BACKGROUND: Understanding the mechanisms of amyloid-beta protein (Abeta) 
production and clearance in the brain has been essential to elucidating the 
etiology of Alzheimer disease (AD). Chronically decreasing brain Abeta levels is 
an emerging therapeutic approach for AD, but no such disease-modifying agents 
have achieved clinical validation. Certain proteases are responsible for the 
catabolism of brain Abeta in vivo, and some experimental evidence suggests they 
could be used as therapeutic tools to reduce Abeta levels in AD. The objective 
of this study was to determine if enhancing the clearance of Abeta in the brain 
by ex vivo gene delivery of an Abeta-degrading protease can reduce amyloid 
plaque burden.
METHODS AND FINDINGS: We generated a secreted form of the Abeta-degrading 
protease neprilysin, which significantly lowers the levels of naturally secreted 
Abeta in cell culture. We then used an ex vivo gene delivery approach utilizing 
primary fibroblasts to introduce this soluble protease into the brains of 
beta-amyloid precursor protein (APP) transgenic mice with advanced plaque 
deposition. Brain examination after cell implantation revealed robust clearance 
of plaques at the site of engraftment (72% reduction, p = 0.0269), as well as 
significant reductions in plaque burden in both the medial and lateral 
hippocampus distal to the implantation site (34% reduction, p = 0.0020; and 55% 
reduction, p = 0.0081, respectively).
CONCLUSIONS: Ex vivo gene delivery of an Abeta-degrading protease reduces 
amyloid plaque burden in transgenic mice expressing human APP. These results 
support the use of Abeta-degrading proteases as a means to therapeutically lower 
Abeta levels and encourage further exploration of ex vivo gene delivery for the 
treatment of Alzheimer disease.