Single Molecule Force Spectroscopy of Chondrocyte α5β1 and α1β1 Integrins

Osteoarthritis (OA) is one of the leading chronic, degenerative diseases in the world, causing joint pain and disability with no known cure. Integrins are known to be important mechanotransducers in many cell types, and to be important for the maintenance of chondrocyte homeostasis (chondrocytes are in turn responsible for the maintenance of articular cartilage). Forces on integrins in the piconewton (pN) range are known to influence both healthy and pathogenic signaling pathways in a variety of other cell types; however, it is not known what specific range of forces on chondrocyte integrins induce these pathways. Here, we are using Atomic Force Microscopy (AFM) to reveal the range of interaction forces between α5β1 and α1β1 integrins and their ligands (fibronectin and type VI collagen, respectively) to elucidate the range of forces withstood by integrins under both basal and rupture conditions. AFM cantilevers mounted with fibronectin or collagen-VI coated silica microbeads were used to measure the binding force versus distance for α5β1 and α1β1 integrins on primary human articular chondrocytes. Single-molecule adhesion forces were found to be higher for α1β1 integrins than for α5β1 integrins (mean 104 pN ± 84 pN [S.D.] vs. 32.2 pN ± 13 pN [S.D.]) based on a Kolmogorov-Smirnov test. Maximum single-molecule binding forces of 140.8 pN and 810.8 pN were observed for α5β1 and α1β1 integrins, respectively. The distribution of rupture forces for α5β1 integrins was nearly lognormal, while α1β1 integrins had a log-bimodal distribution with rupture forces of 45.8 pN and 139.8 pN occurring most frequently. These data indicate a potential mechanical importance of α1β1 integrins in distributing mechanical stresses from the pericellular matrix to intracellular proteins within articular chondrocytes that has not previously been reported.