Regulation of NMDA receptor activity by F-actin and myosin light chain kinase

The postsynaptic density (PSD) at excitatory dendritic synapses comprises a protein complex of glutamate receptors, scaffolding elements, and signalin g enzymes. For example, NMDA receptors (NMDARs) are linked to several prote ins in the PSD, such as PSD-95, and are also tethered via binding proteins such as alpha -actinin directly to filamentous actin of the cytoskeleton. D epolymerization of the cytoskeleton modulates the activity of NMDARs, and, in turn, strong activation of NMDARs can trigger depolymerization of actin. Myosin, the motor protein of muscular contraction and nonmuscle motility, is also associated with NMDARs and the PSD. We show here that constitutivel y active myosin light chain kinase (MLCK) enhances NMDAR-mediated whole-cel l and synaptic currents in acutely isolated CA1 pyramidal and cultured hipp ocampal neurons, whereas inhibitors of MLCK depress these currents. This ML CK-dependent regulation was observed in cell-attached patches but was lost after excision to inside-out patches. Furthermore, the enhancement induced by constitutively active MLCK and the depression of MLCK inhibitors were el iminated after depolymerization of the cytoskeleton. NMDARs and MLCK did no t colocalize in clusters on the dendrites of cultured hippocampal neurons, further indicating that the effects of MLCK are mediated indirectly via act omyosin. Our results suggest that MLCK enhances actomyosin contractility to either increase the membrane tension on NMDARs or to alter physical relati onships between the actin cytoskeleton and the linker proteins of NMDARs.