Single-molecule force spectroscopy of cartilage aggrecan self-adhesion

We investigated self-adhesion between highly negatively charged aggrecan macromolecules extracted from bovine cartilage extracellular matrix by performing atomic force microscopy (AFM) imaging and single-molecule force spectroscopy (SMFS) in saline solutions. By controlling the density of aggrecan molecules on both the gold substrate and the gold-coated tip surface at submonolayer densities, we were able to detect and quantify the Ca²⁺-dependent homodimeric interaction between individual aggrecan molecules at the single-molecule level. We found a typical nonlinear sawtooth profile in the AFM force-versus-distance curves with a molecular persistence length of l_p = 0.31 ± 0.04 nm. This is attributed to the stepwise dissociation of individual glycosaminoglycan (GAG) side chains in aggrecans, which is very similar to the known force fingerprints of other cell adhesion proteoglycan systems. After studying the GAG-GAG dissociation in a dynamic, loading-rate-dependent manner (dynamic SMFS) and analyzing the data according to the stochastic Bell-Evans model for a thermally activated decay of a metastable state under an external force, we estimated for the single glycan interaction a mean lifetime of τ = 7.9 ± 4.9 s and a reaction bond length of x_β = 0.31 ± 0.08 nm. Whereas the x_β-value compares well with values from other cell adhesion carbohydrate recognition motifs in evolutionary distant marine sponge proteoglycans, the rather short GAG interaction lifetime reflects high intermolecular dynamics within aggrecan complexes, which may be relevant for the viscoelastic properties of cartilage tissue.