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b-Site APP-cleaving enzyme (BACE) initiates the processing of the amyloid precursor protein (APP) leading to the generation of b-amyloid, the main component of Alzheimer's disease senile plaques. BACE (Asp2, memapsin 2) is a type I transmembrane aspartyl protease and is responsible for the b-secretase cleavage of APP producing different endoproteolytic fragments referred to as the carboxy-terminal C99, C89 and the soluble ectodomain sAPPb. Here we describe two transgenic mouse lines expressing human BACE in the brain. Overexpression of BACE augments the amyloidogenic processing of APP as demonstrated by decreased levels of full-length APP and increased levels of C99 and C89 in vivo. In mice expressing huBACE in addition to human APP wild-type or carrying the Swedish mutation, the induction of APP processing characterized by elevated C99, C89 and sAPPb, results in increased brain levels of b-amyloid peptides Ab40 and Ab42 at steady-state.

Endoproteolysis of the β-amyloid precursor protein (APP) by β- and γ-secretases generates the toxic amyloid β-peptide (Aβ), which accumulates in the brain of Alzheimer's disease (AD) patients. Here, we established a novel approach to regulate production of Aβ based on intracellular expression of single chain antibodies (intrabodies) raised to an epitope adjacent to the β-secretase cleavage site of human APP. The intrabodies rapidly associated, within the endoplasmic reticulum (ER), with newly synthesized APP. One intrabody remained associated during APP transport along the secretory line, shielded the β-secretase cleavage site and facilitated the alternative, innocuous cleavage operated by α-secretase. Another killer intrabody with an ER retention sequence triggered APP disposal from the ER. The first intrabody drastically inhibited and the second almost abolished generation of Aβ. Intrabodies association with specific substrates rather than with enzymes, may modulate intracellular processes linked to disease with highest specificity and may become instrumental to investigate molecular mechanisms of cellular events.

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