Clearance of extracellular and cell-associated amyloid beta peptide through viral expression of neprilysin in primary neurons

Amyloid beta peptide (A beta), the pathogenic agent of Alzheimer's disease (AD), is a physiological metabolite constantly anabolized and catabolized i n the brain. We previously demonstrated that neprilysin is the major A beta -degrading enzyme in vivo. To investigate whether or not manipulation of n eprilysin activity in the brain would be an effective strategy for regulati ng A beta levels, we expressed neprilysin in primary cortical neurons using a Sindbis viral vector and examined the effect on A beta metabolism. The c orresponding recombinant protein, expressed in the cell bodies and processe s, exhibited thiorphan-sensitive endopeptidase activity, whereas a mutant n eprilysin with an amino acid substitution in the active site did not show a ny such activity. Expression of the wildtype neprilysin, but not the mutant , led to significant decreases in both the A beta 40 and 42 levels in the c ulture media in a dose-dependent manner. Moreover, neprilysin expression al so resulted in reducing cell-associated A beta, which could be more neuroto xic than extracellular A beta. These results indicate that the manipulation of neprilysin activity in neurons, the major source of A beta in the brain , would be a relevant strategy for controlling the A beta levels and thus t he A beta -associated pathology in brain tissues.