A fine balance: Regulation of hippocampal Arc/Arg3.1 transcription, translation and degradation in a rat model of normal cognitive aging

'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.

Supplementation of Regular Diet With Medium-Chain Triglycerides for Procognitive Effects: A Narrative Review

It is now widely accepted that ketosis (a physiological state characterized by elevated plasma ketone body levels) possesses a wide range of neuroprotective effects. There is a growing interest in the use of ketogenic supplements, including medium-chain triglycerides (MCT), to achieve intermittent ketosis without adhering to a strict ketogenic diet. MCT supplementation is an inexpensive and simple ketogenic intervention, proven to benefit both individuals with normal cognition and those suffering from mild cognitive impairment, Alzheimer's disease, and other cognitive disorders. The commonly accepted paradigm underlying MCT supplementation trials is that the benefits stem from ketogenesis and that MCT supplementation is safe. However, medium-chain fatty acids (MCFAs) may also exert effects in the brain directly. Moreover, MCFAs, long-chain fatty acids, and glucose participate in mutually intertwined metabolic pathways. Therefore, the metabolic effects must be considered if the desired procognitive effects require administering MCT in doses larger than 1 g/kg. This review summarizes currently available research on the procognitive effects of using MCTs as a supplement to regular feed/diet without concomitant reduction of carbohydrate intake and focuses on the revealed mechanisms linked to particular MCT metabolites (ketone bodies, MCFAs), highlighting open questions and potential considerations.

Keeping Synapses in Shape: Degradation Pathways in the Healthy and Aging Brain

Synapses maintain their molecular composition, plasticity and function through the concerted action of protein synthesis and removal. The complex and polarized neuronal architecture poses specific challenges to the logistics of protein and organelle turnover since protein synthesis and degradation mainly happen in the cell soma. In addition, post-mitotic neurons accumulate damage over a lifetime, challenging neuronal degradative pathways and making them particularly susceptible to the effects of aging. This review will summarize the current knowledge on neuronal protein turnover mechanisms with a particular focus on the presynapse, including the proteasome, autophagy and the endolysosomal route and their roles in regulating presynaptic proteostasis and function. In addition, the author will discuss how physiological brain aging, which entails a progressive decline in cognitive functions, affects synapses and the degradative machinery.

mGluR5 ablation leads to age-related synaptic plasticity impairments and does not improve Huntington’s disease phenotype

Glutamate receptors, including mGluR5, are involved in learning and memory impairments triggered by aging and neurological diseases. However, each condition involves distinct molecular mechanisms. It is still unclear whether the mGluR5 cell signaling pathways involved in normal brain aging differ from those altered due to neurodegenerative disorders. Here, we employed wild type (WT), mGluR5^−/−, BACHD, which is a mouse model of Huntington’s Disease (HD), and mGluR5^−/−/BACHD mice, at the ages of 2, 6 and 12 months, to distinguish the mGluR5-dependent cell signaling pathways involved in aging and neurodegenerative diseases. We demonstrated that the memory impairment exhibited by mGluR5^−/− mice is accompanied by massive neuronal loss and decreased dendritic spine density in the hippocampus, similarly to BACHD and BACHD/mGluR5^−/− mice. Moreover, mGluR5 ablation worsens some of the HD-related alterations. We also show that mGluR5^−/− and BACHD/mGluR5^−/− mice have decreased levels of PSD95, BDNF, and Arc/Arg3.1, whereas BACHD mice are mostly spared. PSD95 expression was affected exclusively by mGluR5 ablation in the aging context, making it a potential target to treat age-related alterations. Taken together, we reaffirm the relevance of mGluR5 for memory and distinguish the mGluR5 cell signaling pathways involved in normal brain aging from those implicated in HD.

