Arc/Arg3.1: Linking Gene Expression to Synaptic Plasticity and Memory
2006; Cell Press; Volume: 52; Issue: 3 Linguagem: Inglês
10.1016/j.neuron.2006.10.016
ISSN1097-4199
AutoresAnastassios V. Tzingounis, Roger A. Nicoll,
Tópico(s)Retinal Development and Disorders
ResumoArc/Arg3.1 is an effector immediate-early gene implicated in the consolidation of memories. Although cloned a decade ago, the physiological role of Arc/Arg3.1 in the brain has remained elusive. Four papers in this issue of Neuron address this function. These studies show that Arc/Arg3.1 regulates endophilin 3 and dynamin 2, two components of the endocytosis machinery. Genetic ablation of Arc/Arg3.1 in mice or overexpression in culture suggest that Arc/Arg3.1 regulates AMPA receptor trafficking and synaptic plasticity. Finally, Arc/Arg3.1 knockout mice show memory retention deficits. These recent developments provide new insights into the function of this popular activity-dependent neuronal marker. Arc/Arg3.1 is an effector immediate-early gene implicated in the consolidation of memories. Although cloned a decade ago, the physiological role of Arc/Arg3.1 in the brain has remained elusive. Four papers in this issue of Neuron address this function. These studies show that Arc/Arg3.1 regulates endophilin 3 and dynamin 2, two components of the endocytosis machinery. Genetic ablation of Arc/Arg3.1 in mice or overexpression in culture suggest that Arc/Arg3.1 regulates AMPA receptor trafficking and synaptic plasticity. Finally, Arc/Arg3.1 knockout mice show memory retention deficits. These recent developments provide new insights into the function of this popular activity-dependent neuronal marker. The molecular mechanisms for the induction and stabilization of synapse strengthening during learning and memory has become a major focus of cellular neuroscience over the past two decades (Bredt and Nicoll, 2003Bredt D.S. Nicoll R.A. Neuron. 2003; 40: 361-379Abstract Full Text Full Text PDF PubMed Scopus (888) Google Scholar). The recognition that high-frequency stimulation of afferent fibers in hippocampus can lead to long-term changes in synaptic strength (i.e., LTP) provided a means to study synaptic plasticity at the cellular level. Early studies using pharmacological manipulations of receptor and kinase function concluded that LTP, a presumed synaptic correlate of memory (Whitlock et al., 2006Whitlock J.R. Heynen A.J. Shuler M.G. Bear M.F. Science. 2006; 313: 1093-1097Crossref PubMed Scopus (1307) Google Scholar), requires calcium influx through NMDA receptors leading to rapid insertion of AMPA receptors (Malenka and Nicoll, 1999Malenka R.C. Nicoll R.A. Science. 1999; 285: 1870-1874Crossref PubMed Scopus (2163) Google Scholar). However, for LTP or any other form of synaptic plasticity to underlie learning and memory, a persistent modification of synapses is necessary. Most likely, any long-lasting change would depend on rapid induction of gene transcription and subsequent protein synthesis. Therefore, in parallel to studies that focused on the immediate molecular mechanism of the strengthening stimulus, several groups pursued research involving transcription-dependent changes. Initial evidence into the relationship between intense synaptic activity and rapid transient gene expression came from studies involving the immediate-early gene (IEG) c-fos (Morgan et al., 1987Morgan J.I. Cohen D.R. Hempstead J.L. Curran T. Science. 1987; 237: 192-197Crossref PubMed Scopus (1459) Google Scholar). The demonstration that calcium influx, in response to cholinergic receptor activation, could induce c-fos transcription suggested a mechanism to link electrical activity to gene regulation (Greenberg et al., 1986Greenberg M.E. Ziff E.B. Greene L.A. Science. 1986; 234: 80-83Crossref PubMed Scopus (550) Google Scholar). Soon afterwards several papers were published identifying many more activity-induced IEGs and, in particular, linking this induction to LTP-inducing stimuli (Cole et al., 1989Cole A.J. Saffen D.W. Baraban J.M. Worley P.F. Nature. 1989; 340: 474-476Crossref PubMed Scopus (879) Google Scholar). An IEG is defined as a gene that is rapidly and transiently activated at the transcriptional level following robust synaptic, neurotransmitter, or growth factor stimulation (for extensive review of the early IEG literature see Sheng and Greenberg, 1990Sheng M. Greenberg M.E. Neuron. 