The presenilin 1 deltaE9 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 Ca(2+) concentrations (Ca(2+)) in human SH-SY5Y neuroblastoma cells. We demonstrate that PS1-DeltaE9 cells have an enhanced basal PI hydrolysis and Ca(2+) 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 Ca(2+) was also reversed by xestospongin C confirming that this effect was inositol trisphosphate receptor-mediated. Carbachol gave a greater stimulation of Ca(2+) 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 Ca(2+). Ruthenium red only reduced carbachol peak elevations of Ca(2+) 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 Ca(2+) with either ryanodine or the endoplasmic reticulum Ca(2+) ATPase inhibitor cyclopiazonic acid. Immunostaining experiments revealed that for all the cell types PS1 is present at the plasma membrane and co-localizes 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 Ca(2+). This may provide a novel mechanism by which the PS1-DeltaE9 mutation sensitizes cells to apoptotic stimuli and enhanced amyloid beta generation.