Abstract
The biochemical transduction of excitatory synaptic signals occurs in the cytoplasm within dendritic spines. The associated reaction kinetics are shaped by the mobility of the signaling molecules; however, accurate monitoring of diffusional events within the femtoliter-sized spine structures has not yet been demonstrated. Here, we applied two-photon fluorescence correlation spectroscopy and raster image correlation spectroscopy to monitor protein dynamics within spines, revealing that F-actin restricts the mobility of proteins with a molecular mass of >100 kDa. This restriction is transiently removed during actin remodeling at the initial phase of spine structural plasticity. Photobleaching experiments combined with super-resolution imaging indicate that this increase in mobility facilitates molecular interactions, which may modulate the functions of key postsynaptic signaling molecules, such as Tiam1 and CaMKII. Thus, actin polymers in dendritic spines act as precise temporal regulators of molecular diffusion and modulate signal transduction during synaptic plasticity.