Enhanced amyloidogenic processing of amyloid precursor protein and cell death under prolonged endoplasmic reticulum stress in brain endothelial cells.

Cerebral amyloid angiopathy resulting from the deposition of misfolded amyloid beta (Aβ) peptide in the walls of brain's blood vessels is exhibited by the majority of Alzheimer's disease (AD) patients, suggesting that alterations in protein quality control contribute to AD-associated vascular dysfunction. The present work addressed the role of ER stress in the amyloidogenic amyloid precursor protein (APP) processing and subsequent Aβ generation in brain endothelial cells (ECs). For that purpose, the RBE4 cell line was exposed to the classical ER stressors thapsigargin or brefeldin A to mimic the altered ER homeostasis observed in AD. In treated cells, an increase in the levels of markers of ER stress (XBP1 and GRP78) and of the ER stress-induced apoptotic pathway (caspase-12, JNK, and CHOP) was observed concomitantly with the accumulation of reactive oxygen species. Under these conditions, a significant ER-to-mitochondria Ca(2+) transfer was also found, which culminated in mitochondrial Ca(2+) overload and activation of mitochondria-dependent apoptosis. Moreover, it was showed that prolonged ER stress induces intracellular APP accumulation, which colocalizes with the ER chaperone GRP78, and activation of β-secretase, leading to increased intracellular Aβ levels, together with a decrease in secreted Aβ. Finally, it was demonstrated that ER stress-induced changes in Aβ levels and apoptotic cell death can be ameliorated by a blocker of the mitochondrial Bax channel. These observations suggest that chronic ER stress triggers APP accumulation in early comportments along the secretory pathway in brain ECs and increases its amyloidogenic processing and Aβ generation leading to apoptotic cell death.