title: Myosin IIb activity and phosphorylation status determines dendritic spine and post-synaptic density morphology.
abstract: Dendritic spines in hippocampal neurons mature from a filopodia-like precursor into a mushroom-shape with an enlarged post-synaptic density (PSD) and serve as the primary post-synaptic location of the excitatory neurotransmission that underlies learning and memory. Using myosin II regulatory mutants, inhibitors, and knockdowns, we show that non-muscle myosin IIB (MIIB) activity determines where spines form and whether they persist as filopodia-like spine precursors or mature into a mushroom-shape. MIIB also determines PSD size, morphology, and placement in the spine. Local inactivation of MIIB leads to the formation of filopodia-like spine protrusions from the dendritic shaft. However, di-phosphorylation of the regulatory light chain on residues Thr18 and Ser19 by Rho kinase is required for spine maturation. Inhibition of MIIB activity or a mono-phosphomimetic mutant of RLC similarly prevented maturation even in the presence of NMDA receptor activation. Expression of an actin cross-linking, non-contractile mutant, MIIB R709C, showed that maturation into a mushroom-shape requires contractile activity. Loss of MIIB also leads to an elongated PSD morphology that is no longer restricted to the spine tip; whereas increased MIIB activity, specifically through RLC-T18, S19 di-phosphorylation, increases PSD area. These observations support a model whereby myosin II inactivation forms filopodia-like protrusions that only mature once NMDA receptor activation increases RLC di-phosphorylation to stimulate MIIB contractility, resulting in mushroom-shaped spines with an enlarged PSD.
authors: Department of Cell Biology, University of Virginia School of Medicine, Charlottesville, Virginia, United States of America.
authors: Jennifer L
authors: JL
authors: Hodges
authors: Karen
authors: K
authors: Newell-Litwa
authors: Hannelore
authors: H
authors: Asmussen
authors: Miguel
authors: M
authors: Vicente-Manzanares
authors: Alan Rick
authors: AR
authors: Horwitz
mesh_terms: D000818:Animals; D022081:Cell Surface Extensions / Q000648:ultrastructure; D049229:Dendritic Spines / Q000378:metabolism / Q000648:ultrastructure*; D024501:Nonmuscle Myosin Type IIB / Q000378:metabolism*; D010766:Phosphorylation; D057907:Post-Synaptic Density / Q000378:metabolism / Q000648:ultrastructure*; D011554:Pseudopodia; D051381:Rats; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism
chemical_list: D016194:Receptors, N-Methyl-D-Aspartate; D024501:Nonmuscle Myosin Type IIB