Interfering with the Actin Network and Its Effect on Long-Term Potentiation and Synaptic Tagging in Hippocampal CA1 Neurons in Slices in Vitro
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The Journal of Neuroscience, September 30, 2009 • 29(39):12167–12173 • 12167 Cellular/Molecular Interfering with the Actin Network and Its Effect on Long-Term Potentiation and Synaptic Tagging in Hippocampal CA1 Neurons in Slices In Vitro Binu Ramachandran and Julietta U. Frey Department of Neurophysiology, Leibniz Institute for Neurobiology, 39118 Magdeburg, Germany Long-termpotentiation(LTP)isacellularcorrelateformemoryformation,whichrequiresthedynamicchangesoftheactincytoskeleton. As shown by others, the polymerization of the actin network is important for early stages of LTP. Here, we investigated the role of actin dynamics in synaptic tagging and particularly in the induction of protein synthesis-dependent late-LTP in the CA1 region in hippocampal slices in vitro. We found that the inhibition of actin polymerization affects protein synthesis-independent early-LTP, prevents late-LTP, and interferes with synaptic tagging in apical dendrites of hippocampal CA1. The transformation of early-LTP into late-LTP was blocked by the application of the structurally different actin polymerization inhibitors latrunculin A or cytochalasin D. We suggest that the actin network is required for early “housekeeping” processes to induce and maintain early-LTP. Furthermore, inhibition of actin dynamics negatively interacts with the setting of the synaptic tagging complex. We propose actin as a further tag-specific molecule in apical CA1 dendrites where it is directly involved in the tagging/capturing machinery. Consequently, inhibition of the actin network prevents the interaction of tagging complexes with plasticity-related proteins. This results in the prevention of late-LTP by inhibition of the actin network during LTP induction. Introduction The actin network plays an important, multifunctional role in Changes in synaptic efficacy are considered as basic processes of synaptic function as well as during plasticity and, specifically, in information storage in the brain. Currently, there are only two hippocampal LTP. Actin depolymerization blocks both functional cellular phenomena which could underlie such prolonged as well as structural plasticity (Krucker et al., 2000; Fukazawa et al., changes called long-term potentiation (LTP) and long-term de- 2003; Lisman, 2003; Matsuzaki et al., 2004; Okamoto et al., 2004; pression (LTD) (Matthies et al., 1990; Bear and Malenka, 1994; Chen et al., 2007; Tanaka et al., 2008). The actin network is asso- Frey and Frey, 2008). Both, LTP and LTD show distinct temporal ciated with a rapid and persistent reorganization of the spine stages such as a protein synthesis-independent early phase (early- cytoskeleton (Fischer et al., 1998; Okamoto et al., 2004; Lin et al., LTP, early-LTD) and a protein synthesis-dependent late phase 2005). Furthermore, an increase in F-actin content and structural (late-LTP, late-LTD). LTP/LTD maintenance requires the syn- changes within dendritic spines during LTP induction were de- thesis of plasticity-related proteins (PRPs) (Krug et al., 1984; scribed previously (Moshkov and Petrovskaia, 1983; Fifkova´, 1985; Frey et al., 1988; Otani et al., 1989; Nguyen and Kandel, 1996; Pak et al., 2001; Fukazawa et al., 2003; Lisman, 2003; Okamoto et Manahan-Vaughan et al., 2000). How PRPs interact only with al., 2004, 2007; Chen et al., 2007), and it was shown that the actin activated (“tagged”) synapses expressing LTP or LTD is funda- cytoskeleton mediates exocytosis and endocytosis of AMPA- mental to synapse-specificity of the process and can be explained receptors at postsynaptic sites (Malinow et al., 2000; Sheng and by the “synaptic tagging” hypothesis (Frey and Morris, 1997, Lee, 2001). Actin polymerization is also involved in the regula- 1998a,b). Specific stimulation transiently sets a “tag” at activated tion of the synthesis of protein kinase Mzeta (PKMzeta), which synapses and induces PRP synthesis. If both tags and PRPs are enhances synaptic efficacy during late-LTP by recruiting more available within a specific time window, they can interact, thus postsynaptic AMPA receptors (Serrano et al., 2005; Ling et al., transforming a normally transient early-LTP (E-LTP) into late- 2006). PKMzeta has been identified as a PRP required for late- LTP (L-LTP) (Frey and Morris, 1998a; Frey and Frey, 2008). LTP in the apical CA1 dendrites (Sajikumar et al., 2005b). Be- cause the actin network is directly involved in local protein trafficking within the dendritic spine (Kaech et al., 1996; Langford Received April 30, 2009; revised July 31, 2009; accepted Aug. 24, 2009. This work was supported by Deutsche Forschungsgemeinschaft Grant 1034-7 and Sonderforschungsbereich and Molyneaux, 