Aktuelle Neurologie 2005; 32 - P488
DOI: 10.1055/s-2005-919520

GGA1 overexpression alters trafficking and subcellular distribution of the amyloid precursor protein

C.A.F von Arnim 1, I.D Peltan 1, M Deng 1, S Courchesne 1, S.F Lichtenthaler 1, M.C Irizarry 1, B.T Hyman 1
  • 1Ulm; Boston, USA; Munich

Background: The amyloid precursor protein (APP) is cleaved by two sequential proteases, first BACE (beta-site of APP-cleaving enzyme) followed by Gamma-secretase to release the A-beta peptides, that accumulate in the senile plaques in Alzheimer's disease (AD) and the APP intracellular domain (AICD). APP and BACE traffic together. GGA1, a member of the Golgi-localized gamma-ear-containing ARF binding (GGA) protein family interacts with BACE phosphorylation dependent via its VHS domain in the Golgi and influences its subcellular distribution.

Objective: We postulated therefore that GGA might also interact with APP and tested if GGA1 interacts with APP directly and affects APP processing and transport.

Methods: We applied confocal imaging and a novel technique to show close protein-protein vicinity: fluorescence lifetime imaging microscopy (FLIM). BACE and APP maturation and APP processing were assessed by western blot. To detect the APP cleavage products we employed an Alkaline phosphatase based APP shedding assay, A-beta ELISA and a luciferase based transactivation assay.

Results: We show that GGA1 interacts with APP and confines it to the Golgi. Furthermore we found that GGA1 overexpression affects both BACE and APP maturation. GGA1 overexpression in cells co-transfected with BACE and APP reduced both secreted and intracellular A-beta while the beta-cleavage product APP-CTFbeta was increased. In addition, APP intracellular domain release was blunted by GGA1. Deletion of the VHS domain of GGA did not affect this, however all effects could be abolished by further deletion of the GAT domain of GGA1.

Discussion: These results suggest that GGA1 prevented the beta-cleavage products from becoming substrates for gamma-secretase. GGA1 therefore not only regulates BACE trafficking but also affects APP processing and A-beta secretion via a mechanism that is independent of the BACE-GGA interaction via the GGA-VHS domain. It appears likely that GGA1 is one player in a complex pathway regulating APP transport and cleavage. If activity of this or related proteins are changed in AD, they may alter the risk for spontaneous AD progression by subtly altering the kinetics of APP trafficking and processing over long periods of time.