RIAM and RapL regulate distinct signaling events and functional outcomes upon TCR-mediated activation (50.8)

Abstract

Abstract Rap1, a small GTPase of the Ras superfamily, regulates inside-out activation of integrins and cell adhesion. Two new Rap1 effectors, RapL and RIAM, have been implicated in this function. Significant differences in their structure indicate that these molecules may mediate distinct signaling events. RIAM has a coiled-coil region, RA and pleckstrin homology (PH) domains, and proline-rich regions with specific motifs capable of interacting with the EVH1 domains of Ena/VASP proteins, with profilin and with SH3 domain containing proteins. RapL has an RA domain and a C-terminal coiled-coil region. In the present study we investigated the role of RIAM and RapL in regulating signaling and functional events activated via the TCR. Using shRNA we determined that activation of the extracellular signal regulated kinases MEK1/2 and Erk1/2 was impaired by elimination of RIAM, whereas activation of these kinases was unaffected by elimination of RapL. Activation of p38 was unaltered in RIAM-KD but was abrogated in RapL-KD cells. Moreover, knockdown of RIAM but not RapL resulted in impaired activation of Ras and Rap1 due to defective activation of the calcium and diacylglycerole-dependent GEFs, RasGRP1 and CalDAG-GEFI. Strikingly, RIAM-KD resulted in impaired IL-2 production, whereas RapL-KD cells displayed a dramatic increase in IL-2 production upon TCR/CD3-plus-CD28 crosslinking. Thus, RIAM and RapL have distinct roles in regulating signaling and functional outcomes of T cell responses.