Long-Term Environmental Enrichment Relieves Dysfunctional Cognition and Synaptic Protein Levels Induced by Prenatal Inflammation in Older CD-1 Mice

A mounting body of evidence suggests that prenatal inflammation may enhance the rate of age-associated cognitive decline and may involve aberrant amounts of synaptic proteins in the hippocampus, including synaptotagmin-1 (Syt1) and activity-regulated cytoskeleton-associated protein (Arc). However, little is known about the specific impact of adolescent environmental enrichment (EE) on age-associated cognitive decline and the changes in synaptic proteins caused by prenatal inflammation. In this study, CD-1 mice in late pregnancy were given intraperitoneal doses of lipopolysaccharide (LPS, 50 μg/kg) or normal saline. Offspring arising from LPS dams were divided into a LPS group and a LPS plus EE (LPS-E) group. The LPS-E mice were exposed to EE from 2 months of age until the end of the experiment (3 or 15 months old). The Morris water maze (MWM) was used to assess the spatial learning and memory capacities of experimental mice, while western blotting and RNA-scope were used to determine the expression levels of Arc and Syt1 in the hippocampus at the protein and mRNA levels, respectively. Analysis revealed that at 15 months of age, the control mice experienced a reduction in cognitive ability and elevated expression levels of Arc and Syt1 genes when compared to control mice at 3 months of age. The LPS-E group exhibited better cognition and lower protein and mRNA levels of Arc and Syt1 than mice in the LPS group of the same age. However, the enriched environment mitigated but did not counteract, the effects of prenatal inflammation on cognitive and synaptic proteins when tested at either 3 or 15 months of age. Our findings revealed that long-term environmental enrichment improved the expression levels of synaptic proteins in CD-1 mice and that this effect was linked to the dysfunctional cognition caused by prenatal inflammation; this process may also be involved in the reduction of hippocampal Arc and Syt1 gene expression.

Effect of Cardiotonic Steroid Marinobufagenin on Vascular Remodeling and Cognitive Impairment in Young Dahl-S Rats

The hypertensive response in Dahl salt-sensitive (DSS) rats on a high-salt (HS) diet is accompanied by central arterial stiffening (CAS), a risk factor for dementia, and heightened levels of a prohypertensive and profibrotic factor, the endogenous Na/K-ATPase inhibitor marinobufagenin (MBG). We studied the effect of the in vivo administration of MBG or HS diet on blood pressure (BP), CAS, and behavioral function in young DSS rats and normotensive Sprague-Dawley rats (SD), the genetic background for DSS rats. Eight-week-old male SD and DSS rats were given an HS diet (8% NaCl, n = 18/group) or a low-salt diet (LS; 0.1% NaCl, n = 14-18/group) for 8 weeks or MBG (50 µg/kg/day, n = 15-18/group) administered via osmotic minipumps for 4 weeks in the presence of the LS diet. The MBG-treated groups received the LS diet. The systolic BP (SBP); the aortic pulse wave velocity (aPWV), a marker of CAS; MBG levels; spatial memory, measured by a water maze task; and tissue collection for the histochemical analysis were assessed at the end of the experiment. DSS-LS rats had higher SBP, higher aPWV, and poorer spatial memory than SD-LS rats. The administration of stressors HS and MBG increased aPWV, SBP, and aortic wall collagen abundance in both strains vs. their LS controls. In SD rats, HS or MBG administration did not affect heart parameters, as assessed by ECHO vs. the SD-LS control. In DSS rats, impaired whole-heart structure and function were observed after HS diet administration in DSS-HS vs. DSS-LS rats. MBG treatment did not affect the ECHO parameters in DSS-MBG vs. DSS-LS rats. The HS diet led to an increase in endogenous plasma and urine MBG levels in both SD and DSS groups. Thus, the prohypertensive and profibrotic effect of HS diet might be partially attributed to an increase in MBG. The prohypertensive and profibrotic functions of MBG were pronounced in both DSS and SD rats, although quantitative PCR revealed that different profiles of profibrotic genes in DSS and SD rats was activated after MBG or HS administration. Spatial memory was not affected by HS diet or MBG treatment in either SD or DSS rats. Impaired cognitive function was associated with higher BP, CAS, and cardiovascular remodeling in young DSS-LS rats, as compared to young SD-LS rats. MBG and HS had similar effects on the cardiovascular system and its function in DSS and SD rats, although the rate of change in SD rats was lower than in DSS rats. The absence of a cumulative effect of increased aPWV and BP on spatial memory can be explained by the cerebrovascular and brain plasticity in young rats, which help the animals to tolerate CAS elevated by HS and MBG and to counterbalance the profibrotic effect of heightened MBG.

