Missense mutations in the human presenilin-1 (PS1) gene, which is found on chromosome 14, cause early-onset familial Alzheimer's disease (FAD). FAD-linked PS1 variants alter proteolytic processing of the amyloid precursor protein and cause an increase in vulnerability to apoptosis induced by various cell stresses. However, the mechanisms responsible for these phenomena are not clear. Here we report that mutations in PS1 affect the unfolded-protein response (UPR), which responds to the increased amount of unfolded proteins that accumulate in the endoplasmic reticulum (ER) under conditions that cause ER stress. PS1 mutations also lead to decreased expression of GRP78/Bip, a molecular chaperone, present in the ER, that can enable protein folding. Interestingly, GRP78 levels are reduced in the brains of Alzheimer's disease patients. The downregulation of UPR signalling by PS1 mutations is caused by disturbed function of IRE1, which is the proximal sensor of conditions in the ER lumen. Overexpression of GRP78 in neuroblastoma cells bearing PS1 mutants almost completely restores resistance to ER stress to the level of cells expressing wild-type PS1. These results show that mutations in PS1 may increase vulnerability to ER stress by altering the UPR signalling pathway.

The rat homologue of a mitochondrial ATP-dependent protease Lon was cloned from cultured astrocytes exposed to hypoxia. Expression of Lon was enhanced in vitro by hypoxia or ER stress, and in vivo by brain ischemia. These observations suggested that changes in nuclear gene expression (Lon) triggered by ER stress had the potential to impact important mitochondrial processes such as assembly and/or degradation of cytochrome c oxidase (COX). In fact, steady-state levels of nuclear-encoded COX IV and V were reduced, and mitochondrial-encoded subunit II was rapidly degraded under ER stress. Treatment of cells with cycloheximide caused a similar imbalance in the accumulation of COX subunits, and enhanced mRNA for Lon and Yme1, the latter another mitochondrial ATP-dependent protease. Furthermore, induction of Lon or GRP75/mtHSP70 by ER stress was inhibited in PERK (−/−) cells. Transfection studies revealed that overexpression of wild-type or proteolytically inactive Lon promoted assembly of COX II into a COX I–containing complex, and partially prevented mitochondrial dysfunction caused by brefeldin A or hypoxia. These observations demonstrated that suppression of protein synthesis due to ER stress has a complex effect on the synthesis of mitochondrial-associated proteins, both COX subunits and ATP-dependent proteases and/or chaperones contributing to assembly of the COX complex.

An alternative spliced form of the presinilin 2 (PS2) gene (PS2V) lacking exon 5 has previously been reported to be expressed in human brains in sporadic Alzheimer's disease (AD). PS2V encodes the amino-terminal portion of PS2, which contains residues Met 1 -Leu 119 and 5 additional amino acid residues (SSMAG) at its carboxyl terminus. Here we report that PS2V protein impaired the signaling pathway of the unfolded protein response, similarly to familial AD-linked PS1 mutants and caused significant increases in the production of both amyloid ␤ 40 and ␤ 42 . Interestingly, PS2V-encoding protein was expressed in neuropathologically affected neurons of the hippocampal CA1 region and temporal cortex in AD patients. These findings suggest that the aberrant splicing of the PS2 gene may be implicated in the neuropathology of sporadic AD.

Abstract:Mutations in the presenilin-1 (PS-1) and presenilin-2 (PS-2) genes account for the majority of cases of early-onset familial Alzheimer's disease (AD). Alternative splicing forms of the PS-1 and PS-2 gene products have previously been reported in fibroblast and brain tissue from both familial and sporadic AD patients, as well as from normal tissues and cell lines. We demonstrate here unusual alternative splicing of the PS-2 gene that leads to the generation of mRNA lacking exon 5 in human brain tissue. This product was more frequently detected in brain tissue from sporadic AD patients (70.0%; 21 of 30) than from normal age-matched controls (17.6%; three of 17). In cultured neuroblastoma cells, this splice variant was generated in hypoxia but not under other forms of cellular stress. Hypoxia-mediated induction of this splice variant was blocked by pretreatment of neuroblastoma cells with the protein synthesis inhibitor cycloheximide or antioxidants such as N-acetylcysteine and diphenyl iodonium, suggesting that hypoxia-mediated oxidant stress might, at least in part, underlie the alternative splicing of PS-2 mRNA through de novo protein synthesis. Furthermore, the stable transfectants of this splice variant produced the N-terminal part of PS-2 protein (15 kDa) and were more susceptible to cellular stresses than control transfectants. These results suggest the possibility that altered presenilin gene products in stress conditions may also participate in the pathogenesis of AD.