Mechanical Coupling through Endothelial Cell Adhesions Determined Using a Novel Live-Cell Strain Device

Abstract

Mechanotransduction is mediated by cell-matrix adhesion sites in response to extracellular mechanical forces. Cytoskeletal dynamics in single cells have been measured during migration on substrates of varying elastic modulus, but live-cell measurement of structural dynamics during substrate stretch in confluent cell monolayers has been difficult to achieve. We developed a novel stretch device optimized for high-resolution live-cell imaging. The unit assembles onto standard inverted microscopes and applies static or cyclic stretch at physiological magnitudes to cultured cells on elastic membranes. Interchangeable modular indenters enable rapid switching between equibiaxial and uniaxial stretch profiles. In endothelial cell monolayers expressing EGFP-vimentin and paxillin-DsRed2 and subjected to constant equibiaxial or uniaxial stretch, the 2-D strain tensor demonstrated efficient transmission through the extracellular matrix and focal adhesions. Strain transmission to the intermediate filament network was decreased in magnitude, as demonstrated by spatial correlation of vimentin and paxillin displacement vectors, and cells did not align perpendicular to constant uniaxial stretch. During cyclical uniaxial stretch at 1 Hz, strain focusing was increased relative to constant stretch and peaked at 10-15 min after stretch onset. Strain focusing recovered more slowly over a time scale of ∼1 hr, and cells aligned perpendicular to the stretch direction in 6 hr. These observations using the live-cell stretch device demonstrate that sustained strain focusing and mechanical coupling through adhesion sites may be required for endothelial cell morphological adaptation to cyclical uniaxial stretch.