Background: Several lines of evidence suggest that most of the early-onset forms of familial Alzheimer's disease (FAD) are due to inherited mutations borne by a chromosome 14-encoded protein, presenilin 1 (PS1). This is likely related to an increased production of amyloid beta-peptide (A beta) 42, one of the main components of the extracellular deposits called senile plaques that invade human cortical areas during the disease.
Materials and methods: We set up stably transfected HEK293 cells overexpressing wild-type (wt) and various FAD-linked mutated PS1. By Western blot analysis, we examined the influence of specific proteasome inhibitors on PS1-like immunoreactivities. Furthermore, by means of metabolic labeling and immunoprecipitation with A beta 40 and A beta 42-directed specific antibodies, we assessed the effect of the inhibitors on the production of A beta s by wt and mutated PS1-expressing cells transiently transfected with beta APP751.
Results: We show that two distinct proteasome inhibitors, Z-IE (Ot-Bu)A-Leucinal and lactacystin, increase in a time- and dose-dependent manner the immunoreactivities of both wt and mutated PS1. Furthermore, we demonstrate that PS1 is polyubiquitinated in these cells. Other inhibitors, ineffective on the proteasome, fail to protect wt and mutated PS1-like immunoreactivities. We also establish that the FAD-linked mutations of PS1 trigger a selective increased formation of A beta 42 as reflected by higher A beta 42 over total A beta ratios when compared with wtPS1-expressing cells. Interestingly, this augmentation was further amplified by proteasome inhibitors in cells expressing mutated but not wtPS1.
Conclusion: Altogether, our data indicate that PS1 undergoes polyubiquitination in HEK293 cells and that the proteasome contributes to the degradation of wt and FAD-linked PS1, thereby directly influencing the A beta production in human cells.