Abstract
BackgroundUnderstanding the mechanisms of amyloid-β protein (Aβ) production and clearance in the brain has been essential to elucidating the etiology of Alzheimer disease (AD). Chronically decreasing brain Aβ 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 Aβ in vivo, and some experimental evidence suggests they could be used as therapeutic tools to reduce Aβ levels in AD. The objective of this study was to determine if enhancing the clearance of Aβ in the brain by ex vivo gene delivery of an Aβ-degrading protease can reduce amyloid plaque burden.Methods and FindingsWe generated a secreted form of the Aβ-degrading protease neprilysin, which significantly lowers the levels of naturally secreted Aβ in cell culture. We then used an ex vivo gene delivery approach utilizing primary fibroblasts to introduce this soluble protease into the brains of β-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).ConclusionsEx vivo gene delivery of an Aβ-degrading protease reduces amyloid plaque burden in transgenic mice expressing human APP. These results support the use of Aβ-degrading proteases as a means to therapeutically lower Aβ levels and encourage further exploration of ex vivo gene delivery for the treatment of Alzheimer disease.
Highlights
The pathologic hallmarks of Alzheimer disease (AD) are extracellular plaques of amyloid-b protein (Ab) and intraneuronal neurofibrillary tangles of tau protein, both of which accumulate in brain regions mediating memory and cognition [1]
Ex vivo gene delivery of an Ab-degrading protease reduces amyloid plaque burden in transgenic mice expressing human amyloid precursor protein (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
Cleavage of the signal peptide produces a soluble, secreted form of NEP. (C) NEP immunoblot of cellular lysates from Chinese hamster ovary (CHO) cells transfected with empty vector, NEP, or secreted neprilysin (sNEP) constructs. (D) NEP activity assay in which the fluorogenic NEP substrate DAGNPG was incubated with lysates from CHO cells stably transfected with the indicated constructs
Summary
The pathologic hallmarks of Alzheimer disease (AD) are extracellular plaques of amyloid-b protein (Ab) and intraneuronal neurofibrillary tangles of tau protein, both of which accumulate in brain regions mediating memory and cognition [1]. Supporting a therapeutic function for Abdegrading proteases, both transgenic overexpression [15] and direct viral vector injection [14,16,17] of these enzymes have been shown to potently lower Ab levels and plaque burden and reduce Ab-associated neuropathology The applicability of these approaches for AD treatment in humans is uncertain, and alternative methods of gene delivery merit exploration. There are a number of inherited forms of Alzheimer disease which are caused by mutations either in the protein from which Ab is derived, called betaamyloid precursor protein (shortened to APP) or in other proteins that act to release the Ab protein from APP Research on these inherited forms of Alzheimer disease has helped in the understanding of how plaques accumulate, which has subsequently led to new potential approaches to the treatment of Alzheimer disease such as lowering the production of Ab from APP or enhancing clearance of the plaques
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