Sex-dependent responsiveness of hippocampal neurons to sex neurosteroids: A role of Arc/Arg3.1

Sex steroids, such as estradiol (E₂ ) and dihydrotestosterone (DHT), regulate hippocampal plasticity and memory in a sex-dependent manner. Because the activity-regulated cytoskeleton protein Arc/Arg3.1 is essential for long-term memory formation and synaptic plasticity, we investigated the expression of Arc/Arg3.1 with respect to its responsiveness to E₂ and DHT in male and female hippocampal neurons. For the first time, we show that, in hippocampal neurons, Arc/Arg3.1 expression is sex-dependently regulated by sex steroids. No difference in the expression between sexes was observed under control conditions. Using a quantitative real-time polymerase chain reaction, western blot analysis and quantitative immunoreactivity, upregulation of Arc/Arg3.1 protein expression was observed in specifically female hippocampal neurons after application of E₂ to the cultures. Conversely, upregulation of Arc/Arg3.1 was seen in specifically male neurons after application of DHT. A quantitative real-time PCR revealed that the sex-dependency was most pronounced on the mRNA level. Most importantly, the effects of E₂ in cultures of female animals were abolished when neuron-derived E₂ synthesis was inhibited. Our results point to a potentially important role of Arc/Arg3.1 regarding sex-dependency in sex steroid-induced synaptic plasticity in the hippocampus.

Moderately aged OFA rats as a novel model for mild age-related alterations in learning and memory

Aged rodents have been used as preclinical models of age-associated cognitive decline. Most of those models displayed substantial impairments in learning and memory. The initial, more subtle changes that precede more severe losses in cognitive abilities have not been well characterized. Here, we established a model detecting initial subtle cognitive changes by comparing the performance of moderately aged Oncins France Strain A Sprague Dawley rats with young rats in the Morris water maze (MWM) and the Open Field (OF) test. Both age groups improved their performance during the training period at a similar rate; however, the older rats performed worse in several parameters measured in the MWM. Our results suggest that already at the age of 18-20 months rats show changes in their approach to solve the spatial memory task while their ability to learn is not yet diminished. The disparate spatial information processing of the moderately aged rats provides a novel animal model for early age-related cognitive alterations that could be useful to test the effect of early intervention strategies. Moreover, our results suggest that the sensitivity of cognitive tests in the elderly could be substantially enhanced if they assess both the improvement after several trials, and the strategy used to solve a certain task.

Differential Retinoic Acid Signaling in the Hippocampus of Aged Rats with and without Memory Impairment

Retinoic acid (RA), a metabolite of vitamin A, has many physiological functions, and mounting evidence points to important roles in cognition. In vitro experiments indicate that RA is involved in homeostatic synaptic scaling in the hippocampus, which supports overall network stability during learning. It has been previously determined that disrupted RA signaling in the hippocampus causes deterioration of memory, that RA signaling declines with age in brain, and that application of RA reverses this decline. Here, we explore whether RA signaling is altered in an animal model of neurocognitive aging. We used a Morris water maze protocol to study cognitive decline in aged rats, which assesses hippocampus-dependent spatial memory and reveals substantial interindividual differences in aged animals. Aged unimpaired (AU) rats perform on par with young (Y), while aged impaired (AI) animals exhibit spatial memory deficits. We show that the major substrate for RA, retinol binding protein 4 (RBP4), is decreased in AU rats, and retinol cell surface receptor declines with chronological age. Other affected components of RA signaling include selective increases in AI animals in hippocampal synthesis (RALDH1) and catabolism of RA (CYP26B1), RA receptor α, the RA regulated ionotropic glutamate receptor (GluR1), as well as fragile X mental retardation protein (FMRP). The results support the conclusion that, surprisingly, increased RA signaling in the aged hippocampus is associated with poor cognitive outcome.

Age-related changes in expression of lysine48 and lysine63 ubiquitin linkages in dopamine neurons of midbrain in mice

