Stress-relaxation of fibroblasts in collagen matrices triggers ectocytosis of plasma membrane vesicles containing actin, annexins II and VI, and β1 integrin receptors

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Abstract
To learn about the effects of tension on fibroblast func-tion, we have been studying initial cellular responses to stress-relaxation. Human foreskin fibroblasts were cul-tured 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 margins. The actin was found to be located inside 200 nm diam-eter vesicles that were budding from the cell surface. Vesicles isolated from the matrix after stress-relaxation contained prominent 24 kDa, 36 kDa (doublet), 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 predominantly in the Triton-soluble vesicle fraction, was identified as annexin II. Vesicles also contained annexin VI and 11 integrin receptors but not tubulin, vimentin, vinculin or annexin I. The results suggest that stress-relaxation of fibrob-lasts induces a novel ectocytotic process involving tran-sient budding of intact, plasma membrane vesicles from the cell cortex. On the basis of their morphological and biochemical 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.