STRESS-RELAXATION OF FIBROBLASTS IN COLLAGEN MATRICES TRIGGERS ECTOCYTOSIS OF PLASMA-MEMBRANE VESICLES CONTAINING ACTIN, ANNEXIN-II AND ANNEXIN-VI, AND BETA-1 INTEGRIN RECEPTORS

To learn about the effects of tension on fibroblast function, we have been studying initial cellular responses to stress-relaxation. Human f oreskin fibroblasts were cultured in anchored collagen matrices for 2 days, during which time mechanical stress developed. Subsequently, the matrices were dislodged; thereby allowing stress to dissipate. Within 5 min after initiating stress-relaxation, fibroblasts retracted their pseudopodia. At this time, we observed the disappearance of cellular stress fibers and the formation of actin clusters along the cell margi ns. The actin was found to be located inside 200 nm diameter vesicles that were budding from the cell surface. Vesicles isolated from the ma trix after stress-relaxation contained prominent 24 kDa, 36 kDa (doubl et), 45 kDa, and 135 kDa polypeptides. The 45 kDa polypeptide was the major component in the Triton-insoluble vesicle fraction and appeared to be actin. The 36 kDa (doublet) polypeptide, which was found predomi nantly in the Triton-soluble vesicle fraction, was identified as annex in II. Vesicles also contained annexin VI and beta1 integrin receptors but not tubulin, vimentin, vinculin or annexin I. The results suggest that stress-relaxation of fibroblasts induces a novel ectocytotic pro cess involving transient budding of intact, plasma membrane vesicles f rom the cell cortex. On the basis of their morphological and biochemic al features, these vesicles may be analogous to the 'matrix vesicles' released by chondrocytes and could play a role in extracellular matrix remodeling after wound contraction.