Ubiquitination of target proteins is mediated via different ubiquitin lysine (K) linkages and determines the protein fates. In particular, K48 ubiquitin linkage targets proteins for degradation, whereas K63 ubiquitin linkage plays a nondegradative role. Parkinson's disease is an age-onset neurodegenerative disorder, which shows selective loss of dopamine neurons in substantia nigra pars compacta (SNC) and ubiquitinated protein aggregates. However, age-related expression of K48 and K63 ubiquitin linkages in SNC dopamine neurons remains elusive. We thus sought to explore the expression of K48 and K63 ubiquitin linkages in dopamine neurons in SNCs of mice at different ages with morphological and biochemical assays. Here our results indicated that in 5-week-old mice, dopamine neurons presented higher levels of K48 and K63 ubiquitin linkages than nondopamine neural cells. Aging promoted the formation of protein aggregates that are positive for both K48 and K63 ubiquitin linkages, together with tyrosine hydroxylase, a dopamine neuron marker. Moreover, 21-month-old mice showed fewer neural cells and tyrosine hydroxylase positive neurons in the SNCs than younger mice. Through biochemical analysis, the 21-month-old mice were shown to express more K48 ubiquitin linkages and less tyrosine hydroxylase and NeuN than the 5-week-old mice. These results suggest the first time that expression of K48 and K63 ubiquitin lysine linkages in midbrain dopamine neurons is age-related and may be involved in the loss of dopamine neurons.

Structure, Dynamics and Function of the 26S Proteasome

The 26S proteasome is the most complex ATP-dependent protease machinery, of ~2.5 MDa mass, ubiquitously found in all eukaryotes. It selectively degrades ubiquitin-conjugated proteins and plays fundamentally indispensable roles in regulating almost all major aspects of cellular activities. To serve as the sole terminal "processor" for myriad ubiquitylation pathways, the proteasome evolved exceptional adaptability in dynamically organizing a large network of proteins, including ubiquitin receptors, shuttle factors, deubiquitinases, AAA-ATPase unfoldases, and ubiquitin ligases, to enable substrate selectivity and processing efficiency and to achieve regulation precision of a vast diversity of substrates. The inner working of the 26S proteasome is among the most sophisticated, enigmatic mechanisms of enzyme machinery in eukaryotic cells. Recent breakthroughs in three-dimensional atomic-level visualization of the 26S proteasome dynamics during polyubiquitylated substrate degradation elucidated an extensively detailed picture of its functional mechanisms, owing to progressive methodological advances associated with cryogenic electron microscopy (cryo-EM). Multiple sites of ubiquitin binding in the proteasome revealed a canonical mode of ubiquitin-dependent substrate engagement. The proteasome conformation in the act of substrate deubiquitylation provided insights into how the deubiquitylating activity of RPN11 is enhanced in the holoenzyme and is coupled to substrate translocation. Intriguingly, three principal modes of coordinated ATP hydrolysis in the heterohexameric AAA-ATPase motor  were discovered to regulate intermediate functional steps of the proteasome, including ubiquitin-substrate engagement, deubiquitylation, initiation of substrate translocation and processive substrate degradation. The atomic dissection of the innermost working of the 26S proteasome opens up a new era in our understanding of the ubiquitin-proteasome system and has far-reaching implications in health and disease.

Cognitive Reserve in Model Systems for Mechanistic Discovery: The Importance of Longitudinal Studies

The goal of this review article is to provide a resource for longitudinal studies, using animal models, directed at understanding and modifying the relationship between cognition and brain structure and function throughout life. We propose that forthcoming longitudinal studies will build upon a wealth of knowledge gleaned from prior cross-sectional designs to identify early predictors of variability in cognitive function during aging, and characterize fundamental neurobiological mechanisms that underlie the vulnerability to, and the trajectory of, cognitive decline. Finally, we present examples of biological measures that may differentiate mechanisms of the cognitive reserve at the molecular, cellular, and network level.

Experience-dependent persistent Arc expression is reduced in the aged hippocampus

Aging is typically accompanied by both memory decline and changes in hippocampal function. Lasting memory is thought to also require recapitulation of recent memory traces during subsequent rest-a phenomenon termed memory trace reactivation or replay. Replay becomes less synchronized in the CA1 region of aged animals, and while subtle, this deficit may have profound physiological consequences for driving plasticity. Importantly, spike timing changes during replay may impair the induction of plasticity-regulating gene products, such as activity-regulated cytoskeletal protein (Arc). To test this hypothesis, Arc transcription was assessed both during spatial exploration and subsequent memory-related replay in hippocampal CA1 of young and aged animals. A significant age-related difference was observed in the pattern of pyramidal cells expressing Arc during rest, supporting the hypothesis that altered plasticity-related cascade is a major consequence of the changes in coordinated activity that occur during consolidation in older animals.