1990; 4: 477-485Abstract Full Text PDF PubMed Scopus (1923) Google Scholar). The first collection of IEGs encoded transcription factors. This indicated that IEGs might not have a direct role in synapse strengthening but rather would orchestrate the expression of genes encoding proteins that might be more intimately involved in synaptic plasticity and memory formation. This realization led to a search for IEGs that might play such a role. Using subtractive cloning techniques to identify mRNAs from hippocampus induced by the maximal electroconvulsive seizure method (MECS), Paul Worley and colleagues isolated several IEGs that are now collectively known as effector IEGs (see Guzowski, 2002Guzowski J.F. Hippocampus. 2002; 12: 86-104Crossref PubMed Scopus (306) Google Scholar for an extensive review). Effector IEGs encode for growth factors (BDNF, β-activin), signal transduction molecules (Homer 1a, Rheb), metabolic enzymes (COX-2), and cell surface proteins (Arcadlin, Narp). Another such effector IEG that the Worley group identified is Arc (Lyford et al., 1995Lyford G.L. Yamagata K. Kaufmann W.E. Barnes C.A. Sanders L.K. Copeland N.G. Gilbert D.J. Jenkins N.A. Lanahan A.A. Worley P.F. Neuron. 1995; 14: 433-445Abstract Full Text PDF PubMed Scopus (944) Google Scholar). Concomitantly and independently of Worley's group, Dietmar Kuhl also isolated Arc from hippocampus, under the name Arg3.1, using similar cloning methods (Link et al., 1995Link W. Konietzko U. Kauselmann G. Krug M. Schwanke B. Frey U. Kuhl D. Proc. Natl. Acad. Sci. USA. 1995; 92: 5734-5738Crossref PubMed Scopus (542) Google Scholar). Arc/Arg3.1 is expressed in the brain and is rapidly activated by robust patterned synaptic activity, including natural stimuli, seizures, LTP, and memory-related behavioral paradigms (for review see Guzowski, 2002Guzowski J.F. Hippocampus. 2002; 12: 86-104Crossref PubMed Scopus (306) Google Scholar). Localization studies have determined that, following a behavioral experience, Arc/Arg3.1 is selectively expressed in CaMKII-positive glutamatergic neurons in the forebrain (Vazdarjanova et al., 2006Vazdarjanova A. Ramirez-Amaya V. Insel N. Plummer T.K. Rosi S. Chowdhury S. Mikhael D. Worley P.F. Guzowski J.F. Barnes C.A. J. Comp. Neurol. 2006; 498: 317-329Crossref PubMed Scopus (175) Google Scholar) and that Arc/Arg3.1 binds to and is phosphorylated by CaMKII (Donai et al., 2003Donai H. Sugiura H. Ara D. Yoshimura Y. Yamagata K. Yamauchi T. Neurosci. Res. 2003; 47: 399-408Crossref PubMed Scopus (51) Google Scholar). Arc/Arg3.1 protein is found in the postsynaptic density (PSD), copurifies with the NMDA receptor complex (Husi et al., 2000Husi H. Ward M.A. Choudhary J.S. Blackstock W.P. Grant S.G. Nat. Neurosci. 2000; 3: 661-669Crossref PubMed Scopus (994) Google Scholar, Steward and Worley, 2001aSteward O. Worley P.F. Proc. Natl. Acad. Sci. USA. 2001; 98: 7062-7068Crossref PubMed Scopus (186) Google Scholar), and, like many IEGs, its induction in vivo requires NMDA receptor activation (Link et al., 1995Link W. Konietzko U. Kauselmann G. Krug M. Schwanke B. Frey U. Kuhl D. Proc. Natl. Acad. Sci. USA. 1995; 92: 5734-5738Crossref PubMed Scopus (542) Google Scholar, Steward and Worley, 2001bSteward O. Worley P.F. Neuron. 2001; 30: 227-240Abstract Full Text Full Text PDF PubMed Scopus (355) Google Scholar). In a series of elegant experiments, Steward and Worley, 2001aSteward O. Worley P.F. Proc. Natl. Acad. Sci. USA. 2001; 98: 7062-7068Crossref PubMed Scopus (186) Google Scholar, Steward and Worley, 2001bSteward O. Worley P.F. Neuron. 2001; 30: 227-240Abstract Full Text Full Text PDF PubMed Scopus (355) Google Scholar found that newly synthesized Arc/Arg3.1 mRNA is rapidly delivered to dendrites and accumulates selectively in the band of synapses that had been activated. Remarkably, when this experiment was repeated following electroconvulsive seizures, which caused a uniform dendritic expression of Arc/Arg3.1 mRNA, local stimulation caused a band to appear in which preexisting dendritic mRNA was redistributed within minutes from inactive synapses to the activated synapses in an NMDA receptor-dependent manner (Figure 1). However, NMDA receptor activation is not the sole mechanism to induce Arc/Arg3.1 mRNA transcription. In PC12 cells and dissociated hippocampal neurons, membrane depolarization and subsequent calcium influx through voltage-gated calcium channels also induces Arc/Arg3.1 mRNA synthesis. In these reduced systems, cAMP can also trigger synthesis of Arc/Arg3.1 mRNA. Calcium and cAMP both induce Arc/Arg3.1 transcription through PKA. MAPK is also implicated in the cAMP-dependent activation of Arc/Arg3.1 (Waltereit et al., 2001Waltereit R. Dammermann B. Wulff P. Scafidi J. Staubli U. Kauselmann G. Bundman M. Kuhl D. J. Neurosci. 