The morphology of adsorbed extracellular matrix assemblies is critically dependent on solution calcium concentration

The adsorption of proteins to surfaces may alter their biological properties. Understanding and controlling these interactions is important in ultrastructural, biochemical and cellular studies. We have previously demonstrated that both the morphology and biological function of extracellular matrix assemblies such as fibrillin and type VI collagen microfibrils are influenced by surface chemistry. In this study we have employed atomic force microscopy to determine if the morphology of extracellular matrix microfibrils is influenced by solution chemistry. Microfibrils were adsorbed to mica or poly-l-lysine modified mica (mica-PLL) in the presence of 31 μM–1000 μM Ca2+. Although both microfibrillar species adsorbed to mica and mica-PLL at all calcium concentrations, maximal adsorption was observed on mica at 125–250 μM. On mica surfaces fibrillin microfibril morphology varied continuously with calcium concentration from laterally diffuse assemblies at high concentrations to compact assemblies at low concentrations. In contrast, distinct type VI collagen microfibril morphologies were observed at high, intermediate and low calcium concentrations. Similar calcium dependent microfibrillar morphologies were evident on mica-PLL. Therefore physiologically relevant concentrations of solution calcium, independent of surface charge, profoundly influenced both the adsorbed amount and morphology of native extracellular assemblies. These studies highlight the importance not only of surface chemistry but also of solute composition and concentration in influencing the morphology and hence biological function of adsorbed proteins.