P4‐300: The Role of Degradation Pathways in Alzheimer’S Disease Pathogenesis

Abstract

Background:A significant proportion of amyloid-beta (Ab) peptides in Alzheimer's disease (AD) is truncated at the N-terminus. Among these, accumulation of pGlu-modified amyloid (pGlu-Ab3-40/42) has been shown to correlate with disease progression and tau pathology. Therapeutic strategies targeting pGlu-Ab, e.g. inhibitors of glutaminyl cyclase (QC) that catalyzes pGlu-formation, are currently in clinical assessment. However, it is still unclear, which enzyme(s) might be responsible for N-truncation of Ab and thus, generation of QC substrates. Aim of the study was to characterize the potential formation of the precursor(s) of pGlu-Ab3-40/42 by Meprin b in cell culture. Methods:We expressed different APP proteins (wt or familial Alzheimer's mutations) in the cell lines HEK293 and CHO. The APP processing and production was assessed using Western Blot analysis and ELISAs detecting N-truncated or full length Ab. Maldi-TOF mass spectrometry was used to analyze cleavage of APP-derived peptides. Results: An in vitro analysis of cleavage of APP-related peptides suggested specificity of Meprin b for the b-site of APP. The primary cleavage products were Ab(1-x), Ab(2-x) and, to a lesser extent, Ab(3-x). In cell culture, co-expression of APP and Meprin b, but not of its isoenzyme Meprin a, resulted in production of N-truncated Ab peptides, primarily Ab(2-40/42). Thereby, Meprin b cleaved preferably APPwt and not APPswedish, which contrasts to the b-secretase BACE. The cleavage of APP resulted also in minor amounts of Ab(3-40/42), which was revealed by addition of human QC and application of highly sensitive ELISA detecting pGlu-Ab3-40/42. Finally, analysis of human brain samples revealed upregulation of Meprin b, but not Meprin a, in AD. Meprin b positive astrocytes are found in the vicinity of pGlu-Ab containing deposits in human tissue. Conclusions: The data support a BACE-independent processing of APPwt that may contribute to formation of N-truncated Ab. These truncated forms, in turn, might be prone to further post-translational modification. Thus, Meprin b might represent a potential upstream target to suppress formation of pGlu-Ab.