These proteases cleave APP within the extracellular domain, probably in the endosomal-lysosomal pathway following reinternalization of extracellular membrane-bound APP that escapes α-secretase cleavage.26 Action of the putative protease γ-secretase at a second site

releases free Aβ of between 40 and 42 amino acids, depending on the exact site of cleavage. The γ-secretase site is unusual Inhibitors,research,lifescience,medical in that it is buried within the lipid bilayer. Mutations in APP and the formation of Aβ Activity of all three secretases can be found in normal brain. Aβ and APPs can be detected from normal cells, and, in humans, Aβ is detectable by enzyme-linked immunosorbent assay (ELISA) in cerebrospinal fluid (CSF) as well as in Inhibitors,research,lifescience,medical serum. These, then, are not pathological processes per se, but rather they suggest that disease results from a tendency towards the amyloidogenic combination of secretases resulting, over a lifetime, in increased Aβ formation and increased plaque formation. What then are the known influences on these, essentially normal, processes? The first

influence on APP metabolism to be discovered was the mutations in APP. Autosomal dominant AD in a few rare families results from mutations that cluster adjacent to the regions of α-, β-, or γ-secretase cleavage. The first set of mutations to be discovered were those clustering at, or adjacent to, the γ-secretase Inhibitors,research,lifescience,medical cleavage Inhibitors,research,lifescience,medical site (APP717). Expression of these mutated APP cDNAs in cells confirmed that the mutation does indeed alter APP metabolism, as relatively more of the longer forms of Aβ were generated in mutation-carrying cells.27,28 Mutations at the c-terminal end of the Aβ sequence within APP also alter APP metabolism, presumably by

interfering with BACE. These mutations, the double Swedish mutation (APP670/671), also alter APP metabolism in cultured cells, and the amount of Aβ Inhibitors,research,lifescience,medical in serum or CSF of patients carrying either the mutations near the γ- or the β-secretase site is increased.29,30 Two very interesting mutations occur within the Aβ region close to the α-secretasc site. One, at APP693, is associated with a rare disorder, hereditary cerebral Carnitine dehydrogenase hemorrhage with amyloidosis, Dutch type, and the other, at APP692, with presenile dementia and cerebral hemorrhage due to cerebral amyloid angiopathy – a clearly related, but, not identical, disorder. In the APP692 disorder, but not in APP693 disease, there was not only angiopathy but, large plaques and neurofibrillary tangles.31 In cells, the effect of the APP692 mutation is to increase both Aβ40 and Aβ42 secretion, whereas APP693 does not. Thus there is, in the APP mutations, convincing evidence in favor of the amyloid ABT-378 cascade hypothesis – mutations associated with AD increase either all Aβ or the longer and more fibrillogenic forms of Aβ, whereas mutations associated withother disease do not.