Intracellular Aβ1-42 Aggregates Stimulate the Accumulation of Stable, Insoluble Amyloidogenic Fragments of the Amyloid Precursor Protein in Transfected Cells

Abstract We have analyzed the effect of internalized amyloid β-protein (Aβ) 1-42 aggregates on the metabolism of the amyloid precursor protein (APP) in stably transfected 293 cells. The amount of potentially amyloidogenic fragments of APP immunoprecipitated by anti-carboxyl-terminal APP and anti-Aβ antibodies is dramatically enhanced by the treatment of the cells with Aβ1-42, which is resistant to degradation, but not Aβ1-28, which does not accumulate in cells. This accumulation of amyloidogenic carboxyl-terminal fragments is specific, since there is relatively little effect of Aβ1-42 on the amount of the nonamyloidogenic α-secretase carboxyl-terminal fragment. The amyloidogenic fragments accumulate in the same nonionic detergent-insoluble fraction of the cell that contains the internalized Aβ1-42. Western analysis indicates that a subset of the amyloidogenic fragments react with antibodies that recognize a conformation of Aβ that is specifically associated with aggregated forms of Aβ, suggesting that the adoption of this aggregation-related conformation may be an early event which precedes the final processing that produces Aβ. Pulse-chase analysis of the [35S]Met-labeled 16-kDa amyloidogenic fragment indicates that it is relatively stable in Aβ1-42-treated cells, with a half-life of approximately 50 h. This fragment is degraded with a half-life of 30 min in control cells treated with Aβ1-28. In contrast, the turnover of the nonamyloidogenic α-secretase product is not significantly altered by the presence of Aβ1-42. The continuous uptake of Aβ1-42 from the medium is not required for the stimulation of amyloidogenic fragment accumulation, suggesting that the presence of intracellular Aβ1-42 aggregates establishes a new pathway for APP catabolism in cells which leads to the long term stability of the fragments. If these amyloidogenic fragments of APP ultimately give rise to Aβ, then the production of Aβ may be an autocatalytic, “runaway” process in cells containing Aβ1-42 nuclei. It is conceivable that the accumulation of insoluble APP and amyloidogenic fragments of APP in response to Aβ1-42 aggregates may mimic the pathophysiology of dystrophic neurites, where the accumulation of intracellular APP and APP fragments has been documented by immunohistochemistry.