The serpin-enzyme complex receptor recognizes soluble, nontoxic amyloid-beta peptide but not aggregated, cytotoxic amyloid-beta peptide

There is now extensive evidence that amyloid-beta peptide is toxic to neurons and that its cytotoxic effects can be attributed to a domain corresponding to amyloid-beta 25-35, GSNKGAIIGLM. We have shown recently that the serine proteinase inhibitor (serpin)-enzyme complex receptor (SEC-R), a receptor initially identified for binding of alpha 1-antitrypsin (alpha 1-AT) and other serine protease inhibitors, also recognizes the amyloid-beta 25-35 domain, In fact, by recognizing the amyloid-beta 25-35 domain, SEC-R mediates cell surface binding, internalization, and degradation of soluble amyloid-beta peptide, In this study, we examined the possibility that SEC-R mediates the neurotoxic effect of amyloid-beta peptide, A series of peptides based on the sequences of amyloid-beta peptide and alpha 1-AT was prepared soluble in dimethyl sulfoxide or insoluble in water and examined in assays for SEC-R binding, for cytotoxicity in neuronal PC12 cells and murine cortical neurons in primary culture, and for aggregation in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, The results show that amyloid-beta peptide 25-35 and amyloid-a peptide 1-40 prepared soluble in dimethyl sulfoxide compete for binding to SEC-R, are nontoxic, and migrate as monomers in SDS-PAGE analysis, In contrast, the same peptides aged in water did not compete for binding to SEC-R but were toxic and migrated as aggregates in SDS-PAGE, An all-D-amyloid-beta 25-35 peptide was not recognized at all by SEC-R but retained full toxic/aggregating properties, Using a series of deleted, substituted, and chimeric am beta/alpha 1-AT peptides, toxicity correlated well with aggregation but poorly with SEC-R recognition, In a subclone of PC12 cells which developed resistance to the toxic effect of aggregated amyloid-beta 25-35 there was a 2.53 fold increase in the number of SEC-R molecules/cell compared with the parent PC12 cell line, These data show that SEC-R does not mediate the cytotoxic effect of aggregated amyloid-beta peptide, Rather, SEC-R could play a protective role by mediating clearance and catabolism of soluble, monomeric amyloid-beta peptide, if soluble amyloid-beta peptide proves to be an in vivo precursor of the insoluble, toxic peptide.