Decreased levels of NMDA but not AMPA receptors in the lipid-raft fraction of 3xTg-AD model of Alzheimer's disease: Relation to Arc/Arg3.1 protein expression

Memory related molecular signatures: The pivots for memory consolidation and Alzheimer's related memory decline

Age-related cognitive decline is the major cause of concern due to its 70% more incidence than dementia cases worldwide. Moreover, aging is also the major risk factor of Alzheimer's disease (AD), associated with progressive memory loss. Approx. 13 million people will have Alzheimer-related memory decline by 2050. Learning and memory is the fundamental process of brain functions. However, the mechanism for the same is still under investigation. Thus, it is critical to understand the process of memory consolidation in the brain and extrapolate its understanding to the memory decline mechanism. Research on learning and memory has identified several molecular signatures such as Protein kinase M zeta (PKMζ), Calcium/calmodulin-dependent protein kinase II (CaMKII), Brain-derived neurotrophic factor (BDNF), cAMP-response element binding protein (CREB) and Activity-regulated cytoskeleton-associated protein (Arc) crucial for the maintenance and stabilization of long-term memory in the brain. Interestingly, memory decline in AD has also been linked to the abnormality in expressing these memory-related molecular signatures. Hence, in the present consolidated review, we explored the role of these memory-related molecular signatures in long-term memory consolidation. Additionally, the effect of amyloid-beta toxicity on these molecular signatures is discussed in detail.

Effect of pharmacological manipulations on Arc function

Activity-regulated cytoskeleton-associated protein (Arc) is a brain-enriched immediate early gene that regulates important mechanisms implicated in learning and memory. Arc levels are controlled through a balance of induction and degradation in an activity-dependent manner. Arc further undergoes multiple post-translational modifications that regulate its stability, localization and function. Recent studies demonstrate that these features of Arc can be pharmacologically manipulated. In this review, we discuss some of these compounds, with an emphasis on drugs of abuse and psychotropic drugs. We also discuss inflammatory states that regulate Arc.

Cocaine-Induced Synaptic Redistribution of NMDARs in Striatal Neurons Alters NMDAR-Dependent Signal Transduction

The consequence of repeated cocaine exposure and prolonged abstinence on glutamate receptor expression in the nucleus accumbens has been extensively studied. However, the early effects of cocaine on NMDAR signaling remain unknown. NMDAR signaling depends on the subunit composition, subcellular localization, and the interaction with proteins at the postsynaptic density (PSD), where NMDARs and other proteins form supercomplexes that are responsible for the signaling pathways activated by NMDAR-induced Ca²⁺ influx. Here, we investigated the effect of cocaine on NMDAR subunit composition and subcellular localization after both intraperitoneal non-contingent cocaine and response-contingent intravenous cocaine self-administration in mice. We found that repeated cocaine exposure, regardless of the route or contingency of drug administration, decreases NMDAR interactions with the PSD and synaptic lipid rafts in the accumbens shell and dorsal striatum. We provide evidence that cocaine triggers an early redistribution of NMDARs from synaptic to extrasynaptic sites, and that this adaptation has implications in the activation of downstream signaling pathways. Thus, consistent with a loss of NMDAR function, cocaine-induced ERK phosphorylation is attenuated. Because early NMDAR activity contributes to the initiation of lasting addiction-relevant neuroadaptations, these data may hold clues into cellular mechanisms responsible for the development of cocaine addiction.

Alterations of NMDA and AMPA receptors and their signaling apparatus in the hippocampus of mouse offspring induced by developmental arsenite exposure

Loss of cognitive function due to arsenic exposure is a serious health concern in many parts of the world, including China. The present study aims to determine the molecular mechanism of arsenic-induced neurotoxicity and its consequent effect on downstream signaling pathways of mouse N-methyl-D-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). Drinking water containing 0, 25, 50 or 100 mg/L arsenite was provided each day to mother mice throughout gestation period until postnatal day (PND) 35 to expose the newborn mice to arsenite during early developmental period. The effect of arsenite in the expressions of different components of NMDAR (NR1, NR2A, NR2B) and AMPAR (GluR1, GluR2, GluR3), including calcium/calmodulin-dependent protein kinase II (CaMKII) and phosphorylated-CaMKII (p-CaMKII), at PND 7, 14, 21 and 35 was estimated and analyzed from the hippocampus of mice. A significant inhibition in the protein and mRNA expressions of NR1, NR2A, NR2B and GluR1 was observed in mice exposed to 50 mg/L arsenite since PND 7. Down regulation of GluR2 and GluR3 both at mRNA and protein levels was observed in mice exposed to 50 mg/L arsenite till PND 14. Moreover, both CaMKII as well as p-CaMKII expressions were significantly limited since PND 7 in 50 mg/L arsenite exposed mice group. Findings form this study suggested that the previously reported impairment in learning and memorizing abilities in later stage due to early life arsenite exposure is associated with the alterations of NMDARs, AMPARs, CaMKII and p-CaMKII expressions.

