INTEGRIN DYNAMICS ON THE TAIL REGION OF MIGRATING FIBROBLASTS

Cell migration is a complex process that can be considered as a repeat ed cycle of lamellipod extension and attachment, cytoskeletal contract ion, and tail detachment, While lamellipodial and cytoskeletal phenome na are currently the focus of considerable research on cell migration, under many conditions locomotion appears to be rate-limited by events at the cell rear, especially release of cell/substratum adhesions, To study the mechanism of tail detachment, we have developed a novel exp erimental system that permits observation of integrin dynamics on the ventral surface of migrating fibroblasts. Photoactivatable caged fluor escein is to a non-adhesion-perturbing anti-avian-beta 1 subunit antib ody, which labels integrins on chicken fibroblasts migrating on a lami nin-coated glass coverslip, Ultraviolet light is focused through a pin hole to photoactivate the caged fluorophore in a 10-mu m-diameter spot at the rear of a polarized cell, The fate of integrins initially pres ent in this spot is monitored using a cooled CCD camera to follow the movement of fluorescent intensity as a function of time over a 2 to 3 hour period. We find that a substantial fraction of the integrins is l eft behind on the substratum as the cell detaches and locomotes, while another fraction collects into vesicles which are transported along t he cell body as the cell migrates, As aggregates rip from the cell mem brane, the integrin-cytoskeletal bonds are preferentially fractured re sulting in 81+/-15% of the integrin remaining attached to the substrat um, We additionally find that adhesions sometimes disperse into integr ins which can form new adhesions at other locations in the cell, Adhes ions along the cell edge can release from the substrate and translocat e with the cell, They either disperse in the cell membrane, rip from t he cell membrane and remain attached to the substratum, or form a new aggregate, These observations indicate that the behavior of integrins at the cell rear is much more dynamic than previously appreciated, sug gesting that an important locus for regulation of motility may reside in this region.