Intrinsic signaling function of APP as a novel target of three V642 mutations linked to familial Alzheimer's disease.

APP695 is a transmembrane precursor of Abeta amyloid. In familial Alzheimer's disease (FAD), three mutations V642I/F/G were discovered in APP695, which has been suggested by multiple studies to be a cell surface signaling receptor. We previously reported that normal APP695 encodes a potential GO-linked receptor with ligand-regulated function and that expression of the three FAD mutants (FAD-APPs), not normal APP, induces cellular outputs by GO-dependent mechanisms. This suggests that FAD-APPs are constitutively active GO-linked receptors. Here, we provide direct evidence for this notion. Reconstitution of either recombinant FAD-APP with GO vesicles induced activation of GO, which was inhibitable by pertussis toxin, sensitive to Mg2+ and proportional in quantity to the reconstituted amounts of FAD-APP. Consistent with the dominant inheritance of this type of FAD, this function was dominant over normal APP, because little activation was observed in APP695-GO vesicles. Experiments with antibody competition and sequence deletion indicated that His657-Lys676 of FAD-APP, which has been specified as the ligand-dependent GO-coupling domain of normal APP, was responsible for this constitutive activation, confirming that the three FAD-APPs are mutationally activated APP695. This study identifies the intrinsic signaling function of APP to be a novel target of hereditary Alzheimer's disease mutations, providing an in vitro system for the screening of potential FAD inhibitors.