Microdistribution of substratum-bound ligands affects cell function: hepatocyte spreading on PEO-tethered galactose.

Cell functions are regulated by signal transduction processes triggered by binding interactions with extracellular ligands, including those coupled to insoluble matrices as well as those diffusing in free solution. Whereas receptor interactions with freely diffusible soluble ligands are quantitatively governed by the mean ligand concentration, interactions with spatially constrained substratum-bound ligands may be affected not only by the mean ligand concentration, but also by the ligand spatial microdistribution. To probe this hypothesis we have generated surfaces presenting galactose ligands at different concentrations and different capabilities for micromobility--and thus spatial microdistribution--by means of polyethylene oxide tethering and assayed for hepatocyte spreading mediated by asialoglycoprotein receptor/cytoskeleton linkages. We demonstrate that spreading is not uniquely determined by the mean galactose concentration presented by the culture substrate. Rather, the ability of primary hepatocytes to spread is additionally specified by a combination of ligand concentration and tether length. Our results suggest spreading results when monovalent ligands possess sufficient mobility to form effectively multivalent bonds with the receptor; that is, when the substratum-bound ligands to cluster in spatial microdomains.