Genetic Inhibition of Phosphorylation of the Translation Initiation Factor eIF2α Does Not Block Aβ-Dependent Elevation of BACE1 and APP Levels or Reduce Amyloid Pathology in a Mouse Model of Alzheimer's Disease

beta-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) initiates the production of beta-amyloid (A beta), the major constituent of amyloid plaques in Alzheimer's disease (AD). BACE1 is elevated similar to 2-3 fold in AD brain and is concentrated in dystrophic neurites near plaques, suggesting BACE1 elevation is A beta-dependent. Previously, we showed that phosphorylation of the translation initiation factor eIF2 alpha de-represses translation of BACE1 mRNA following stress such as energy deprivation. We hypothesized that stress induced by A beta might increase BACE1 levels by the same translational mechanism involving eIF2 alpha phosphorylation. To test this hypothesis, we used three different genetic strategies to determine the effects of reducing eIF2 alpha phosphorylation on A beta-dependent BACE1 elevation in vitro and in vivo: 1) a two-vector adeno-associated virus (AAV) system to express constitutively active GADD34, the regulatory subunit of PP1c eIF2 alpha phosphatase; 2) a non-phosphorylatable eIF2 alpha S51A knockin mutation; 3) a BACE1-YFP transgene lacking the BACE1 mRNA 59 untranslated region (UTR) required for eIF2 alpha translational regulation. The first two strategies were used in primary neurons and 5XFAD transgenic mice, while the third strategy was employed only in 5XFAD mice. Despite very effective reduction of eIF2 alpha phosphorylation in both primary neurons and 5XFAD brains, or elimination of eIF2 alpha-mediated regulation of BACE1-YFP mRNA translation in 5XFAD brains, A beta-dependent BACE1 elevation was not decreased. Additionally, robust inhibition of eIF2 alpha phosphorylation did not block A beta-dependent APP elevation in primary neurons, nor did it reduce amyloid pathology in 5XFAD mice. We conclude that amyloid-associated BACE1 elevation is not caused by translational derepression via eIF2 alpha phosphorylation, but instead appears to involve a post-translational mechanism. These definitive genetic results exclude a role for eIF2 alpha phosphorylation in A beta-dependent BACE1 and APP elevation. We suggest a vicious pathogenic cycle wherein Ab42 toxicity induces peri-plaque BACE1 and APP accumulation in dystrophic neurites leading to exacerbated A beta production and plaque progression.