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Reducing Amyloid Plaque Burden via Ex Vivo Gene Delivery of an Ab-Degrading Protease: A Novel Therapeutic Approach to Alzheimer Disease
Matthew L. Hemming1, Michaela Patterson2, Casper Reske-Nielsen2, Ling Lin2, Ole Isacson2, Dennis J. Selkoe1*
1 Center for Neurologic Diseases, Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts, United States ofAmerica, 2 Neuroregeneration Laboratories, McLean Hospital and Harvard University Udall Parkinson’s Disease Research Center of Excellence, Belmont, Massachusetts, United States of America Funding: This work was supported by National Institutes of Health Grants AG12749 (DJS) and NS39793 (OI), the Stern Foundation (OI), the Anti-Aging Foundation (OI), and the Harvard Center for Neurodegeneration andRepair (MLH). DJS is a consultant to Elan. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Academic Editor: Sam Gandy, Farber Institute, United States of America Citation: Hemming ML, Patterson M, Reske-Nielsen C, Lin L, Isacson O, et al.(2007) Reducing amyloid plaque burden via ex vivo gene delivery of an Ab-degrading protease: A novel therapeutic approach to Alzheimer disease. PLoS Med 4(8): e262. doi:10. 1371/journal.pmed.0040262 Received: March 19, 2007 Accepted: July 18, 2007 Published: August 28, 2007 Copyright: Ó 2007 Hemming et al. This is an open-access article distributed under the terms of the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Abbreviations: Ab, amyloid bprotein; ACE, angiotensin-converting enzyme; AD, Alzheimer disease; APP, b-amyloid precursor protein; CHO, Chinese hamster ovary; DAGNPG, 3dansyl-D-Ala-Gly-p-(nitro)-Phe-Gly; GFAP, glial fibrillary acidic protein; HEK, humanembryonic kidney; NEP, neprilysin; SEM, standard error of the mean; sNEP, secreted neprilysin * To whom correspondence should be addressed. E-mail: dselkoe@rics.

Understanding the mechanisms of amyloid-b protein (Ab) production and clearance in the brain has been essential to elucidating the etiology of Alzheimer disease (AD). Chronically decreasing brain Ablevels is an emerging therapeutic approach for AD, but no such diseasemodifying agents have achieved clinical validation. Certain proteases are responsible for the catabolism of brain Ab in vivo, and some experimental evidence suggests they could be used as therapeutic tools to reduce Ab levels in AD. The objective of this study was to determine if enhancing the clearance of Ab in the brain by exvivo gene delivery of an Ab-degrading protease can reduce amyloid plaque burden.

Methods and Findings
We generated a secreted form of the Ab-degrading protease neprilysin, which significantly lowers the levels of naturally secreted Ab in cell culture. We then used an ex vivo gene delivery approach utilizing primary fibroblasts to introduce this soluble protease into the brains of bamyloidprecursor 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 ¼ 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 ¼ 0.0020; and 55% reduction, p ¼ 0.0081,respectively).

Ex vivo gene delivery of an Ab-degrading protease reduces amyloid plaque burden in transgenic mice expressing human APP. These results support the use of Ab-degrading proteases as a means to therapeutically lower Ab levels and encourage further exploration of ex vivo gene delivery for the treatment of Alzheimer disease.
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