Age-Related Memory Impairment Is Associated with Increased zif268 Protein Accumulation and Decreased Rpt6 Phosphorylation

Aging is associated with cognitive decline, including impairments in the ability to accurately form and recall memories. Some behavioral and brain changes associated with aging are evident as early as middle age, making the understanding of associated neurobiological mechanisms essential to aid in efforts aimed at slowing cognitive decline throughout the lifespan. Here, we found that both 15-month-old and 22-month-old rats showed impaired memory recall following trace fear conditioning. This behavioral deficit was accompanied by increased zif268 protein accumulation relative to 3-month-old animals in the medial prefrontal cortex, the dorsal and ventral hippocampi, the anterior and posterior retrosplenial cortices, the lateral amygdala, and the ventrolateral periaqueductal gray. Elevated zif268 protein levels corresponded with decreases in phosphorylation of the Rpt6 proteasome regulatory subunit, which is indicative of decreased engagement of activity-driven protein degradation. Together, these results identify several brain regions differentially impacted by aging and suggest that the accumulation of proteins associated with memory retrieval, through reduced proteolytic activity, is associated with age-related impairments in memory retention.

Effects of Prenatal Exposure to Inflammation Coupled With Stress Exposure During Adolescence on Cognition and Synaptic Protein Levels in Aged CD-1 Mice

Age-associated impairment of spatial learning and memory (AISLM) presents substantial challenges to our health and society. Increasing evidence has indicated that embryonic exposure to inflammation accelerates the AISLM, and this can be attributable, at least partly, to changed synaptic plasticity associated with the activities of various proteins. However, it is still uncertain whether social psychological factors affect this AISLM and/or the expression of synaptic protein-associated genes. Synaptotagmin-1 (Syt1) and activity-regulated cytoskeleton-associated protein (Arc) are two synaptic proteins closely related to cognitive functions. In this study, pregnant CD-1 mice received daily intraperitoneal injections of lipopolysaccharide (LPS) (50 μg/kg) or normal saline at days 15-17 of gestation, and half of the offspring of each group were then subjected to stress for 28 days in adolescence. The Morris water maze (MWM) test was used to separately evaluate spatial learning and memory at 3 and 15 months of age, while western blotting and RNAscope assays were used to measure the protein and mRNA levels of Arc and Syt1 in the hippocampus. The results showed that, at 15 months of age, control mice had worse cognitive ability and higher protein and mRNA levels of Arc and Syt1 than their younger counterparts. Embryonic exposure to inflammation or exposure to stress in adolescence aggravated the AISLM, as well as the age-related increase in Arc and Syt1 expression. Moreover, the hippocampal protein and mRNA levels of Arc and Syt1 were significantly correlated with the performance in the learning and memory periods of the MWM test, especially in the mice that had suffered adverse insults in early life. Our findings indicated that prenatal exposure to inflammation or stress exposure in adolescence exacerbated the AISLM and age-related upregulation of Arc and Syt1 expression, and these effects were linked to cognitive impairments in CD-1 mice exposed to adverse factors in early life.

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.

Perturbations of Ubiquitin-Proteasome-Mediated Proteolysis in Aging and Alzheimer's Disease

The ubiquitin-proteasome pathway (UPP) has multiple roles in the normal nervous system, including the development of synaptic connections and synaptic plasticity. Research over the past several years has indicated a role for the UPP in aging without any overt pathology in the brain. In addition, malfunction of the UPP is implicated in Alzheimer’s disease (AD) and dementia associated with it. In this mini review article, we assess the literature on the role of protein degradation by the UPP in aging and in AD with special emphasis on dysregulation of the UPP and its contribution to cognitive decline and impairment.

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.

Dissociable effects of advanced age on prefrontal cortical and medial temporal lobe ensemble activity

The link between age-related cellular changes within brain regions and larger scale neuronal ensemble dynamics critical for cognition has not been fully elucidated. The present study measured neuron activity within medial prefrontal cortex (PFC), perirhinal cortex (PER), and hippocampal subregion CA1 of young and aged rats by labeling expression of the immediate-early gene Arc. The proportion of cells expressing Arc was quantified at baseline and after a behavior that requires these regions. In addition, PER and CA1 projection neurons to PFC were identified with retrograde labeling. Within CA1, no age-related differences in neuronal activity were observed in the entire neuron population or within CA1 pyramidal cells that project to PFC. Although behavior was comparable across age groups, behaviorally driven Arc expression was higher in the deep layers of both PER and PFC and lower in the superficial layers of these regions. Moreover, age-related changes in activity levels were most evident within PER cells that project to PFC. These data suggest that the PER-PFC circuit is particularly vulnerable in advanced age.

