NMDA Receptor-Arc Signaling Is Required for Memory Updating and Is Disrupted in Alzheimer's Disease.

BACKGROUND: Memory deficits are central to many neuropsychiatric diseases. During acquisition of new information, memories can become vulnerable to interference, yet mechanisms that underlie interference are unknown. METHODS: We describe a novel transduction pathway that links the NMDA receptor (NMDAR) to AKT signaling via the immediate early gene Arc and evaluate its role in memory. The signaling pathway is validated using biochemical tools and transgenic mice, and function is evaluated in assays of synaptic plasticity and behavior. The translational relevance is evaluated in human postmortem brain. RESULTS: Arc is dynamically phosphorylated by CaMKII (calcium/calmodulin-dependent protein kinase II) and binds the NMDAR subunits NR2A/NR2B and a previously unstudied PI3K (phosphoinositide 3-kinase) adapter p55PIK (PIK3R3) in vivo in response to novelty or tetanic stimulation in acute slices. NMDAR-Arc-p55PIK recruits p110α PI3K and mTORC2 (mechanistic target of rapamycin complex 2) to activate AKT. NMDAR-Arc-p55PIK-PI3K-mTORC2-AKT assembly occurs within minutes of exploratory behavior and localizes to sparse synapses throughout hippocampal and cortical regions. Studies using conditional (Nestin-Cre) p55PIK deletion mice indicate that NMDAR-Arc-p55PIK-PI3K-mTORC2-AKT functions to inhibit GSK3 and mediates input-specific metaplasticity that protects potentiated synapses from subsequent depotentiation. p55PIK conditional knockout mice perform normally in multiple behaviors including working memory and long-term memory tasks but exhibit deficits indicative of increased vulnerability to interference in both short-term and long-term paradigms. The NMDAR-AKT transduction complex is reduced in postmortem brain of individuals with early Alzheimer's disease. CONCLUSIONS: A novel function of Arc mediates synapse-specific NMDAR-AKT signaling and metaplasticity that contributes to memory updating and is disrupted in human cognitive disease.

Arc Oligomerization Is Regulated by CaMKII Phosphorylation of the GAG Domain: An Essential Mechanism for Plasticity and Memory Formation.

Arc is a synaptic protein essential for memory consolidation. Recent studies indicate that Arc originates in evolution from a Ty3-Gypsy retrotransposon GAG domain. The N-lobe of Arc GAG domain acquired a hydrophobic binding pocket in higher vertebrates that is essential for Arc's canonical function to weaken excitatory synapses. Here, we report that Arc GAG also acquired phosphorylation sites that can acutely regulate its synaptic function. CaMKII phosphorylates the N-lobe of the Arc GAG domain and disrupts an interaction surface essential for high-order oligomerization. In Purkinje neurons, CaMKII phosphorylation acutely reverses Arc's synaptic action. Mutant Arc that cannot be phosphorylated by CaMKII enhances metabotropic receptor-dependent depression in the hippocampus but does not alter baseline synaptic transmission or long-term potentiation. Behavioral studies indicate that hippocampus- and amygdala-dependent learning requires Arc GAG domain phosphorylation. These studies provide an atomic model for dynamic and local control of Arc function underlying synaptic plasticity and memory.

Screening for Mutations in the TBX1 Gene on Chromosome 22q11.2 in Schizophrenia.

A higher-than-expected frequency of schizophrenia in patients with 22q11.2 deletion syndrome suggests that chromosome 22q11.2 harbors the responsive genes related to the pathophysiology of schizophrenia. The TBX1 gene, which maps to the region on chromosome 22q11.2, plays a vital role in neuronal functions. Haploinsufficiency of the TBX1 gene is associated with schizophrenia endophenotype. This study aimed to investigate whether the TBX1 gene is associated with schizophrenia. We searched for mutations in the TBX1 gene in 652 patients with schizophrenia and 567 control subjects using a re-sequencing method and conducted a reporter gene assay. We identified six SNPs and 25 rare mutations with no association with schizophrenia from Taiwan. Notably, we identified two rare schizophrenia-specific mutations (c.-123G>C and c.-11delC) located at 5' UTR of the TBX1 gene. The reporter gene assay showed that c.-123C significantly decreased promoter activity, while c.-11delC increased promoter activity compared with the wild-type. Our findings suggest that the TBX1 gene is unlikely a major susceptible gene for schizophrenia in an ethnic Chinese population for Taiwan, but a few rare mutations in the TBX1 gene may contribute to the pathogenesis of schizophrenia in some patients.

Rare mutations and hypermethylation of the ARC gene associated with schizophrenia.

Activity-regulated cytoskeleton-associated protein (ARC), which interacts with the N-methyl-d-aspartate receptor (NMDAR) complex, is a critical effector molecule downstream of multiple neuronal signaling pathways. Dysregulation of the ARC/NMDAR complex can disrupt learning, memory, and normal brain functions. This study examined the role of ARC in susceptibility to schizophrenia. We used a resequencing strategy to identify the variants of ARC in 1078 subjects, including patients with schizophrenia and normal controls. We identified 16 known SNPs and 27 rare mutations. SNP-based analysis showed no association of ARC with schizophrenia. In addition, the rare mutations did not increase the burden in patients compared with controls. However, one patient-specific allele in the putative ARC promoter region and seven patient-specific mutants in ARC exon regions significantly reduced the reporter gene activity compared with ARC wild-type. Methylation of a putative ARC promoter attenuated reporter activity in vitro, suggesting that ARC expression is regulated by DNA methylation. Pyrosequencing revealed eight hypermethylated CpG sites in the putative ARC promoter region in 64 schizophrenic patients compared with 63 controls. Taken together, our results suggest that both rare variants and epigenetic regulation of ARC contribute to the pathogenesis of schizophrenia in some patients.

