Fibroblast spreading induced by connective tissue stretch involves intracellular redistribution of α- and β-actin

Mechanical stretching of connective tissue occurs with normal movement and postural changes, as well as treatments including physical therapy, massage and acupuncture. Connective tissue fibroblasts were recently shown to respond actively to short-term mechanical stretch (minutes to hours) with reversible cytoskeletal remodeling, characterized by extensive cell spreading and lamellipodia formation. In this study, we have examined the effect of tissue stretch on the distribution of alpha- and beta-actin in subcutaneous tissue fibroblasts ex vivo. Normal fibroblasts uniformly exhibited alpha-smooth muscle actin (alpha-SMA) immunoreactivity. Unlike cultured fibroblasts and smooth muscle cells, alpha-SMA in these fibroblasts was not in F-actin form (indicated by lack of phalloidin co-localization) nor was it organized into distinct stress fibers. The lack of stress fibers and fibronexus was confirmed by electron microscopy, indicating that these cells were not myofibroblasts. In unstretched tissue, the pattern of alpha-actin was diffuse and granular. With tissue stretch (30 min), alpha-actin formed a star-shaped pattern centered on the nucleus, while beta-actin extended throughout the cytoplasm including lamellipodia and cell cortex. This dual response pattern of alpha- and beta-actin may be an important component of cellular mechanotransduction mechanisms relevant to physiologic and therapeutic mechanical forces applied to connective tissue.