Familial Alzheimer's Disease Mutations in Presenilins: Effects on Endoplasmic Reticulum Calcium Homeostasis and Correlation with Clinical Phenotypes

Abstract

Mutations in presenilins (PS1 and PS2) are responsible for approximately 40% of all early onset familial Alzheimer’s disease (FAD) monogenic cases. Presenilins (PSs) function as the catalytic subunit of γ-secretase and support cleavage of the amyloid precursor protein (APP). We previously discovered that PSs also function as passive endoplasmic reticulum (ER) calcium (Ca2+) leak channels and that most FAD mutations in PSs affected their ER Ca2+ leak function. To further validate the relevance of our findings to human disease, we here performed Ca2+ imaging experiments with lymphoblasts established from FAD patients. We discovered that most FAD mutations in PSs disrupted ER Ca2+ leak function and resulted in increased ER Ca2+ pool in human lymphoblasts. However, we found that a subset of PS1 FAD mutants supported ER Ca2+ leak activity, as ER Ca2+ pool was unaffected in lymphoblasts. Most of the “functional” mutations for ER Ca2+ leak were clustered in the exon 8–9 area of PSEN1 gene and segregated with the cotton wool plaques and spastic paraparesis (CWP/SP) clinical phenotype occasionally observed in PS1 FAD patients. Our findings with the “functional” and “non-functional” PS1 FAD mutants were confirmed in Ca2+ rescue experiments with PS double-knockout (DKO) mouse embryonic fibroblasts. Based on the combined effects of the PS1 FAD mutations on ER Ca2+ leak and γ-secretase activities we propose a model that explains the heterogeneity observed in FAD. The proposed model has implications for understanding the pathogenesis of both familial and sporadic AD.

Familial Alzheimer’s disease–associated presenilin-1 alters cerebellar activity and calcium homeostasis

“…Our results indicate that altered Ca 2+ homeostasis in the cerebella of these patients might be yet another consequence, which does not necessarily depend on Aβ-or pTau-related processes. Finally, our data are in line with a study in which FAD-derived lymphoblasts showed altered ER Ca 2+ homeostasis (34). However, in this lymphoblast study, PS1 E280G mutation showed preserved ER Ca 2+ leak function.…”

Lack of Evidence for Presenilins as Endoplasmic Reticulum Ca2+ Leak Channels

“…Subsequent reports employed D1-ER measurements in transfected primary hippocampal neurons from triple transgenic mice (harboring the PS1M146V-KIN mutation), but only a qualitative description was provided without quantitative analyses of leak rates (30). The use of the AI as an indirect estimate of [Ca 2ϩ ] ER to infer conclusions regarding the role of PS in ER Ca 2ϩ permeability has been the basis of the primary quantitative analysis in the previous studies (26,28,30,31). When we measured the AI, we observed the phenomenon reported in these studies; PS DKO and hPS1M146L expressing fibroblasts had an enhanced area under their whole cell ionomycin-induced Ca 2ϩ release versus time curve compared with WT hPS1-and hPS1D257A-expressing cells.…”

“…However, this indirect measurement is compromised by strong influences of Ca 2ϩ clearance and buffering. Mag-Fura 2 was measured in permeabilized PS DKO and WT MEF cells (31) and in PS DKO transiently and stably expressing FAD mutants (26,27) to quantify [Ca 2ϩ ] ER . However, Mag-Fura 2 may compartmentalize into many distinct intracellular compartments.…”

Presenilins as endoplasmic reticulum calcium leak channels and Alzheimer’s disease pathogenesis

“…We found that many FAD mutations in PS1 disrupt this Ca 2+ leak function [56][57][58], resulting in overloaded ER Ca 2+ stores and exaggerated ER Ca 2+ release in double PS knock-out fibroblasts [56][57][58], cultured hippocampal neurons from PS double-knockout mice and PS1-M146V mutant neurons [51] and lymphoblasts from FAD patients [58]. These observations suggest that PS1 plays a pivotal role in deranged Ca 2+ in AD and they also provide further support for the contribution of PS1 to Ca 2+ homeostasis in neurons.…”

“…Could this variant AD somehow be explained by the altered ER Ca 2+ signaling caused by PS1-FAD mutations? Over a series of experiments [56][57][58], we have tested a total of 23 FAD mutations in PS1, 1 FAD mutation in PS2 and 3 FTD implicated PS1 mutations. We concluded that 14 FAD mutations abolished ER Ca 2+ leak function of PS ( Figure 2A).…”