APPLICATION OF B-SECRETASE FOR PRESYMPTOMATIC DIAGNOSIS OF ALZHEIMERS DISEASE

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The processing of β-amyloid precursor protein (APP) and generation of β-amyloid (Aβ) essentially are associated with the pathophysiology of Alzheimer's disease (AD). As the proteases responsible for the process in the human brain have yet to be clarified, we searched for activities capable of cleaving native brain APP in human hippocampus. 40 kDa proteolytic activity that degrades native brain APP in vitro was purified and characterized, and following molecular analysis clarified as being a novel protease belonging to the carboxypeptidase B (CPB) family. PC12 cells overexpressing the cDNA encoding this protease generate a major 12 kDa β-amyloid-bearing peptide in cytosol, which peptide was also detected in a cell-free system using purified brain APP as substrate. although the protease is homologous to plasma CPB synthesized in liver, it has specific domain such as C-terminal 14 amino acid residues. Western analysis, cDNA-cloning process, and Northern analysis suggested a brain-specific expression of this protease. An immunohistochemical study showed that the protease is expressed in various neuronal perikarya, including that of pyramidal neurons of the hippocampus, ependymal-choroid plexus cells, and in a portion of the microglia of normal brains. In brains of patients with sporadic AD, decreased neuronal expression and a cluster of microglia with protease immunoreactivity associated with its extracellular deposition being detected. These findings suggest that brain CPB has a physiological function in APP processing and may have significance in AD pathophysiology.