2001; 21: 5484-5493Crossref PubMed Google Scholar). A more recent study, also using cultured dissociated hippocampal neurons as well as slice cultures, suggested that Arc/Arg3.1 transcription is under the control of both NMDA and AMPA receptors. NMDA receptor activation increases Arc/Arg3.1 levels while AMPA receptor activation decreases them, surprisingly, via a pertussis toxin-sensitive G protein (Rao et al., 2006Rao V.R. Pintchovski S.A. Chin J. Peebles C.L. Mitra S. Finkbeiner S. Nat. Neurosci. 2006; 9: 887-895Crossref PubMed Scopus (131) Google Scholar). A critical goal of neuroscience is to relate the activity of neural circuits to behavior. The unraveling of this relationship was for several decades the purview of electrophysiology, but this has recently changed due to the rapid emergence of technologies integrating molecular biology and imaging techniques. Because elevated expression of Arc/Arg3.1 mRNA in the dendrites of neurons is readily observed following neural activation, Arc/Arg3.1 expression has been used as a marker of neuronal activity throughout the brain. In 1999, Guzowski et al. published a seminal paper describing a new approach to the study of neural networks using Arc/Arg3.1 as a marker (Guzowski et al., 1999Guzowski J.F. McNaughton B.L. Barnes C.A. Worley P.F. Nat. Neurosci. 1999; 2: 1120-1124Crossref PubMed Scopus (754) Google Scholar). The technique is referred to as cellular compartment analysis of temporal activity by fluorescent in-situ hybridization, or catFISH. Although catFISH does not provide real-time information on the activity of neural ensembles, it provides knowledge on the activity history of neurons with both temporal and cellular resolution. At the heart of this technique are the different time courses of nuclear Arc/Arg3.1 pre-mRNA and cytoplasmic Arc/Arg3.1 mRNA (Wallace et al., 1998Wallace C.S. Lyford G.L. Worley P.F. Steward O. J. Neurosci. 1998; 18: 26-35Crossref PubMed Google Scholar). Within 2 min after the induction of a behavioral experience or other Arc/Arg3.1-inducing stimuli, Arc/Arg3.1 pre-mRNA appears in neuronal nuclei, and by around 20 min it has disappeared from the nucleus as the processed mRNA moves to the cytoplasm (Wallace et al., 1998Wallace C.S. Lyford G.L. Worley P.F. Steward O. J. Neurosci. 1998; 18: 26-35Crossref PubMed Google Scholar). Detection of cytoplasmic Arc/Arg3.1 mRNA using catFISH occurs ∼20–45 min postinduction. Therefore, after two behavioral experiences separated by at least 20 min, neurons that are active during both experiences have both nuclear and cytoplasmic Arc/Arg3.1 mRNA, while neurons active in only one of the two experiences would have either nuclear or cytoplasmic Arc/Arg3.1 mRNA expression (see Guzowski et al., 2001Guzowski J.F. McNaughton B.L. Barnes C.A. Worley P.F. Curr. Opin. Neurobiol. 2001; 11: 579-584Crossref PubMed Scopus (77) Google Scholar, for a detailed review of catFISH and its more recent variant, double-label catFISH). Thus, Arc/Arg3.1 catFISH can determine the activity history of an individual neuron, the spatial distribution of thousands of activated neurons, and the visualization of neural ensembles activated by two distinct behavioral experiences. In addition, catFISH bypasses the inability to monitor the neural activity from many distinct anatomical regions, a shortcoming of electrophysiological techniques. Many recent studies using either conventional Arc/Arg3.1 in situ hybridization or Arc/Arg3.1 catFISH display the utility of Arc/Arg3.1 as an activity neuronal marker. For example, Bruce McNaughton, Carol Barnes, and colleagues have shown that sequential exposure of rats to two distinct environments leads to activation of different neuronal ensembles in the hippocampus (Guzowski et al., 1999Guzowski J.F. McNaughton B.L. Barnes C.A. Worley P.F. Nat. Neurosci. 1999; 2: 1120-1124Crossref PubMed Scopus (754) Google Scholar). Using catFISH, this group also probed the relationship between the hippocampus and the parietal and gustatory cortices during different behavioral experiences (Burke et al., 2005Burke S.N. Chawla M.K. Penner M.R. Crowell B.E. Worley P.F. Barnes C.A. McNaughton B.L. Neuron. 2005; 45: 667-674Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar). Furthermore, with Arc/Arg3.1 as a reporter, Tagawa et al. suggested that neuronal plasticity in the mouse visual cortex differs from that in cats and monkeys (Tagawa et al., 2005Tagawa Y. Kanold P.O. Majdan M. Shatz C.J. Nat. Neurosci. 2005; 8: 380-388Crossref PubMed Scopus (189) Google Scholar), while Zou and Buck used Arc/Arg3.1 to study the coding of odor mixtures in the cortex (Zou and Buck, 2006Zou Z. Buck L.B. Science. 