CA2+ AND CALCINEURIN-DEPENDENT RECYCLING OF AN INTEGRIN TO THE FRONT OF MIGRATING NEUTROPHILS

CHEMOATTRACTANTS stimulate neutrophil migration by activating signalling pathways(1) including repeated transient increases in intracellular free calcium, [Ca2+](i) (refs 2, 3). A motile neutrophil sends out many pseudopods, some of which adhere to the substrate; to continue moving forward the cell must release these attachments(4,5). Adhesion can be actively regulated, and neutrophils in which [Ca2+](i) transients are inhibited become stuck on fibronectin or vitronectin(6,7), extracellular matrix proteins that neutrophils encounter in vivo. Function-blocking antibodies to beta 3 integrins or the alpha v beta 3 heterodimer restore motility on vitronectin to [Ca2+](i)-buffered cells (B, Hendey, M.A.L., E. Marcantonio and F.R.M., manuscript submitted), indicating that an alpha v beta 3-like integrin is responsible for the [Ca2+](i)-sensitive adhesion. We show that the density of alpha v beta 3 integrins in the adherent membrane of neutrophils migrating on vitronectin is much higher at the leading edge than at the rear, but [Ca2+](i) buffering or inhibition of Ca2+-calmodulin-activated protein phosphatase 2B (calcineurin) leads to accumulation of alpha v beta 3 on the adherent surface at the rear of the cell. We show that the polarized distribution of alpha v beta 3 integrins in migrating neutrophils is maintained by [Ca2+](i)-dependent release of adhesion followed by endocytosis of these integrins and recycling to the leading edge.