Mid‐life environmental enrichment increases synaptic density in CA1 in a mouse model of Aβ‐associated pathology and positively influences synaptic and cognitive health in healthy ageing

Early‐life cognitive enrichment may reduce the risk of experiencing cognitive deterioration and dementia in later‐life. However, an intervention to prevent or delay dementia is likely to be taken up in mid to later‐life. Hence, we investigated the effects of environmental enrichment in wildtype mice and in a mouse model of Aβ neuropathology (APP_SWE/PS1_dE9) from 6 months of age. After 6 months of housing in standard laboratory cages, APP_SWE/PS1_dE9 (n = 27) and healthy wildtype (n = 21) mice were randomly assigned to either enriched or standard housing. At 12 months of age, wildtype mice showed altered synaptic protein levels and relatively superior cognitive performance afforded by environmental enrichment. Environmental enrichment was not associated with alterations to Aβ plaque pathology in the neocortex or hippocampus of APP_SWE/PS1_dE9 mice. However, a significant increase in synaptophysin immunolabeled puncta in the hippocampal subregion, CA1, in APP_SWE/PS1_dE9 mice was detected, with no significant synaptic density changes observed in CA3, or the Fr2 region of the prefrontal cortex. Moreover, a significant increase in hippocampal BDNF was detected in APP_SWE/PS1_dE9 mice exposed to EE, however, no changes were detected in neocortex or between Wt animals. These results demonstrate that mid to later‐life cognitive enrichment has the potential to promote synaptic and cognitive health in ageing, and to enhance compensatory capacity for synaptic connectivity in pathological ageing associated with Aβ deposition.     

Reduction of hippocampal N‐acetyl aspartate level in aged APPSwe/PS1dE9 transgenic mice is associated with degeneration of CA3 pyramidal neurons

Age‐related metabolic changes in the hippocampus of APP_Swe/PS1_dE9 mice were measured with long echo‐time in vivo ¹H‐magnetic resonance spectroscopy (¹H‐MRS). Thioflavine S staining and Nissl staining were used to characterize deposition of Aβ aggregates and neuronal degeneration in the transgenic animals, respectively. The results showed that the APP_Swe/PS1_dE9 mice had significantly decreased hippocampal N‐acetyl aspartate (NAA)/total creatine (tCr) level at 16 months of age, which was associated with degeneration of and intracellular deposition of thioflavine S‐positive materials in hippocampal CA3 pyramidal neurons. The results of this study provide direct evidence showing association among Aβ pathology (intracellular deposition of thioflavine S‐positive materials), neuronal degeneration, and metabolic changes observable with in vivo ¹H‐MRS in the hippocampus of APP_Swe/PS1_dE9 mice. © 2010 Wiley‐Liss, Inc.     

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.     

Essential role of hippocampal noradrenaline in the regulation of spatial working memory and TDP‐43 tissue pathology

Extensive loss of noradrenaline‐containing neurons and fibers is a nearly invariant feature of Alzheimer's Disease (AD). However, the exact noradrenergic contribution to cognitive and histopathological changes in AD is still unclear. Here, this issue was addressed following selective lesioning and intrahippocampal implantation of embryonic noradrenergic progenitors in developing rats. Starting from about 3 months and up to 12 months post‐surgery, animals underwent behavioral tests to evaluate sensory‐motor, as well as spatial learning and memory, followed by post‐mortem morphometric analyses. At 9 months, Control, Lesioned and Lesion + Transplant animals exhibited equally efficient sensory‐motor and reference memory performance. Interestingly, working memory abilities were seen severely impaired in Lesion‐only rats and fully recovered in Transplanted rats, and appeared partly lost again 2 months after ablation of the implanted neuroblasts. Morphological analyses confirmed the almost total lesion‐induced noradrenergic neuronal and terminal fiber loss, the near‐normal reinnervation of the hippocampus promoted by the transplants, and its complete removal by the second lesion. Notably, the noradrenergic‐rich transplants normalized also the nuclear expression of the transactive response DNA‐binding protein 43 (TDP‐43) in various hippocampal subregions, whose cytoplasmic (i.e., pathological) occurrence appeared dramatically increased as a result of the lesions. Thus, integrity of ascending noradrenergic inputs to the hippocampus may be required for the regulation of specific aspects of learning and memory and to prevent TDP‐43 tissue pathology.     

