The role of amyloid precursor protein in amyloid-beta- mediated synaptic dysfunction

Amyloid precursor protein (APP) is a type one transmembrane protein and has two mammalian homologues, amyloid precursor-like protein 1 (APLP1) and amyloid precursor-like protein 2 (APLP2). APP is the parent molecule to amyloid-[beta] (A[beta]), the amyloidogenic species found in the plaques of people with Alzheimer's disease (AD). Several lines of evidence suggest A[beta] to lie at the center of AD pathology, with converging evidence to indicate that synapses are the site of the initial damage. Recent studies have shown that APP may be necessary for the toxicity induced by A[beta], in part by cleavage of a caspase site on the intracellular domain of the APP protein and the subsequent release of the toxic molecule, C31. This caspase cleavage is shown to induce APP-mediated A[beta] toxicity in cell culture models, however assays were based on contribution to cell death. Thus, the physiologic relevance of the cleavage event has never been tested and in particular, whether this pathway contributes to synaptic damage is unclear. Here, we seek to test the role of caspase cleavage of APP in A[beta]- induced synaptic damage and to test the specificity of this event by testing whether caspase cleavage of APLP2, the protein most homologous to APP, also contributes to these A[beta]-driven synaptic changes. Additionally, because APP dimerization was shown to be necessary for the Aβ-induced caspase cleavage of APP and subsequent release of C31, we wanted to test the effects of dimerization on APP proteolysis and A[beta] productio

Induced Dimerization of the Amyloid Precursor Protein Leads to Decreased Amyloid-β Protein Production*

The amyloid precursor protein (APP) plays a central role in Alzheimer disease (AD) pathogenesis because sequential cleavages by β- and γ-secretase lead to the generation of the amyloid-β (Aβ) peptide, a key constituent in the amyloid plaques present in brains of AD individuals. In several studies APP has recently been shown to form homodimers, and this event appears to influence Aβ generation. However, these studies have relied on APP mutations within the Aβ sequence itself that may affect APP processing by interfering with secretase cleavages independent of dimerization. Therefore, the impact of APP dimerization on Aβ production remains unclear. To address this question, we compared the approach of constitutive cysteine-induced APP dimerization with a regulatable dimerization system that does not require the introduction of mutations within the Aβ sequence. To this end we generated an APP chimeric molecule by fusing a domain of the FK506-binding protein (FKBP) to the C terminus of APP. The addition of the synthetic membrane-permeant drug AP20187 induces rapid dimerization of the APP-FKBP chimera. Using this system we were able to induce up to 70% APP dimers. Our results showed that controlled homodimerization of APP-FKBP leads to a 50% reduction in total Aβ levels in transfected N2a cells. Similar results were obtained with the direct precursor of β-secretase cleavage, C99/SPA4CT-FKBP. Furthermore, there was no modulation of different Aβ peptide species after APP dimerization in this system. Taken together, our results suggest that APP dimerization can directly affect γ-secretase processing and that dimerization is not required for Aβ production