'Arc'-hitecture of normal cognitive aging

Arc is an effector immediate-early gene that is critical for forming long-term memories. Since its discovery 25 years ago, it has repeatedly surprised us with a number of intriguing properties, including the transport of its mRNA to recently-activated synapses, its master role in bidirectionally regulating synaptic strength, its evolutionary retroviral origins, its ability to mediate intercellular transfer between neurons via extracellular vesicles (EVs), and its exceptional regulation-both temporally and spatially. The current review discusses how Arc has been used as a tool to identify the neural networks involved in cognitive aging and how Arc itself may contribute to cognitive outcome in aging. In addition, we raise several outstanding questions, including whether Arc-containing EVs in peripheral blood might provide a noninvasive biomarker for memory-related synaptic failure in aging, and whether rectifying Arc dysregulation is likely to be an effective strategy for bending the arc of aging toward successful cognitive outcomes.

Survey of the Arc Epigenetic Landscape in Normal Cognitive Aging

Aging is accompanied by aberrant gene expression that ultimately affects brain plasticity and the capacity to form long-term memories. Immediate-early genes (IEGs) play an active role in these processes. Using a rat model of normal cognitive aging, we found that the expression of Egr1 and c-Fos was associated with chronological age, whereas Arc was more tightly linked to cognitive outcomes in aging. More specifically, constitutive Arc expression was significantly elevated in aged rats with memory impairment compared to cognitively intact aged rats and young adult animals. Since alterations in the neuroepigenetic mechanisms that gate hippocampal gene expression are also associated with cognitive outcome in aging, we narrowed our focus on examining potential epigenetic mechanisms that may lead to aberrant Arc expression. Employing a multilevel analytical approach using bisulfite sequencing, chromatin immunoprecipitations, and micrococcal nuclease digestion, we identified CpG sites in the Arc promoter that were coupled to poor cognitive outcomes in aging, histone marks that were similarly coupled to spatial memory deficits, and nucleosome positioning that also varied depending on cognitive status. Together, these findings paint a diverse and complex picture of the Arc epigenetic landscape in cognitive aging and bolster a body of work, indicating that dysfunctional epigenetic regulation is associated with memory impairment in the aged brain.

Cortical network dynamics are coupled with cognitive aging in rats

Changes in neuronal network activity and increased interindividual variability in memory are among the most consistent features of growing older. Here, we examined the relationship between these hallmarks of aging. Young and aged rats were trained on a water maze task where aged individuals reliably display an increased range of spatial memory capacities relative to young. Two weeks later, neuronal activity was induced pharmacologically with a low dose of pilocarpine and control animals received vehicle. Activity levels were proxied by quantifying the immediate early gene products Arc and c-Fos. While no relationship was observed between basal, resting activity, and individual differences in spatial memory in any brain region, pilocarpine-induced marker expression was tightly coupled with memory in all areas of the prefrontal cortex (PFC) and hippocampus examined. The nature of this association, however, differed across regions and in relation to age-related cognitive outcome. Specifically, in the medial PFC, induced activity was greatest in aged rats with cognitive impairment and correlated with water maze performance across all subjects. In the hippocampus, the range of induced marker expression was comparable between groups and similarly coupled with memory in both impaired and unimpaired aged rats. Together the findings highlight that the dynamic range of neural network activity across multiple brain regions is a critical component of neurocognitive aging.

Isolation and Quantification Brain Region-Specific and Cell Subtype-Specific Histone (De)Acetylation in Cognitive Neuroepigenetics

The acetylation of histone tails, which relaxes compact chromatin structure and enhances the accessibility of DNA to regulatory proteins, has emerged as a key mechanism for regulating gene expression. These modifications in turn play critical roles in forming long-term memories. Chromatin immunoprecipitation (ChIP) experiments have enabled the identification of specific histone modifications and the genes most closely associated with active memory formation. Problematically, however, the majority of these studies analyze diverse populations of cell homogenates obtained from the gross dissection of large brain regions. The protocol outlined here uses methods to ascribe gene-specific histone modifications (via specific antibodies and RT-qPCR) to specific cell subtypes (via specific antibodies and cell sorting) in discrete memory-related brain regions (via microdissection) to more precisely identify the role of histone acetylation and deacetylation in cognitive neuroepigenetics.

