IRON LEVELS MODULATE ALPHA-SECRETASE CLEAVAGE OF AMYLOID PRECURSOR PROTEIN

The amyloid precursor protein (APP) is a membrane-spanning glycoprotei n that is the source of beta A4 peptides, which aggregate in Alzheimer 's disease to form senile plaques. APP is cleaved within the beta A4 s equence to release a soluble N-terminal derivative (APP(sol)), which h as a wide range of trophic and protective functions. In the current st udy we have examined the hypothesis that iron availability may modulat e expression or processing of APP, whose mRNA contains, based on seque nce homology, a putative iron response element (IRE). Radiolabeled APP and its catabolites were precipitated from lysates and conditioned me dium of stably transfected HEK 293 cells using antibodies selective fo r C-terminal, beta A4, and N-terminal domains. The relative abundance of the different APP catabolites under different conditions of iron av ailability was determined by quantitative densitometry after separatio n by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The da ta show a specific effect on the production of APP(sol). Using standar d conditions previously established for IRE studies, it was found that iron chelation reduces APP(sol) production, whereas iron level elevat ion augments it. No changes were observed in levels of immature and ma ture APP holoprotein or in the C-terminal alpha-secretase derivative C 83, beta A4, and p3 peptides. The specificity for modulatory changes i n APP(sol) suggests that iron acts at the level of alpha-secretase act ivity. In addition to its modulatory effects, iron at very high levels was found to inhibit maturation of APP and production of its downstre am catabolites without blocking formation of immature APP. The data es tablish a potential physiological role for iron in controlling the pro cessing of APP. If APP(sol) were to function trophically, as suggested by other studies, the current conclusion suggests that changes in iro n and iron-regulating proteins in Alzheimer's disease could contribute to neuronal degeneration by decreasing the production of APP(sol).