Local, persistent activation of Rho GTPases during plasticity of single dendritic spines

Abstract

Even brief neuronal activity can result in long-term potentiation (LTP) of synapses, which is associated with enlargement of dendritic spines on the neuron at the receiving end of neurotransmission. Using live imaging of fluorescently tagged signalling proteins in individual spines, Ryohei Yasuda and colleagues show that transient activation of the Ca2+/calmodulin-dependent kinase (CaMKII) is translated into short-term versus long-term and long-range (dendritic) versus short-range (spine-confined) signalling, depending on which small GTPase of the Rho family is activated. The technique is bringing the study of the cellular bases of learning and memory down to the nanometre scale. Even brief neuronal activity can result in long-term potentiation of synapses, which is associated with enlargement of dendritic spines on the neuron at the receiving end of neurotransmission. Using live imaging of fluorescently tagged signalling proteins in individual dendritic spines, this study shows that transient activation of the Ca2+/calmodulin-dependent kinase is translated into short-term versus long-term and long-range (dendritic) versus short-range (spine-confined) signalling, depending on which small GTPase of the Rho family is activated. The technique is bringing the study of the cellular bases of learning and memory down to the nanometre scale. The Rho family of GTPases have important roles in the morphogenesis of the dendritic spines1,2,3 of neurons in the brain and synaptic plasticity4,5,6,7,8,9 by modulating the organization of the actin cytoskeleton10. Here we used two-photon fluorescence lifetime imaging microscopy11,12,13 to monitor the activity of two Rho GTPases—RhoA and Cdc42—in single dendritic spines undergoing structural plasticity associated with long-term potentiation in CA1 pyramidal neurons in cultured slices of rat hippocampus. When long-term volume increase was induced in a single spine using two-photon glutamate uncaging14,15, RhoA and Cdc42 were rapidly activated in the stimulated spine. These activities decayed over about five minutes, and were then followed by a phase of persistent activation lasting more than half an hour. Although active RhoA and Cdc42 were similarly mobile, their activity patterns were different. RhoA activation diffused out of the stimulated spine and spread over about 5 µm along the dendrite. In contrast, Cdc42 activation was restricted to the stimulated spine, and exhibited a steep gradient at the spine necks. Inhibition of the Rho–Rock pathway preferentially inhibited the initial spine growth, whereas the inhibition of the Cdc42–Pak pathway blocked the maintenance of sustained structural plasticity. RhoA and Cdc42 activation depended on Ca2+/calmodulin-dependent kinase (CaMKII). Thus, RhoA and Cdc42 relay transient CaMKII activation13 to synapse-specific, long-term signalling required for spine structural plasticity.