2006; 311: 1477-1481Crossref PubMed Scopus (128) Google Scholar). Lastly, Temple et al. used experience-induced Arc/Arg3.1 to visualize neural circuit remodeling after brain injury (Temple et al., 2003Temple M.D. Worley P.F. Steward O. J. Neurosci. 2003; 23: 2779-2788PubMed Google Scholar). The general picture that emerged from these studies was that Arc/Arg3.1 localizes to synapses in an activity-dependent manner. For many years after its discovery, Arc/Arg3.1 was searching for a function beyond that of an activity-dependent neuronal marker. A series of recently published papers, including the ones published in this issue of Neuron, demonstrate that Arc/Arg3.1 functions to regulate AMPA receptor trafficking. The study of Chowdhury et al., 2006Chowdhury S. Shepherd J.D. Okuno H. Lyford G. Petralia R.S. Plath N. Kuhl D. Huganir R.L. Worley P.F. Neuron. 2006; 52 (this issue): 445-459Abstract Full Text Full Text PDF PubMed Scopus (532) Google Scholar establishes a direct link between endocytosis and Arc/Arg3.1. Using several biochemical assays, they showed that Arc/Arg3.1 directly interacts with endophilin 3 and dynamin 2, two proteins involved in the endocytosis of membrane vesicles. Arc/Arg3.1 binds to dynamin 2 and endophilin 3 through distinct nonoverlapping domains. Dynamin 2 is a member of the dynamin GTPase superfamily (Praefcke and McMahon, 2004Praefcke G.J. McMahon H.T. Nat. Rev. Mol. Cell Biol. 2004; 5: 133-147Crossref PubMed Scopus (1059) Google Scholar) and is involved in the scission of invaginated clathrin-coated vesicles from the plasma membrane. Dynamins bind directly to phospholipids through a pleckstrin homology domain (PH) and are involved in protein-protein interactions through a proline-rich domain (PRD) (Hinshaw, 2000Hinshaw J.E. Annu. Rev. Cell Dev. Biol. 2000; 16: 483-519Crossref PubMed Scopus (564) Google Scholar). Perhaps unexpectedly, binding of Arc/Arg3.1 to dynamin 2 is through the PH domain and not through the PRD. Endophilin 3 is a cytoplasmic Src-homology 3 (SH3)-containing protein found at the PSD. Endophilins interact with dynamin and amphiphysin to mediate/regulate clathrin-mediated vesicle recycling (Conner and Schmid, 2003Conner S.D. Schmid S.L. Nature. 2003; 422: 37-44Crossref PubMed Scopus (2848) Google Scholar). Endophilins also contain an N-terminal Bin/amphiphysin/Rvs (N-BAR) domain that is implicated in the recognition of curved membranes. BAR domains are banana-shaped domains that usually bind to negatively charged membranes (McMahon and Gallop, 2005McMahon H.T. Gallop J.L. Nature. 2005; 438: 590-596Crossref PubMed Scopus (1472) Google Scholar). Like the Arc/Arg3.1-dynamin 2 interaction, Arc/Arg3.1 binds to endophilin not via the typical protein-protein interaction SH3 domain but instead through the BAR domain. As the regulation of AMPA receptor exocytosis and endocytosis underlies synaptic plasticity in many brain regions (Bredt and Nicoll, 2003Bredt D.S. Nicoll R.A. Neuron. 2003; 40: 361-379Abstract Full Text Full Text PDF PubMed Scopus (888) Google Scholar), Chowdhury et al. tested whether Arc/Arg3.1 could regulate AMPA receptor surface expression (Chowdhury et al., 2006Chowdhury S. Shepherd J.D. Okuno H. Lyford G. Petralia R.S. Plath N. Kuhl D. Huganir R.L. Worley P.F. Neuron. 2006; 52 (this issue): 445-459Abstract Full Text Full Text PDF PubMed Scopus (532) Google Scholar). Using dissociated hippocampal neurons, the authors show that overexpression (∼16 hr) of Arc/Arg3.1 downregulates surface expression of AMPA receptors (∼50%) through an increased rate of AMPA receptor endocytosis. The interaction of Arc/Arg3.1 with dynamin 2 and endophilin 3 is critical for the trafficking phenotype since mutant Arc/Arg3.1 that cannot bind to either dynamin 2 or endophilin 3 has no effect on AMPA receptor surface expression. However, Arc/Arg3.1 overexpression is also accompanied by a significant loss of total AMPA receptor protein (∼30%) through an unknown mechanism. In support of the conclusion that Arc/Arg3.1 regulates AMPA receptor surface expression, cultured hippocampal neurons prepared from Arc/Arg3.1 knockout mice showed a 2-fold increase in AMPA receptor surface expression, increased miniature EPSCs amplitudes (mEPSCs), and a decreased rate of endocytosis (Shepherd et al., 2006Shepherd J.D. Rumbaugh G. Wu J. Chowdhury S. Plath N. Kuhl D. Huganir R.L. Worley P.F. Neuron. 2006; 52 (this issue): 475-484Abstract Full Text Full Text PDF PubMed Scopus (535) Google Scholar). Furthermore, in an accompanying paper by Rial Verde et al., 2006Rial Verde E.M. Lee-Osbourne J. Worley P.F. Malinow R. Cline H.T. Neuron. 