Cell death and mechanoprotection by filamin A in connective tissues after challenge by applied tensile forces

Cells in mechanically challenged environments must cope with high amplitude forces to maintain cell viability and tissue homeostasis. Currently, force-induced cell death and the identity of mechanoprotective factors are not defined. We examined death in cultured periodontal fibroblasts, connective tissue cells that are exposed to heavy applied forces in vivo. Static tensile forces (0.48 piconewtons/μm² cell area) were applied through magnetite beads coated with collagen or bovine serum albumin. There was a time-dependent increase of the percentage of propidium iodide-permeable cells in force-loaded cultures incubated with collagen but not bovine serum albumin beads, indicating a role for integrins. Cells exhibited reduced mitochondrial membrane potential, increased caspase-3 activation, nuclear condensation, terminal deoxynucleotidyl transferase nick end labeling staining, and detachment from the culture dish. The caspase-3 inhibitor acetyl-Asp-Glu-Val-Asp-aldehyde reduced detachment 3-fold. There was a rapid (<10-s) decrease in plasma membrane potential after force application, which, in filamin A-deficient melanoma cells, contributed to irreversible cell depolarization. In fibroblast cultures, cells with increased permeability to propidium iodide exhibited ∼2-fold less filamin A content than impermeable cells. Fibroblasts transfected with antisense filamin A constructs or with filamin A constructs without an actin-binding domain exhibited 2-3-fold increased proportions of dead cells relative to controls. We conclude that high amplitude forces delivered through integrins can promote apoptosis in a proportion of cells and that filamin A confers mechanoprotection by preventing membrane depolarization.