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.

FK506-Binding Protein 12.6/1b, a Negative Regulator of [Ca2+], Rescues Memory and Restores Genomic Regulation in the Hippocampus of Aging Rats

Hippocampal overexpression of FK506-binding protein 12.6/1b (FKBP1b), a negative regulator of ryanodine receptor Ca²⁺ release, reverses aging-induced memory impairment and neuronal Ca²⁺ dysregulation. Here, we tested the hypothesis that FKBP1b also can protect downstream transcriptional networks from aging-induced dysregulation. We gave hippocampal microinjections of FKBP1b-expressing viral vector to male rats at either 13 months of age (long-term, LT) or 19 months of age (short-term, ST) and tested memory performance in the Morris water maze at 21 months of age. Aged rats treated ST or LT with FKBP1b substantially outperformed age-matched vector controls and performed similarly to each other and young controls (YCs). Transcriptional profiling in the same animals identified 2342 genes with hippocampal expression that was upregulated/downregulated in aged controls (ACs) compared with YCs (the aging effect). Of these aging-dependent genes, 876 (37%) also showed altered expression in aged FKBP1b-treated rats compared with ACs, with FKBP1b restoring expression of essentially all such genes (872/876, 99.5%) in the direction opposite the aging effect and closer to levels in YCs. This inverse relationship between the aging and FKBP1b effects suggests that the aging effects arise from FKBP1b deficiency. Functional category analysis revealed that genes downregulated with aging and restored by FKBP1b were associated predominantly with diverse brain structure categories, including cytoskeleton, membrane channels, and extracellular region. Conversely, genes upregulated with aging but not restored by FKBP1b associated primarily with glial–neuroinflammatory, ribosomal, and lysosomal categories. Immunohistochemistry confirmed aging-induced rarefaction and FKBP1b-mediated restoration of neuronal microtubular structure. Therefore, a previously unrecognized genomic network modulating diverse brain structural processes is dysregulated by aging and restored by FKBP1b overexpression.

SIGNIFICANCE STATEMENT Previously, we found that hippocampal overexpression of FK506-binding protein 12.6/1b (FKBP1b), a negative regulator of intracellular Ca²⁺ responses, reverses both aging-related Ca²⁺ dysregulation and cognitive impairment. Here, we tested whether hippocampal FKBP1b overexpression also counteracts aging changes in gene transcriptional networks. In addition to reducing memory deficits in aged rats, FKBP1b selectively counteracted aging-induced expression changes in 37% of aging-dependent genes, with cytoskeletal and extracellular structure categories highly associated with the FKBP1b-rescued genes. Our results indicate that, in parallel with cognitive processes, a novel transcriptional network coordinating brain structural organization is dysregulated with aging and restored by FKBP1b.

Hypertension-induced synapse loss and impairment in synaptic plasticity in the mouse hippocampus mimics the aging phenotype: implications for the pathogenesis of vascular cognitive impairment

Strong epidemiological and experimental evidence indicates that hypertension has detrimental effects on the cerebral microcirculation and thereby promotes accelerated brain aging. Hypertension is an independent risk factor for both vascular cognitive impairment (VCI) and Alzheimer's disease (AD). However, the pathophysiological link between hypertension-induced cerebromicrovascular injury (e.g., blood-brain barrier disruption, increased microvascular oxidative stress, and inflammation) and cognitive decline remains elusive. The present study was designed to characterize neuronal functional and morphological alterations induced by chronic hypertension and compare them to those induced by aging. To achieve that goal, we induced hypertension in young C57BL/6 mice by chronic (4 weeks) infusion of angiotensin II. We found that long-term potentiation (LTP) of performant path synapses following high-frequency stimulation of afferent fibers was decreased in hippocampal slices obtained from hypertensive mice, mimicking the aging phenotype. Hypertension and advanced age were associated with comparable decline in synaptic density in the stratum radiatum of the mouse hippocampus. Hypertension, similar to aging, was associated with changes in mRNA expression of several genes involved in regulation of neuronal function, including down-regulation of Bdnf, Homer1, and Dlg4, which may have a role in impaired synaptic plasticity. Collectively, hypertension impairs synaptic plasticity, reduces synaptic density, and promotes dysregulation of genes involved in synaptic function in the mouse hippocampus mimicking the aging phenotype. These hypertension-induced neuronal alterations may impair establishment of memories in the hippocampus and contribute to the pathogenesis and clinical manifestation of both vascular cognitive impairment (VCI) and Alzheimer's disease (AD).