2006; 52 (this issue): 461-474Abstract Full Text Full Text PDF PubMed Scopus (305) Google Scholar, overexpression of Arc/Arg3.1 in organotypic hippocampal slice cultures induced a reduction of both AMPA receptor surface expression and AMPA receptor-mediated synaptic transmission (∼30% loss), assayed either through evoked synaptic stimulation or through recordings of mEPSCs. The Arc/Arg3.1 phenotype was selective, as no change was found in either NMDA receptor- or GABA-mediated synaptic transmission. Although these studies have provided persuasive evidence that Arc/Arg3.1 regulates basal AMPA receptor surface expression and synaptic transmission, recordings from acute hippocampal slices prepared from Arc/Arg3.1 knockout mice do not fully corroborate these conclusions. Arc/Arg3.1 knockout mice and wild-type mice have similar input-output curves, a measure of basal synaptic transmission, and identical mEPSC amplitudes and frequency (Plath et al., 2006Plath N. Ohana O. Dammermann B. Errington M.L. Schmitz D. Gross C. Mao X. Engelsberg A. Mahlke C. Welzl H. et al.Neuron. 2006; 52 (this issue): 437-444Abstract Full Text Full Text PDF PubMed Scopus (594) Google Scholar). Perhaps the reason for this discrepancy lies in the different experimental approaches. For example, in vivo Arc/Arg3.1 is expressed predominantly following a bout of robust synaptic activity such as that given to induce LTP. However, high-frequency stimulation not only activates Arc/Arg3.1 but also several other signaling cascades that would regulate AMPA receptor trafficking in parallel. In the in vitro culture assays, Arc/Arg3.1 overexpression might overshadow the contribution of other AMPA receptor trafficking regulatory mechanisms, possibly leading to a more extreme phenotype. In further support of this notion, Adesnik et al., 2005Adesnik H. Nicoll R.A. England P.M. Neuron. 2005; 48: 977-985Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar demonstrated that the basal recycling of native AMPA receptors is slow (∼16 hr) compared to the turnover rate estimated using overexpressed AMPA receptors (Passafaro et al., 2001Passafaro M. Piech V. Sheng M. Nat. Neurosci. 2001; 4: 917-926Crossref PubMed Scopus (521) Google Scholar). Furthermore, the limiting factor for AMPA anchoring in synapses is the presence of MAGUK molecules such as PSD-95 (Elias et al., 2006Elias G.M. Funke L. Stein V. Grant S.G. Bredt D.S. Nicoll R.A. Neuron. 2006; 52: 307-320Abstract Full Text Full Text PDF PubMed Scopus (292) Google Scholar, Schnell et al., 2002Schnell E. Sizemore M. Karimzadegan S. Chen L. Bredt D.S. Nicoll R.A. Proc. Natl. Acad. Sci. USA. 2002; 99: 13902-13907Crossref PubMed Scopus (555) Google Scholar). Previously, Worley and colleagues had suggested that Arc/Arg3.1 (S. Chowdhury et al., 2002, Soc. Neurosci., abstract #746.13) might associate with PSD-95 through its SH3-GK domain. If this is indeed the case, Arc/Arg3.1 overexpression might bind to and deplete PSD-95 molecules in the synapse, causing a reduction in the synaptic AMPA receptors levels. This would be independent of the Arc/Arg3.1-induced changes in endocytosis. Further studies are needed to distinguish between the different possibilities. Currently, there are several forms of synaptic plasticity, all of which may contribute to memory formation: LTP, LTD, and more recently homeostatic synaptic plasticity. These various forms of plasticity typically involve insertion or removal of AMPA receptors from the synapse (Bredt and Nicoll, 2003Bredt D.S. Nicoll R.A. Neuron. 2003; 40: 361-379Abstract Full Text Full Text PDF PubMed Scopus (888) Google Scholar). It has long been suggested that Arc/Arg3.1 might contribute directly to the longer phases of LTP and consequently memory formation because paradigms that induce robust synapse strengthening lead to the rapid induction of Arc/Arg3.1. In 2000, Guzowski et al. provided the first experimental evidence that Arc/Arg3.1 regulates the late phases of LTP (Guzowski et al., 2000Guzowski J.F. Lyford G.L. Stevenson G.D. Houston F.P. McGaugh J.L. Worley P.F. Barnes C.A. J. Neurosci. 2000; 20: 3993-4001Crossref PubMed Google Scholar). The authors delivered Arc/Arg3.1 antisense oligonucleotides (AOD) into the hippocampus of awake behaving rats in a manner that blocked the transient increase of Arc/Arg3.1 mRNA/protein following high-frequency stimulation. This manipulation had little effect on LTP during the first 4 hr, but LTP decayed subsequently and by the fifth day had returned to baseline. Consistent with the electrophysiological findings, rats treated with Arc/Arg3.1 AODs soon after a spatial water task showed impaired memory consolidation, while rats treated 8 hr after the learning task, a time when poststimulation Arc/Arg3.1 levels are in decline, showed no learning deficits. More recently, McIntyre et al., 2005McIntyre C.K. Miyashita T. Setlow B. Marjon K.D. Steward O. Guzowski J.F. McGaugh J.L. Proc. Natl. Acad. Sci. USA. 2005; 102: 10718-10723Crossref PubMed Scopus (197) Google Scholar reported that infusion of AODs in dorsal hippocampus impairs memory retention of an inhibitory avoidance task and that memory-altering drugs injected in the basolateral amygdala could alter Arc/Arg3.1 protein levels in dorsal hippocampus. Interestingly, Kelly and Deadwyler, 2003Kelly M.P. Deadwyler S.A. J. Neurosci. 