Cytohesin-1 controls the activation of RhoA and modulates integrin-dependent adhesion and migration of dendritic cells

Abstract

Adhesion and motility of mammalian leukocytes are essential requirements for innate and adaptive immune defense mechanisms. We show here that the guanine nucleotide exchange factor cytohesin-1, which had previously been demonstrated to be an important component of beta-2 integrin activation in lymphocytes, regulates the activation of the small GTPase RhoA in primary dendritic IntroductionIntegrin-mediated adhesion is an essential function of most metazoan cells, and plays a key role in, for example, organ development, the maintenance of tissue architecture, blood clotting, and many aspects of leukocyte biology. The mammalian integrin family consists of 24 heterodimeric cell surface receptors that bear distinct functional binding specificities for extracellular matrix proteins and plasma membrane counterreceptors. 1 Immune cells have highly specific requirements for timing and location of cell-cell and -matrix contacts, which need to be finely tuned to variable conditions of homeostasis and inflammation.An important hallmark of integrin function in the immune system is the rapid adhesion of leukocytes to activated endothelia, a process that is essential for immune cell egress from the vasculature into either lymphoid organs or into infected tissues. To perform such functions, integrins on leukocytes have to be activated by a process that has been termed "inside-out" signaling. It is now generally believed that integrins on circulating, resting leukocytes are conformationally restricted, so that they normally cannot bind to their protein ligands, such as ICAM-1 or VCAM-1. However, if a leukocyte becomes tethered to an endothelial cell, for example, by selectin-mediated adhesive "rolling," immune cell chemokine receptors such as CCR7 or CXCR4 may be activated by chemokines present on the activated vessel wall. 2,3 This, in turn, results in intracellular, heterotrimeric G-protein-coupled signal transduction, which enables very fast conformational changes and ligand binding of cell surface integrins, resulting in a robust firm adhesion, the so-called integrin-mediated leukocyte "arrest." 3 Shear forces imposed by the blood flow apparently play an important role in the regulation of this process. 4 The mode of activation of immune cell integrins is incompletely understood. Several intracellular proteins that directly or indirectly interact with the cytoplasmic domains of the leukocyte-specific beta-2 integrins, such as LFA-1, have been implicated in this function. These include the actin-and integrinbinding protein talin, which is important for the general coupling of integrin-mediated cell adhesion to the actin cytoskeleton, 5-7 the GTPase Rap1 7-9 and its effector proteins RAPL and RIAM, 9-11 the ADAP/Vav/SLP76 complex in T cells, 12-14 the Rho GTPases RhoA 15 and CdC42, 16 and, recently, the so-called kindlin proteins. 17,18 Our laboratory has identified the guanine nucleotide exchange factor (GEF) for ADP-ribosylation factor (ARF) GTPases, cytohesin-1, as an LFA-1 interacting protein. Cytohesin-1 binds specifical...

The lymphoid chemokine CCL21 triggers LFA‐1 adhesive properties on human dendritic cells

“…12 Furthermore, the second CCR7 ligand CCL19 was recently found to induce a general b2-integrin activation epitope on mature DCs. 18 Therefore, we investigated whether CCL21 was responsible for the upregulation of the LFA-1 activation epitope NKI-L16 on human DCs.…”

“…However, the contribution of integrins in DC (trans-) migration remains controversial as it was shown that knockdown of all integrins in murine DCs did not impair the migration of DCs from the periphery to the secondary lymphoid organs. 38 Nevertheless, Quast et al 18 have shown that although phenotypically similar, migration of integrin-null and integrin-bearing murine DCs within the interstitium is molecularly distinct. While integrin-bearing DCs require cytohesin-1 and RhoA signaling, integrin-null DCs use a cytohesin-1-independent, RhoA-dependent signaling pathway for migration.…”

LFA-1 dynamics on the membrane of leukocytes : a single dye tracing study

“…Furthermore, the type of diffusion of LFA-1 subpopulations did not seem to change upon differentiation. Therefore, it is tempting to believe that imDCs are still potentially active and might be able to switch to an active state where LFA-1 mediated adhesion is required, which is in line with recent studies on the role of LFA-1 in DCs [15,16,18,227].…”

“…On monocytes, LFA-1 plays a similar role [12,13]. Upon differentiation of monocytes towards immature DCs [14], LFA-1 functionality is downregulated [3], although recent findings suggest that LFA-1 does play a role during certain stages of the DC life cycle [15][16][17][18]. In addition, LFA-1 on lymphocytes acts as a co-receptor by binding to ICAM-1 on mature DCs during the immunological synapse [19,3].…”

“…Also, a recent study showed involvement of ICAM-1 (intercellular adhesion molecule 1) in homing to the lymph nodes, suggesting an active role of LFA-1 in DC migration [17]. Finally, it was demonstrated in vivo that β 2 integrins play a role in the ameboid migration mode of DCs [18]. Above that, the fundamental question of how changes in organization and conformational state relate to mobility prompted us to study LFA-1 mobility on cell types where LFA-1 is less active and homogeneously distributed over the cell surface.…”