Linda Younkin

Age-Dependent Changes in Brain, CSF, and Plasma Amyloid β Protein in the Tg2576 Transgenic Mouse Model of Alzheimer's Disease

The accumulation of amyloid β protein (Aβ) in the Tg2576 mouse model of Alzheimers disease (AD) was evaluated by ELISA, immunoblotting, and immunocytochemistry. Changes in Aβ begin at 6–7 months as SDS-insoluble forms of Aβ42 and Aβ40 that require formic acid for solubilization appear. From 6 to 10 months, these insoluble forms increase exponentially. As insoluble Aβ appears, SDS-soluble Aβ decreases slightly, suggesting that it may be converting to an insoluble form. Our data indicate that it is full-length unmodified Aβ that accumulates initially in Tg2576 brain. SDS-resistant Aβ oligomers and most Aβ species that are N-terminally truncated or modified develop only in older Tg2576 mice, in which they are present at levels far lower than in human AD brain. Between 6 and 10 months, when SDS-insoluble Aβ42 and Aβ40 are easily detected in every animal, histopathology is minimal because only isolated Aβ cores can be identified. By 12 months, diffuse plaques are evident. From 12 to 23 months, diffuse plaques, neuritic plaques with amyloid cores, and biochemically extracted Aβ42 and Aβ40 increase to levels like those observed in AD brains. Coincident with the marked deposition of Aβ in brain, there is a decrease in CSF Aβ and a substantial, highly significant decrease in plasma Aβ. If a similar decline occurs in human plasma, it is possible that measurement of plasma Aβ may be useful as a premorbid biomarker for AD.

Impaired synaptic plasticity and learning in aged amyloid precursor protein transgenic mice

We investigated synaptic communication and plasticity in hippocampal slices from mice overexpressing mutated 695-amino-acid human amyloid precursor protein (APP695SWE), which show behavioral and histopathological abnormalities simulating Alzheimers disease. Although aged APP transgenic mice exhibit normal fast synaptic transmission and short term plasticity, they are severely impaired in in-vitro and in-vivo long-term potentiation (LTP) in both the CA1 and dentate gyrus regions of the hippocampus. The LTP deficit was correlated with impaired performance in a spatial working memory task in aged transgenics. These deficits are accompanied by minimal or no loss of presynaptic or postsynaptic elementary structural elements in the hippocampus, suggesting that impairments in functional synaptic plasticity may underlie some of the cognitive deficits in these mice and, possibly, in Alzheimers patients.

The Relationship between Aβ and Memory in the Tg2576 Mouse Model of Alzheimer's Disease

Transgenic mice expressing mutant amyloid precursor proteins (APPs) have provided important new information about the pathogenesis of Alzheimers disease (AD) histopathology. However, the molecular basis of memory loss in these mice is poorly understood. One of the major impediments has been the difficulty of distinguishing between age-dependent and age-independent behavioral changes. To address this issue we studied in parallel two lines of APP transgenic mice expressing comparable levels of mutant and wild-type human APP. This enabled us to identify age-independent behavioral deficits that were not specifically related to mutant APP expression. When mice with age-independent deficits were eliminated, we detected memory loss in transgenic mice expressing mutant APP (Tg2576 mice) starting at ∼6 months, which coincided with the appearance of detergent-insoluble Aβ aggregates (Aβinsol). Genetically accelerating the formation of Aβinsol resulted in an earlier onset of memory decline. A facile interpretation of these results, namely that memory loss and Aβinsol were closely connected, was rejected when we extended our analysis to include older mice. No obvious correspondence between memory and Aβinsol was apparent in a combined group of old and young mice unless the mice were stratified by age, whereupon inverse correlations between memory and Aβinsol became evident. These results suggested that Aβinsol is a surrogate marker for small assemblies of Aβ that disrupt cognition and occur as intermediates during Aβinsol formation, and they are the first descriptive in vivo data supporting their role in impairing memory. These studies also provide a methodological framework within which to investigate these Aβ assemblies in vivo.

