Early impairment of long-term depression in the perirhinal cortex of a mouse model of Alzheimer's disease

Expression of tiRNA and tRF in APP/PS1 transgenic mice and the change of related proteins expression

Background

Transcriptomics, such as that of non-coding RNA (ncRNA), which include microRNA (miRNA), circular RNA, and the transfer RNA (tRNA)-derived fragments (tiRNA and tRF) in Alzheimer's disease (AD) have attracted much attention recently. The tiRNA and tRFs are produced when the tRNA splits at specific sites. The expression change and related function of tiRNA and tRFs in AD has not been fully investigated.

Methods

In our study, APP/PS1 transgenic mice (AD mice model) and healthy control mice were used to discover the differentially expressed tiRNA and tRFs with high-throughput sequencing. Among the differentially expressed tiRNA and tRFs, we chose two tRFs (tRF-Thr-CGT-003 and tRF-Leu-CAA-004) and predicted the target messenger RNAs (mRNAs) with miRanda and Target Scan. The target mRNAs of tRF-related function and pathways were analyzed, then we performed quantitative reverse transcription polymerase chain reaction (RT-qPCR) and western blot to validate the related target mRNAs and pathways.

Results

A total of 27 significantly different tiRNA and tRFs were detected between wild type (WT) and APP/PS1 groups, including 14 up-regulated and 13 down-regulated. Through analyzing the target mRNAs of all differentially expressed tiRNA and tRFs with GO enrichment, we found the target mRNAs could take part in the learning and memory biological process, synapse organization, cognition biological process, synaptic transmission, amyloid-β (Aβ) metabolic process, and so on. We then chose three differentially expressed tRFs for further qPCR validation and passed two tRFs: tRF-Thr-CGT-003 and tRF-Leu-CAA-004, that were found to regulate the calcium regulation-related proteins (the voltage-gated calcium channel γ2 subunit and the RYR1 endoplasmic reticulum calcium released protein) and the retinol metabolism-related proteins (retinoic acid metabolic enzymes CYP2S1, CYP2C68, CYP2S1).

Conclusions

The APP expression and presenilin mutation in APP/PS1 mice could cause tiRNA and tRFs expression change. Among the differentially expressed tiRNA and tRFs, we found some tRFs took part in the voltage-gated calcium channel γ2 subunit expression and regulation, influencing the neuron calcium homeostasis. Moreover, we also found the tRFs may participate in the regulation of retinol metabolism. Our findings suggest that the dysregulated tiRNA and tRFs may be beneficially exploited as potential diagnostic biomarkers and/or therapeutic targets of AD.

α1-Adrenergic Receptors in Neurotransmission, Synaptic Plasticity, and Cognition

α₁-adrenergic receptors are G-Protein Coupled Receptors that are involved in neurotransmission and regulate the sympathetic nervous system through binding and activating the neurotransmitter, norepinephrine, and the neurohormone, epinephrine. There are three α₁-adrenergic receptor subtypes (α_1A, α_1B, α_1D) that are known to play various roles in neurotransmission and cognition. They are related to two other adrenergic receptor families that also bind norepinephrine and epinephrine, the β- and α₂-, each with three subtypes (β₁, β₂, β₃, α_2A, α_2B, α_2C). Previous studies assessing the roles of α₁-adrenergic receptors in neurotransmission and cognition have been inconsistent. This was due to the use of poorly-selective ligands and many of these studies were published before the characterization of the cloned receptor subtypes and the subsequent development of animal models. With the availability of more-selective ligands and the development of animal models, a clearer picture of their role in cognition and neurotransmission can be assessed. In this review, we highlight the significant role that the α₁-adrenergic receptor plays in regulating synaptic efficacy, both short and long-term synaptic plasticity, and its regulation of different types of memory. We will also present evidence that the α₁-adrenergic receptors, and particularly the α_1A-adrenergic receptor subtype, are a potentially good target to treat a wide variety of neurological conditions with diminished cognition.