Amyloid β‐protein fragments 25–35 and 31–35 potentiate long‐term depression in hippocampal CA1 region of rats in vivo

Amyloid β‐protein (Aβ) is thought to be responsible for the deficit of learning and memory in Alzheimer's disease (AD), possibly through interfering with synaptic plasticity in the brain. It has been reported that Aβ fragments suppress the long‐term potentiation (LTP) of synaptic transmission. However, it is unclear whether Aβ fragments can regulate long‐term depression (LTD), an equally important form of synaptic plasticity in the brain. The present study investigates the effects of Aβ fragments on LTD induced by low frequency stimulation (LFS) in the hippocampus in vivo. Our results showed that (1) prolonged 1–10 Hz of LFS all effectively elicited LTD, which could persist for at least 2 h and be reversed by high frequency stimulation (HFS); (2) the effectiveness of LTD induction depended mainly on the number of pulses but not the frequency of LFS; (3) pretreatment with Aβ fragment 25–35 (Aβ_25–35, 12.5 and 25 nmol) did not change baseline field excitatory postsynaptic potentials but dose‐dependently potentiated LTD; (4) Aβ fragment 31–35 (Aβ_31–35), a shorter Aβ fragment than Aβ_25–35, also dose‐dependently strengthened LFS‐induced hippocampal LTD. Thus, the present study demonstrates the enhancement of hippocampal LTD by Aβ in in vivo condition. We propose that Aβ‐induced potentiation of LTD, together with the suppression of LTP, will result in the impairment of cognitive function of the brain. Synapse 63:206–214, 2009. © 2008 Wiley‐Liss, Inc.     

Selenium positively affects the proteome of 3 × Tg‐AD mice cortex by altering the expression of various key proteins: unveiling the mechanistic role of selenium in AD prevention

Selenium (Se) deficiency is believed to be involved in pathogenesis of Alzheimer's disease (AD) due to failure of antioxidant system. Its supplementation may restore the antioxidant system and compensate the impairments caused by AD. Present study reveals the effect of Se on the proteomic changes in cortex within triple transgenic male AD mice (3 × Tg‐AD) after 4 months sodium selenate supplementation. Using iTRAQ comparative proteomics approach, 142 proteins found significant alterations with 96 down‐regulated and 46 up‐regulated proteins in the cortices of AD mice in comparison with the wild non‐transgenic type mice. On treatment with sodium selenate, 41 proteins showed reverse expression, that is, thirty three proteins were down‐regulated in AD mice but up‐regulated in selenate treated AD mice while eight up‐regulated proteins in AD mice showed lower expression in selenate treated mice. OmicsBean bioinformatics analysis revealed that Se positively affected the proteins vital in biological process, structural cores, and molecular functions, which include metabolic proteins, structural proteins, signaling molecules, oxidative stress balancers, and proteosomal degradation proteins. Results of mass spectrometry (MS) were further confirmed by Western blot analysis of five important proteins, prompting the authenticity of the MS results. This paper fills the protein‐based molecular gap between AD and Se‐treatment, and it provides a full view of Se in reversing the change of cortical protein levels during AD formation.     

Inhibition of PDE2A,but not PDE9A,modulates presynaptic short‐term plasticity measured by paired‐pulse facilitation in the CA1 region of the hippocampus

