Myosin IIb controls actin dynamics underlying the dendritic spine maturation

Precise control of the formation and development of dendritic spines is critical for synaptic plasticity. Consequently, abnormal spine development is linked to various neurological disorders. The actin cytoskeleton is a structural element generating specific changes in dendritic spine morphology. Although mechanisms underlying dendritic filopodia elongation and spine head growth are relatively well understood, it is still not known how spine heads are enlarged and stabilized during dendritic spine maturation. By using rat hippocampal neurons, we demonstrate that the size of the stable actin pool increases during the neuronal maturation process. Simultaneously, the treadmilling rate of the dynamic actin pool increases. We further show that myosin IIb controls dendritic spine actin cytoskeleton by regulating these two different pools of F-actin via distinct mechanisms. The findings indicate that myosin IIb stabilizes the stable F-actin pool through actin cross-linking. Simultaneously, activation of myosin IIb contractility increases the treadmilling rate of the dynamic pool of actin. Collectively, these data show that myosin IIb has a major role in the regulation of actin filament stability in dendritic spines, and elucidate the complex mechanism through which myosin IIb functions in this process. These new insights into the mechanisms underlying dendritic spine maturation further the model of dendritic spine morphogenesis.