Transgenic expression of ß1 antibody in brain neurons impairs age-dependent amyloid deposition in APP23 mice.

Heterologous expression of the functional amyloid beta (Aß) antibody ß1 in the central nervous system was engineered to maximize antibody exposure in the brain and assess the effects on Aß production and accumulation in these conditions. A single open reading frame encoding the heavy and light chains of ß1 linked by the mouth and foot virus peptide 2A was expressed in brain neurons of transgenic mice. Two of the resulting BIN66 transgenic lines were crossed with APP23 mice, which develop severe central amyloidosis. Brain concentrations at steady-state 5 times greater than those found after peripheral ß1 administration were obtained. Similar brain and plasma ß1 concentrations indicated robust antibody efflux from the brain. In preplaque mice, ß1 formed a complex with Aß that caused a modest Aß increase in brain and plasma. At 11 months of age, ß1 expression reduced amyloid by 97% compared with age-matched APP23 mice. Interference of ß1 with ß-secretase cleavage of amyloid precursor protein was relatively small. Our data suggest that severely impaired amyloid formation was primarily mediated by a complex of ß1 with soluble Aß, which might have prevented Aß aggregation or favored transport out of the brain.

The Swedish APP mutation alters the effect of genetically reduced BACE1 expression on the APP processing.

Inhibition of ß-secretase (BACE1) is a key therapeutic approach in Alzheimer's disease (AD), as BACE1 initiates amyloid-ß (Aß) cleavage from the ß-amyloid precursor protein (APP). As Aß reductions in mice lacking one BACE1 allele diverged considerably between studies we investigated the effect of BACE1 knock-out in more detail. With both BACE1 alleles the Swedish mutation (APP23 mice) increased APP processing and shifted it towards the ß-secretase pathway as compared with non-mutated APP expressed at a similar level (APP51/16 mice). This effect was much smaller then observed in cell culture. An about 50% decrease in BACE1 enzyme activity resulted in a sub-proportional Aß reduction with the Swedish mutation (-20%) and even less for non-mutated APP (-16%). In wild-type mice, the Aß reduction may be even further diminished. Other metabolites of the ß-secretase pathway decreased accordingly while the alternative α-secretase pathway increased. Complete BACE1 deletion strongly enhanced these changes. The remaining Aß signal also described by others can be explained by assay cross-reactivity with other APP metabolites supporting BACE1 as the major ß-secretase. Our data indicate that BACE1 is in excess over APP at the cleavage site(s). Alterations in APP expression or substrate properties, therefore, quantitatively change its cleavage and Aß generation.

Dynamics of Abeta turnover and deposition in different beta-amyloid precursor protein transgenic mouse models following gamma-secretase inhibition.

Human beta-amyloid precursor protein (APP) transgenic mice are commonly used to test potential therapeutics for Alzheimer's disease. We have characterized the dynamics of beta-amyloid (Abeta) generation and deposition following gamma-secretase inhibition with compound LY-411575 [N(2)-[(2S)-2-(3,5-difluorophenyl)-2-hydroxyethanoyl]-N(1)-[(7S)-5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]azepin-7-yl]-L-alaninamide]. Kinetic studies in preplaque mice distinguished a detergent-soluble Abeta pool in brain with rapid turnover (half-lives for Abeta40 and Abeta42 were 0.7 and 1.7 h) and a much more stable, less soluble pool. Abeta in cerebrospinal fluid (CSF) reflected the changes in the soluble brain Abeta pool, whereas plasma Abeta turned over more rapidly. In brain, APP C-terminal fragments (CTF) accumulated differentially. The half-lives for gamma-secretase degradation were estimated as 0.4 and 0.1 h for C99 and C83, respectively. Three different APP transgenic lines responded very similarly to gamma-secretase inhibition regardless of the familial Alzheimer's disease mutations in APP. Amyloid deposition started with Abeta42, whereas Abeta38 and Abeta40 continued to turn over. Chronic gamma-secretase inhibition lowered amyloid plaque formation to a different degree in different brain regions of the same mice. The extent was inversely related to the initial amyloid load in the region analyzed. No evidence for plaque removal below baseline was obtained. gamma-Secretase inhibition led to a redistribution of intracellular Abeta and an elevation of CTFs in neuronal fibers. In CSF, Abeta showed a similar turnover as in preplaque animals demonstrating its suitability as marker of newly generated, soluble Abeta in plaque-bearing brain. This study supports the use of APP transgenic mice as translational models to characterize Abeta-lowering therapeutics.