Moderating effect of mode of delivery on the genetics of intelligence: Explorative genome-wide analyses in ALSPAC

INTRODUCTION

Intelligence is a core construct of individual differences in cognitive abilities and a strong predictor of important life outcomes. Within recent years, rates of cesarean section have substantially increased globally, though little is known about its effect on neurodevelopmental trajectories. Thus, we aimed to investigate the influence of delivery by cesarean section on the genetics of intelligence in children.

METHODS

Participants were recruited through the Avon Longitudinal Study of Parents and Children (ALSPAC). Intelligence was measured by the Wechsler Intelligence Scale for Children (WISC). Genotyping was performed using the Illumina Human Hap 550 quad genome-wide SNP genotyping platform and was followed by imputation using MACH software. Genome-wide interaction analyses were conducted using linear regression.

RESULTS

A total of 2,421 children and 2,141,747 SNPs were subjected to the genome-wide interaction analyses. No variant reached genome-wide significance. The strongest interaction was observed at rs17800861 in the GRIN2A gene (β = -3.43, 95% CI = -4.74 to -2.12, p = 2.98E-07). This variant is predicted to be located within active chromatin compartments in the hippocampus and may influence binding of the NF-kappaB transcription factor.

CONCLUSIONS

Our results may indicate that mode of delivery might have a moderating effect on genetic disposition of intelligence in children. Studies of considerable sizes (>10,000) are likely required to more robustly detect variants governing such interaction. In summary, the presented findings prompt the need for further studies aimed at increasing our understanding of effects various modes of delivery may have on health outcomes in children.

Implication of the APP Gene in Intellectual Abilities

Background:

Cognitive functions are highly heritable and polygenic, though the source of this genetic influence is unclear. On the neurobiological level, these functions rely on effective neuroplasticity, in which the activity-regulated cytoskeleton associated protein (ARC) plays an essential role.

Objectives:

To examine whether the ARC gene complex may contribute to the genetic components of intellectual function given the crucial role of ARC in brain plasticity and memory formation.

Methods:

The ARC complex was tested for association with intelligence (IQ) in children from the Avon Longitudinal Study of Parents and Children (ALSPAC, N = 5,165). As Alzheimer’s disease (AD) shares genetics with cognitive functioning, the association was followed up in an AD sample (17,008 cases, 37,154 controls).

Results:

The ARC complex revealed association with verbal and total IQ (empirical p = 0.027 and 0.041, respectively) in the ALSPAC. The strongest single variant signal (rs2830077; empirical p = 0.018), within the APP gene, was confirmed in the AD sample (p = 2.76E-03). Functional analyses of this variant showed its preferential binding to the transcription factor CP2.

Discussion:

This study implicates APP in childhood IQ. While follow-up studies are needed, this observation could help elucidate the etiology of disorders associated with cognitive dysfunction, such as AD.

