α5β1 and αvβ3 integrins employ two distinct adhesion strengthening modes to respond to fibronectin stiffness within seconds of initiating adhesion

The complex interplay between extracellular matrix stiffness and actomyosin contractility regulates long-term integrin-based adhesion and signaling in mammalian cells. However, how cells sense and respond to the stiffness of the environment during the first minutes of initiating adhesion remains elusive. Here, we show that fibroblasts upon initiating adhesion to fibronectin switch between two distinct mechanosensitive adhesion modes. The first mode of "slow" adhesion strengthening, shared between 5{beta}1 and v{beta}3 integrins and depends modestly on fibronectin stiffness. Fibroblasts adapt this slow adhesion strengthening mode, which is independent of actomyosin contractility and intracellular signaling, on soft fibronectin substrates (<5 kPa). On stiff fibronectin substrates (>5 kPa), however, 5{beta}1 integrins but not v{beta}3 integrins switch to a "fast" adhesion strengthening mode that considerably strengthens adhesion within seconds. The switch to the fast mode depends on myosin II-mediated contractility and a mechanosensitive signaling hub that includes the 5{beta}1 integrin-FN catch bond, paxillin, and focal adhesion kinase. The mechanistic findings highlight the similarities and differences of integrin-type specific adhesion strengthening and intracellular regulation, which depend on mechanotransduction in fibroblasts during adhesion initiation.