SORLA regulates processing of amyloid precursor protein via interaction

D i s s e r t a t i o n zur Erlangung des akademischen Grades des Doktors der Naturwissenschaften d o c t o r r e r u m n a t u r a l i u m Summary Alzheimer disease (AD) is the most common form of neurodegenerative diseases. Only little is known about the control mechanisms which affect APP metabolism and that cause Alzheimer´s Disease. Gene expression profiling studies revealed reduced levels of Sorla mRNA in brain autopsies from patients suffering from AD compared with healthy individuals. Based on this observation, reduced SORLA activity may be considered a causal event in AD development. The mammalian receptor SORLA is a type-1 membrane protein that shares structural similarity to members of the group of VPS10P domain-containing receptors. In my studies, I have been able to uncover the molecular mechanisms how this receptor impacts on APP metabolism and progression. I showed that SORLA binds directly to APP and that both proteins co-immunopre-cipitate from cell lysats, suggesting a direct interaction between SORLA and APP. Overexpression of SORLA in CHO and SH-SY5Y cell lines caused sequestration of APP in the Golgi compartment and protection from proteolytic processing as deduced from intracellular accumulation of mature APP. In addition, SORLA over-expression decreased the activity of both amyloidogenic and non-amyloidogenic processing pathway for APP. This inhibitory effect may either be attributed to an inhibition of the interaction of APP with the respective proteases or by controlling the transport of APP to the distinct subcellular compartments less favorable for pro-teolytic breakdown. Furthermore, the crucial role of SORLA in APP trafficking and processing fates has been established by site-directed mutagenesis of Sorla cDNA sequence to generate trafficking deficient receptor variants. Using these mutants, I was able to elucidate two trafficking routes for SORLA, and their dependence on interaction of the cy-toplasmic domain of the receptor with the adaptor proteins GGA1 and PACS1. Finally , I identified the functional relevance of these sorting pathways for regulation of APP trafficking and metabolism. SORLA Δcd and SORLA acidic variants were predominantly localized to the cell surface and showed defects in endocytosis leading to accelerated Aβ production. In contrast, the SORLA GGA variant failed to undergo II shuttling between TGN and endosomal compartments. Cells overexpressing this receptor mutant displayed increased sAPPα and decreased Aβ secretion, suggesting a shift from the amyloidogenic to the non-amyloidogenic pathway. As well as …