Studies of the cell biological function of the Amyloid Precursor Protein (APP) family in Drosophila melanogaster and mammals

Abstract

Alzheimer�s disease (AD) is the most common neurodegenerative disorder affecting cognitive functions of the brain, and is pathologically characterized by extracellular Amyloid plaques and intracellular neurofibrillary tangles are the main pathological features of AD. Amyloid-beta, the main protein constituent of Amyloid plaques, is derived from the Amyloid Precursor Protein (APP) by proteolytic processing. APP is a type I transmembrane protein, which resembles a cell surface receptor, and consists of a large ectodomain and a short cytoplasmic tail. While Abeta generation from APP is well investigated, the physiological function of APP is still incompletely understood. Nevertheless, a diverse set of APP functions has been proposed, including cell-cell and cell-matrix interactions and intracellular signalling. In this study, the cell adhesion properties of APP and its mammalian paralogues, APLP1 and APLP2, were investigated in vivo and in vitro. Using Drosophila melanogaster as a model system, it has been shown that expression of APP, APLP1, or APLP2 in the Drosophila wing leads to cell adhesion defects, which was evident from detached cell layers and incomplete wing development. It was further revealed that the induced, so called blistered wing phenotype depends on the extracellular domain and membrane anchoring of APP, and the phenotype is additionally modulated by proteolytic conversion of APP. Interestingly, the most pronounced defects in wing development were caused by overexpression of APLP2, which were shown to be caused by interference of APLP2 with Wingless signaling via genetic interaction with the Drosophila Glypican Dally. Furthermore, using the Drosophila model organism, genetic interactions of the APP intracellular domain with two novel putative interaction partners, Numb and Disabled-2, were identified. The interacting domain of APP was mapped, and binding was verified by biochemical analysis. The results obtained with the Drosophila model system for cell adhesion properties of APP family proteins were extended to an in vitro cell aggregation assay, where APP, APLP1, or APLP2 were shown to mediate homo- and heterotypic cell interaction. The intercellular interaction of APP family proteins is highly specific, as a mutant of APP lacking the extracellular domain failed to promote cell clustering. These data strongly suggest that APP family proteins form trans-dimers and contribute to cellular interactions via formation of homo- and heterotypic, trans-cellular complexes. Interestingly, the in vivo phenotype strength induced by APP family proteins in Drosophila correlates with the cell aggregation data, suggesting that trans-cellular interaction of APP family proteins is crucial for both phenomena. Additionally, APLP1 and APLP2 were shown to mediate cellular uptake of their corresponding secreted fragments, supporting a hitherto not observed receptor-like function. Moreover, a genetic interdependence and molecular interaction of APP and APLP1 in synaptically enriched membrane compartments was found in this study, corroborating a functional role for APP family proteins in the connectivity of pre- and postsynaptic membranes. Taken together, previously not described homo- and hetero-trans-dimerization of APP family proteins seems to be involved in cell adhesion and represents an important feature required for their physiological function.