2003; 23: 6443-6451Crossref PubMed Google Scholar have also reported that animals that are either overtrained or slow in their acquisition of a behavioral task have higher Arc/Arg3.1 mRNA levels compared to control animals. In agreement with previous antisense studies, Arc/Arg3.1 knockout mice have several memory deficits, as they do not form either long-term spatial, fear, or taste memories (Plath et al., 2006Plath N. Ohana O. Dammermann B. Errington M.L. Schmitz D. Gross C. Mao X. Engelsberg A. Mahlke C. Welzl H. et al.Neuron. 2006; 52 (this issue): 437-444Abstract Full Text Full Text PDF PubMed Scopus (594) Google Scholar). In addition, Arc/Arg3.1 also participates in the processing of visual experience by the visual cortex. Wang et al. showed that Arc/Arg3.1 regulates orientation selectivity in the visual cortex by using in vivo two-photon microscopy in knockin mice that have the Arc/Arg3.1 open reading frame replaced with green fluorescent protein (GFP) (Wang et al., 2006Wang K.H. Majewska A. Schummers J. Farley B. Hu C. Sur M. Tonegawa S. Cell. 2006; 126: 389-402Abstract Full Text Full Text PDF PubMed Scopus (176) Google Scholar). These data are consistent with previous work that showed a strong correlation between Arc/Arg3.1 expression and visual experience (Tagawa et al., 2005Tagawa Y. Kanold P.O. Majdan M. Shatz C.J. Nat. Neurosci. 2005; 8: 380-388Crossref PubMed Scopus (189) Google Scholar). How does Arc/Arg3.1 affect memory formation? Recent work has suggested that recycling of early endosomes contributes to the stable expression of LTP (Park et al., 2004Park M. Penick E.C. Edwards J.G. Kauer J.A. Ehlers M.D. Science. 2004; 305: 1972-1975Crossref PubMed Scopus (545) Google Scholar), while LTD requires clathrin-mediated endocytosis (Carroll et al., 1999Carroll R.C. Lissin D.V. von Zastrow M. Nicoll R.A. Malenka R.C. Nat. Neurosci. 1999; 2: 454-460Crossref PubMed Scopus (370) Google Scholar). As the study by Chowdhury et al. established that Arc/Arg3.1 could regulate AMPA receptor endocytosis and early endosome recycling, it might not be surprising that genetic ablation of Arc/Arg3.1 leads to impaired late-phase LTP and LTD (Plath et al., 2006Plath N. Ohana O. Dammermann B. Errington M.L. Schmitz D. Gross C. Mao X. Engelsberg A. Mahlke C. Welzl H. et al.Neuron. 2006; 52 (this issue): 437-444Abstract Full Text Full Text PDF PubMed Scopus (594) Google Scholar, Rial Verde et al., 2006Rial Verde E.M. Lee-Osbourne J. Worley P.F. Malinow R. Cline H.T. Neuron. 2006; 52 (this issue): 461-474Abstract Full Text Full Text PDF PubMed Scopus (305) Google Scholar). The close relationship between Arc/Arg3.1 and LTD is further supported by the study of Rial Verde et al., 2006Rial Verde E.M. Lee-Osbourne J. Worley P.F. Malinow R. Cline H.T. Neuron. 2006; 52 (this issue): 461-474Abstract Full Text Full Text PDF PubMed Scopus (305) Google Scholar. In neurons, clathrin-mediated endocytosis occurs in hotspots located perisynaptically through a series of protein-protein and protein-lipid interactions (Blanpied et al., 2002Blanpied T.A. Scott D.B. Ehlers M.D. Neuron. 2002; 36: 435-449Abstract Full Text Full Text PDF PubMed Scopus (270) Google Scholar, Racz et al., 2004Racz B. Blanpied T.A. Ehlers M.D. Weinberg R.J. Nat. Neurosci. 2004; 7: 917-918Crossref PubMed Scopus (161) Google Scholar). For clathrin-mediated endocytosis and LTD to take place, the recruitment of the adaptor protein 2 complex (AP2) is necessary. Previous work has shown that peptides that inhibit the interaction between GluR2 and AP2 prevent LTD (see Bredt and Nicoll, 2003Bredt D.S. Nicoll R.A. Neuron. 2003; 40: 361-379Abstract Full Text Full Text PDF PubMed Scopus (888) Google Scholar, for review). Similarly, preventing the interaction of GluR2 and AP2 also interferes with the ability of Arc/Arg3.1 to downregulate AMPA-mediated synaptic transmission, while overexpression of Arc/Arg3.1 in slice cultures leads to the selective surface downregulation of GluR2/GluR3-containing AMPA receptors. Furthermore, Arc/Arg3.1 overexpression occludes LTD in slice culture, and phosphatase inhibitors that prevent LTD also inhibit the ability of Arc/Arg3.1 to modulate AMPA-mediated synaptic transmission (Rial Verde et al., 2006Rial Verde E.M. Lee-Osbourne J. Worley P.F. Malinow R. Cline H.T. Neuron. 