INTRACELLULAR A-BETA-1-42 AGGREGATES STIMULATE THE ACCUMULATION OF STABLE, INSOLUBLE AMYLOIDOGENIC FRAGMENTS OF THE AMYLOID PRECURSOR PROTEIN IN TRANSFECTED CELLS

We have analyzed the effect of internalized amyloid beta-protein (A beta) 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 immunoprecipitatedby anti-carboxyl-terminal APP and anti-A beta antibodies is dramatically enhanced by the treatment of the cells with A beta 1-42, which is resistant to degradation, but not A beta 1-28, which does not accumulate in cells. This accumulation of amyloidogenic carboxyl-terminal fragments is specific, since there is relatively little effect of A beta 1-42 on the amount of the nonamyloidogenic alpha-secretase carboxyl-terminal fragment. The amyloidogenic fragments accumulate in the same nonionic detergent-insoluble fraction of the cell that contains the internalized A beta 1-42. Western analysis indicates that a subset of the amyloidogenic fragments react with antibodies that recognize a conformation of A beta that is specifically associated with aggregated forms of A beta, suggesting that the adoption of this aggregation-related conformation may be an early event which precedes the final processing that produces A beta. Pulse-chase analysis of the [S-35]Met-labeled 16-kDa amyloidogenic fragment indicates that it is relatively stable in A beta 1-42-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 A beta 1-28. In contrast, the turnover of the nonamyloidogenic alpha-secretase product is not significantly altered by the presence of A beta 1-42, The continuous uptake of A beta 1-42 from the medium is not required for the stimulation of amyloidogenic fragment accumulation, suggesting that the presence of intracellular A beta 1-42 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 A beta, then the production of A beta may be an autocatalytic, ''runaway'' process in cells containing A beta 1-42 nuclei. It is conceivable that the accumulation of insoluble APP and amyloidogenic fragments of APP in response to A beta 1-42 aggregates may mimic the pathophysiology of dystrophic neurites, where the accumulation of intracellular APP and APP fragments has been documented by immunohistochemistry.