integrin-binding site. This inhibition is released by binding of the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) to the talin head, which disrupts the autoinhibitory interaction between the rod domain and integrin binding region on the talin head and orients talin on the plasma membrane enabling b integrin tail binding. Talin activity has been shown to be regulated by activation of the small GTPase Ras-related protein 1 (Rap1), a known regulator of cell adhesion. Rap1-guanosine triphosphate (GTP)–interacting adapter molecule (RIAM) is a Rap1 effector molecule, which localizes to the leading edge of spreading cells and is expressed in many tissues. It belongs to the Mig-10/RIAM/lamellipodin (MRL) protein family, a group of multidomain adapter proteins involved in the regulation of actin dynamics, cell adhesion and migration, and cell growth. Knockdown of RIAM abrogated Rap1-induced adhesion to integrin ligands, whereas RIAM overexpression increased cell spreading and adhesion to b1 and b2 integrin ligands. RIAM was identified in signaling complexes downstreamof the T-cell receptor andCCchemokine receptor 7, suggesting a role in integrin aLb2 inside-out signaling. The molecular mechanism that underlies RIAM-mediated