Background: Homer is a postsynaptic scaffold protein that links various synaptic signaling proteins, including the type I metabotropic glutamate receptor subunits 1alpha and 5, the inositol 1,4,5-trisphosphate receptor, Shank and Cdc42 small GTPase. Overexpression of Homer induces changes in dendritic spine morphology in cultured hippocampal neurons. However, the molecular basis underpinning Homer-mediated spine morphogenesis remains unclear. In this study, we aimed to elucidate the structural and functional properties of the interaction between Cupidin/Homer2 and two actin-cytoskeletal regulators, Cdc42 small GTPase and Drebrin.
Results: Cupidin/Homer2 interacted with activated Cdc42 small GTPase via the Cdc42-binding domain that resides around amino acid residues 191-283, within the C-terminal coiled-coil domain. We generated a Cupidin deletion mutant lacking amino acids 191-230 (CPDDelta191-230), which showed decrease Cdc42-binding ability but maintained self-multimerization ability. Cupidin suppressed Cdc42-induced filopodia-like protrusion formation in HeLa cells, whereas CPDDelta191-230 failed to do so. In cultured hippocampal neurons, Cupidin was targeted to dendritic spines, whereas CPDDelta191-230 was distributed in dendritic shafts as well as spines. Overexpression of CPDDelta191-230 decreased the number of synapses and reduced the amplitudes of miniature excitatory postsynaptic currents in hippocampal neurons. Cupidin interacted with a dendritic spine F-actin-binding protein, Drebrin, which possesses two Homer ligand motifs, via the N-terminal EVH-1 domain. CPDDelta191-230 overexpression decreased Drebrin clustering in the dendritic spines of hippocampal neurons.
Conclusion: These results indicate that Cupidin/Homer2 interacts with the dendritic spine actin regulators Cdc42 and Drebrin via its C-terminal and N-terminal domains, respectively, and that it may be involved in spine morphology and synaptic properties.