Mechanisms regulating the dimerization of the Amyloid Precursor Protein and their role in its trafficking and processing

(eng) Alzheimer’s disease is the most common neurodegenerative disorder. One of the major neuropathological lesions characterizing AD is the senile plaque. The senile plaques are found in brains of AD patients and are built-up by accumulation of Aβ peptide. Aβ derives from the large amyloid precursor protein (APP) by the sequential cleavage by the β- and γ-secretases. Ten APP isoforms, produced by pre-mRNA alternative splicing have been identified. The two major isoforms of human APP, APP695 and APP751, differ by the presence of a KPI domain (Kunitz-type Protease Inhibitor) in the extracellular region. Several studies have evidenced increased expression of APP-KPI isoforms in AD brains, associated with disease progression. The cellular and molecular mechanisms regulating APP processing are still poorly understood. Dimerization has recently been proposed as a key regulator of APP cleavage and function. We investigated the mechanism of APP dimerization in living cells by using a Bimolecular Fluorescence Complementation approach (BiFC) coupled to FACS analysis and vital confocal imaging. We found that dimerization of APP was increased by the presence of the KPI domain in the extracellular region, whereas the three GxxxG motifs at the transmembrane domain play only a marginal role. Moreover, we showed that the native folding of KPI is critical for APP dimerization. We detected dimers of both APP isoforms in the secretory pathway, particularly in the endoplasmic reticulum-Golgi intermediate compartment and the trans-Golgi network compartment. The influence of the native folding of the KPI domain in the trafficking of the APP dimers was demonstrated by analyzing two KPI mutants. These KPI mutations led to retention of the APP dimers in the endoplasmic reticulum. Finally, we demonstrated that increased dimerization was associated with enhanced non-amyloidogenic processing. Altogether, the correlation between dimerization, subcellular localization and processing suggests that dimerization impacts on APP trafficking in the secretory compartments, with major consequences on its processing.