Effect of γ-secretase inhibitors on muscarinic receptor-mediated calcium signaling in human salivary epithelial cells

Altered intracellular Ca2+ signaling has been observed in cells derived from Alzheimer’s disease patients, and a possible link between γ-secretase activity and the content of intracellular Ca2+ stores has been suggested. To test this hypothesis we studied the effects of several γ-secretase inhibitors on muscarinic receptor-mediated intracellular calcium release in the human salivary gland cell line HSG. Although several inhibitors in the peptide aldehyde class partially blocked carbachol-induced Ca2+ transients, these effects did not appear to be due to γ-secretase inhibition, and overall we found no evidence that inhibition of γ-secretase activity had any significant effect on agonist-induced intracellular calcium release in HSG cells. In complementary experiments with presenilin-null cells we found that the reconstitution of γ-secretase activity by transfection with wild-type presenilin 1 likewise had no significant effect on thapsigargin-induced Ca2+ release. In a test of the specific hypothesis that the level of APP intracellular domain (AICD), the intracellular fragment of the β-amyloid precursor protein (APP) resulting from γ-secretase cleavage, can modulate the Ca2+ content of the endoplasmic reticulum, we were unable to demonstrate any effect of APP small interfering RNA on the magnitude of carbachol-induced intracellular calcium release in HSG cells. Together our data cast considerable doubt on the hypothesis that there is a direct link between γ-secretase activity and the content of intracellular Ca2+ stores.