Extracellular heat shock proteins in neurodegenerative diseases: New perspectives

One pathological hallmark of neurodegenerative diseases and CNS trauma is accumulation of insoluble, hydrophobic molecules and protein aggregations found both within and outside cells. These may be the consequences of an inadequate or overburdened cellular response to stresses resulting from potentially toxic changes in extra- and intracellular environments. The upregulated expression of heat shock proteins (HSPs) is one example of a highly conserved cellular response to both internal and external stress. Intracellularly these proteins act as chaperones, playing vital roles in the folding of nascent polypeptides, the translocation of proteins between subcellular locations, and the disaggregation of misfolded or aggregated proteins in an attempt to maintain cellular proteostasis during both homeostatic and stressful conditions. While the predominant study of the HSPs has focused on their intracellular chaperone functions, it remains unclear if all neuronal populations can mount a complete stress response. Alternately, it is now well established that some members of this family of proteins can be secreted by nearby, non-neuronal cells to act in the extracellular environment. This review addresses the current literature detailing the use of exogenous and extracellular HSPs in the treatment of cellular and animal models of neurodegenerative disease. These findings offer a new measure of therapeutic potential to the HSPs, but obstacles must be overcome before they can be efficiently used in a clinical setting.

Atrophy in Distributed Networks Predicts Cognition in Alzheimer's Disease and Type 2 Diabetes

BACKGROUND

Alzheimer's disease (AD) and type 2 diabetes (T2DM) are common causes of cognitive decline among older adults and share strong epidemiological links. Distinct patterns of cortical atrophy are observed in AD and T2DM, but robust comparisons between structure-function relationships across these two disease states are lacking.

OBJECTIVE

To compare how atrophy within distributed brain networks is related to cognition across the spectrum of cognitive aging.

METHODS

The relationship between structural MRI changes and cognition was studied in 22 mild-to-moderate AD, 28 T2DM, and 27 healthy participants. Cortical thickness measurements were obtained from networks of interest (NOIs) matching the limbic, default, and frontoparietal resting-state networks. Composite cognitive scores capturing domains of global cognition, memory, and executive function were created. Associations between cognitive scores and the NOIs were assessed using linear regression, with age as a covariate. Within-network General Linear Model (GLM) analysis was run in Freesurfer 6.0 to visualize differences in patterns of cortical atrophy related to cognitive function in each group. A secondary analysis examined hemispheric differences in each group.

RESULTS

Across all groups, cortical atrophy within the limbic NOI was significantly correlated with Global Cognition (p = 0.009) and Memory Composite (p = 0.002). Within-network GLM analysis and hemispheric analysis revealed qualitatively different patterns of atrophy contributing to cognitive dysfunction between AD and T2DM.

CONCLUSION

Brain network atrophy is related to cognitive function across AD, T2DM, and healthy participants. Differences in cortical atrophy patterns were seen between AD and T2DM, highlighting neuropathological differences.

Combined machine learning and diffusion tensor imaging reveals altered anatomic fiber connectivity of the brain in primary open-angle glaucoma

Parameters derived from diffusion tensor imaging (DTI) have been found to be significantly altered in the optic tracts, optic nerves, and optic radiations in patients with primary open-angle glaucoma (POAG). In this study, DTI-derived parameters were further constructed into fiber connectivity, and we investigated anatomical fiber connectivity changes within and beyond the visual pathway in POAG patients. DTI and T1-weighted magnetic resonance images were acquired in 18 POAG patients and 26 healthy controls (HC). White matter tracts based on the Brodmann atlases (BA) were constructed using the deterministic fiber tracking method. The mean fractional anisotropy (FA), fiber number (FN), and mean fiber length (FL) were measured and then evaluated using two-sample t-tests between POAG and HC. The fiber connectivity between regions was taken as the features for classifying HC and POAG using a machine learning method known as naïve Bayesian classification. The mean FA decreased in connections between visual cortex BA17/BA18 and cortex BA23/BA25/BA35/BA36, while it increased in the connections between cortex BA3/BA7/BA9 and BA5/BA6/BA45/BA25 in POAG. Classification using fibers where a significant difference in FN had been identified produced better accuracy (ACC = 0.89) than using FA or FL (ACC = 0.77 and 0.75, respectively). The FN of individual fiber connections with higher accuracy and significant changes in POAG involved brain regions associated with vision (BA19), depression (BA10/BA46/BA25), and memory (BA29). These findings strengthen the hypothesis that POAG involves changes in anatomical connectivity within and beyond the visual pathway. Classification using the machine learning method reveals that mean FN has the potential to be used as a biomarker for detecting white matter microstructure changes in POAG.