2006; 52 (this issue): 461-474Abstract Full Text Full Text PDF PubMed Scopus (305) Google Scholar). Arc/Arg3.1's role is also extended to a newly discovered form of plasticity—homeostatic synaptic plasticity, or synaptic scaling (Davis, 2006Davis G.W. Annu. Rev. Neurosci. 2006; 29: 307-323Crossref PubMed Scopus (395) Google Scholar). In cortical or hippocampal cultures, chronic blockade of neural activity leads to a pronounced uniform enhancement of mEPSC amplitudes (Turrigiano et al., 1998Turrigiano G.G. Leslie K.R. Desai N.S. Rutherford L.C. Nelson S.B. Nature. 1998; 391: 892-896Crossref PubMed Scopus (1524) Google Scholar). The mechanism underlying this phenomenon is unclear. It has recently been reported (Stellwagen and Malenka, 2006Stellwagen D. Malenka R.C. Nature. 2006; 440: 1054-1059Crossref PubMed Scopus (1147) Google Scholar) that block of neural activity by TTX (voltage-gated sodium channel blocker) decreases glutamate release, causing the release of the cytokine TNF-α from glia cells. This in turn, upregulates AMPA receptor-mediated synaptic events through an unknown mechanism. In cultured neurons with increased basal activity, Arc/Arg3.1 levels are relatively high. Application of TTX in these cultured neurons decreases Arc/Arg3.1 activity (Shepherd et al., 2006Shepherd J.D. Rumbaugh G. Wu J. Chowdhury S. Plath N. Kuhl D. Huganir R.L. Worley P.F. Neuron. 2006; 52 (this issue): 475-484Abstract Full Text Full Text PDF PubMed Scopus (535) Google Scholar). The decrease in Arc/Arg3.1 protein levels correlates with a uniform increase of AMPA receptor surface expression and mEPSC amplitudes, while overexpression of Arc/Arg3.1 correlates with a uniform decrease of AMPA receptor surface expression and decrease in mEPSC amplitude. Most importantly, high Arc/Arg3.1 protein levels induced through overexpression blocked homeostatic synaptic plasticity (Shepherd et al., 2006Shepherd J.D. Rumbaugh G. Wu J. Chowdhury S. Plath N. Kuhl D. Huganir R.L. Worley P.F. Neuron. 2006; 52 (this issue): 475-484Abstract Full Text Full Text PDF PubMed Scopus (535) Google Scholar). In agreement with the previous findings, cultured neurons from Arc/Arg3.1 knockout mice have increased AMPA receptor surface expression, and subsequent treatment with TTX does not lead to homeostatic synaptic plasticity. Together these data provide persuasive evidence that Arc/Arg3.1 is implicated in homeostatic synaptic plasticity. Future studies are necessary to establish the link between TNF-α and Arc/Arg3.1. The discovery of Arc/Arg3.1 a decade ago raised great promise of being able to link gene expression to synaptic plasticity and behavior. However, until now the most exciting finding involving Arc/Arg3.1 was based on its ability to identify recently activated neurons. The recent papers, especially those appearing in this issue of Neuron, go a long way toward filling in the gap between our knowledge of the expression of Arc/Arg3.1 and its postulated role in behavior. The linkage of Arc/Arg3.1 to synaptic plasticity is compelling. It is induced by the same stimuli that induce synaptic plasticity. It requires the activation of NMDA receptors and is found exclusively at activated synapses, as are most forms of synaptic plasticity. The time course of Arc/Arg3.1 protein expression correlates well with the time course of the late phase of LTP, which requires protein synthesis. Finally, just as with synaptic plasticity, Arc/Arg3.1 is proposed to mediate its effects by controlling the trafficking of AMPA receptors, and learning and memory tasks are impaired in mice lacking Arc/Arg3.1. As is the case with any rapidly evolving field the recent studies on Arc/Arg3.1 raise more questions than they answer and suggest directions for future research. First, if the sole function of Arc/Arg3.1, which is induced by LTP-triggering stimuli, is to increase the rate of AMPA receptor endocytosis, then it is difficult to explain its role during LTP, which requires net insertion of AMPA receptors. Why is LTP lost in the Arc/Arg3.1 knockout mice? One might actually predict that it would be enhanced. Second, LTD is impaired in the Arc/Arg3.1 knockout mice, and overexpression of Arc/Arg3.1 occludes LTD, even though the stimulation protocols that induce LTD are not known to significantly affect Arc/Arg3.1 mRNA and protein levels. Therefore, the loss of LTD in the Arc/Arg3.1 knockout mice may