Cell death and signal transduction pathways in Alzheimer's disease : the role of presenilin 1

Sammanfattning: Mutated presenilins (PSs) may cause familial Alzheimer's disease (FAD) by altering neuronal signal transduction pathways, by increasing AP production or by triggering a number of proapoptotic mechanisms. The present thesis explores mechanisms by which PSs regulate signal transduction and cell death with relevance to AD. Paper I explored the complex proteolytic processing of wild-type (WT) and FAD presenilin 1 (PS1) exon 9 deleted mutant (deltaE9 PS1) during apoptosis. PS1 was actively processed by both caspases and the proteasome during apoptosis in SH-SY5Y neuroblastoma cells non-transfected (NT) or transfected with WT PS1. Proteasome inhibitors blocked the degradation of full-length WT PS1 and caspase cleaved PS1 fragments. In contrast, the Nterminal and C-terminal fragments generated by PS1 endoproteolysis and the truncated fullength deltaE9 PS1 were not degraded by the proteasome during apoptosis. Paper II aimed to investigate whether PS1 cleavage by caspases is an early or a late event in the activation of apoptotic cascades. Overexpression of deltaE9 PS1 sensitized SH-SY5Y neuroblastoma cells to death induced by calcium ionophore A23187. We found that PS1 alternative cleavage is an early apoptotic event in deltaE9 PS1 transfected cells, simultaneous with minimal caspase-3 activation and preceding cleavage of poly(ADP-ribose) polymerase (PARP) and gelsolin. PS1 deltaE9 perturbed Ca2+ homeostasis and buffering in SH-SY5Y cells, this being at least one of the mechanisms by which mutated PS1 sensitized cells to apoptosis. We concluded that alternative cleavage of PS1 is an early apoptotic event; therefore it may play a role for the regulation of the proteolytic cascades during apoptosis. Papers III and IV aimed to characterize the role of PS1 in cholinergic muscarinic receptor signal transduction pathways. We studied the effect of three FAD PS1 mutants (deltaE9, M146V and L250S) and two dominant negative PS1 mutants (D257A and D385N) on basal and carbachol-stimulated phosphoinositide (PI) hydrolysis and intracellular calcium concentrations ([Ca2+]i) in SH-SY5Y neuroblastoma cells. We found a significant increase in basal PI hydrolysis in PS1 deltaE9 and PS1 M146V, but not in PS1 L250S, cells. All PS1 deltaE9, PS1 M146V and PS1 L250S cells showed a significant increase in carbachol-induced [Ca2+]i as compared to non-transfected or wild-type PS1 transfected cells. The elevated carbachol-induced [Ca2+]i signals were reversed by PLC inhibition and by ryanodine receptor blockade in all mutant PS1 cell lines. In PS1 M146V and PS1 L250S cells, pharmacological gamma-secretase inhibition was also able to reverse the elevated carbachol-induced [Ca2+]i signals. Cells expressing dominant negative PS1 had attenuated carbachol-stimulated PI hydrolysis and [Ca2+]i responses. In NT or WT PS1 cells pharmacological inhibition of gamma-secretase attenuated carbachol-stimulated PI hydrolysis and [Ca2+]i responses to levels found in PS1 dominant negative cells. The findings of these reports suggest that PS1 can regulate PLC activity and that this function is gamma-secretase activity dependent. In conclusion, these studies provide evidence that PS1 regulates both cholinergic signal transduction pathways and apoptotic cell death and that PS1 mutations are responsible for both sensitizing neurons to death and altering neuronal signal transduction.