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 c orresponding to amyloid-beta 25-35, GSNKGAIIGLM. We have shown recentl y that the serine proteinase inhibitor (serpin)-enzyme complex recepto r (SEC-R), a receptor initially identified for binding of alpha 1-anti trypsin (alpha 1-AT) and other serine protease inhibitors, also recogn izes the amyloid-beta 25-35 domain, In fact, by recognizing the amyloi d-beta 25-35 domain, SEC-R mediates cell surface binding, internalizat ion, and degradation of soluble amyloid-beta peptide, In this study, w e examined the possibility that SEC-R mediates the neurotoxic effect o f 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 bindi ng, for cytotoxicity in neuronal PC12 cells and murine cortical neuron s in primary culture, and for aggregation in sodium dodecyl sulfate-po lyacrylamide gel electrophoresis (SDS-PAGE) analysis, The results show that amyloid-beta peptide 25-35 and amyloid-a peptide 1-40 prepared s oluble in dimethyl sulfoxide compete for binding to SEC-R, are nontoxi c, 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 c himeric am beta/alpha 1-AT peptides, toxicity correlated well with agg regation but poorly with SEC-R recognition, In a subclone of PC12 cell s 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 mole cules/cell compared with the parent PC12 cell line, These data show th at SEC-R does not mediate the cytotoxic effect of aggregated amyloid-b eta peptide, Rather, SEC-R could play a protective role by mediating c learance 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.