Oriented endocytic recycling of α5β1 in motile neutrophils

Abstract

AbstractDuring cell migration, integrin attachments to the substratum provide the means to generate the traction and force necessary to achieve locomotion. Once the cell has moved over these attachments, however, it is equally important that integrins detach from the substratum. The fate of integrins after detachment may include release from the cell, lateral diffusion across the cell surface, or endocytosis and redelivery to the cell surface. Polymorphonuclear neutrophils (PMNs) become stuck on the extracellular matrix proteins fibronectin and vitronectin when their intracellular free calcium concentration ([Ca++]i) is buffered. Taking advantage of this feature of PMN migration, we investigated the fate of integrins to differentiate among various models of migration. We demonstrate that 5β1, one of the fibronectin-binding integrins, is responsible for immobilization of [Ca++]i-buffered PMNs on fibronectin. We find that 5 and β1 are in endocytic vesicles in PMNs and that 5 colocalizes with a marker for an endocytic recycling compartment. When [Ca++]i is buffered, 5 and β1 become concentrated in clusters in the rear of the adherent cells, suggesting that [Ca++]i transients are required for 5β1 detachment from the substratum. Inhibition of 5β1 detachment by buffering [Ca++]i results in the depletion of 5 from both endocytic vesicles and the recycling compartment, providing compelling evidence that integrins are normally recycled by way of endocytosis and intracellular trafficking during cell migration. This model is further refined by our demonstration that the endocytic recycling compartment reorients to retain its localization just behind the leading lamella as PMNs migrate, indicating that membrane recycling during neutrophil migration has directionality.