Aβ and the dementia syndrome: Simple versus complex perspectives

BACKGROUND

The amyloid cascade hypothesis (ACH) has dominated strategy in dementia research for decades despite evidence of its limitations including known heterogeneity of the dementia syndrome in the population and the narrow focus on a single molecule - the amyloid beta protein (Aβ) as causal for all Alzheimer-type dementia. Other hypotheses relevant to Aβ are the presenilin (PS) hypothesis (PSH) relating to the involvement of PS in the generation of Aβ, and the amyloid precursor protein (APP) matrix approach (AMA), relating to the complex and dynamic breakdown of APP, from which Aβ derives.

MATERIALS AND METHODS

In this article we explore perspectives relating to complex disorders occurring mainly in older populations through a detailed case study of the role of Aβ in AD.

RESULTS

Scrutiny of the evidence generated so far reveals and a lack of understanding of the wider APP proteolytic system and how narrow research into the dementia syndrome has been to date. Confounding factors add significant limitations to the understanding of the current evidence base.

CONCLUSIONS

A better characterisation of the entire APP proteolytic system in the human brain is urgently required to place Aβ in its complex physiological context. From a molecular perspective, a combination of the alternative hypotheses, the PSH and the AMA may better describe the complexity of the APP proteolytic system leading to new therapeutic approaches. The reductionist approach is widespread throughout biomedical research and this example highlights how neglect of complexity can undermine investigations of complex disorders, particularly those arising in the oldest in our populations.

Muscarinic Receptor-Dependent Long Term Depression in the Perirhinal Cortex and Recognition Memory are Impaired in the rTg4510 Mouse Model of Tauopathy

Neurodegenerative diseases affecting cognitive dysfunction, such as Alzheimer's disease and fronto-temporal dementia, are often associated impairments in the visual recognition memory system. Recent evidence suggests that synaptic plasticity, in particular long term depression (LTD), in the perirhinal cortex (PRh) is a critical cellular mechanism underlying recognition memory. In this study, we have examined novel object recognition and PRh LTD in rTg4510 mice, which transgenically overexpress tau_P301L. We found that 8-9 month old rTg4510 mice had significant deficits in long- but not short-term novel object recognition memory. Furthermore, we also established that PRh slices prepared from rTg4510 mice, unlike those prepared from wildtype littermates, could not support a muscarinic acetylcholine receptor-dependent form of LTD, induced by a 5 Hz stimulation protocol. In contrast, bath application of the muscarinic agonist carbachol induced a form of chemical LTD in both WT and rTg4510 slices. Finally, when rTg4510 slices were preincubated with the acetylcholinesterase inhibitor donepezil, the 5 Hz stimulation protocol was capable of inducing significant levels of LTD. These data suggest that dysfunctional cholinergic innervation of the PRh of rTg4510 mice, results in deficits in synaptic LTD which may contribute to aberrant recognition memory in this rodent model of tauopathy.

Young Blood Plasma Administration to Fight Alzheimer's Disease?

Despite decades of intensive research, no drugs can cure or even stabilize Alzheimer's disease (AD). Current pharmacological treatments only partially mask the symptoms while the disease progresses within the brain. Finding a preventive measure or a cure for people with AD is indeed a worldwide urgent priority. A recent interesting study by T. Wyss-Coray's research group provides the first evidence that exposure to young blood or plasma can reverse some AD-related molecular and behavioral alterations. Heterochronic parabiosis (shared blood circulation) of AD transgenic mice with young healthy mice did not reduce amyloidosis and microglial activation in AD mice, but reversed the loss of synaptophysin and calbindin (critical synaptic proteins, indicators of cognitive decline in AD) in the dentate gyrus, and the abnormal expression, in the hippocampus, of many genes involved in key neuronal signaling pathways. Moreover, repeated intravenous administration of plasma from young healthy mice to AD mice reversed the excessive phosphorylation of hippocampal extracellular signal-regulated kinase (ERK), and improved spatial working memory and associative memory. Although observations in mouse models of AD might not necessarily extrapolate to humans, this preclinical study provides the first demonstration that young plasma has potential therapeutic properties, by ameliorating aspects of the disease that are present in AD patients. Clinical trials are already under way. If young plasma transfusion will be effective in AD patients, it will be important to identify the key factors responsible for the positive effects, as they might lead to the development of molecule interventions with a better efficacy/risk profile.