Arc Interacts with the Integral Endoplasmic Reticulum Protein, Calnexin

Activity-regulated cytoskeleton-associated protein, Arc, is a major regulator of long-term synaptic plasticity and memory formation. Here we reveal a novel interaction partner of Arc, a resident endoplasmic reticulum transmembrane protein, calnexin. We show an interaction between recombinantly-expressed GST-tagged Arc and endogenous calnexin in HEK293, SH-SY5Y neuroblastoma and PC12 cells. The interaction was dependent on the central linker region of the Arc protein that is also required for endocytosis of AMPA-type glutamate receptors. High-resolution proximity-ligation assays (PLAs) demonstrate molecular proximity of endogenous Arc with the cytosolic C-terminus, but not the lumenal N-terminus of calnexin. In hippocampal neuronal cultures treated with brain-derived neurotrophic factor (BDNF), Arc interacted with calnexin in the perinuclear cytoplasm and dendritic shaft. Arc also interacted with C-terminal calnexin in the adult rat dentate gyrus (DG). After induction of long-term potentiation (LTP) in the perforant path projection to the DG of adult anesthetized rats, enhanced interaction between Arc and calnexin was obtained in the dentate granule cell layer (GCL). Although Arc and calnexin are both implicated in the regulation of receptor endocytosis, no modulation of endocytosis was detected in transferrin uptake assays. Previous work showed that Arc interacts with multiple protein partners to regulate synaptic transmission and nuclear signaling. The identification of calnexin as a binding partner further supports the role of Arc as a hub protein and extends the range of Arc function to the endoplasmic reticulum, though the function of the Arc/calnexin interaction remains to be defined.

Common variants in the ARC gene are not associated with cognitive abilities

INTRODUCTION

The Activity-Regulated Cytoskeleton-associated (ARC) gene encodes a protein that is critical for the consolidation of synaptic plasticity and long-term memory formation. Given ARC's key role in synaptic plasticity, we hypothesized that genetic variations in ARC may contribute to interindividual variability in human cognitive abilities or to attention-deficit hyperactivity disorder (ADHD) susceptibility, where cognitive impairment often accompanies the disorder.

METHODS

We tested whether ARC variants are associated with six measures of cognitive functioning in 670 healthy subjects in the Norwegian Cognitive NeuroGenetics (NCNG) by extracting data from its Genome-Wide Association Study (GWAS). In addition, the Swedish Betula sample of 1800 healthy subjects who underwent similar cognitive testing was also tested for association with 19 tag SNPs.

RESULTS

No ARC variants show association at the study-wide level, but several markers show a trend toward association with human cognitive functions. We also tested for association between ARC SNPs and ADHD in a Norwegian sample of cases and controls, but found no significant associations.

CONCLUSION

This study suggests that common genetic variants located in ARC do not account for variance in human cognitive abilities, though small effects cannot be ruled out.

Balancing Arc synthesis, mRNA decay, and proteasomal degradation: maximal protein expression triggered by rapid eye movement sleep-like bursts of muscarinic cholinergic receptor stimulation

Cholinergic signaling induces Arc/Arg3.1, an immediate early gene crucial for synaptic plasticity. However, the molecular mechanisms that dictate Arc mRNA and protein dynamics during and after cholinergic epochs are little understood. Using human SH-SY5Y neuroblastoma cells, we show that muscarinic cholinergic receptor (mAchR) stimulation triggers Arc synthesis, whereas translation-dependent RNA decay and proteasomal degradation strictly limit the amount and duration of Arc expression. Chronic application of the mAchR agonist, carbachol (Cch), induces Arc transcription via ERK signaling and release of calcium from IP(3)-sensitive stores. Arc translation requires ERK activation, but not changes in intracellular calcium. Proteasomal degradation of Arc (half-life ∼37 min) was enhanced by thapsigargin, an inhibitor of the endoplasmic calcium-ATPase pump. Similar mechanisms of Arc protein regulation were observed in cultured rat hippocampal slices. Functionally, we studied the impact of cholinergic epoch duration and temporal pattern on Arc protein expression. Acute Cch treatment (as short as 2 min) induces transient, moderate Arc expression, whereas continuous treatment of more than 30 min induces maximal expression, followed by rapid decline. Cholinergic activity associated with rapid eye movement sleep may function to facilitate long term synaptic plasticity and memory. Employing a paradigm designed to mimic intermittent rapid eye movement sleep epochs, we show that application of Cch in a series of short bursts generates persistent and maximal Arc protein expression. The results demonstrate dynamic, multifaceted control of Arc synthesis during mAchR signaling, and implicate cholinergic epoch duration and repetition as critical determinants of Arc expression and function in synaptic plasticity and behavior.