KIF21B binds Myosin Va for Spine Entry and regulates Actin Dynamics to control Homeostatic Synaptic Downscaling

Abstract

Homeostatic synaptic plasticity adjusts the strength of synapses to restrain neuronal activity within a physiological range. Postsynaptic GKAP controls the bidirectional synaptic scaling of AMPA receptors (AMPARs) however how chronic activity triggers postsynaptic protein remodeling to downscale synaptic transmission is barely understood. Here we report that the microtubule-dependent kinesin motor KIF21B interacts with GKAP and likewise enters dendritic spines in a myosin Va- and activity-dependent manner. We observed that under conditions of chronic activity KIF21B regulates actin dynamics in spines, triggers spine removal of GluA2-containing AMPA receptors, and mediates homeostatic synaptic downscaling of AMPA receptor-mediated mEPSC amplitudes. Our data highlight a myosin-kinesin interaction that enables the entry of the microtubule-dependent motor KIF21B into actin-rich spine compartments. A slow actin turnover rate might be beneficial for efficient protein removal from excitatory synapses, suggesting a functional role of KIF21B in a GKAP- and AMPA receptor-dependent mechanism, underlying homeostatic downscaling of neuronal firing.