Detection of a soluble form of BACE-1 in human cerebrospinal fluid by a sensitive activity assay

Background: Formation of deposits of the insoluble amyloid beta-peptide is believed to be causally related with neurodegeneration in Alzheimer disease (AD). The beta-peptide originates from a larger amyloid precursor protein (APP) by the action of proteolytic enzymes. The first proteolytic event leading to amyloid formation is the cleavage of APP by the membrane-bound aspartyl protease BACE-1, also known as memapsin-2. Inhibition of BACE-1 is thought to be a therapeutic approach to AD. Measuring BACE-1 activity in biological samples would be useful to elucidate the mechanism of AD and for development of AD drugs. Methods: We developed a sensitive and specific activity assay for BACE-1. The assay is based on a genetically engineered proenzyme that is specifically activated by BACE-1. The resulting active enzyme is measured with a chromogenic substrate. The use of 2 coupled reactions produces a detection limit as low as 0.4 pmol/L. Results: The assay detected BACE-1 activity in extracts of human brain tissue as well as, unexpectedly, in human cerebrospinal fluid (CSF). Gel electrophoresis and Western blotting identified the BACE-1 present in CSF as a truncated soluble form of the originally membrane-bound BACE-1. Conclusion: Detection of the soluble form of BACE-1 in CSF, a relatively easily accessible biological fluid, may be useful for monitoring the effects of drug candidates in vivo and may have diagnostic or prognostic applications. (c) 2006 American Association for Clinical Chemistry.