APP-BP1 mediates APP-induced apoptosis and DNA synthesis and is increased in Alzheimer's disease brain

Abstract

APP-BP1, first identified as an amyloid precursor protein (APP) binding protein, is the regulatory subunit of the activating enzyme for the small ubiquitin-like protein NEDD8. We have shown that APP-BP1 drives the S- to M-phase transition in dividing cells, and causes apoptosis in neurons (Chen, Y., D.L. McPhie, J. Hirschberg, and R.L. Neve. 2000. J. Biol. Chem. 275:8929–8935). We now demonstrate that APP-BP1 binds to the COOH-terminal 31 amino acids of APP (C31) and colocalizes with APP in a lipid-enriched fraction called lipid rafts. We show that coexpression of a peptide representing the domain of APP-BP1 that binds to APP, abolishes the ability of overexpressed APP or the V642I mutant of APP to cause neuronal apoptosis and DNA synthesis. A dominant negative mutant of the NEDD8 conjugating enzyme hUbc12, which participates in the ubiquitin-like pathway initiated by APP-BP1, blocks neuronal apoptosis caused by APP, APP(V642I), C31, or overexpression of APP-BP1. Neurons overexpressing APP or APP(V642I) show increased APP-BP1 protein levels in lipid rafts. A similar increase in APP-BP1 in lipid rafts is observed in the Alzheimer's disease brain hippocampus, but not in less-affected areas of Alzheimer's disease brain. This translocation of APP-BP1 to lipid rafts is accompanied by a change in the subcellular localization of the ubiquitin-like protein NEDD8, which is activated by APP-BP1.

Drosophila homolog of APP-BP1 (dAPP-BP1) interacts antagonistically with APPL during Drosophila development

“…Chen et al 4 demonstrated that neurons expressing APP or APP (V642I) show an increase in APP-BP1 levels in lipid rafts, which is also observed in the hippocampal region of AD patients. This could explain why an elevated level of membrane APP increases the physical interaction between APP and APP-BP1 required for reducing APP-induced toxicity.…”

“…1 The binding site has been identified as the carboxyl-terminal 31 amino-acid residues of APP. 4 This suggests the possibility of a similar interaction between dAPP-BP1 and APPL. Interaction was examined by an in vitro binding assay using a bacterially expressed GST-tagged APPLC63 (APPL carboxyl-terminal 63 amino-acid residues), GST, and in vitro expressed 35 S-methionine-labeled dAPP-BP1.…”

“…3 In AD patients, brain APP-BP1 protein levels in lipid rafts are increased. 4 Well-characterized substrates described for the NEDD8 conjugation pathway (neddylation) mainly include only members of the cullin family of proteins. Every cullin family member is known to be modified by NEDD8, 5 except for APC2.…”

The emerging roles of protein homeostasis‐governing pathways in Alzheimer's disease

“…Involvement of APP‐BP1 and the formation of Aβ plaques have been proposed (Chen, Liu, McPhie, Hassinger, & Neve, 2003). It is well known that APP undergoes γ‐secretase cleavage to produce Aβ.…”

“…These regulatory mechanisms suggest that when the expression level of APP‐BP1 is elevated, the levels of Aβ would decrease accordingly. Nevertheless, this idea remains controversial with the observation that APP‐BP1 expression has been shown to increase in AD‐affected regions of the brain (Chen et al., 2003). In an important way, overexpression of APP‐BP1 has been shown to trigger cytoplasmic relocalization of NEDD8 within neuronal cells, which may erroneously incorporate into the ubiquitin chains of degradation substrates and disrupt the subsequent proteolytic activities instead of enhancing the ubiquitination activity (Chen et al., 2012).…”