Phosphodiesterase (PDE) inhibitors are currently considered promising therapeutic targets for treatment of cognitive impairment in diseases such as Schizophrenia and Alzheimer's disease. Inhibitors of PDE2A and PDE9A have emerged as potential candidates shown to improve synaptic plasticity and memory function in animals. However, the functional relevance of their putative different localization in the neuron is not understood. Thus, this study aims at elucidating potential presynaptic effects of PDE2A inhibition in comparison to the inhibition of PDE9A. For this purpose, we used paired‐pulse facilitation (PPF), a model of short‐term synaptic plasticity related to presynaptic function. First, we performed a series of experiments to validate the model in acute rat hippocampal slices using several reference substances including calcium channel blockers, glutamatergic receptor antagonists, and GPCR agonists. Second, we analysed the effect of PDE2A and PDE9A inhibition and their role regulating the influence that the second messengers cAMP and cGMP exert on basal transmission. Our results show that the interplay between the adenylyl cyclase activator forskolin, the soluble guanylyl cyclase activator BAY 41‐8543 and the PDE2A inhibitor PF‐999 reveals a primarily presynaptic mechanism of action of PDE2A inhibition. On the contrary, inhibition of PDE9A did not alter PPF under similar conditions. In conclusion, these data provide new evidence supporting a role of PDE2A modulating short‐term synaptic plasticity. Moreover, this function of PDE2A is suggested to rely on an active modulation of the cAMP hydrolysis as a response to changes in cGMP levels at the presynaptic level. Synapse 69:484–496, 2015 . © 2015 Wiley Periodicals, Inc.     

Frequency and distribution of TUNEL‐positive neurons in brains of dementia with Lewy bodies: Comparison with those in brains of Alzheimer's disease

The present study investigated the frequency and distribution of TUNEL‐positive neurons in brains of dementia with Lewy bodies (DLB) in comparison with those in brains of Alzheimer's disease (AD), Down syndrome (DS) and non‐demented elderly persons. In DLB brains, TUNEL‐positive neurons were increased in frequency compared with those in non‐demented elderly brains, and showed a distribution similar to those in AD and DS brains. DLB cases with TUNEL‐positive neurons showing severe Lewy pathology were all neocortical type, while DLB cases of the limbic type showing mild Lewy pathology did not demonstrate TUNEL‐positive neurons. In addition, we investigated the relationships between TUNEL‐positive neurons and pathological hallmarks of DLB or AD brains. TUNEL‐positive neurons had no Lewy bodies or neurofibrillary tangles, and were not located within amyloid deposits. These findings suggest that neuronal damage showing DNA fragmentations occurs in DLB brains as well as in AD and DS brains, and that it is accelerated by progression of Lewy pathology as well as Alzheimer pathology, although it is not directly related to their pathological hallmarks.     

Increased levels of NMDA receptor NR2A subunits at pre‐ and postsynaptic sites of the hippocampal CA1: An early response to conditional double knockout of presenilin 1 and 2

Greater than 90% of familial Alzheimer's disease (AD) is linked to mutations of presenilin (PS), and the loss of PS function altogether within mouse brains by conditional double knockout of the PS 1 and 2 genes (PS‐cDKO) leads to age‐dependent emergence of AD phenotypes, including neurodegeneration and reduced synaptic plasticity in the hippocampal CA1. The goal of our study was to identify the ultrastructural and molecular changes at synapses in the hippocampal CA1 of this PS‐cDKO mouse model of AD. We examined the asymmetric (excitatory) synapses formed on apical dendrites of CA1 pyramidal neurons at 2 months postnatal, an age when AD‐like symptoms emerge but brain morphology, as assessed by light microscopy, is still normal. Our quantitative electron microscopic analyses confirm that PS‐cDKO hippocampi at 2 months postnatal do not yet exhibit synapse losses or spine size alterations. However, immunocytochemistry reveals that the same region exhibits a 28% increase in the proportion of spines labeled for the NR2A subunits of NMDA receptors (NMDAR), with a 31% increase specifically at postsynaptic densities and a concomitant reduction of these subunits at nonsynaptic sites within spine heads. In contrast, no change in levels or the distribution pattern of NR2B subunit levels were detected within spine heads. Presynaptically, NR2A levels are elevated at axo‐spinous junctions and these may contribute to the timing‐dependent, long‐term depression. These observations point to an early‐onset trapping of NMDAR at synapses that are subtle but may underlie the reduced synaptic plasticity at 2 months of age and excitotoxicity at later stages. J. Comp. Neurol. 517:512–523, 2009. © 2009 Wiley‐Liss, Inc.