Differential utilization of upstream AUGs in the β-secretase mRNA suggests that a shunting mechanism regulates translation

beta-Secretase [also known as the beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1)] is an enzyme involved in the production of Abeta-amyloid plaques in the brains of patients with Alzheimer's disease. The enhanced production of this enzyme occurs without corresponding changes in BACE1 mRNA levels. The complex 5' leader of BACE1 mRNA contains three upstream ORFs (uORFs) preceding the BACE1 initiation codon. In this study, we investigated how this 5' leader affects translation efficiency as a first step in understanding the enhanced production of the enzyme in the disease. Using reporter constructs in transfected mammalian cell lines and cell-free lysates, we showed that the translation mediated by the BACE1 5' leader is cap-dependent and inhibited by cis-acting segments contained within the 5' leader. Disruption of the uORFs had no effect on translation in B104 cells, which was surprising because the first two AUGs reside in contexts able to function as initiation codons. Possible mechanisms to explain how ribosomes bypass the uORFs, including reinitiation, leaky scanning, and internal initiation of translation were found to be inconsistent with the data. The data are most consistent with a model in which ribosomes shunt uORF-containing segments of the 5' leader as the ribosomes move from the 5' end of the mRNA to the initiation codon. In PC12 cells, however, the second uORF appears to be translated. We hypothesize that the translation efficiency of the BACE1 initiation codon may be increased in patients with Alzheimer's disease by molecular mechanisms that enhance shunting or increase the relative accessibility the BACE1 initiation coclon.