Altered intrinsic excitability of hippocampal CA1 pyramidal neurons in aged PDAPP mice

Amyloidopathy involves the accumulation of insoluble amyloid β (Aβ) species in the brain's parenchyma and is a key histopathological hallmark of Alzheimer's disease (AD). Work on transgenic mice that overexpress Aβ suggests that elevated Aβ levels in the brain are associated with aberrant epileptiform activity and increased intrinsic excitability (IE) of CA1 hippocampal neurons. In this study we examined if similar changes could be observed in hippocampal CA1 pyramidal neurons from aged PDAPP mice (20-23 month old, Indiana mutation: V717F on APP gene) compared to their age-matched wild-type littermate controls. Whole-cell current clamp recordings revealed that sub-threshold intrinsic properties, such as input resistance, resting membrane potential and hyperpolarization activated "sag" were unaffected, but capacitance was significantly decreased in the transgenic animals. No differences between genotypes were observed in the overall number of action potentials (AP) elicited by 500 ms supra-threshold current stimuli. PDAPP neurons, however, exhibited higher instantaneous firing frequencies after accommodation in response to high intensity current injections. The AP waveform was narrower and shorter in amplitude in PDAPP mice: these changes, according to our in silico model of a CA1/3 pyramidal neuron, depended on the respective increase and reduction of K(+) and Na(+) voltage-gated channels maximal conductances. Finally, the after-hyperpolarization, seen after the first AP evoked by a +300 pA current injection and after 50 Hz AP bursts, was more pronounced in PDAPP mice. These data show that Aβ-overexpression in aged mice altered the capacitance, the neuronal firing and the AP waveform of CA1 pyramidal neurons. Some of these findings are consistent with previous work on younger PDAPP; they also show important differences that can be potentially ascribed to the interaction between amyloidopathy and ageing. Such a change of IE properties over time underlies that the increased incidence of seizure observed in AD patients might rely on different mechanistic pathways during progression of the disease.

Oxidative Stress during the Progression of β-Amyloid Pathology in the Neocortex of the Tg2576 Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, characterized by progressive neurodegeneration. Pathogenetic mechanisms, triggered by β-amyloid (Aβ) accumulation, include oxidative stress, derived from energy homeostasis deregulation and involving mitochondria and peroxisomes. We here addressed the oxidative stress status and the elicited cellular response at the onset and during the progression of Aβ pathology, studying the neocortex of Tg2576 model of AD. Age-dependent changes of oxidative damage markers, antioxidant enzymes, and related transcription factors were analysed in relation to the distribution of Aβ peptide and oligomers, by a combined molecular/morphological approach. Nucleic acid oxidative damage, accompanied by defective antioxidant defences, and decreased PGC1α expression are already detected in 3-month-old Tg2576 neurons. Conversely, PPARα is increased in these cells, with its cytoplasmic localization suggesting nongenomic, anti-inflammatory actions. At 6 months, when intracellular Aβ accumulates, PMP70 is downregulated, indicating impairment of fatty acids peroxisomal translocation and their consequent harmful accumulation. In 9-month-old Tg2576 neocortex, Aβ oligomers and acrolein deposition correlate with GFAP, GPX1, and PMP70 increases, supporting a compensatory response, involving astroglial peroxisomes. At severe pathological stages, when senile plaques disrupt cortical cytoarchitecture, antioxidant capacity is gradually lost. Overall, our data suggest early therapeutic intervention in AD, also targeting peroxisomes.

