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It is well established that the proteolytic processing of the ␤-amyloid precursor protein (APP) generates ␤-amyloid (A␤), which plays a central role in the pathogenesis of Alzheimer's disease (AD). In contrast, the physiological role of APP and of its numerous proteolytic fragments and the question of whether a loss of these functions contributes to AD are still unknown. To address this question, we replaced the endogenous APP locus by gene-targeted alleles and generated two lines of knock-in mice that exclusively express APP deletion variants corresponding either to the secreted APP ectodomain (APPs␣) or to a C-terminal (CT) truncation lacking the YENPTY interaction motif (APP⌬CT15). Interestingly, the ⌬CT15 deletion resulted in reduced turnover of holoAPP, increased cell surface expression, and strongly reduced A␤ levels in brain, likely because of reduced processing in the endocytic pathway. Most importantly, we demonstrate that in both APP knock-in lines the expression of APP N-terminal domains either grossly attenuated or completely rescued the prominent deficits of APP knock-out mice, such as reductions in brain and body weight, grip strength deficits, alterations in circadian locomotor activity, exploratory activity, and the impairment in spatial learning and long-term potentiation. Together, our data suggest that the APP C terminus is dispensable and that APPs␣ is sufficient to mediate the physiological functions of APP assessed by these tests.

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Chronic use of nonsteroidal anti-inflammatory drugs (NSAIDs) is associated with a lower risk of developing Alzheimer's disease. Recent evidence indicates that some NSAIDs specifically inhibit secretion of the amyloidogenic A␤42 peptide in cultured cells and mouse models of Alzheimer's disease. The reduction of A␤42 peptides is not mediated by inhibition of cyclooxygenases (COX) but the molecular mechanism underlying this novel activity of NSAIDs has not been further defined. We now demonstrate that NSAIDs efficiently reduce the intracellular pool of A␤42 in cell-based studies and selectively decrease A␤42 production in a cell-free assay of ␥-secretase activity. Moreover, we find that presenilin-1 (PS1) mutations, which affect ␥-secretase activity, differentially modulate the cellular A␤42 response to NSAID treatment. Overexpression of the PS1-M146L mutation enhances the cellular drug response to A␤42 lowering NSAIDs as compared with cells expressing wildtype PS1. In contrast, expression of the PS1-⌬Exon9 mutation strongly diminishes the A␤42 response, showing that PS1 mutations can modulate the cellular drug response to NSAID treatment both positively and negatively. Enhancement of the NSAID drug response was also observed with overexpression of the APP V717F mutation but not with Swedish mutant APP, which affects ␤-secretase cleavage. In sum, these results strongly suggest that NSAIDs represent a founding group of compounds that lower A␤42 production by direct modulation of ␥-secretase activity or its substrate.Despite considerable advances in the understanding of Alzheimer's disease (AD) 1 pathology, therapeutic interventions that may halt or reverse the underlying disease process are not available (1, 2). Numerous epidemiological studies support the finding that chronic intake of nonsteroidal anti-inflammatory drugs (NSAIDs) can decrease the risk for AD by more than 50% (3-6). This protective effect of NSAIDs has generally been ascribed to a diminution of deleterious inflammatory processes in the AD brain (3). However, recent findings suggest a direct impact of some NSAIDs on the amyloid pathology in AD. Treatment of various cultured cells with the NSAIDs sulindac sulfide, ibuprofen, indomethacin, and flurbiprofen specifically inhibited the release of the amyloidogenic A␤42 peptide (7,8). A␤42 is a proteolytic fragment derived from the ␤-amyloid precursor protein (APP) by ␤-and subsequent ␥-secretase cleavage activities and is believed to play a central role in AD pathology (9, 10). Short term administration of ibuprofen to APP transgenic mice lowered brain levels of A␤42 and chronic high dose ibuprofen treatment significantly reduced amyloid plaque numbers and plaque-associated pathology in aging APP transgenic mice (7, 11). The reduction in A␤42 levels was achieved without affecting other APP processing pathways, like secretion of the soluble APP ectodomain (APPs), and is not the result of enhanced degradation or cell-mediated clearance of A␤42. Importantly, within the concentration range tested, A␤42-lower...