Arc

Arc, for activity-regulated cytoskeleton-associated protein (also known as Arg3.1), is a plasticity protein first characterized in 1995. Arc is a member of the immediate-early gene (IEG) family, a rapidly activated class of genes functionally defined by their ability to be transcribed in the presence of protein synthesis inhibitors. Arc mRNA is localized to activated synaptic sites in an NMDA receptor-dependent manner, where the newly translated protein is believed to play a critical role in learning and memory-related molecular processes. Arc is widely considered to be an important protein in neurobiology because of its activity regulation, localization, and utility as a marker for plastic changes in the brain. Along with other IEGs such as zif268 and Homer 1a, Arc is also a significant tool for systems neuroscience as illustrated by the development of the cellular compartment analysis of temporal activity by fluorescence in situ hybridization, or catFISH technique (see fluorescent in situ hybridization).

A number of promoter and enhancer regions have been identified that mediate activity-dependent Arc transcription: a serum response element (SRE; see serum response factor) at ~1.5 kb upstream of the initiation site;[8][9] a second SRE at ~6.5 kb;[9] and a synaptic activity response element (SARE) sequence at ~7 kb upstream that contains binding sites for cyclic AMP response element-binding protein (CREB), myocyte enhancer factor 2 (MEF2), and SRF.

Changes in Arc mRNA and/or protein are correlated with a number of behavioral paradigms including cued fear conditioning,[20] contextual fear conditioning,[21] spatial memory,[22][23] operant conditioning,[24][25] and inhibitory avoidance.[5] The mRNA is notably upregulated following electrical stimulation in LTP-induction procedures such as high frequency stimulation (HFS),[22] and is massively and globally induced by maximal electroconvulsive shock (MECS).[1][3] The Arc transcript is the first known IEG that is entirely dependent upon activation of the mitogen-activated protein kinase or MAP kinase (MAPK) cascade,[8] a pathway important for regulation of cell growth and survival.[26] Extracellular signaling to neuronal dendrites activates postsynaptic sites to increase Arc levels through a wide variety of signaling molecules, including mitogens such as epidermal growth factor (EGF),[1] nerve growth factor (NGF),[1] and brain-derived neurotrophic factor (BDNF),[12] glutamate acting at NMDA receptors,[3][4] dopamine through activation of the D1 receptor subtype,[27][28] and dihydroxyphenylglycine (DHPG).[29] The common factor for these signaling molecules involves activation of cyclic-AMP and its downstream target protein kinase A (PKA). As such, direct pharmacological activation of cAMP by forskolin or 8-Br-cAMP robustly increases Arc levels[8][28] while H89, a PKA antagonist, blocks these effects[28] as does further downstream blockade of mitogen-activated protein kinase kinase [sic] (MEK).[8] Note that the MAPK cascade is a signaling pathway involving multiple kinases acting sequentially [MAPKKK--> MAPKK--> MAPK]. MAPK is able to enter the nucleus and perform its phosphotransferase activity on a number of gene regulatory components[30] that have implications for the regulation of immediate-early genes. Several transcription factors are known to be involved in regulating the Arc gene (see above), including serum response factor (SRF),[8][31] CREB,[31] MEF2,[31] and zif268.[32]

Following transcription, Arc mRNA is transported out of the nucleus and localized to neuronal dendrites[1] and activated synapses,[33] a process dependent on the 3' UTR,[11] polymerization of actin,[34] and ERK phosphorylation.[34] The mRNA (and aggregate protein) is carried along microtubules radiating out from the nucleus by kinesin (specifically KIF5)[35] and likely translocated into dendritic spines by the actin-based motor protein myosin-Va.[36] Arc has been shown to be associated with polyribosomes at synaptic sites,[37] and is translated in isolated synaptoneurosomal fractions[38] in vitro indicating that the protein is likely locally translated in vivo. Synaptically localized Arc protein interacts with dynamin and endophilin, proteins involved in clathrin-mediated endocytosis, and facilitates the removal of AMPA receptors from the plasma membrane.[16] Consistent with this, increased Arc levels reduce AMPA currents,[39] while Arc KOs display increases in surface AMPA expression.[40]

The Activity-Regulated Cytoskeletal-Associated Protein (Arc/Arg3.1) Is Required for Reconsolidation of a Pavlovian Fear Memory

The activity-regulated cytoskeletal-associated protein (Arc/Arg3.1) is an immediate-early gene that has been widely implicated in synaptic plasticity and in the consolidation of a variety of hippocampal- and amygdala-dependent memory tasks. The functional role of Arc/Arg3.1 in memory reconsolidation processes, however, has not been systematically studied. In the present study, we examined the role of Arc/Arg3.1 in the reconsolidation of an amygdala-dependent auditory pavlovian fear memory. We show that Arc/Arg3.1 protein is regulated in the lateral nucleus of the amygdala (LA) by retrieval of an auditory fear memory. Next, we show that antisense knockdown of Arc/Arg3.1 in the LA impairs fear memory reconsolidation of both a recent (1-d-old) as well as a well-consolidated (2-week-old) fear memory; that is, post-retrieval short-term memory, tested at 3 h after retrieval, is intact, whereas post-retrieval long-term memory, tested ∼24 h after retrieval, is significantly impaired. The effect of Arc/Arg3.1 knockdown was observed to be time limited and specific to an actively reactivated fear memory. Moreover, the reconsolidation deficit induced by Arc/Arg3.1 knockdown was not found to be sensitive to spontaneous recovery, reinstatement, or a shift in the testing context, suggesting that our behavioral effects are not attributable to facilitated extinction. Collectively, our findings provide the first comprehensive look at the functional role of Arc/Arg3.1 in memory reconsolidation processes in the mammalian brain.