text: Quantitative analysis of type I collagen fibril regulation by lumican and decorin using AFM.
  Lumican and decorin, two members of the small leucine-rich repeat proteoglycan (SLRP) family, have been implicated as regulators of collagen I fibril structure in different tissues. Both proteoglycans consist of a core protein and a glycosaminoglycan (GAG) chain, but quantitative information regarding the precise role of the protein and GAG moieties in regulating collagen structure is still limited. In this study, we used AFM imaging and a model system of aligned collagen I nanofibrils to investigate the role of lumican and decorin on collagen I fibril structure with high resolution. When co-assembled with collagen I, recombinant lumican or decorin proteins lacking the GAG chains decreased collagen fibril width to values below <100nm and increased interfibrillar spacing in a dose-dependent manner. At lower concentrations, lumican appeared to have a stabilizing effect on newly-formed collagen fibrils, while at higher concentrations both lumican and decorin inhibited collagen fibrillogenesis. GAG-containing decorin also increased interfibrillar spacing, decreased fibril width and ultimately inhibited fibrillogenesis, but these effects required lower concentrations compared to recombinant decorin, indicating that the decorin core protein alone cannot compensate for the full regulatory and structural contribution of the GAG chain during collagen I fibrillogenesis. Using a 2D autocorrelation approach, we furthermore analyzed and compared the effects of recombinant and glycosylated decorin on collagen ultrastructure, providing a quantitative measure for the observed structural differences. AFM analysis of ordered fibrillar collagen arrays in combination with quantitative autocorrelation image analysis thus provides a useful tool for investigating SLRP-dependent nanoscale effects on collagen fibril structure.