Modified Huang-Lian-Jie-Du Decoction Ameliorates Aβ Synaptotoxicity in a Murine Model of Alzheimer's Disease

Alzheimer's disease (AD) is a common neurodegenerative disease, characterized by cognitive dysfunction; however, the therapeutic strategies are not fully understood. Huang-Lian-Jie-Du-Decoction (HLJDD) is a famous traditional Chinese herbal formula that has been widely used clinically to treat dementia. Recently, according to previous study and our clinical practice, we generate a new modification of HLJDD (named modified-HLJDD). In this study, we indicated that modified-HLJDD attenuated learning and memory deficiencies in Aβ_1-42 oligomer-induced AD model, and we confirmed the exact metabolites in modified-HLJDD solution, as compared with HLJDD by UHPLC-Q-TOF-MS. Using GC-Q-TOF/MS-based metabolomics, we identified adenosine as the potential significant metabolite, responsible for modified-HLJDD regulating energy metabolism and synaptic plasticity in AD model. We also revealed that the potential underlying mechanism of modified-HLJDD in AD model may involve NMDA receptor-mediated glutamatergic transmission and adenosine/ATPase/AMPK cascade. Moreover, we also indicated the differential gut microbiota which mainly involved Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria at the phylum level upon modified-HLJDD treatment in AD model. Based on the correlation of metabolomic analysis with microbiome analysis, we clarified that Dorea is the most affected microbiota with adenosine upon modified-HLJDD treatment in AD model. Thus, our study suggests that modified-HLJDD may serve as a potential therapeutic drug in treating AD.

Vulnerability and resilience to Alzheimer's disease: early life conditions modulate neuropathology and determine cognitive reserve

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with a high prevalence among the elderly and a huge personal and societal impact. Recent epidemiological studies have indicated that the incidence and age of onset of sporadic AD can be modified by lifestyle factors such as education, exercise, and (early) stress exposure. Early life adversity is known to promote cognitive decline at a later age and to accelerate aging, which are both primary risk factors for AD. In rodent models, exposure to 'negative' or 'positive' early life experiences was recently found to modulate various measures of AD neuropathology, such as amyloid-beta levels and cognition at later ages. Although there is emerging interest in understanding whether experiences during early postnatal life also modulate AD risk in humans, the mechanisms and possible substrates underlying these long-lasting effects remain elusive.

METHODS

We review literature and discuss the role of early life experiences in determining later age and AD-related processes from a brain and cognitive 'reserve' perspective. We focus on rodent studies and the identification of possible early determinants of later AD vulnerability or resilience in relation to early life adversity/enrichment.

RESULTS

Potential substrates and mediators of early life experiences that may influence the development of AD pathology and cognitive decline are: programming of the hypothalamic-pituitary-adrenal axis, priming of the neuroinflammatory response, dendritic and synaptic complexity and function, overall brain plasticity, and proteins such as early growth response protein 1 (EGR1), activity regulated cytoskeleton-associated protein (Arc), and repressor element-1 silencing transcription factor (REST).

CONCLUSIONS

We conclude from these rodent studies that the early postnatal period is an important and sensitive phase that influences the vulnerability to develop AD pathology. Yet translational studies are required to investigate whether early life experiences also modify AD development in human studies, and whether similar molecular mediators can be identified in the sensitivity to develop AD in humans.

Infant avoidance training alters cellular activation patterns in prefronto-limbic circuits during adult avoidance learning: II. Cellular imaging of neurons expressing the activity-regulated cytoskeleton-associated protein (Arc/Arg3.1)

Positive and negative feedback learning is essential to optimize behavioral performance. We used the two-way active avoidance (TWA) task as an experimental paradigm for negative feedback learning with the aim to test the hypothesis that neuronal ensembles activate the activity-regulated cytoskeletal (Arc/Arg3.1) protein during different phases of avoidance learning and during retrieval. A variety of studies in humans and other animals revealed that the ability of aversive feedback learning emerges postnatally. Our previous findings demonstrated that rats, which as infants are not capable to learn an active avoidance strategy, show improved avoidance learning as adults. Based on these findings, we further tested the hypothesis that specific neuronal ensembles are "tagged" during infant TWA training and then reactivated during adult re-exposure to the same learning task. Using cellular imaging by immunocytochemical detection of Arc/Arg3.1, we observed that, compared to the untrained control group, (1) only in the dentate gyrus the density of Arc/Arg3.1-expressing neurons was elevated during the acquisition phase of TWA learning, and (2) this increase in Arc/Arg3.1-expressing neurons was not specific for the TWA learning task. With respect to the effects of infant TWA training we found that compared to the naïve non-pretrained group (a) the infant pretraining group displayed a higher density of Arc/Arg3.1-expressing neurons in the anterior cingulate cortex during acquisition on training day 1, and (b) the infant pretraining group displayed elevated density of Arc/Arg3.1-expressing neurons in the dentate gyrus during retrieval on test day 5. Correlation analysis for the acquisition phase revealed for the ACd that the animals which showed the highest number of avoidances and the fastest escape latencies displayed the highest density of Arc/Arg3.1-expressing neurons. Taken together, we are the first to use the synaptic plasticity protein Arc/Arg3.1 to label neuronal ensembles which are involved in different phases of active avoidance learning and whose activity patterns are changing in response to previous learning experience during infancy. Our results indicate (1) that, despite the inability to learn an active avoidance response in infancy, lasting memory traces are formed encoding the subtasks that are learned in infancy (e.g., the association of the CS and UCS, escape strategy), which are encoded in the infant brain by neuronal ensembles, which alter their synaptic connectivity via activation of specific synaptic plasticity proteins such as Arc/Arg3.1 and Egr1, and (2) that during adult training these memories can be retrieved by reactivating these neuronal ensembles and their synaptic circuits and thereby accelerate learning.