Focus on hematology Oriented endocytic recycling of a5b1 in motile neutrophils

During 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 a5b1, one of the fibronectin-binding integrins, is responsible for immobilization of [Ca]i-buffered PMNs on fibronectin. We find that a5 and b1 are in endocytic vesicles in PMNs and that a5 colocalizes with a marker for an endocytic recycling compartment. When [Ca]i is buffered, a5 and b1 become concentrated in clusters in the rear of the adherent cells, suggesting that [Ca]i transients are required for a5b1 detachment from the substratum. Inhibition of a5b1 detachment by buffering [Ca]i results in the depletion of a5 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. (Blood. 2000; 95:2471-2481)