The presenilin 1 ΔE9 mutation gives enhanced basal phospholipase C activity and a resultant increase in intracellular calcium concentrations

We studied effects of the familial Alzheimer's disease presenilin 1 (PS1) exon 9 deletion (PS1-DeltaE9) mutation on basal and carbachol-stimulated phosphoinositide (PI) hydrolysis and intracellular Ca2+ concentrations ([Ca2+](i)) in human SH-SY5Y neuroblastoma cells. We demonstrate that PS1-DeltaE9 cells have an enhanced basal PI hydrolysis and [Ca2+](i) as compared with both wild type PS1 (PS1-WT) and nontransfected (NT) cells. Both were reversed by the phospholipase C (PLC) inhibitor neomycin. The PS1-DeltaE9-related high basal [Ca2+](i) was also reversed by xestospongin C confirming that this effect was inositol trisphosphate receptor-mediated. Carbachol gave a greater stimulation of [Ca2+](i) in PS1-DeltaE9 cells that took longer to return to basal as compared with responses seen in NT and PS1-WT cells. This long tail-off effect seen in PS1-DeltaE9 cells after carbachol stimulation was reversed by xestospongin C and dantrolene, suggesting that it was mediated by inositol trisphosphate receptor and ryanodine receptor amplification of Ca2+. Ruthenium red only reduced carbachol peak elevations of [Ca2+](i) in NT and PS1-WT cells and not in PS1-DeltaE9 cells. No significant between cell type differences were seen for basal and carbachol-stimulated [Ca2+](i) with either ryanodine or the endoplasmic reticulum Ca2+ ATPase inhibitor cyclopiazonic acid. Immunostaining experiments revealed that for all the cell types PSI is present at the plasma membrane and colocalizes with N-cadherin, a component of the cell-cell adhesion complex. Immunoblotting of cell extracts for PLC-beta1 showed that, compared with NT and PS1-WT cells, the PS1-DeltaE9 transfectants gave a relative increase in levels of the calpain generated N-terminal fragment (100 kDa) over full-length (150 kDa) PLC-beta1. Our results suggest that the PS1-DeltaE9 mutation causes upstream changes in PI signaling with enhanced basal PLC activity as a primary effect that leads to a higher [Ca2+](i). This may provide a novel mechanism by which the PS1-DeltaE9 mutation sensitizes cells to apoptotic stimuli and enhanced amyloid beta generation.