Rap1: selective activator of β2 integrins?

In T cells, Rap1 is well-recognized as a major mediator of inside-out signaling to integrins following stimulation by antigen and chemokines. Ghandour and colleagues have identified a selective role for Rap1 in human T cells in chemokine-dependent adhesion via LFA-1, but not VLA-4. Their study highlights the specificity of Rap1 for LFA-1 in SDF-1α–mediated adhesion, and shows that PLC and the Rap1 exchanger CalDAG-GEFI are required.

Two major types of integrins are highly expressed in T lymphocytes: lymphocyte-function–associated antigen-1 (LFA-1, or αLβ2) and the very late antigens (including VLA-4 or α4β1). LFA-1 binds intracellular adhesion molecules (ICAM-1, -2, or -3), and VLAs bind to both vascular cell-adhesion molecule-1 (VCAM-1) and fibronectin to regulate T-cell/antigen recognition, extravasation, and homing to target organs. These interactions are enhanced by a variety of stimuli that increase both affinity and avidity of the integrins for their ligands in a process termed “inside-out signaling.”¹ 

One intracellular small G protein, Rap1, a member of the Ras superfamily, is activated by these diverse stimuli and is thought to couple these signals to integrin activation.²  Other signaling proteins have been implicated as well, including phospholipase C (PLC), phosphoinositol-3 kinase (PI3-K), protein kinase C (PKC), and intracellular second messengers calcium and diacylglycerol (DAG). Recent research has focused on determining how these diverse signaling pathways converge on Rap1, and whether the mode of Rap1 activation dictates the selectivity of Rap1's actions for specific integrins.

Ghandour and colleagues have begun to answer these questions by examining the specific integrins activated by the chemokine stromal cell-derived factor 1α (SDF-1α, also called CXCL12). In human T cells, SDF-1α activates adhesion to ICAM via LFA-1 and to VCAM via VLA-4. Surprisingly, Rap1 was required only for SDF-1α's enhancement of adhesion to ICAM, a finding that was mimicked by the phorbol ester PMA, a potent agonist of PKC. However, PKC inhibition blocked only adhesion through VLA-4. Taken together, these data demonstrate that Rap1 can be activated in a PKC-independent manner to activate LFA-1, and that PKC can act independently of Rap1 to stimulate VLA-4. In contrast, other studies that show a requirement for Rap1 in chemokine- and VLA-4 – dependent adhesion have utilized murine lymphocytes^3,4  and human Jurkat cells.⁵  It will be important to further characterize these species differences. Interestingly, in this study, adhesion through both LFA-1 and VLA-4 required PLC. The Rap1 exchanger CalDAG-GEF1 is activated by direct binding of calcium and DAG, making it highly responsive to PLC.⁶  Using a knockdown approach, the authors show that CalDAG-GEF1 is required for the LFA-1–dependent effects of both SDF-1α and PMA (see figure).

The study identifies key questions for future research. How does PLC/PKC activation by chemokines activate VLA-4 independently of Rap1? Are the small G proteins Rac and Rho involved, as suggested by others?⁷  Is there a Rap1-independent role for PKD1, a kinase that is activated by PKC and associates with β1 integrins?⁵  How is the pool of Rap1 activated by SDF-1α restricted to the activation of LFA-1? Do specific adaptor proteins, including adhesion- and degranulation-promoting adaptor protein (ADAP) and Src kinase-associated phosphoprotein of 55kDa (SKAP-55), or the selective recruitment of Rap1 effectors RapL and/or RAIM, contribute to this specificity?⁸