Alzheimer's disease (AD) is characterized neuropathologically by the extracellular deposition of the 4 kDa b-amyloid peptide (Ab), a 39-43 amino acid peptide that is neurotoxic (Borchelt et al. 1996) and accumulates in neuritic plaques and in cerebral and meningeal microvessels (Wahrle et al. 2002). Amyloid precursor protein (APP) is the precursor of Ab and can be processed via alternative pathways. A nonamyloidogenic secretory pathway includes cleavage of APP to soluble APP (sAPPa) by a-secretases from the a disintegrin and metalloprotease (ADAM) family of proteases within the Ab peptide sequence, thus precluding the formation of Ab. In contrast, the formation of the amyloidogenic Ab peptides is regulated by the sequential action of b-and c-secretases (Checler 1995;Nunan and Small 2000). A primary strategy proposed to treat AD is to prevent the formation of Ab peptides, and their deposition as senile plaques in the brain. Recently, Postina et al. (2004) showed that activation of a-secretase significantly reduces AD-like pathology in an animal model of AD. This raises the possibility that a pharmacological approach to increase a-secretase activity and sAPPa levels, and thus reduce Ab formation may be suitable for AD treatment (Bandyopadhyay et al. 2007).This approach is consistent with the observation that profound changes occur in APP processing with aging and the development of AD. In particular, a reduction in the activity of the non-amyloidogenic pathway leading to Received February 7, 2008; revised manuscript received March 6, 2008; accepted March 28, 2008. Address correspondence and reprint requests to Laurent Désiré, Department of Neurology, Exonhit Therapeutics, 63 Bd Massena, 75013 Paris, France. E-mail: laurent.desire@exonhit.comAbbreviations used: Ab, beta amyloid; ADAM, a disintegrin and metalloprotease; AD, Alzheimer's Disease; APP, amyloid precursor protein; BACE, beta-site APP-cleaving enzyme; DAPT, N-[N-(3,5-difluorophenacetyl-L-alanyl)]-S-phenylglycine t-butyl ester; IP, immunoprecipitation; LTP, long-term potentiation; PDE4, phosphodiesterase 4; PTX, picrotoxin; sAPPa, soluble APP ectodomain; SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis; TAPI-1, TNF-a Protease Inhibitor-1. AbstractPharmacological modulation of the GABA A receptor has gained increasing attention as a potential treatment for central processes affected in Alzheimer disease (AD), including neuronal survival and cognition. The proteolytic cleavage of the amyloid precursor protein (APP) through the a-secretase pathway decreases in AD, concurrent with cognitive impairment. This APP cleavage occurs within the b-amyloid peptide (Ab) sequence, precluding formation of amyloidogenic peptides and leading to the release of the soluble N-terminal APP fragment (sAPPa) which is neurotrophic and procognitive. In this study, we show that at nanomolar-low micromolar concentrations, etazolate, a selective GABA A receptor modulator, stimulates sAPPa production in rat cortical neurons and in guinea pig brains. Etazolate (20 ...

␤-Amyloid peptides (A␤) that form the senile plaques of Alzheimer disease consist mainly of 40-and 42-amino acid (A␤ 40 and A␤ 42) peptides generated from the cleavage of the amyloid precursor protein (APP). Generation of A␤ involves ␤-secretase and ␥-secretase activities and is regulated by membrane trafficking of the proteins involved in A␤ production. Here we describe a new small molecule, EHT 1864, which blocks the Rac1 signaling pathways. In vitro, EHT 1864 blocks A␤ 40 and A␤ 42 production but does not impact sAPP␣ levels and does not inhibit ␤-secretase. Rather, EHT 1864 modulates APP processing at the level of ␥-secretase to prevent A␤ 40 and A␤ 42 generation. This effect does not result from a direct inhibition of the ␥-secretase activity and is specific for APP cleavage, since EHT 1864 does not affect Notch cleavage. In vivo, EHT 1864 significantly reduces A␤ 40 and A␤ 42 levels in guinea pig brains at a threshold that is compatible with delaying plaque accumulation and/or clearing the existing plaque in brain. EHT 1864 is the first derivative of a new chemical series that consists of candidates for inhibiting A␤ formation in the brain of AD patients. Our findings represent the first pharmacological validation of Rac1 signaling as a target for developing novel therapies for Alzheimer disease. Alzheimer disease (AD)2 is the most common neurodegenerative disorder marked by progressive loss of memory and cognitive ability. The pathology of AD is characterized by the presence of amyloid plaques (1), intracellular neurofibrillary tangles, and pronounced cell death. The ␤-amyloid peptide (A␤) (2) is the main constituent of senile plaques found in AD brains. Furthermore, extracellular A␤ 42 appears toxic to neurons in vitro and in vivo (reviewed in Ref.3). A␤ is generated by proteolysis of an integral membrane protein, the amyloid precursor protein (APP), via at least two post-translational pathways. The amyloidogenic cleavage of APP is a sequential processing of APP initiated by ␤-secretase (BACE), which cleaves APP within the luminal domain or at the cell surface, generating the N terminus of A␤ (4). This cleavage generates several membrane-bound proteolytic C-terminal fragments (CTFs), such as the 99-residue ␤-CTF (also called C99), as well as the secreted APP ectodomain sAPP␤. The C terminus of A␤ is subsequently generated by intramembranous cleavage of CTFs by ␥-secretase, producing either A␤ 40 or A␤ 42. The cleavages at residues 40 -42 are referred to as ␥-cleavage, and the cleavages at residues 49 -52 are referred to as ⑀-cleavage (5). The nonamyloidogenic cleavage of APP, which precludes A␤ generation, is mediated by ␣-secretase, a disintegrin and metalloproteinase 10, and a disintegrin and metalloproteinase 17, in a reaction believed to occur primarily on the plasma membrane. This proteolytic cleavage by ␣-secretase occurs within the A␤ region and produces soluble APP (sAPP␣), the dominant processing product, and the residual membrane-bound 10-kDa CTF (CTF␣, also called C83). Like C99, C83 is a subs...

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