Rap1 and its effector RIAM are required for lymphocyte trafficking

Abstract

• Rap1 and its effector RIAM are required for integrinmediated T-cell adhesion and homing to lymph nodes, but not for T-cell development.• RIAM regulates the activation of lymphocyte functionassociated antigen 1 and very late antigen 4 on lymphocytes, but not aIIbb3 on platelets.Regulation of integrins is critical for lymphocyte adhesion to endothelium and trafficking through secondary lymphoid organs. Inside-out signaling to integrins is mediated by the small GTPase Rap1. Two effectors of Rap1 regulate integrins, RapL and Rap1 interacting adaptor molecule (RIAM). Using mice conditionally deficient in both Rap1a and Rap1b and mice null for RIAM, we show that the Rap1/RIAM module is not required for T-or B-cell development but is essential for efficient adhesion to intercellular adhesion molecule (ICAM) 1 and vascular cell adhesion molecule (VCAM) 1 and for proper trafficking of lymphocytes to secondary lymphoid organs. Interestingly, in RIAM-deficient mice, whereas peripheral lymph nodes (pLNs) were depleted of both B and T cells and recirculating B cells were diminished in the bone barrow (BM), the spleen was hypercellular, albeit with a relative deficiency of marginal zone B cells. The abnormality in lymphocyte trafficking was accompanied by defective humoral immunity to T-cell-dependent antigens. Platelet function was intact in RIAM-deficient animals. These in vivo results confirm a role for RIAM in the regulation of some, but not all, leukocyte integrins and suggest that RIAM-regulated integrin activation is required for trafficking of lymphocytes from blood into pLNs and BM, where relatively high shear forces exist in high endothelial venules and sinusoids, respectively. (Blood. 2015;126(25):2695-2703

D120 and K152 within the PH Domain of T Cell Adapter SKAP55 Regulate Plasma Membrane Targeting of SKAP55 and LFA-1 Affinity Modulation in Human T Lymphocytes

“…Hence, it appears as if Rap1 is not required for this basally induced adhesion. Still, the TCR-mediated augmentation of adhesion was found to be dependent on Rap1 in both cells expressing wild-type SKAP55 as well as cells expressing the D120K mutant, which is in line with previously reported data showing that Rap1 deficiency reduces TCR-mediated adhesion but not adhesion in nonstimulated T cells (5).…”

“…We show that the isolated PH domain of SKAP55 has a preference for phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P 3 ] over phosphatidylinositol 4,5-bisphosphate [PI(4,5)P 2 ] binding in vitro. Interestingly, however, in T cells, plasma membrane (PM) recruitment of the PH domain does not depend on PI(3,4,5)P 3 formation.…”

The Rap1-RIAM pathway prefers β2 integrins

“…1 The molecular mechanisms by which talin binds integrin tails and triggers integrin activation are now well accepted, 1 analyses of an independently generated RIAM knockout mouse. These new studies confirm that RIAM-deficient mice are viable and fertile, 3,4 and that they lack platelet defects, 3 but both also show that the absence of RIAM produces severe defects in b2 integrin-mediated leukocyte adhesion and trafficking. In fact, despite normal cell surface integrin levels, the RIAM-null phenotype resembles the leukocyte adhesion deficiency seen in b2 integrin null mice.…”

“…1 Yet, the RIAM knockout mouse is viable and fertile and exhibits no platelet adhesion or aggregation defects, 2 casting doubt on the in vivo role of RIAM. In this issue of Blood, Su et al 3 and Klapproth et al 4 now show that RIAM is required for b2 integrin-dependent leukocyte adhesion and trafficking in vitro and in vivo, but apparently not for all Rap-and talin-meditated activation of b1 and b3 integrins.…”

Ena/VASP proteins regulate activated T-cell trafficking by promoting diapedesis during transendothelial migration

“…For example, our previous work (6) has indicated that the cytoskeletal effector myosin-IIA affects both T-cell motility and diapedesis, and deletion of mDia1, another actin effector protein, both impairs thymocyte development and produces defects in chemotactic migration and in vivo homing of naïve T cells (11). Similarly, deletion of the cytoskeletal regulators Rap1, RIAM, talin, or RAPL impair chemokine-stimulated ICAM-1 adhesion and naïve T-cell trafficking (52)(53)(54), whereas CRK proteins regulate T-cell adhesion, chemotaxis, and diapedesis, leading to reduced T-cell trafficking selectively to inflamed tissues (55). In contrast, recent reports show that Kindlin-3 is not required for diapedesis, although it has been shown to play an important role in adhesion and CNS trafficking more generally (56,57).…”