RNAi-mediated knock-down of Dab and Numb attenuate Aβ levels via γ-secretase mediated APP processing

Amyloid-β-protein (Aβ), the key component of senile plaques in Alzheimer's disease (AD) neuropathology, was first isolated from meningovascular amyloid deposits in AD and Down's syndrome [1,2], and has also been reported to be the subunit of the plaque amyloid [2-4]. The current amyloid hypothesis of AD states that the imbalance between Aβ generation and Aβ clearance is the basis of AD neuropathogenesis. Aβ is generated from amyloid precursor protein (APP). Specifically, APP is first hydrolyzed by β-secretase to generate a 99-residue membrane-associated C-terminus fragment (APP-C99) [5-8]. APP-C99 is further cleaved to release a ~4-kDa peptide, Aβ, and the amyloid precursor protein intracellular domain (AICD). This cleavage is achieved by an unusual form of proteolysis in which the protein is cleaved within the transmembrane domain (at residue +40 or +42) by γ-secretase [9-11]. α-secretase cleaves the majority of APP in the middle of the Aβ region of APP. This cleavage will preclude Aβ generation, lead to the release of a large ectodomain (α-APPs), and leave behind a carboxy-terminus fragment of 83 amino acids (APP-C83) in the membrane. γ-Secretase cleaves APP-C83 to produce p3, an amino-terminally truncated form of Aβ [12,13], [see review in [14]].The cleavage of the APP cytoplasmic tail by γ-secretase generates AICD, which contains the strongly conserved YENPTY-motif. The YENPTY-sequence is a consensus motif for the binding of adaptor proteins that possess a phosphotyrosine-binding domain (PTB) present in several APP adaptor proteins, such as X11, Fe65, ShcC, Numb, Dab and JIP families [see review in [15]]. We have previously reported that RNAi knock-down of X11α, ShcC and Fe65 in H4 human neuroglioma cells lower Aβ levels [16,17].Dab (encoded by gene DAB), the PTB-containing APP adaptor protein, can bind to and interact with the YENPTY-motif of APP [18,19]. Dab has been reported to function as an adaptor molecule in signal transduction process [20,21]. Numb (enco