Intrinsic excitability changes induced by acute treatment of hippocampal CA1 pyramidal neurons with exogenous amyloid β peptide

Accumulation of beta‐amyloid (Aβ) peptides in the human brain is a canonical pathological hallmark of Alzheimer's disease (AD). Recent work in Aβ‐overexpressing transgenic mice indicates that increased brain Aβ levels can be associated with aberrant epileptiform activity. In line with this, such mice can also exhibit altered intrinsic excitability (IE) of cortical and hippocampal neurons: these observations may relate to the increased prevalence of seizures in AD patients. In this study, we examined what changes in IE are produced in hippocampal CA1 pyramidal cells after 2–5 h treatment with an oligomeric preparation of synthetic human Aβ 1–42 peptide. Whole cell current clamp recordings were compared between Aβ‐(500 nM) and vehicle‐(DMSO 0.05%) treated hippocampal slices obtained from mice. The soluble Aβ treatment did not produce alterations in sub‐threshold intrinsic properties, including membrane potential, input resistance, and hyperpolarization activated “sag”. Similarly, no changes were noted in the firing profile evoked by 500 ms square current supra‐threshold stimuli. However, Aβ 500 nM treatment resulted in the hyperpolarization of the action potential (AP) threshold. In addition, treatment with Aβ at 500 nM depressed the after‐hyperpolarization that followed both a single AP or 50 Hz trains of a number of APs between 5 and 25. These data suggest that acute exposure to soluble Aβ oligomers affects IE properties of CA1 pyramidal neurons differently from outcomes seen in transgenic models of amyloidopathy. However, in both chronic and acute models, the IE changes are toward hyperexcitability, reinforcing the idea that amyloidopathy and increased incidence in seizures might be causally related in AD patients. © 2014 The Authors Hippocampus Published by Wiley Periodicals, Inc.

Parallel age-associated changes in brain and plasma neuronal pentraxin receptor levels in a transgenic APP/PS1 rat model of Alzheimer's disease

Neuronal pentraxin receptor (NPR) is a synaptic protein implicated in AMPA receptor trafficking at excitatory synapses. Since glutamate neurotransmission is disrupted in Alzheimer's disease (AD), NPR levels measured from plasma represent a potential biomarker for synaptic dysfunction associated with AD. We sought to determine the relationship between AD pathology and brain and plasma NPR levels by examining age-associated NPR levels in these compartments in a transgenic APP/PS1 rat model of AD. NPR levels in cortical homogenate were similar in wild-type (Wt) and APP/PS1 rats at 3 months of age (prior to Aβ plaque deposition), but significantly increased in APP/PS1 rats by 9 and 18-20 months of age (after the onset of plaque deposition). These age-dependent differences were driven by proportional increases in NPR in membrane-associated cortical fractions. Genotype-related differences in NPR expression were also seen in the hippocampus, which exhibits significant Aβ pathology, but not in the cerebellum, which does not. Plasma analyses revealed increased levels of a 26 kDa NPR fragment in APP/PS1 rats relative to Wt rats by 18-20 months of age, which correlated with the levels of full-length NPR in cortex. Our findings indicate that cerebral accumulation of NPR and Aβ occurs with similar temporal and regional patterns in the APP/PS1 model, and suggest that a 26 kDa plasma NPR fragment may represent a peripheral biomarker of this process.

In search of a recognition memory engram

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The role of the perirhinal cortex in familiarity discrimination is reviewed.

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Behavioural, pharmacological and electrophysiological evidence is considered.

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The cortex is found to be essential for memory acquisition, retrieval and storage.

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The evidence indicates that perirhinal synaptic weakening is critically involved.

A large body of data from human and animal studies using psychological, recording, imaging, and lesion techniques indicates that recognition memory involves at least two separable processes: familiarity discrimination and recollection. Familiarity discrimination for individual visual stimuli seems to be effected by a system centred on the perirhinal cortex of the temporal lobe. The fundamental change that encodes prior occurrence within the perirhinal cortex is a reduction in the responses of neurones when a stimulus is repeated. Neuronal network modelling indicates that a system based on such a change in responsiveness is potentially highly efficient in information theoretic terms. A review is given of findings indicating that perirhinal cortex acts as a storage site for recognition memory of objects and that such storage depends upon processes producing synaptic weakening.

New hope from an old drug: fighting Alzheimer's disease with the cancer drug bexarotene (targretin)?

