Contribution of Kunitz Protease Inhibitor and Transmembrane Domains to Amyloid Precursor Protein Homodimerization

Background: The two major isoforms of the human amyloid precursor protein (APP) are APP695 and APP751. They differ by the insertion of a Kunitz-type protease inhibitor (KPI) sequence in the extracellular domain of APP751. APP-KPI isoforms are increased in Alzheimer's disease brains, and they could be associated with disease progression. Recent studies have shown that APP processing to A beta is regulated by homodimerization, which involves both extracellular and juxtamembrane/transmembrane (JM/TM) regions. Objective: Our aim is to understand the mechanisms controlling APP dimerization and the contribution of the ectodomain and JM/TM regions to this process. Methods: We used bimolecular fluorescence complementation approaches coupled to fluorescence-activated cell sorting analysis to measure the dimerization level of different APP isoforms and APP C-terminal fragments (C99) mutated in their JM/TM region. Results: APP751 was found to form significantly more homodimers than APP695. Mutation of dimerization motifs in the TM domain of APP or C99 did not significantly affect fluorescence complementation. Conclusion: These findings indicate that the KPI domain plays a major role in APP dimerization. They set the basis for further investigation of the relation between dimerization, metabolism and function of APP. Copyright (C) 2012 S. Karger AG, Basel