Cognition and Synaptic-Plasticity Related Changes in Aged Rats Supplemented with 8- and 10-Carbon Medium Chain Triglycerides

Brain glucose hypometabolism is a common feature of Alzheimer’s disease (AD). Previous studies have shown that cognition is improved by providing AD patients with an alternate energy source: ketones derived from either ketogenic diet or supplementation with medium chain triglycerides (MCT). Recently, data on the neuroprotective capacity of MCT-derived medium chain fatty acids (MCFA) suggest 8-carbon and 10-carbon MCFA may have cognition-enhancing properties which are not related to ketone production. We investigated the effect of 8 week treatment with MCT8, MCT10 or sunflower oil supplementation (5% by weight of chow diet) in 21 month old Wistar rats. Both MCT diets increased ketones plasma similarly compared to control diet, but MCT diets did not increase ketones in the brain. Treatment with MCT10, but not MCT8, significantly improved novel object recognition memory compared to control diet, while social recognition increased in both MCT groups. MCT8 and MCT10 diets decreased weight compared to control diet, where MCFA plasma levels were higher in MCT10 groups than in MCT8 groups. Both MCT diets increased IRS-1 (612) phosphorylation and decreased S6K phosphorylation (240/244) but only MCT10 increased Akt phosphorylation (473). MCT8 supplementation increased synaptophysin, but not PSD-95, in contrast MCT10 had no effect on either synaptic marker. Expression of Ube3a, which controls synaptic stability, was increased by both MCT diets. Cortex transcription via qPCR showed that immediate early genes related to synaptic plasticity (arc, plk3, junb, egr2, nr4a1) were downregulated by both MCT diets while MCT8 additionally down-regulated fosb and egr1 but upregulated grin1 and gba2. These results demonstrate that treatment of 8- and 10-carbon length MCTs in aged rats have slight differential effects on synaptic stability, protein synthesis and behavior that may be independent of brain ketone levels.

Age-related changes in Egr1 transcription and DNA methylation within the hippocampus

Aged animals show functional alterations in hippocampal neurons that lead to deficits in synaptic plasticity and changes in cognitive function. Transcription of immediate-early genes (IEGs), including Egr1, is necessary for processes such as long-term potentiation and memory consolidation. Here, we show an age-related reduction in the transcription of Egr1 in the dentate gyrus following spatial behavior, whereas in the area CA1, Egr1 is reduced at rest, but its transcription can be effectively driven by spatial behavior to levels equivalent to those observed in adult animals. One mechanism possibly contributing to these aging-related changes is an age-associated, CpG site-specific change in methylation in DNA associated with the promoter region of the Egr1 gene. Our results add to a growing body of work demonstrating that complex transcriptional and epigenetic changes in the hippocampus significantly contribute to brain and cognitive aging. © 2016 Wiley Periodicals, Inc.

Decreased Npas4 and Arc mRNA Levels in the Hippocampus of Aged Memory-Impaired Wild-Type But Not Memory Preserved 11β-HSD1 Deficient Mice

Mice deficient in the glucocorticoid-regenerating enzyme 11β-HSD1 resist age-related spatial memory impairment. To investigate the mechanisms and pathways involved, we used microarrays to identify differentially expressed hippocampal genes that associate with cognitive ageing and 11β-HSD1. Aged wild-type mice were separated into memory-impaired and unimpaired relative to young controls according to their performance in the Y-maze. All individual aged 11β-HSD1-deficient mice showed intact spatial memory. The majority of differentially expressed hippocampal genes were increased with ageing (e.g. immune/inflammatory response genes) with no genotype differences. However, the neuronal-specific transcription factor, Npas4, and immediate early gene, Arc, were reduced (relative to young) in the hippocampus of memory-impaired but not unimpaired aged wild-type or aged 11β-HSD1-deficient mice. A quantitative reverse transcriptase-polymerase chain reaction and in situ hybridisation confirmed reduced Npas4 and Arc mRNA expression in memory-impaired aged wild-type mice. These findings suggest that 11β-HSD1 may contribute to the decline in Npas4 and Arc mRNA levels associated with memory impairment during ageing, and that decreased activity of synaptic plasticity pathways involving Npas4 and Arc may, in part, underlie the memory deficits seen in cognitively-impaired aged wild-type mice.