Despite decades of research, there is no cure for Alzheimer disease (AD), and current pharmacological treatments only partially mask the symptoms while the disease progresses within the brain. AD is associated with impaired clearance of β-amyloid (Aβ) from the brain, a process facilitated by apolipoprotein E (ApoE), whose expression is transcriptionally regulated by the ligand-activated nuclear receptors peroxisome proliferator-activated receptor-γ (PPARγ) and liver X receptor (LXR), in conjunction with retinoid X receptor (RXR). A very interesting study performed by G.E. Landreth's group in three murine models of AD has shown that the RXR agonist bexarotene (Targretin), Food and Drug Administration (FDA) approved and used since 1999 for the treatment of cutaneous T cell lymphoma, promotes a fast ApoE-dependent clearance of soluble Aβ peptides from the brain, reduces Aβ plaques, and stimulates the reversal of cognitive, social, and olfactory deficits. Four independent studies tried to replicate these observations; the clearance of soluble Aβ peptides and the reversal of cognitive deficits were replicated in two studies, but all of the studies failed to replicate the reduction of Aβ plaques. In a second report, G.E. Landreth's group formulates some hypotheses to explain these discrepancies. Although observations in mouse models of AD might not necessarily extrapolate to humans, bexarotene is a very interesting potential drug against AD; phase I and II clinical trials are under way.

Regular exercise prevents non-cognitive disturbances in a rat model of Alzheimer's disease

Previously, we reported that in a rat model of sporadic Alzheimer's disease (AD) generated by exogenous administration of Aβ₁₋₄₂ (250 pmol/d for 2 wk) via mini-osmotic pump, the animals exhibited learning and memory impairment, which could be attributed to the deleterious alterations in the levels of cognition-related signalling molecules. We showed that 4 wk of treadmill exercise totally prevented these impairments. Here, we evaluated the effect of exercise on non-cognitive function and basal synaptic transmission in the Cornu Ammonis 1 (CA1) area using the same AD model. Our results indicated that the anxiety behaviour of Aβ-treated rats was prevented by 4 wk of treadmill exercise. Exercised/Aβ-infused rats spent a longer time in the centre area of the open field (OF), elevated plus maze (EPM) paradigms and the light area of the light-dark (LD) box, which were similar to those of control and exercise rats. Furthermore, under basal conditions the aberrant up-regulation of calcineurin (PP2B) and reduction of phosphorylated Ca²⁺/calmodulin dependent protein kinase II (p-CaMKII) levels induced by AD-like pathology were normalised by the exercise regimen. We conclude that regular exercise may exert beneficial effects on both cognitive and non-cognitive functions in this AD model.

Dysfunctional synapse in Alzheimer's disease - A focus on NMDA receptors

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly. Alterations capable of causing brain circuitry dysfunctions in AD may take several years to develop. Oligomeric amyloid-beta peptide (Aβ) plays a complex role in the molecular events that lead to progressive loss of function and eventually to neurodegeneration in this devastating disease. Moreover, N-methyl-D-aspartate (NMDA) receptors (NMDARs) activation has been recently implicated in AD-related synaptic dysfunction. Thus, in this review we focus on glutamatergic neurotransmission impairment and the changes in NMDAR regulation in AD, following the description on the role and location of NMDARs at pre- and post-synaptic sites under physiological conditions. In addition, considering that there is currently no effective ways to cure AD or stop its progression, we further discuss the relevance of NMDARs antagonists to prevent AD symptomatology. This review posits additional information on the role played by Aβ in AD and the importance of targeting the tripartite glutamatergic synapse in early asymptomatic and possible reversible stages of the disease through preventive and/or disease-modifying therapeutic strategies. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Impairments of synaptic plasticity in aged animals and in animal models of Alzheimer's disease

Aging is associated with a gradual decline in cognitive functions, and more dramatic cognitive impairments occur in patients affected by Alzheimer's disease (AD). Electrophysiological and molecular studies performed in aged animals and in animal models of AD have shown that cognitive decline is associated with significant modifications in synaptic plasticity (i.e., activity-dependent changes in synaptic strength) and have elucidated some of the cellular mechanisms underlying this process. Morphological studies have revealed a correlation between the quality of memory performance and the extent of structural changes of synaptic contacts occurring during memory consolidation. We briefly review recent experimental evidence here.