Atomic Force Microscopy-Based Nanoscopy of Chondrogenically Differentiating Human Adipose-Derived Stem Cells: Nanostructure and Integrin β1 Expression.

Integrin β1 is known to be involved in differentiation, migration, proliferation, wound repair, tissue development, and organogenesis. In order to analyze the binding probability between integrin β1 ligand and cluster of differentiation 29 (CD29) receptors, atomic force microscopy (AFM) was used to detect native integrin β1-coupled receptors on the surface of human adipose-derived stem cells (hADSc). The binding probability of integrin β1 ligand-receptor interaction was probed by integrin β1-functionalized tips on hADSc during early chondrogenic differentiation at the two-dimensional cell culture level. Cell morphology and ultrastructure of hADSc were measured by AFM, which demonstrated that long spindled cells became polygonal cells with decreased length/width ratios and increased roughness during chondrogenic induction. The binding of integrin β1 ligand and CD29 receptors was detected by β1-functionalized tips for living hADSc. A total of 1200 curves were recorded at 0, 6, and 12 days of chondrogenic induction. Average rupture forces were, respectively, 61.8 ± 22.2 pN, 60 ± 20.2 pN, and 67.2 ± 22.0 pN. Rupture events were 19.58 ± 1.74%, 28.03 ± 2.05%, and 33.4 ± 1.89%, respectively, which demonstrated that binding probability was increased between integrin β1 ligand and receptors on the surface of hADSc during chondrogenic induction. Integrin β1 and the β-catenin/SOX signaling pathway were correlated during chondrogenic differentiation. The results of this investigation imply that AFM offers kinetic and visual insight into the changes in integrin β1 ligand-CD29 receptor binding on hADSc during chondrogenesis. Changes in cellular morphology, membrane ultrastructure, and the probability of ligand-transmembrane receptor binding were demonstrated to be useful markers for evaluation of the chondrogenic differentiation process.