BACE1 Deficiency Rescues Memory Deficits and Cholinergic Dysfunction in a Mouse Model of Alzheimer's Disease

beta-site APP cleaving enzyme 1 (BACE1) is the beta-secretase enzyme required for generating pathogenic beta-amyloid (Abeta) peptides in Alzheimers disease (AD). BACE1 knockout mice lack Abeta and are phenotypically normal, suggesting that therapeutic inhibition of BACE1 may be free of mechanism-based side effects. However, direct evidence that BACE1 inhibition would improve cognition is lacking. Here we show that BACE1 null mice engineered to overexpress human APP (BACE1(-/-).Tg2576(+)) are rescued from Abeta-dependent hippocampal memory deficits. Moreover, impaired hippocampal cholinergic regulation of neuronal excitability found in the Tg2576 AD model is ameliorated in BACE1(-/-).Tg2576(+) bigenic mice. The behavioral and electrophysiological rescue of deficits in BACE1(-/-).Tg2576(+) mice is correlated with a dramatic reduction of cerebral Abeta40 and Abeta42 levels and occurs before amyloid deposition in Tg2576 mice. Our gene-based approach demonstrates that lower Abeta levels are beneficial for AD-associated memory impairments, validating BACE1 as a therapeutic target for AD.

Environmental Enrichment Mitigates Cognitive Deficits in a Mouse Model of Alzheimer's Disease

Epidemiological studies suggest that individuals with greater education or more cognitively demanding occupations have diminished risk of developing dementia. We wanted to test whether this effect could be recapitulated in rodents using environmental enrichment, a paradigm well documented to attenuate behavioral deficits induced by various pathological insults. Here, we demonstrate that learning and memory deficits observed in a transgenic mouse model of Alzheimers disease can be ameliorated by enrichment. Female transgenic mice overexpressing amyloid precursor protein and/or presenilin-1 and nontransgenic controls were placed into enriched or standard cages at 2 months of age and tested for cognitive behavior after 6 months of differential housing. Enrichment significantly improved performance of all genotypes in the radial water maze and in the classic and repeated-reversal versions of the Morris water maze. However, enrichment did not benefit all genotypes equally. Mice overproducing amyloid-β (Aβ), particularly those with amyloid deposits, showed weaker memory for the platform location in the classic Morris water maze and learned new platform positions in the repeated-reversals task less quickly than their nontransgenic cagemates. Nonetheless, enrichment normalized the performance of Aβ-overproducing mice to the level of standard-housed nontransgenic mice. Moreover, this functional preservation occurred despite increased neuritic plaque burden in the hippocampus of double-transgenic animals and elevated steady-state Aβ levels, because both endogenous and transgene-derived Aβ are increased in enriched animals. These results demonstrate that the generation of Aβ in vivo and its impact on the function of the nervous system can be strongly modulated by environmental factors.

18-Month study of intravenous immunoglobulin for treatment of mild Alzheimer disease.

Intravenous immunoglobulin (IVIg) has been proposed as a potential agent for Alzheimers disease (AD) immunotherapy because it contains antibodies against beta-amyloid (Abeta). We carried out an open label dose-ranging study in 8 mild AD patients in which IVIg was added to approved AD therapies for 6 months, discontinued, and then resumed for another 9 months. Infusions were generally well-tolerated. Anti-Abeta antibodies in the serum from AD patients increased in proportion to IVIg dose and had a shorter half-life than anti-hepatitis antibodies and total IgG. Plasma Abeta levels increased transiently after each infusion. Cerebrospinal fluid Abeta decreased significantly at 6 months, returned to baseline after washout and decreased again after IVIg was re-administered for an additional 9 months. Mini-mental state scores increased an average of 2.5 points after 6 months, returned to baseline during washout and remained stable during subsequent IVIg treatment. Our findings confirm and extend those obtained by Dodel et al. [Dodel, R.C., Du, Y., Depboylu, C., Hampel, H., Frolich, L., Haag, A., Hemmeter, U., Paulsen, S., Teipel, S.J., Brettschneider, S., Spottke, A., Nolker, C., Moller, H.J., Wei, X., Farlow, M., Sommer, N., Oertel, W.H., 2004. Intravenous immunoglobulins containing antibodies against beta-amyloid for the treatment of Alzheimers disease. J. Neurol. Neurosurg. Psychiatry 75, 1472-1474] from a 6-month trial of IVIg in 5 AD patients and justify further studies of IVIg for treatment of AD.

