Cluster-Bildung, Prozessierung und Internalisierung des Amyloid Precursor Protein (APP) an der Plasmamembran
Biophysik
Zellbiologie
Zusammenfassung der Projektergebnisse
Deposits of neurotoxic amyloid β-peptides (Aβ) in the brain are associated with Alzheimer’s disease (AD). The peptides derive from the amyloid precursor protein (APP) after subsequent β- and γ-cleavage mediated by secretases. β-/γ-cleavage is circumvented when APP is cleaved first by an α-secretase, yielding shorter and harmless peptides. Because α-cleavage occurs at the plasma membrane and β-cleavage after internalization in endosomes, a better understanding of APP dynamics at the plasma membrane could be relevant for shifting APP processing from the amyloidogenic (β/γ) to the non-amyloidogenic (α/γ) cleavage pathway. We previously found that at the plasma membrane, APP forms clusters via a five amino acid segment that is identical to the Aβ N-terminal region. In the same region, mutations associated with familial AD occur, as well as a protective mutation. We hypothesized that the pathogenic mutations favor β-/γ-cleavage because APP forms tighter packed and/or more clusters making APP less accessible to α-secretases. Moreover, stronger clustering could accelerate internalization, shortening the residence time of APP at the plasma membrane, and consequently the chance of being cleaved by α-secretases. In contrast, α-processing of the protective mutation could be enhanced because it is less clustered. Apart from these issues, for a better understanding of plasmalemmal APP processing, we planned characterizing the association between APP and α-secretases. Studying a pathogenic and a protective mutant, we occasionally observed trends as expected (e.g. less and more α-processing of the pathogenic and the protective mutant, respectively), but effects were small and the variability between experiments was so large that even after many repetitions we did not obtain statistically significant data, which is why we could not convincingly proof our hypothesis. Analyzing by microscopy the association between APP and the α- secretases ADAM10 and ADAM17, we found no difference between the secretases regarding signal overlap and distance between APP and secretase signal maxima. However, by crosslinking of APP in the native membrane we could show that APP is physically linked to the secretase ADAM10 but not to ADAM17, explaining the known dominant role of ADAM10 in α-processing of APP. In addition, we could identify the APP transmembrane segment as the domain being required for the association with ADAM10 and α-processing. So far, to the best of our knowledge there are no reports on immunoprecipitation experiments characterizing the interaction between APP and α-secretases, probably because during the long immunoprecipitation procedure the APP substrate is cleaved. Our cross-linking assay can be used for studying more details of APP-secretase interactions possibly creating knowledge for developing an AD treatment based on promoting α-processing.