well be an indirect effect. Third, how is Arc/Arg3.1 protein sequestered at active synapses? Is it through its interaction with CaMKII (Donai et al., 2003Donai H. Sugiura H. Ara D. Yoshimura Y. Yamagata K. Yamauchi T. Neurosci. Res. 2003; 47: 399-408Crossref PubMed Scopus (51) Google Scholar) and/or PSD-95? What is the nature of the NMDA receptor-induced tag at activated synapses? Fourth, how does Arc/Arg3.1 selectively control the endocytosis of AMPA receptors but not NMDA receptors? If it doesn't directly interact with the receptors, what intermediary proteins are involved? Fifth, evidence indicates that Arc/Arg3.1 is critically involved in both LTP/LTD and synaptic homeostasis. These two processes appear to have quite distinct mechanisms, and, thus, it is unclear mechanistically how Arc/Arg3.1 can be critical for both forms of plasticity. Sixth, the enhancement of LTP for the first 30 min and then a block thereafter in the Arc/Arg3.1 KO mouse is intriguing but is difficult to explain with the currently available data. Seventh, it is unclear why the Arc/Arg3.1 knockout doesn't have reduced basal synaptic transmission. This problem is reminiscent of a number of knockout mice in which synaptic plasticity is altered without any change in basal transmission. Given the sudden upsurge of interest in Arc/Arg3.1 and the influx of such a large amount of new and exciting data on the role of Arc/Arg3.1, we can be certain that answers to many of these questions will soon be forthcoming. Arc/Arg3.1 Interacts with the Endocytic Machinery to Regulate AMPA Receptor TraffickingChowdhury et al.NeuronNovember 09, 2006In BriefArc/Arg3.1 is an immediate-early gene whose mRNA is rapidly transcribed and targeted to dendrites of neurons as they engage in information processing and storage. Moreover, Arc/Arg3.1 is known to be required for durable forms of synaptic plasticity and learning. Despite these intriguing links to plasticity, Arc/Arg3.1's molecular function remains enigmatic. Here, we demonstrate that Arc/Arg3.1 protein interacts with dynamin and specific isoforms of endophilin to enhance receptor endocytosis. Arc/Arg3.1 selectively modulates trafficking of AMPA-type glutamate receptors (AMPARs) in neurons by accelerating endocytosis and reducing surface expression. Full-Text PDF Open ArchiveArc/Arg3.1 Mediates Homeostatic Synaptic Scaling of AMPA ReceptorsShepherd et al.NeuronNovember 09, 2006In BriefHomeostatic plasticity may compensate for Hebbian forms of synaptic plasticity, such as long-term potentiation (LTP) and depression (LTD), by scaling neuronal output without changing the relative strength of individual synapses. This delicate balance between neuronal output and distributed synaptic weight may be necessary for maintaining efficient encoding of information across neuronal networks. Here, we demonstrate that Arc/Arg3.1, an immediate-early gene (IEG) that is rapidly induced by neuronal activity associated with information encoding in the brain, mediates homeostatic synaptic scaling of AMPA type glutamate receptors (AMPARs) via its ability to activate a novel and selective AMPAR endocytic pathway. Full-Text PDF Open ArchiveArc/Arg3.1 Is Essential for the Consolidation of Synaptic Plasticity and MemoriesPlath et al.NeuronNovember 09, 2006In BriefArc/Arg3.1 is robustly induced by plasticity-producing stimulation and specifically targeted to stimulated synaptic areas. To investigate the role of Arc/Arg3.1 in synaptic plasticity and learning and memory, we generated Arc/Arg3.1 knockout mice. These animals fail to form long-lasting memories for implicit and explicit learning tasks, despite intact short-term memory. Moreover, they exhibit a biphasic alteration of hippocampal long-term potentiation in the dentate gyrus and area CA1 with an enhanced early and absent late phase. Full-Text PDF Open ArchiveIncreased Expression of the Immediate-Early Gene Arc/Arg3.1 Reduces AMPA Receptor-Mediated Synaptic TransmissionRial Verde et al.NeuronNovember 09, 2006In BriefArc/Arg3.1 is an immediate-early gene whose expression levels are increased by strong synaptic activation, including synapse-strengthening activity patterns. Arc/Arg3.1 mRNA is transported to activated dendritic regions, conferring the distribution of Arc/Arg3.1 protein both temporal correlation with the inducing stimulus and spatial specificity. Here, we investigate the effect of increased Arc/Arg3.1 levels on synaptic transmission. Surprisingly, Arc/Arg3.1 reduces the amplitude of synaptic currents mediated by AMPA-type glutamate receptors (AMPARs). Full-Text PDF Open Archive
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