The ␤-amyloid peptide (A␤) is the major constituent of the amyloid core of senile plaques found in the brain of patients with Alzheimer disease. A␤ is produced by the sequential cleavage of the amyloid precursor protein (APP) by ␤-and ␥-secretases. Cleavage of APP by ␥-secretase also generates the APP intracellular C-terminal domain (AICD) peptide, which might be involved in regulation of gene transcription. APP contains three Gly-XXX-Gly (GXXXG) motifs in its juxtamembrane and transmembrane (TM) regions. Such motifs are known to promote dimerization via close apposition of TM sequences. We demonstrate that pairwise replacement of glycines by leucines or isoleucines, but not alanines, in a GXXXG motif led to a drastic reduction of A␤40 and A␤42 secretion. ␤-Cleavage of mutant APP was not inhibited, and reduction of A␤ secretion resulted from inhibition of ␥-cleavage. It was anticipated that decreased ␥-cleavage of mutant APP would result from inhibition of its dimerization. Surprisingly, mutations of the GXXXG motif actually enhanced dimerization of the APP C-terminal fragments, possibly via a different TM ␣-helical interface. Increased dimerization of the TM APP C-terminal domain did not affect AICD production.The progressive deposition of ␤-amyloid peptide (A␤) 3 leading to the formation of senile plaques is an invariant feature of Alzheimer disease. A␤ is a 39 -43-amino acid peptide, with two major isoforms of 40 and 42 amino acids (1, 2). A␤ is produced by the amyloidogenic cleavage of its precursor, the amyloid precursor protein or APP (3).The amyloidogenic processing of APP is initiated by ␤-cleavage within the lumenal/extracellular domain of the protein.The ␤-cleavage of APP is performed by the BACE proteins (BACE1 and -2) that are integral membrane proteins belonging to the aspartyl protease family (4 -8). ␤-Cleavage produces a 99-amino acid, membrane-anchored APP C-terminal fragment (␤CTF), which is further cleaved by the ␥-secretase activity to generate A␤. The ␥-secretase activity is contained in a high molecular weight multiprotein complex formed at least by the following proteins: a presenilin (PS1 or PS2), nicastrin (Nct), Pen-2, and Aph-1 (9). The activity of the ␥-secretase complex is also required for the generation of the intracellular fragment named (APP intracellular C-terminal domain (AICD). AICD was shown to translocate to the nucleus (10, 11), and there is growing experimental evidence suggesting a role for AICD in the regulation of gene transcription (12-17) even if the identity of APP target genes remains a matter of debate (18). The ␥-secretase complex, therefore, plays a central role in the onset and progression of Alzheimer disease not only because proteolysis of ␤CTF controls the production of A␤, but it also controls the intracellular signaling associated with APP, which in turn might regulate the expression of genes involved in the disease. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "ad...

Alzheimer disease (AD), the most frequent cause of dementia, is characterized by an important neuronal loss. A typical histological hallmark of AD is the extracellular deposition of ␤-amyloid peptide (A␤), which is produced by the cleavage of the amyloid precursor protein (APP). Most of the gene mutations that segregate with the inherited forms of AD result in increasing the ratio of A␤42/A␤40 production. A␤42 also accumulates in neurons of AD patients. Altogether, these data strongly suggest that the neuronal production of A␤42 is a critical event in AD, but the intraneuronal A␤42 toxicity has never been demonstrated. Here, we report that the long term expression of human APP in rat cortical neurons induces apoptosis. Although APP processing leads to production of extracellular A␤1-40 and soluble APP, these extracellular derivatives do not induce neuronal death. On the contrary, neurons undergo apoptosis as soon as they accumulate intracellular A␤1-42 following the expression of full-length APP or a C-terminal deleted APP isoform. The inhibition of intraneuronal A␤1-42 production by a functional ␥-secretase inhibitor increases neuronal survival. Therefore, the accumulation of intraneuronal A␤1-42 is the key event in the neurodegenerative process that we observed.A clear diagnosis of AD 1 can be performed by correlating clinical findings and postmortem examination of brain sections. AD is characterized by a massive neuronal loss in vulnerable brain regions (1, 2). Two typical hallmarks of AD are neurofibrillary tangles and senile plaques (3). The major constituent of the amyloid core of senile plaques is the ␤-amyloid or A␤ peptide. The A␤ peptide is a 39 -43-amino acid peptide produced from a larger precursor, the amyloid precursor protein or APP (4). Among the ten identified isoforms of human APP (5), eight contain the A␤ sequence. The isoform that is mainly expressed in the human brain is a 695-amino acid protein known as APP695 (4). APP is processed by the non-amyloidogenic pathway, where ␣Ϫsecretase activity (6) produces soluble forms of APP, and by the amyloidogenic pathway, where ␤-secretase (7) and ␥-secretase activities allow the release of A␤. Several identified mutations in the APP and the presenilins genes segregate with inherited forms of AD known as early onset familial Alzheimer disease or FAD (8, 9). Most of these mutations result in an increased production of the A␤ ending at position 42 (10). In vitro studies have shown that A␤42 rapidly aggregates into fibrils and that extracellular fibrillar A␤ peptides induce apoptosis in cultured neurons (11). On the other hand, recent reports have demonstrated an intraneuronal accumulation of A␤42 in AD-vulnerable brain regions (12, 13). Intraneuronal A␤42 accumulation has also been reported in transgenic mice expressing FAD proteins (14) as well as in transgenic mice showing accelerated neurodegeneration without extracellular amyloid deposition (15). Altogether, these data support the idea that A␤42 accumulation and neuronal loss are closely correlated. N...