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Motile chick skeletal fibroblasts adhere to a laminin substrate by means of clustered beta 1 integrins. These integrin "macroaggregates" are similar to classic focal contacts but do not appear dark under interference-reflection microscopy. They contain alpha 5 integrin and are associated with extracellular fibronectin. To study their behavior during cell movement, time-lapse, low-light video microscopy was used to image integrins on living cells tagged with a fluorescent anti-beta 1 integrin antibody. Integrin macroaggregates remain fixed with respect to the substratum, despite the fact that they fluctuate in size, density, and shape over a period of minutes. Upon detachment of the cell rear, as much as 85% of the beta 1 integrin density of a macroaggregate remains behind on the substrate, along with both alpha 5 integrin and fibronectin. Release of the cell rear does not involve cleavage of the beta 1 integrin cytoplasmic domain from the remainder of the protein. These results indicate that cell motility does not require regulated detachment of integrin receptors from the substrate. On the other hand, cytoskeletal components and a variable fraction of the integrins are carried forward with the cell during detachment, suggesting that some type of cortical disassembly process does occur. Integrin macroaggregate structures are not recycled intact after detachment of the cell rear from the substrate. They do not persist on the cell surface, nor can they be seen to be engulfed by vesicles; yet, some of the individual integrins that make up these macroaggregates are eventually transported forward by both vesicular and cell-surface routes. Antibody-tagged integrins accumulate in dense patches at the lateral edges and dorsal surface of the cell, and move forward on the cell surface. The tagged integrins also enter cytoplasmic vesicles, which move forward within the cytoplasm. Macroaggregates generally form and grow at the cell front; however, application of fluorescent antibody causes integrins to disappear from the leading edge. Therefore, it has not been possible to directly visualize the recycling of the forward moving tagged integrins into new macroaggregates at the cell front. Surprisingly, under these conditions cells move normally despite the absence of any delivery of tagged integrin to the leading edge, indicating that recycling of integrins to the lamella is not required for apparently normal motility.
Regen et al. (Tue,) studied this question.