Downregulation of AβPP Enhances Both Calcium Content of Endoplasmic Reticulum and Acidic Stores and the Dynamics of Store Operated Calcium Channel Activity

The amyloid-beta protein precursor (A beta PP) is a type-1 transmembrane protein involved in Alzheimer's disease (AD). It has become increasingly evident that A beta PP, its protein-protein interactions, and its proteolytical fragments may affect calcium homeostasis and vice versa. In addition, there is evidence that calcium dysregulation contributes to AD. To study the role of A beta PP in calcium homeostasis, we downregulated its expression in SH-SY5Y cells using shRNA (SH-SY5Y/A beta PP-) or increased expression of A beta PP695 by transfection (SH-SY5Y/A beta PP+). The levels of cytosolic Ca2+ after treatment with thapsigargin, monensin, activation of capacitative calcium entry (CCE), and treatment with SKF, a store operated channel (SOCs) inhibitor, were measured by fura-2AM fluorimetry. SH-SY5Y/A beta PP+ cells show reduced response to thapsigargin and reduced CCE, although this reduction is not statistically significant. On the other hand, we found that, relative to SH-SY5Y, SH-SY5Y/A beta PP-cells show a significant increase in the response to thapsigargin but not in CCE and their SOCs were more susceptible to SKF inhibition. Additionally, downregulation of A beta PP resulted in increased response to monensin that induces calcium release from acidic stores. The increase of calcium release from the endoplasmic reticulum and the acidic stores, when A beta PP is downregulated, could be attributed to elevated Ca2+ content or to a dysregulation of Ca2+ transfer through their membranes. These data, along with already existing evidence regarding the role of A beta PP in calcium homeostasis and the early occurring structural and functional abnormalities of endosomes, further substantiate the role of A beta PP in calcium homeostasis and in AD.