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Individuals who have Down syndrome (DS) frequently develop early onset Alzheimer's disease (AD), a neurodegenerative condition caused by the buildup of aggregated amyloid-β (Aβ) and tau proteins in the brain. Aβ is produced by amyloid precursor protein (APP), a gene located on chromosome 21. People who have DS have three copies of chromosome 21 and thus also an additional copy of APP; this genetic change drives the early development of AD in these individuals. Here we use a combination of next-generation mouse models of DS (Tc1, Dp3Tyb, Dp(10)2Yey and Dp(17)3Yey) and a knockin mouse model of Aβ accumulation (AppNL-F>) to determine how chromosome 21 genes, other than APP, modulate APP/Aβ in the brain when in three copies. Using both male and female mice, we demonstrate that three copies of other chromosome 21 genes are sufficient to partially ameliorate Aβ accumulation in the brain. We go on to identify a subregion of chromosome 21 that contains the gene(s) causing this decrease in Ab accumulation and investigate the role of two lead candidate genes, Dyrk1a and Bace2. Thus, an additional copy of chromosome 21 genes, other than APP, can modulate APP/Aβ in the brain under physiological conditions. This work provides critical mechanistic insight into the development of disease and an explanation for the typically later age of onset of dementia in people who have AD in DS, compared with those who have familial AD caused by triplication of APP. [ABSTRACT FROM AUTHOR]

Impairment of episodic memory, a class of memory for spatiotemporal context of an event, is an early symptom of Alzheimer's disease. Both spatial and temporal information are encoded and represented in the hippocampal neurons, but how these representations are impaired under amyloid β (Aβ) pathology remains elusive. We performed chronic imaging of the hippocampus in awake male amyloid precursor protein (App) knock-in mice behaving in a virtual reality environment to simultaneously monitor spatiotemporal representations and the progression of Aβ depositions. We found that temporal representation is preserved, whereas spatial representation is significantly impaired in the App knock-in mice. This is because of the overall reduction of active place cells, but not time cells, and compensatory hyperactivation of remaining place cells near Aβ aggregates. These results indicate the differential impact of Aβ aggregates on two major modalities of episodic memory, suggesting different mechanisms for forming and maintaining these two representations in the hippocampus. [ABSTRACT FROM AUTHOR]

Intraneuronal calcium stimulates the calpain-dependent conversion of p35 to p25, a CDK5 activator. It is widely believed that amyloid β peptide (Aβ) induces this conversion that, in turn, has an essential role in Alzheimer's disease pathogenesis. However, in vivo studies on p25 generation used transgenic mice overexpressing mutant amyloid precursor protein (APP) and presenilin (PS). Here, using single App knock-in mice, we show that p25 generation is an artifact caused by membrane protein overexpression. We show that massive Aj342 accumulation without overexpression of APP or presenilin does not produce p25, whereas p25 generation occurred with APP/PS overexpression and in postmortem mouse brain. We further support this finding using mice deficient for calpastatin, the sole calpain-specific inhibitor protein. Thus, the intracerebral environment of the APP/PS mouse brain and postmortem brain is an unphysiological state. [ABSTRACT FROM AUTHOR]