The rule of Myosin II and Rap2 in synaptic structure and function

Abstract

Synapses, the connections between neurons, exhibit both structural and functional plasticity, and these changes could underlie learning and memory. Two synaptic phenomena that have been studied extensively are Hebbian plasticity and changes in dendritic spine morphology. Recent proteomics studies have uncovered many proteins that reside in the synapse and could play critical roles in these processes. Among these are the molecular motor myosin II and the Ras family GTPase Rap2. Myosin II can move and contract actin filaments in non-neuronal cells, and it represents a novel way to alter spine structure, which is classically thought to occur through actin polymerization and depolymerization. Rap GTPases are the closest relatives to Ras, which is well established as a positive regulator of spines and synaptic transmission. In vitro evidence indicates that Raps could act antagonistically to Ras in neurons, inhibiting spine growth and synaptic strength. To study myosin II's role in dendritic spine morphology and synaptic function, we inhibited myosin II function either pharmacologically or genetically in dissociated hippocampal neurons. Knockdown of myosin II by RNA interference resulted in loss of mature mushroom-shaped spines, and an increase in thin, filopodia-like structures. Treatment with blebbistatin, a chemical inhibitor of myosin II, phenocopied this result. Live imaging revealed that mature spines unravel into filopodia within tens of minutes of myosin II inhibition by blebbistatin. Furthermore, blebbistatin treatment led to decreases in levels of the glutamatergic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), as well as impairment of synaptic transmission.

(cont.) To study Rap2 neuronal function in vivo, we created transgenic mice that expressed constitutively active Rap2 (Rap2V12) in postnatal forebrain. Consistent with an inhibitory role for Rap at synapses, Rap2V12 mice exhibited reduced levels of phospho-ERK (pERK) and a reduction in spine density and length. Behaviorally these mice were hyperactive and showed impairments in spatial learning. In addition, Rap2V12 mice showed normal acquisition of fear memories, but were defective in the extinction of contextual fear. Fear extinction has been associated with several psychiatric disorders, including posttraumatic stress disorder (PTSD), and Rap2V12 mice might offer a potential therapeutic model for such diseases.