Alzheimer's disease (AD) is a progressive neurodegenerative disorder for which numerous mouse models have been generated. In both AD patients and mouse models, there is increasing evidence that neuronal dysfunction occurs before the accumulation of ␤-amyloid (A␤)-containing plaques and neurodegeneration. Characterization of the timing and nature of preplaque dysfunction is important for understanding the progression of this disease and to identify pathways and molecular targets for therapeutic intervention. Hence, we have examined the progression of dysfunction at the morphological, functional, and behavioral levels in the Tg2576 mouse model of AD. Our data show that decreased dendritic spine density, impaired long-term potentiation (LTP), and behavioral deficits occurred months before plaque deposition, which was first detectable at 18 months of age. We detected a decrease in spine density in the outer molecular layer of the dentate gyrus (DG) beginning as early as 4 months of age. Furthermore, by 5 months, there was a decline in LTP in the DG after perforant path stimulation and impairment in contextual fear conditioning. Moreover, an increase in the A␤42͞A␤40 ratio was first observed at these early ages. However, total amyloid levels did not significantly increase until Ϸ18 months of age, at which time significant increases in reactive astrocytes and microglia could be observed. Overall, these data show that the perforant path input from the entorhinal cortex to the DG is compromised both structurally and functionally, and this pathology is manifested in memory defects long before significant plaque deposition. ␤-amyloid ͉ cognitionA lzheimer's disease (AD), a progressive neurodegenerative disease of the elderly, is the most common cause of dementia. Characteristic pathologies develop in the brain of AD patients, including senile plaques composed of ␤-amyloid (A␤), neurofibrillary tangles composed of intracellular hyperphosphorylated microtubule-associated protein tau, as well as dystrophic neurites, diminished synaptic densities, and the loss of neuronal function (1). The amyloid hypothesis suggests that accumulation of A␤ fragments 1-40 and 1-42 is primarily responsible for AD pathology, and that it is the imbalance of A␤ production and A␤ clearance that appears to give rise to neurofibrillary tangle formation and the cognitive impairments associated with AD (2, 3).To better understand disease progression, human amyloid precursor protein (APP) transgenic mouse lines expressing various mutations identified from patients with familial AD have been developed to model the effect of A␤ production and deposition on glial and neuronal structure and function and on cognitive performance (4-6). These models have become crucial to understanding the role of A␤ in AD pathology and for testing novel therapeutic strategies. To test the effects of candidate therapeutic treatments, it is necessary to recognize the type, extent, and onset of pathologies in each model. Variability across models largely reflects the backgroun...

Background The cerebrospinal fluid (CSF) biomarkers amyloid beta 1–42, total tau, and phosphorylated tau are used increasingly for Alzheimer’s disease (AD) research and patient management. However, there are large variations in biomarker measurements among and within laboratories. Methods Data from the first nine rounds of the Alzheimer’s Association quality control program was used to define the extent and sources of analytical variability. In each round, three CSF samples prepared at the Clinical Neurochemistry Laboratory (Mölndal, Sweden) were analyzed by single-analyte enzyme-linked immunosorbent assay (ELISA), a multiplexing xMAP assay, or an immunoassay with electrochemoluminescence detection. Results A total of 84 laboratories participated. Coefficients of variation (CVs) between laboratories were around 20% to 30%; within-run CVs, less than 5% to 10%; and longitudinal within-laboratory CVs, 5% to 19%. Interestingly, longitudinal within-laboratory CV differed between biomarkers at individual laboratories, suggesting that a component of it was assay dependent. Variability between kit lots and between laboratories both had a major influence on amyloid beta 1–42 measurements, but for total tau and phosphorylated tau, between-kit lot effects were much less than between-laboratory effects. Despite the measurement variability, the between-laboratory consistency in classification of samples (using prehoc-derived cutoffs for AD) was high (>90% in 15 of 18 samples for ELISA and in 12 of 18 samples for xMAP). Conclusions The overall variability remains too high to allow assignment of universal biomarker cutoff values for a specific intended use. Each laboratory must ensure longitudinal stability in its measurements and use internally qualified cutoff levels. Further standardization of laboratory procedures and improvement of kit performance will likely increase the usefulness of CSF AD biomarkers for researchers and clinicians.

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Transgenic mice (Tg2576) overexpressing the Swedish mutation of the human amyloid precursor protein display biochemical, pathological, and behavioral markers consistent with many aspects of Alzheimer's disease, including impaired hippocampal function. Impaired, hippocampal-dependent, contextual fear conditioning (CFC) is observed in mice as young as 20 weeks of age. This impairment can be attenuated after treatment before training with the phosphodiesterase-4 inhibitor rolipram (0.1 mg/kg, i.p.). A rolipram-associated improvement is also observed in the littermate controls, suggesting that the effect of rolipram is independent of ␤-amyloid. Acute treatment before training (but not after training or before testing) with the ␥-secretase inhibitor (GSI) N-[N-(3,5-difluorophenacetyl)-Lalanyl]-S-phenylglycine-t-butylester (DAPT), at a dose that reduces brain concentrations of ␤-amyloid (100 mg/kg), attenuates the impairment in 20-to 65-week-old Tg2576 mice. Importantly, DAPT had no effect on performance of control littermates. These data are supportive of a role of ␤-amyloid in the impairment of CFC in Tg2576 mice. Furthermore, they suggest that acute treatment with GSI may provide improved cognitive functioning as well as disease-modifying effects in Alzheimer's disease.

The presenilin containing ␥-secretase complex is responsible for the regulated intramembraneous proteolysis of the amyloid precursor protein (APP), the Notch receptor, and a multitude of other substrates. ␥-Secretase catalyzes the final step in the generation of A␤ 40 and A␤ 42 peptides from APP. Amyloid ␤-peptides (A␤ peptides) aggregate to form neurotoxic oligomers, senile plaques, and congophilic angiopathy, some of the cardinal pathologies associated with Alzheimer's disease. Although inhibition of this protease acting on APP may result in potentially therapeutic reductions of neurotoxic A␤ peptides, nonselective inhibition of the enzyme may cause severe adverse events as a result of impaired Notch receptor processing. Here, we report the preclinical pharmacological profile of GSI-953 (begacestat), a novel thiophene sulfonamide ␥-secretase inhibitor (GSI) that selectively inhibits cleavage of APP over Notch. This GSI inhibits A␤ production with low nanomolar potency in cellular and cell-free assays of ␥-secretase function, and displaces a tritiated analog of GSI-953 from enriched ␥-secretase enzyme complexes with similar potency. Cellular assays of Notch cleavage reveal that this compound is approximately 16-fold selective for the inhibition of APP cleavage. In the human APP-overexpressing Tg2576 transgenic mouse, treatment with this orally active compound results in a robust reduction in brain, plasma, and cerebral spinal fluid A␤ levels, and a reversal of contextual fear-conditioning deficits that are correlated with A␤ load. In healthy human volunteers, oral administration of a single dose of GSI-953 produces dosedependent changes in plasma A␤ levels, confirming pharmacodynamic activity of GSI-953 in humans.This research was supported by Wyeth Research. Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.