MicroRNA-339-5p Down-regulates Protein Expression of β-Site Amyloid Precursor Protein-Cleaving Enzyme 1 (BACE1) in Human Primary Brain Cultures and Is Reduced in Brain Tissue Specimens of Alzheimer Disease Subjects*

Background: BACE1 is the rate-limiting enzyme in the synthesis of A from amyloid precursor protein. Results: Human miR-339-5p negatively regulates BACE1 and A in human brain cultures and is reduced in AD specimens. Conclusion: Human miR-339-5p physiologically regulates human BACE1 protein expression and A and is dysregulated in the AD brain. Significance: miR-339-5p represents a novel drug target in AD. Alzheimer disease (AD) results, in part, from the excess accumulation of the amyloid- (A) peptide as neuritic plaques in the brain. The short A peptide is derived from the large transmembrane A precursor protein (APP). The rate-limiting step in the production of A from APP is mediated by the -site APP-cleaving enzyme 1 (BACE1). Dysregulation of BACE1 levels leading to excess A deposition is implicated in sporadic AD. Thus, elucidating the full complement of regulatory pathways that control BACE1 expression is key to identifying novel drug targets central to the A-generating process. MicroRNAs (miRNAs) are expected to participate in this molecular network. Here, we identified a known miRNA, miR-339-5p, as a key contributor to this regulatory network. Two distinct miR-339-5p target sites were predicted in the BACE1 3-UTR by in silico analyses. Co-transfection of miR-339-5p with a BACE1 3-UTR reporter construct resulted in significant reduction in reporter expression. Mutation of both target sites eliminated this effect. Delivery of the miR-339-5p mimic also significantly inhibited expression of BACE1 protein in human glioblastoma cells and human primary brain cultures. Delivery of target protectors designed against the miR-339-5p BACE1 3-UTR target sites in primary human brain cultures significantly elevated BACE1 expression. Finally, miR-339-5p levels were found to be significantly reduced in brain specimens isolated from AD patients as compared with age-matched controls. Therefore, miR-339-5p regulates BACE1 expression in human brain cells and is most likely dysregulated in at least a subset of AD patients making this miRNA a novel drug target.