Dimeric Amyloid β Protein Rapidly Accumulates in Lipid Rafts followed by Apolipoprotein E and Phosphorylated Tau Accumulation in the Tg2576 Mouse Model of Alzheimer's Disease

To investigate lipid rafts as a site where amyloid β protein (Aβ) oligomers might accumulate and cause toxicity in Alzheimers disease (AD), we analyzed Aβ in the Tg2576 transgenic mouse model of AD. Aβ was highly concentrated in lipid rafts, which comprise a small fraction of brain volume but contain 27% of brain Aβ42 and 24% of Aβ40 in young mice. In the Tg2576 model, memory impairment begins at 6 months before amyloid plaques are visible. Here we show that Aβ dimers appear in lipid rafts at 6 months and that raft Aβ, which is primarily dimeric, rapidly accumulates reaching levels >500× those in young mice by 24–28 months. A similar large accumulation of dimeric Aβ was observed in lipid rafts from AD brain. In contrast to extracellular amyloid fibrils, which are SDS-insoluble, virtually all Aβ in lipid rafts is SDS soluble. Coupled with recent studies showing that synthetic and naturally occurring Aβ oligomers can inhibit hippocampal long-term potentiation, the in vivo age-dependent accumulation of SDS-soluble Aβ dimers in lipid rafts at the time when memory impairment begins in Tg2576 mice provides strong evidence linking Aβ oligomers to memory impairment. After dimeric Aβ began to accumulate in lipid rafts of the Tg2576 brain, apolipoprotein E (ApoE) and then phosphorylated tau accumulated. A similar increase in ApoE and a large increase in phosphorylated tau was observed in lipid rafts from AD brain. These findings suggest that lipid rafts may be an important site for interaction between dimeric Aβ, ApoE, and tau.

GENETIC VARIATION IN PCDH11X IS ASSOCIATED WITH SUSCEPTIBILITY TO LATE ONSET ALZHEIMER'S DISEASE

By analyzing late-onset Alzheimers disease (LOAD) in a genome-wide association study (313,504 SNPs, three series, 844 cases and 1,255 controls) and evaluating the 25 SNPs with the most significant allelic association in four additional series (1,547 cases and 1,209 controls), we identified a SNP (rs5984894) on Xq21.3 in PCDH11X that is strongly associated with LOAD in individuals of European descent from the United States. Analysis of rs5984894 by multivariable logistic regression adjusted for sex gave global P values of 5.7 × 10−5 in stage 1, 4.8 × 10−6 in stage 2 and 3.9 × 10−12 in the combined data. Odds ratios were 1.75 (95% CI = 1.42–2.16) for female homozygotes (P = 2.0 × 10−7) and 1.26 (95% CI = 1.05–1.51) for female heterozygotes (P = 0.01) compared to female noncarriers. For male hemizygotes (P = 0.07) compared to male noncarriers, the odds ratio was 1.18 (95% CI = 0.99–1.41).

Nox2-derived radicals contribute to neurovascular and behavioral dysfunction in mice overexpressing the amyloid precursor protein

Alterations in cerebrovascular regulation related to vascular oxidative stress have been implicated in the mechanisms of Alzheimers disease (AD), but their role in the amyloid deposition and cognitive impairment associated with AD remains unclear. We used mice overexpressing the Swedish mutation of the amyloid precursor protein (Tg2576) as a model of AD to examine the role of reactive oxygen species produced by NADPH oxidase in the cerebrovascular alterations, amyloid deposition, and behavioral deficits observed in these mice. We found that 12- to 15-month-old Tg2576 mice lacking the catalytic subunit Nox2 of NADPH oxidase do not develop oxidative stress, cerebrovascular dysfunction, or behavioral deficits. These improvements occurred without reductions in brain amyloid-β peptide (Aβ) levels or amyloid plaques. The findings unveil a previously unrecognized role of Nox2-derived radicals in the behavioral deficits of Tg2576 mice and provide a link between the neurovascular dysfunction and cognitive decline associated with amyloid pathology.

APP Processing and Synaptic Plasticity in Presenilin-1 Conditional Knockout Mice

We have developed a presenilin-1 (PS1) conditional knockout mouse (cKO), in which PS1 inactivation is restricted to the postnatal forebrain. The PS1 cKO mouse is viable and exhibits no gross abnormalities. The carboxy-terminal fragments of the amyloid precursor protein differentially accumulate in the cerebral cortex of cKO mice, while generation of beta-amyloid peptides is reduced. Expression of Notch downstream effector genes, Hes1, Hes5, and Dll1, is unaffected in the cKO cortex. Although basal synaptic transmission, long-term potentiation, and long-term depression at hippocampal area CA1 synapses are normal, the PS1 cKO mice exhibit subtle but significant deficits in long-term spatial memory. These results demonstrate that inactivation of PS1 function in the adult cerebral cortex leads to reduced Abeta generation and subtle cognitive deficits without affecting expression of Notch downstream genes.