A Critical Role for Myosin IIB in Dendritic Spine Morphology and Synaptic Function
ABSTRACT Dendritic spines show rapid motility and plastic morphology, which may mediate information storage in the brain. It is presently believed that polymerization/depolymerization of actin is the primary determinant of spine motility and morphogenesis. Here, we show that myosin IIB, a molecular motor that binds and contracts actin filaments, is essential for normal spine morphology and dynamics and represents a distinct biophysical pathway to control spine size and shape. Myosin IIB is enriched in the postsynaptic density (PSD) of neurons. Pharmacologic or genetic inhibition of myosin IIB alters protrusive motility of spines, destabilizes their classical mushroom-head morphology, and impairs excitatory synaptic transmission. Thus, the structure and function of spines is regulated by an actin-based motor in addition to the polymerization state of actin.
Full-textDOI: · Available from: Richard Weinberg, Jan 07, 2014
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ABSTRACT: In the vertebrate central nervous system, exploratory filopodia transiently form on dendritic branches to sample the neuronal environment and initiate new trans-neuronal contacts. While much is known about the molecules that control filopodia extension and subsequent maturation into functional synapses, the mechanisms that regulate initiation of these dynamic, actin-rich structures have remained elusive. Here, we find that filopodia initiation is suppressed by recruitment of ArhGAP44 to actin-patches that seed filopodia. Recruitment is mediated by binding of a membrane curvature-sensing ArhGAP44 N-BAR domain to plasma membrane sections that were deformed inward by acto-myosin mediated contractile forces. A GAP domain in ArhGAP44 triggers local Rac-GTP hydrolysis, thus reducing actin polymerization required for filopodia formation. Additionally, ArhGAP44 expression increases during neuronal development, concurrent with a decrease in the rate of filopodia formation. Together, our data reveals a local auto-regulatory mechanism that limits initiation of filopodia via protein recruitment to nanoscale membrane deformations.eLife Sciences 12/2014; 3. DOI:10.7554/eLife.03116 · 8.52 Impact Factor
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ABSTRACT: The morphology of neuronal dendritic spines is a critical indicator of synaptic function. It is regulated by several factors, including the intracellular actin/myosin cytoskeleton and trans-cellular N-cadherin adhesions. To examine the mechanical relationship between these molecular components, we performed quantitative live imaging experiments in primary hippocampal neurons. We found that actin turnover and structural motility were lower in dendritic spines than in immature filopodia, and increased upon expression of a non-adhesive N-cadherin mutant, resulting in an inverse relationship between spine motility and actin enrichment. Furthermore, the pharmacological stimulation of myosin II induced the rearward motion of actin structures in spines, revealing that myosin II exerts tension on the actin network. Strikingly, the formation of stable spine-like structures enriched in actin was induced at contacts between dendritic filopodia and N-cadherin coated beads or micropatterns. Finally, computer simulations of actin dynamics mimicked various experimental conditions, pointing to the actin flow rate as an important parameter controlling actin enrichment in dendritic spines. Together, these data demonstrate that a clutch-like mechanism between N-cadherin adhesions and the actin flow underlies the stabilization of dendritic filopodia into mature spines, a mechanism which may have important implications in synapse initiation, maturation and plasticity in the developing brain. © 2015 by The American Society for Cell Biology.Molecular Biology of the Cell 01/2015; 26(5). DOI:10.1091/mbc.E14-06-1086 · 4.55 Impact Factor
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ABSTRACT: Class-18 myosins are most closely related to conventional class-2 nonmuscle myosins (NM2). Surprisingly, the purified head domains of Drosophila, mouse, and human myosin 18A (M18A) lack actin-activated ATPase activity and the ability to translocate actin filaments, suggesting that the functions of M18A in vivo do not depend on intrinsic motor activity. M18A has the longest coiled coil of any myosin outside of the class-2 myosins, suggesting that it might form bipolar filaments similar to conventional myosins. To address this possibility, we expressed and purified full-length mouse M18A using the baculovirus/Sf9 system. M18A did not form large bipolar filaments under any of the conditions tested. Instead, M18A formed an ∼65-nm-long bipolar structure with two heads at each end. Importantly, when NM2 was polymerized in the presence of M18A, the two myosins formed mixed bipolar filaments, as evidenced by cosedimentation, electron microscopy, and single-molecule imaging. Moreover, super-resolution imaging of NM2 and M18A using fluorescently tagged proteins and immunostaining of endogenous proteins showed that NM2 and M18A are present together within individual filaments inside living cells. Together, our in vitro and live-cell imaging data argue strongly that M18A coassembles with NM2 into mixed bipolar filaments. M18A could regulate the biophysical properties of these filaments and, by virtue of its extra N- and C-terminal domains, determine the localization and/or molecular interactions of the filaments. Given the numerous, fundamental cellular and developmental roles attributed to NM2, our results have far-reaching biological implications. Copyright © 2015 Elsevier Ltd. All rights reserved.