Talin 1 and paxillin facilitate distinct steps in rapid VLA-4-mediated adhesion strengthening to vascular cell adhesion molecule 1.

VLA-4 (alpha4beta1) is a key integrin in lymphocytes, interacting with endothelial vascular cell adhesion molecule 1 (VCAM-1) on blood vessels and stroma. To dissect the contribution of the two cytoskeletal VLA-4 adaptor partners paxillin and talin to VLA-4 adhesiveness, we transiently knocked them down in Jurkat T cells and primary resting human T cells by small interfering RNA silencing. Paxillin was required for VLA-4 adhesiveness to low density VCAM-1 under shear stress conditions and was found to control mechanical stability of bonds mediated by the alpha4 subunit but did not affect the integrin affinity or avidity to VCAM-1 in shear-free conditions. Talin 1 maintained VLA-4 in a high affinity conformation, thereby promoting rapid VLA-4 adhesion strengthening to VCAM-1 under both shear stress and shear-free conditions. Talin 1, but not paxillin, was required for VLA-4 to undergo optimal stimulation by the prototypic chemokine, CXCL12, under shear stress conditions. Interestingly, talin 1 and paxillin played the same distinct roles in VLA-4 adhesions of primary T lymphocytes, although VLA-4 affinity to VCAM-1 was at least 200-fold lower in these cells than in Jurkat cells. Collectively, our results suggest that whereas paxillin is a mechanical regulator of VLA-4 bonds generated in the absence of chemokine signals and low VCAM-1 occupancy, talin 1 is a versatile VLA-4 affinity regulator implicated in both spontaneous and chemokine-triggered rapid adhesions to VCAM-1.