1998), it could also exert its action during the Grant 779-B4 to J.U.F. We are grateful to Dr. Jeffrey Vernon for his very helpful input on this manuscript and to Dr. capture of PRPs at the tagged synapse. Here, we further investi- Sajikumar Sreedharan for his help to train B.R. in in vitro slice techniques, as well as for his technical support and gate the possible specific action of actin network function on discussions. We are also very grateful to Diana Koch for her excellent technical assistance. processes required for late-LTP in the hippocampal CA1 in vitro. CorrespondenceshouldbeaddressedtoDr.JuliettaU.Frey,DepartmentofNeurophysiology,LeibnizInstitutefor Neurobiology, Brenneckestrasse 6, 39118 Magdeburg, Germany. E-mail: [email protected]. We suggest that the actin network interacts with late-LTP and DOI:10.1523/JNEUROSCI.2045-09.2009 specifically with the PRP-capturing process at synaptic tags (Frey Copyright © 2009 Society for Neuroscience 0270-6474/09/2912167-07$15.00/0 and Morris, 1998a; Martin and Kosik, 2002). By using structur- 12168 • J. Neurosci., September 30, 2009 • 29(39):12167–12173 Ramachandran and Frey • Actin and Processes of Late-LTP ally different actin assembly inhibitors, latrunculin A or cytochalasin D, we show that actin function is involved in early-LTP, and it is essential for late- LTP by interacting with the synaptic tagging machinery. Materials and Methods All experiments were performed in accordance with the European Community Council Direc- tive of 24 November 1986 (86/609/EEC). We also certify that formal approval to conduct the experiments has been obtained from the ani- mal subject review board of our institution/ local government which can be provided upon request. All efforts were made to minimize the number of animals used and their suffering. Slice preparation and electrophysiology. Trans- verse hippocampal slices (112; 400 m) were prepared from 112 7-week-old male Wistar rats as described previously (Frey et al., 1988; Sajikumar et al., 2005a). The rats were killed rapidly by a single blow to the back of the neck using a metallic rod (cervical dislocation) and immediately decapitated. Slices were incu- Figure 1. The role of the actin network in long-term potentiation, i.e., late-LTP (time courses of field EPSP recordings). bated in an interface chamber at 32°C, and the A, Schematic representation of a transversal rat hippocampal slice with electrode locations to stimulate two separate synaptic incubation medium [artificial cerebrospinal inputsS1andS2instratumradiatumoftheCA1region.ThelocationofrecordingelectrodesforthePSaswellasthefieldEPSPfrom a single, stimulated neuronal population are provided. The analog traces are representative examples of recorded potentials. B, fluid (ACSF)] contained (in mM) 124 NaCl, 4.9 KCl, 1.2 KH PO , 2.0 MgSO , 2.0 CaCl , 24.6 The time course of the slope of the field EPSP after induction of late-LTP in S1 by STET (filled circles; tetanization are symbolized as 2 4 4 2 arrows) is shown. Open circles represent recordings from the control synaptic pathway S2 (n ϭ 7). C, Induction of late-LTP in the NaHCO3, 10.0 D-glucose. The carbogen con- sumption was 32 L/h, and the flow rate of ACSF presenceoftheactinpolymerizationinhibitorlatrunculinA(boxLatA).LatrunculinAwasapplied30minbeforeSTET(3arrows)of ϭ was 0.7 ml/min (the entire procedure from kill- S1(filledcircles;wash-outofthedrug:30minafterthefirsttetanization;opencirclescontrolinput;n 7).D,Inductionoflate-LTP ϭ ing the animal after placing the slices into the of synaptic input S1 (filled circles) in the presence of actin polymerization inhibitor cytochalasin D (n 7). Cytochalasin D (box Cyt recording chamber took ϳ3–5 min). In all D)wasapplied30minbeforeSTET,anditwaswashed-out30minafterthefirsttetanization(opencircles:controlinputS2).Analog experiments, two monopolar lacquer-coated, traces represent typical field EPSPs 30 min before the normal time point of tetanization (dashed line); 30 min (solid line), and 6 h stainless-steel electrodes (5M⍀; AM-Systems) (dotted line) after tetanization of input S1. The analog traces for S2 were recorded at the same time points, however, without were positioned within the stratum radiatum tetanization. Calibration: 2 mV, 5 ms. Arrows indicate the time point of tetanization of synaptic input S1 (STET are symbolized as 3 of the CA1 region for stimulating two separate arrows). Filled boxes represent drug application. Lat A, latrunculin A; Cyt D, cytochalasin D. independent synaptic inputs, S1 and S2. For recording the field EPSP (measured as its slope function) and the popu- structurally different actin polymerization inhibitor, cytochalasin D lation spike (PS) amplitude, two electrodes (resistance, 5 M⍀; AM- (Calbiochem) was used at a concentration of 0.1 M. Cytochalasin D was Systems) were placed in the CA1 apical dendritic and cell body layer,