Metabotropic Glutamate Receptors Induce a Form of LTP Controlled by Translation and Arc Signaling in the Hippocampus.

Activity-dependent bidirectional modifications of excitatory synaptic strength are essential for learning and storage on new memories. Research on bidirectional synaptic plasticity has largely focused on long-term potentiation (LTP) and long-term depression (LTD) mechanisms that rely on the activation of NMDA receptors. In principle, metabotropic glutamate receptors (mGluRs) are also suitable to convert synaptic activity into intracellular signals for synaptic modification. Indeed, dysfunction of a form of LTD that depends on Type I mGluRs (mGluR-LTD), but not NMDARs, has been implicated in learning deficits in aging and mouse models of several neurological conditions, including Fragile X syndrome and Alzheimer's disease. To determine whether mGluR activation can also induce LTP in the absence of NMDAR activation, we examined in hippocampal slices from rats and mice, an NMDAR-independent form of LTP previously characterized as dependent on voltage-gated Ca(2+) channels. We found that this form of LTP requires activation of Type I mGluRs and, like mGluR-LTD but unlike NMDAR-dependent plasticity, depends crucially on protein synthesis controlled by fragile X mental retardation protein and on Arc signaling. Based on these observations, we propose the coexistence of two distinct activity-dependent systems of bidirectional synaptic plasticity: one that is based on the activity of NMDARs and the other one based on the activation of mGluRs. SIGNIFICANCE STATEMENT: Bidirectional changes of synaptic strength are crucial for the encoding of new memories. Currently, the only activity-dependent mechanism known to support such bidirectional changes are long-term potentiation (LTP) and long-term depression (LTD) forms that relay on the activation of NMDA receptors. Metabotropic glutamate receptors (mGluRs) are, in principle, also suitable to trigger bidirectional synaptic modifications. However, only the mGluR-dependent form of LTD has been characterized. Here we report that an NMDAR-independent form of LTP, initially characterized as dependent on voltage-gated Ca(2+) channels, also requires the activation of mGluRs. These finding suggest the coexistence of two distinct activity-dependent systems of bidirectional synaptic plasticity: one that is based on the activity of NMDARs and the other one based on the activation of mGluRs.

Resequencing of early growth response 2 (EGR2) gene revealed a recurrent patient-specific mutation in schizophrenia.

Abnormal myelination is considered as part of the pathophysiology of schizophrenia. We resequenced the genomic DNA of the EGR2, which has a specific function in the myelination of peripheral nervous system, in 543 schizophrenic patients and 554 non-psychotic controls. We identified six known SNPs, which were not associated with schizophrenia. Nevertheless, we discovered 24 rare mutations, some of them were patient-specific, including a recurrent mutation (p.P173_Y174insP), which might be associated with the pathogenesis of schizophrenia.

Structural basis of arc binding to synaptic proteins: implications for cognitive disease.

Arc is a cellular immediate-early gene (IEG) that functions at excitatory synapses and is required for learning and memory. We report crystal structures of Arc subdomains that form a bi-lobar architecture remarkably similar to the capsid domain of human immunodeficiency virus (HIV) gag protein. Analysis indicates Arc originated from the Ty3/Gypsy retrotransposon family and was "domesticated" in higher vertebrates for synaptic functions. The Arc N-terminal lobe evolved a unique hydrophobic pocket that mediates intermolecular binding with synaptic proteins as resolved in complexes with TARPγ2 (Stargazin) and CaMKII peptides and is essential for Arc's synaptic function. A consensus sequence for Arc binding identifies several additional partners that include genes implicated in schizophrenia. Arc N-lobe binding is inhibited by small chemicals suggesting Arc's synaptic action may be druggable. These studies reveal the remarkable evolutionary origin of Arc and provide a structural basis for understanding Arc's contribution to neural plasticity and disease.

Genetic and functional analyses of early growth response (EGR) family genes in schizophrenia.

OBJECTIVE: Early growth response genes (EGR1, 2, 3, and 4) encode a family of nuclear proteins that function as transcriptional regulators. They are involved in the regulation of synaptic plasticity, learning, and memory, and are implicated in the pathogenesis of schizophrenia. METHODS: We conducted a genetic association analysis of 14 SNPs selected from the EGR1, 2, 3, and 4 genes of 564 patients with schizophrenia and 564 control subjects. We also conducted Western blot analysis and promoter activity assay to characterize the EGR genes associated with schizophrenia RESULTS: We did not detect a true genetic association of these 14 SNPs with schizophrenia in this sample. However, we observed a nominal over-representation of C/C genotype of rs9990 of EGR2 in female schizophrenia as compared to female control subjects (p=0.012, uncorrected for multiple testing). Further study showed that the average mRNA level of the EGR2 gene in the lymphoblastoid cell lines of female schizophrenia patients was significantly higher than that in female control subjects (p=0.002). We also detected a nominal association of 4 SNPs (rs6747506, rs6718289, rs2229294, and rs3813226) of the EGR4 gene that form strong linkage disequilibrium with schizophrenia in males. Reporter gene assay showed that the haplotype T-A derived from rs6747506 and rs6718289 at the promoter region had significantly reduced promoter activity compared with the haplotype A-G. CONCLUSION: Our data suggest a tendency of gender-specific association of EGR2 and EGR4 in schizophrenia, with an elevated expression of EGR2 in lympoblastoid cell lines of female schizophrenia patients and a reduced EGR4 gene expression in male schizophrenia patients.