Laboratory for Proteolytic Neuroscience 　 Background of the study Sporadic Alzheimer's disease(AD), similar to familial AD, presents pathological changes such as the deposition of amyloid peptide (A). However, no evidence that A production is increased in sporadic AD has been appeared. We assumed that A accumulation could be attributed to a decrease in degrading ability (Fig). Detailed studies on the mechanism underlying A production have been performed, whereas the catabolic mechanism of A degradation has not been clarified at all. With this as a background, this study was initiated. Results Two methods were employed to study A degradation in brain. One was analysis of catabolic process of a multiply radiolabeled A injected into the brain using a high-pressure liquid chromatography. The other was measurement of endogenous A levels using a specific and sensitive emzyme-linked immunosorbent assay. Using these methods, the following were clarified. (1) The degradation of the radiolabeled A was markedly decreased in the neprilysin-knockout mouse brain, indicating that neprilysin is the major A-degrading enzyme . (2) The amount of endogenous A in brain was increased approximately 2-fold in the neprilysin-knockout mouse, indicating that neprilysin plays a role in degradation of endogenous A. (3) In both aforementioned methods, A degradation was partially inhibited in the neprilysin-heterozygously knockout mouse brain. The A levels in brain were inversely correlated with the neprilysin gene-dose, indicating that even a partial down-regulation of neprilysin activity will promote A deposition in the brain. (4) The A levels in the brain of the neprilysin-knockout mouse were the highest in the hippocampus and the lowest in the cerebellum. This finding correlated well with severity of A pathology of AD brains. Furthermore, McGeer et al. (University of British Columbia) studied the expression pattern of neprilysin in AD brains based on results of our previous study, and reported that the expression level of neprilysin was selectively decreased in the hippocampus, which agrees with our present results. Prospect of future studies The results of our present study suggest a possibility that a decrease in the activity or expression level of neprilysin in brain with aging results in A deposition, thus causing AD. In the future, the following developments are expected: (1) Prediction of new genetic risk factors. The expression of neprilysin genes is tissue-specifically controlled. Neurons express type 1 mRNA, therefore, it is assumed that gene mutation or polymorphism in a promoter region regulating type 1 mRNA expression will influence the onset of AD. (2) Identification of risk factors and prevention of AD by eliminating risk factors. A factor that decreases the activity or expression level of neprilysin can be a risk factor for AD. To identify risk factors and eliminate them would reduce the risk of onset. (3) Inhibition of the onset by controlling the activity or expression level of neprilysin. If the neprilysin expression level in brain can be increased by gene therapy or transcriptional regulation, the onset of both sporadic and familial AD could be inhibited. (4) Diagnosis of AD before its onset. If the expression level of neprilysin is decreased with the aging of the brain, the risk of onset can be predicted by determining the amount of neprilysin in an individual. Conclusion Japanese researchers studying proteases have been tackled the question, "How can the longevity of individual proteins in an organism be determined?" for several decades. This study represents a part of the effort to obtain an answer to this classic biochemical question. 　 Iwata, N., Tsubuki, S., Takaki, Y., Shirotani,K., Lu,B.,Gerard,N.P., Gerard, C.,Hama,E.,Lee,H,-J.,Saodo,T.C.(2001). Metabolic regulation of brain Ab by neprilysin. Science, 292, 1550-1552.