Age-dependent Memory Impairment Research Articles

Altered Histone Acetylation Is Associated with Age-Dependent Memory Impairment in Mice

As the human life span increases, the number of people suffering from cognitive decline is rising dramatically. The mechanisms underlying age-associated memory impairment are, however, not understood. Here we show that memory disturbances in the aging brain of the mouse are associated with altered hippocampal chromatin plasticity. During learning, aged mice display a specific deregulation of histone H4 lysine 12 (H4K12) acetylation and fail to initiate a hippocampal gene expression program associated with memory consolidation. Restoration of physiological H4K12 acetylation reinstates the expression of learning-induced genes and leads to the recovery of cognitive abilities. Our data suggest that deregulated H4K12 acetylation may represent an early biomarker of an impaired genome-environment interaction in the aging mouse brain.

Spatial Learning Impairment, Enhanced CDK5/p35 Activity, and Downregulation of NMDA Receptor Expression in Transgenic Mice Expressing Tau-Tubulin Kinase 1

Tau-tubulin kinase-1 (TTBK1) is involved in phosphorylation of tau protein at specific Serine/Threonine residues found in paired helical filaments, suggesting its role in tauopathy pathogenesis. We found that TTBK1 levels were upregulated in brains of human Alzheimer' disease (AD) patients compared with age-matched non-AD controls. To understand the effects of TTBK1 activation in vivo, we developed transgenic mice harboring human full-length TTBK1 genomic DNA (TTBK1-Tg). Transgenic TTBK1 is highly expressed in subiculum and cortical pyramidal layers, and induces phosphorylated neurofilament aggregation. TTBK1-Tg mice show significant age-dependent memory impairment as determined by radial arm water maze test, which is associated with enhancement of tau and neurofilament phosphorylation, increased levels of p25 and p35, both activators of cyclin-dependent protein kinase 5 (CDK5), enhanced calpain I activity, and reduced levels of hippocampal NMDA receptor types 2B (NR2B) and D. Enhanced CDK5/p35 complex formation is strongly correlated with dissociation of F-actin from p35, suggesting the inhibitory mechanism of CDK5/p35 complex formation by F-actin. Expression of recombinant TTBK1 in primary mouse cortical neurons significantly downregulated NR2B in a CDK5- and calpain-dependent manner. These data suggest that TTBK1 in AD brain may be one of the underlying mechanisms inducing CDK5 and calpain activation, NR2B downregulation, and subsequent memory dysfunction.

Reverse of Age-Dependent Memory Impairment and Mitochondrial DNA Damage in Microglia by an Overexpression of Human Mitochondrial Transcription Factor A in Mice

Mitochondrial DNA (mtDNA) is highly susceptible to injury induced by reactive oxygen species (ROS). During aging, mutations of mtDNA accumulate to induce dysfunction of the respiratory chain, resulting in the enhanced ROS production. Therefore, age-dependent memory impairment may result from oxidative stress derived from the respiratory chain. Mitochondrial transcription factor A (TFAM) is now known to have roles not only in the replication of mtDNA but also its maintenance. We herein report that an overexpression of TFAM in HeLa cells significantly inhibited rotenone-induced mitochondrial ROS generation and the subsequent NF-kappaB (nuclear factor-kappaB) nuclear translocation. Furthermore, TFAM transgenic (TG) mice exhibited a prominent amelioration of an age-dependent accumulation of lipid peroxidation products and a decline in the activities of complexes I and IV in the brain. In the aged TG mice, deficits of the motor learning memory, the working memory, and the hippocampal long-term potentiation (LTP) were also significantly improved. The expression level of interleukin-1beta (IL-1beta) and mtDNA damages, which were predominantly found in microglia, significantly decreased in the aged TG mice. The IL-1beta amount markedly increased in the brain of the TG mice after treatment with lipopolysaccharide (LPS), whereas its mean amount was significantly lower than that of the LPS-treated aged wild-type mice. At the same time, an increased mtDNA damage in microglia and an impaired hippocampal LTP were also observed in the LPS-treated aged TG mice. Together, an overexpression of TFAM is therefore considered to ameliorate age-dependent impairment of the brain functions through the prevention of oxidative stress and mitochondrial dysfunctions in microglia.

