Differential effects of familial Alzheimer's disease-causing mutations on amyloid precursor protein (APP) trafficking, proteolytic conversion, and synaptogenic activity

The amyloid precursor protein (APP) is a key player in Alzheimer`s disease (AD) and the precursor of the A beta peptide, which is generated by consecutive cleavages of beta- and gamma-secretases. Familial Alzheimer's disease (FAD) describes a hereditary subgroup of AD that represents a low percentage of AD cases with an early onset of the disease. Different APP FAD mutations are thought to have qualitatively different effects on its proteolytic conversion. However, few studies have explored the pathogenic and putative physiological differences in more detail. Here, we compared different FAD mutations, located at the beta- (Swedish), alpha- (Flemish, Arctic, Iowa) or gamma-secretase (Iberian) cleavage sites. We examined heterologous expression of APP WT and FAD mutants in non-neuronal cells and their impact on presynaptic differentiation in contacting axons of co-cultured neurons. To decipher the underlying molecular mechanism, we tested the subcellular localization, the endocytosis rate and the proteolytic processing in detail by immunoprecipitation-mass spectrometry. Interestingly, we found that only the Iberian mutation showed altered synaptogenic function. Furthermore, the APP Iowa mutant shows significantly decreased alpha-secretase processing which is in line with our results that APP carrying the Iowa mutation was significantly increased in early endosomes. However, most interestingly, immunoprecipitation-mass spectrometry analysis revealed that the amino acid substitutions of APP FAD mutants have a decisive impact on their processing reflected in altered A beta profiles. Importantly, N-terminally truncated A beta peptides starting at position 5 were detected preferentially for APP Flemish, Arctic, and Iowa mutants containing amino acid substitutions around the alpha-secretase cleavage site. The strongest change in the ratio of A beta 40/A beta 42 was observed for the Iberian mutation while APP Swedish showed a substantial increase in A beta 1-17 peptides. Together, our data indicate that familial AD mutations located at the alpha-, beta-, and gamma-secretase cleavage sites show considerable differences in the underlying pathogenic mechanisms.