Interaction of membrane-bound Fc gamma receptors with the cytoskeletal matrix

Binding of ligands to cell surface receptors may induce an interaction of the receptors with the cell cytoskeleton. This interaction may decrease the solubility of the receptors in nonionic detergents. We studied effect of binding of various 125I-labeled immunoglobulin ligands to Fc gamma receptors on guinea pig peritoneal macrophages and human placental syncytiotrophoblast plasma membranes on an interaction of these receptors with the cytoskeletal matrix. The receptor-cytoskeleton association was evaluated by measurement radioactivity of bound ligands in pellets and supernatants obtained after lysis of cells or membranes in a nonionic detergent NP-40. Binding of soluble immune complexes or crosslinking of IgG bound induces much stronger insolubilization of the receptors than binding of monomeric or aggregated IgG. It shows that the interaction of the receptors with the cytoskeletal matrix strongly depends on the degree of cross-linking of the Fc gamma receptors by ligands bound. The observed effects were IgG Fc region-specific. Isolated, purified putative Fc gamma receptors from guinea pig peritoneal macrophages and from human placental syncytiotrophoblast plasma membranes do not interact with free G or F actin. We also studied association of the guinea pig peritoneal macrophage Fc gamma receptor with the cytoskeleton, before and after shedding of macrophage membrane proteins. The results obtained showed that the macrophages have only one class of Fc gamma receptors interacting with the cytoskeletal matrix. Effect of a cytoskeleton-destabilizing buffer and DNAse I on release of the receptors from the cytoskeleton suggests that insolubilization of ligand-Fc gamma R complexes was caused, at least partially, by an interaction of the receptors with actin filaments in the cytoskeleton. The results presented in this paper suggest that the cytoskeleton might play a role in transmission of signals from Fc gamma receptors to the cells. They underline the role of immune complexes as physiological ligands for Fc receptors and correlate well with activation of cells via their Fc receptors (e.g. superoxide burst) observed by other authors after treatment of the cells with immune complexes, but not with monomeric or aggregated IgG.