Mutations in the genes for amyloid precursor protein (APP) and presenilins (PS1, PS2) increase production of ␤-amyloid 42 (A␤ 42 ) and cause familial Alzheimer's disease (FAD). Transgenic mice that express FAD mutant APP and PS1 overproduce A␤ 42 and exhibit amyloid plaque pathology similar to that found in AD, but most transgenic models develop plaques slowly. To accelerate plaque development and investigate the effects of very high cerebral A␤ 42 levels, we generated APP/PS1 double transgenic mice that coexpress five FAD mutations (5XFAD mice) and additively increase A␤ 42 production. 5XFAD mice generate A␤ 42 almost exclusively and rapidly accumulate massive cerebral A␤ 42 levels. Amyloid deposition (and gliosis) begins at 2 months and reaches a very large burden, especially in subiculum and deep cortical layers. Intraneuronal A␤ 42 accumulates in 5XFAD brain starting at 1.5 months of age (before plaques form), is aggregated (as determined by thioflavin S staining), and occurs within neuron soma and neurites. Some amyloid deposits originate within morphologically abnormal neuron soma that contain intraneuronal A␤. Synaptic markers synaptophysin, syntaxin, and postsynaptic density-95 decrease with age in 5XFAD brain, and large pyramidal neurons in cortical layer 5 and subiculum are lost. In addition, levels of the activation subunit of cyclin-dependent kinase 5, p25, are elevated significantly at 9 months in 5XFAD brain, although an upward trend is observed by 3 months of age, before significant neurodegeneration or neuron loss. Finally, 5XFAD mice have impaired memory in the Y-maze. Thus, 5XFAD mice rapidly recapitulate major features of AD amyloid pathology and may be useful models of intraneuronal A␤ 42 -induced neurodegeneration and amyloid plaque formation.

Cerebral deposition of amyloid β peptide (Aβ) is an early and critical feature of Alzheimer's disease. Aβ generation depends on proteolytic cleavage of the amyloid precursor protein (APP) by two unknown proteases: β-secretase and γ-secretase. These proteases are prime therapeutic targets. A transmembrane aspartic protease with all the known characteristics of β-secretase was cloned and characterized. Overexpression of this protease, termed BACE (for beta-site APP-cleaving enzyme) increased the amount of β-secretase cleavage products, and these were cleaved exactly and only at known β-secretase positions. Antisense inhibition of endogenous BACE messenger RNA decreased the amount of β-secretase cleavage products, and purified BACE protein cleaved APP-derived substrates with the same sequence specificity as β-secretase. Finally, the expression pattern and subcellular localization of BACE were consistent with that expected for β-secretase. Future development of BACE inhibitors may prove beneficial for the treatment of Alzheimer's disease.

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The amyloid hypothesis has yielded a series of well-validated candidate drug targets with potential for the treatment of Alzheimer disease (AD). Three proteases that are involved in the processing of amyloid precursor protein-α-secretase, β-secretase and γ-secretase-are of particular interest as they are central to the generation and modulation of amyloid-β peptide and can be targeted by small compounds in vitro and in vivo. Given that these proteases also fulfill other important biological roles, inhibiting their activity will clearly be inherently associated with mechanism-based toxicity. Carefully determining a suitable therapeutic window and optimizing the selectivity of the drug treatments towards amyloid precursor protein processing might be ways of overcoming this potential complication. Secretase inhibitors are likely to be the first small-molecule therapies aimed at AD modification that will be fully tested in the clinic. Success or failure of these first-generation AD therapies will have enormous consequences for further drug development efforts for AD and possibly other neurodegenerative conditions.

Animal models of human diseases that accurately recapitulate clinical pathology are indispensable for understanding molecular mechanisms and advancing preclinical studies. The Alzheimer's disease (AD) research community has historically used first‐generation transgenic (Tg) mouse models that overexpress proteins linked to familial AD (FAD), mutant amyloid precursor protein (APP), or APP and presenilin (PS). These mice exhibit AD pathology, but the overexpression paradigm may cause additional phenotypes unrelated to AD. Second‐generation mouse models contain humanized sequences and clinical mutations in the endogenous mouse App gene. These mice show Aβ accumulation without phenotypes related to overexpression but are not yet a clinical recapitulation of human AD. In this review, we evaluate different APP mouse models of AD, and review recent studies using the second‐generation mice. We advise AD researchers to consider the comparative strengths and limitations of each model against the scientific and therapeutic goal of a prospective preclinical study.

The ␤-amyloid (A␤) peptide is the major constituent of amyloid plaques in Alzheimer's disease (AD) brain and is likely to play a central role in the pathogenesis of this devastating neurodegenerative disorder. The ␤-secretase, ␤-site amyloid precursor protein cleaving enzyme (BACE1; also called Asp2, memapsin 2), is the enzyme responsible for initiating A␤ generation. Thus, BACE is a prime drug target for the therapeutic inhibition of A␤ production in AD. Since its discovery 10 years ago, much has been learned about BACE. This review summarizes BACE properties, describes BACE translation dysregulation in AD, and discusses BACE physiological functions in sodium current, synaptic transmission, myelination, and schizophrenia. The therapeutic potential of BACE will also be considered. This is a summary of topics covered at a symposium held at the 39th annual meeting of the Society for Neuroscience and is not meant to be a comprehensive review of BACE.BACE: the ␤-secretase in Alzheimer's disease Although the etiology of Alzheimer's disease (AD) is not completely understood, the study of disease genes that cause AD has revealed important clues about the pathogenesis of this disorder. Familial AD (FAD) cases are caused by autosomal dominant mutations in the genes for amyloid precursor protein (APP) and the presenilins (PS1 and PS2) (Sisodia and St George-Hyslop, 2002). These mutations increase production of the 42-aa-long, fibrillogenic form of A␤ (A␤ 42 ), relative to A␤ 40 . In addition, patients with APP gene duplications or individuals with Down's syndrome (trisomy 21), who have increased dosage of the APP gene (located on chromosome 21), develop early-onset AD and overproduce A␤ 42 (Hardy, 2006). These findings, along with a large body of evidence from other sources (Selkoe, 2008), strongly suggest that A␤ 42 plays a central, early role in AD pathogenesis. Thus, therapeutic strategies to lower cerebral A␤ 42 levels are expected to be beneficial for the treatment or prevention of AD.A␤ is produced through the endoproteolysis of APP, a large type 1 transmembrane protein. Cleavage of APP by two proteases, the ␤-and ␥-secretases, is required to liberate A␤ from APP (Tanzi and Bertram, 2005). The ␤-secretase cuts APP first to generate the N terminus of A␤, thus producing a membrane bound C-terminal fragment called C99. Then, ␥-secretase cleaves C99 to release the mature A␤ peptide. A third protease, ␣-secretase, cuts APP within the A␤ domain, thus precluding A␤ formation.