Clearance of amyloid-beta peptide by neuronal and non-neuronal cells: proteolytic degradation by secreted and membrane associated proteases.

Deposition of amyloid-beta peptide (Abeta) in the brain is an early and invariant feature of all forms of Alzheimer's disease (AD). As for all proteins or peptides, the steady-state level of Abeta peptide is determined not only by its production, but also by its degradation. So, overactive proteases involved in generating Abeta from amyloid precursor protein or underactive Abeta-degrading enzymes could lead to abnormal Abeta deposition in the brain. Since in the sporadic forms of AD (90% of all AD cases) an impaired clearance of Abeta appears to be at the origin of its aggregation and tissue deposition, we have investigated its proteolytic degradation by several neuronal and non-neuronal cells. In this report, we show that these cell types exhibit a similar profile of Abeta-degradation by cell-surface and secreted proteases which were respectively characterized as metallo- and serine proteases. By using a combination of the liquid chromatography/on-line mass spectrometry, we demonstrate that: (i)-the membrane associated protease(s) hydrolizes Abeta40 essentially at Lys(28) Gly(29), Phe(19) Phe(20) and Val(18) Phe(19) bonds; and (ii)-the secreted protease(s) cleaves the generating fragments Abeta (1-28), Abeta (1-19), Abeta (1-18) at His(14) Gln(15) bond and also Abeta (1-28) at Phe(20) Ala(21) and Asp(23) Val(24) sites. This is the first time our results define a proteolytic degradation process of Abeta40 that appears to be independent of the cell type and may represent a general pattern of its enzymatic clearance.