Neurocognitive Aging and the Hippocampus across Species

There is extensive evidence that aging is associated with impairments in episodic memory. Many of these changes have been ascribed to neurobiological alterations to the hippocampal network and its input pathways. A cross-species consensus is beginning to emerge suggesting that subtle synaptic and functional changes within this network may underlie the majority of age-related memory impairments. In this review we survey convergent data from animal and human studies that have contributed significantly to our understanding of the brain-behavior relationships in this network, particularly in the aging brain. We utilize a cognitive as well as a neurobiological perspective and synthesize data across approaches and species to reach a more detailed understanding of age-related alterations in hippocampal memory function.

Experience Modulates the Effects of Histone Deacetylase Inhibitors on Gene and Protein Expression in the Hippocampus: Impaired Plasticity in Aging

The therapeutic potential of histone deacetylase inhibitor (HDACi) treatment has attracted considerable attention in the emerging area of cognitive neuroepigenetics. The possibility that ongoing cognitive experience importantly regulates the cell biological effects of HDACi administration, however, has not been systematically examined. In an initial experiment addressing this issue, we tested whether water maze training influences the gene expression response to acute systemic HDACi administration in the young adult rat hippocampus. Training powerfully modulated the response to HDACi treatment, increasing the total number of genes regulated to nearly 3000, including many not typically linked to neural plasticity, compared with <300 following HDACi administration alone. Although water maze training itself also regulated nearly 1800 genes, the specific mRNAs, gene networks, and biological pathways involved were largely distinct when the same experience was provided together with HDACi administration. Next, we tested whether the synaptic protein response to HDACi treatment is similarly dependent on recent cognitive experience, and whether this plasticity is altered in aged rats with memory impairment. Whereas synaptic protein labeling in the young hippocampus was selectively increased when HDACi administration was provided in conjunction with water maze training, combined treatment had no effect on synaptic proteins in the aged hippocampus. Our findings indicate that ongoing experience potently regulates the molecular consequences of HDACi treatment and that the interaction of recent cognitive experience with histone acetylation dynamics is disrupted in the aged hippocampus.

SIGNIFICANCE STATEMENT

The possibility that interventions targeting epigenetic regulation could be effective in treating a range of neurodegenerative disorders has attracted considerable interest. Here we demonstrate in the rat hippocampus that ongoing experience powerfully modifies the molecular response to one such intervention, histone deacetylase inhibitor (HDACi) administration. A single learning episode dramatically shifts the gene expression profile induced by acute HDACi treatment, yielding a qualitatively distinct hippocampal transcriptome compared with the influence of behavioral training alone. The downstream synaptic protein response to HDACi administration is similarly experience-dependent, and we report that this plasticity is disrupted in the aged hippocampus. The findings highlight that accommodating the modulatory influence of ongoing experience represents a challenge for therapeutic development in the area of cognitive neuroepigenetics.

Head west or left, east or right: interactions between memory systems in neurocognitive aging

Cognitive aging is accompanied by decline in multiple domains of memory. Here, we developed a T-maze task that required rats to learn competing hippocampal, and striatal navigation strategies in succession, across days. A final session increased demands on cognitive flexibility and required within-day switching between strategies, emphasizing capacities that engage the prefrontal cortex. Background characterization in young and aged rats used a water maze protocol optimized for individual differences in hippocampal integrity. Consistent with earlier work, young adults acquired place strategies in the T-maze faster than response, whereas the opposite was observed in aged rats with impaired spatial memory. The novel result was that aged animals with preserved spatial memory displayed a qualitatively distinct pattern, acquiring place and response strategies equally rapidly, without disruption when switching between them. Subsequent in situ hybridization for the plasticity-related immediate-early gene Arc revealed that while increasing demands on cognitive flexibility and within-day strategy switching potently engaged the prefrontal cortex in young adult and aged-impaired rats, Arc expression was insensitive in aged rats with normal spatial memory and superior switching abilities. Together, the results indicate that cognitive aging is an emergent property of the interactions between memory systems, and that successful cognitive outcomes reflect a distinct neuroadaptive process rather than a slower rate of aging.