Hyperphosphorylated tau in parahippocampal cortex impairs place learning in aged mice expressing wild-type human tau

To investigate how tau affects neuronal function during neurofibrillary tangle (NFT) formation, we examined the behavior, neural activity, and neuropathology of mice expressing wild-type human tau. Here, we demonstrate that aged (>20 months old) mice display impaired place learning and memory, even though they do not form NFTs or display neuronal loss. However, soluble hyperphosphorylated tau and synapse loss were found in the same regions. Mn-enhanced MRI showed that the activity of the parahippocampal area is strongly correlated with the decline of memory as assessed by the Morris water maze. Taken together, the accumulation of hyperphosphorylated tau and synapse loss in aged mice, leading to inhibition of neural activity in parahippocampal areas, including the entorhinal cortex, may underlie place learning impairment. Thus, the accumulation of hyperphosphorylated tau that occurs before NFT formation in entorhinal cortex may contribute to the memory problems seen in Alzheimer's disease (AD).

Synaptosomal glutamate release and uptake in mice lacking the cellular prion protein

Glutamate plays a central role in the fast excitatory synaptic transmission and is a key neurotransmitter involved in several neurophysiological processes. Glutamate levels on the synaptic cleft are related to neural excitability, neuroplasticity, and neuronal damage associated with excitotoxicity. Mice lacking the cellular prion protein (PrP c) gene (Prnp) present a decreased astrocytic glutamate uptake in cultures, higher neuronal excitability in vitro and sensitivity to pro-convulsant drugs in vivo, and age-dependent memory impairment. Here, we investigate if PrP c might be involved in neuronal uptake and release of glutamate. For this purpose, we compared synaptosomal preparations from the cerebral cortex, entorhinal cortex, hippocampus, cerebellum, and olfactory bulb of 3- or 9-month-old PrP c null mice and with respective wild-type controls. Although we observed differences in synaptosomal glutamate release and uptake regarding the age of mice and the brain structure studied, these differences were similar for PrP c null mice and their respective wild-type controls. Therefore, despite a possible correlation between neuronal glutamate transporters, excitability, and neuronal damage, our results suggest that PrP c expression is not critical for neuronal glutamate transport.

Age-dependent reduction of γ oscillations in the mouse hippocampus in vitro

Normal brain ageing is associated with a decline in hippocampal memory functions. Neuronal oscillations in the γ frequency band have been implicated in various cognitive tasks. In this study we test the effect of normal brain ageing on γ oscillations in the mouse hippocampus in vitro. γ Oscillations were evoked by either 10 μM carbachol or 100 nM kainate in ventral hippocampus slices from young (>5 month) and aged (>22 month) C57Bl/J6 mice. In slices from young mice carbachol-induced γ oscillations were more regular and more coherent than those induced by kainate. Compared with young, the power in the 20–80 Hz frequency range in area CA3 of slices from aged mice was reduced to 14% for kainate-induced oscillations and to 7% for carbachol-induced oscillations, whereas waveform, dominant frequency and coherence of the oscillation were unchanged. Local network properties were assessed by paired-pulse stimulation of Schaffer collateral/commissural fibers. The excitatory synaptic response in stratum radiatum of CA3 was reduced, in correlation with the antidromic population spike, but functional inhibition in CA3 and CA1 was unaffected. Changes in local network properties could not explain the reduced γ oscillation strength. Since oscillations driven by two different pathways are similarly affected with age, an age-dependent effect on tonic depolarizing drive of principal cells is unlikely to explain the current results. Other mechanisms, including a change with age in the use-dependent modulation of synaptic strength, should account for the impaired γ oscillations in the aged hippocampus that may contribute to age-dependent memory impairment.

Normal induction but accelerated decay of LTP in APP + PS1 transgenic mice

Mice carrying mutated human APPswe and PS1 (A246E) transgenes (A/P mice) show age-dependent memory impairment in hippocampus-dependent tasks. Moreover, the mice show normal learning in the water maze within a day but impairment across days. We recorded LTP in a slice preparation (CA1) and in chronically implanted animals (dentate gyrus, or DG) at 17–18 months of age. The genotypes did not differ in the basal synaptic transmission. Also, LTP induction and its maintenance over 60 min did not differ between A/P and control mice. However, the fEPSP enhancement in vivo decayed to 77% of its maximum in 24 h in A/P mice while remaining at 96% in control mice. The time course of the LTP decay in the A/P mice corresponds to their behavioral impairment and indicates that Aβ accumulation in the dentate gyrus may interfere with the signal transduction pathways responsible for memory consolidation.