A Kinetic Analysis of the Mechanism of β-Amyloid Induced G Protein Activation

Abstract

β-Amyloid (Aβ) is the primary protein component of senile plaques found in Alzheimer's disease. In an aggregated (amyloid fibril, protofibril, or low molecular weight oligomer) state, Aβ has been consistently shown to be toxic to neurons, but the molecular mechanism of this toxicity is poorly understood. We have previously shown that Aβ activates a G i/o protein, and that inhibition of this specific G protein activation attenuated Aβ-induced cell toxicity. In the present study, we use a kinetic analysis to examine the mechanism of Aβ-induced G protein activation. Using synthetic Aβ(1–40) and phospholipid vesicles containing purified G 0α subunits, we examined the relationship between Aβ concentration, G 0α subunit concentration, GTP concentration and rate of GTP hydrolysis experimentally. We found that at low concentrations of Aβ (less than 10 μM), Aβ increased the rate of GTP hydrolysis over the rate of hydrolysis in the absence of peptide, however, at high concentrations of Aβ, significantly decreased rates of GTP hydrolysis were observed. We postulated several molecular level mechanisms for the observed rate behavior, from those mechanisms derived rate equations, and then tested the mechanisms against our experimental rate data. Based on our results, we identified a plausible mechanism for Aβ-induced G protein activation which is consistent with available experimental data. This work demonstrates the utility of an engineering approach to examining steps in the mechanism of Aβ-induced cell toxicity and could provide insight into our understanding of the mechanism of Alzheimer's disease.