Calcium dyshomeostasis and intracellular signalling in Alzheimer's disease

The amyloid precursor protein protects PC12 cells against endoplasmic reticulum stress-induced apoptosis

Endoplasmic reticulum (ER) stress is believed to play an important role in neurodegenerative disorders such as Alzheimer's disease. In the present study, we investigated the effect of the human amyloid precursor protein (APP) on the ER stress response in PC12 cells. Tunicamycin, an inhibitor of N-glycosylation, rapidly induced the expression of the ER-resident chaperone Bip/grp78, a known target gene of the unfolded protein response. Prolonged treatment with tunicamycin (>/= 12 h) resulted in the activation of executioner caspases 3 and 7. Interestingly, PC12 cells overexpressing human wild-type APP (APPwt) showed increased resistance to tunicamycin-induced apoptosis compared with empty vector-transfected controls. This neuroprotective effect was significantly diminished in cells expressing the Swedish mutation of APP (KM670/671NL). Similar effects were observed when ER stress was induced with brefeldin A, an inhibitor of ER-to-Golgi protein translocation. Of note, APP-mediated neuroprotection was not associated with altered expression of Bip/grp78 or transcription factor C/EBP homologous protein-10 (CHOP/GADD153), suggesting that APP acted either downstream or independently of ER-to-nucleus signaling. Our data indicate that APP plays an important physiological role in protecting neurons from the consequences of prolonged ER stress, and that APP mutations associated with familial Alzheimer's disease may impair this protective activity.

Endoplasmic reticulum: a primary target in various acute disorders and degenerative diseases of the brain

Changes in neuronal calcium activity in the various subcellular compartments have divergent effects on affected cells. In the cytoplasm and mitochondria, where calcium activity is normally low, a prolonged excessive rise in free calcium levels is believed to be toxic, in the endoplasmic reticulum (ER), in contrast, calcium activity is relatively high and severe stress is caused by a depletion of ER calcium stores. Besides its role in cellular calcium signaling, the ER is the site where membrane and secretory proteins are folded and processed. These calcium-dependent processes are fundamental to normal cell functioning. Under conditions of ER dysfunction unfolded proteins accumulate in the ER lumen, a signal responsible for activation of the unfolded protein response (UPR) and the ER-associated degradation (ERAD). UPR is characterized by activation of two ER-resident kinases, PKR-like ER kinase (PERK) and IRE1. PERK induces phosphorylation of the eukaryotic initiation factor (eIF2alpha), resulting in a shut-down of translation at the initiation step. This stress response is needed to block new synthesis of proteins that cannot be correctly folded, and thus to protect cells from the effect of unfolded proteins which tend to form toxic aggregates. IRE1, on the other hand, is turned after activation into an endonuclease that cuts out a sequence of 26 bases from the coding region of xbp1 mRNA. Processed xbp1 mRNA is translated into the respective protein, an active transcription factor specific for ER stress genes such as grp78. In acute disorders and degenerative diseases, the ER calcium pool is a primary target of toxic metabolites or intermediates, such as oxygen free radicals, produced during the pathological process. Affected neurons need to activate the entire UPR to cope with the severe form of stress induced by ER dysfunction. This stress response is however hindered under conditions where protein synthesis is suppressed to such an extent that processed xbp1 mRNA is not translated into the processed XBP1 protein (XBP1(proc)). Furthermore, activation of ERAD is important for the degradation of unfolded proteins through the ubiquitin/proteasomal pathway, which is impaired in acute disorders and degenerative diseases, resulting in further ER stress. ER functioning is thus impaired in two different ways: first by the direct action of toxic intermediates, produced in the course of the pathological process, hindering vital ER reactions, and second by the inability of cells to fully activate UPR and ERAD, leaving them unable to withstand the severe form of stress induced by ER dysfunction.

Neuronal ageing from an intraneuronal perspective: roles of endoplasmic reticulum and mitochondria

The nature of brain ageing and the age-dependent decline in cognitive functions remains poorly understood. Physiological brain ageing is characterised by mild mental dysfunctions, whereas age-dependent neurodegeneration, as illustrated by Alzheimer disease (AD), results rapidly in severe dementia. These two states of the aged brain, the physiological and the pathological, are fundamentally different as the latter stems from significant neuronal loss, whereas the former develops without significant neuronal demise. In this paper, we review the changes in neuronal Ca(2+) homeostasis that occur during brain ageing, and conclude that normal, physiological ageing is characterised mainly by a decrease of neuronal homeostatic reserve, defined as the capacity to respond effectively to functional and metabolic stressors, but does not reach the trigger required to induce neuronal death. In contrast, during neurodegenerative states, Ca(2+) homeostasis is affected early during the pathological process and result in significant neuronal demise. We also review recent evidence suggesting that the endoplasmic reticulum (ER) might play an important role in controlling the balance between healthy and pathological neuronal ageing.

Triple-transgenic model of Alzheimer's disease with plaques and tangles: intracellular Abeta and synaptic dysfunction

The neuropathological correlates of Alzheimer's disease (AD) include amyloid-beta (Abeta) plaques and neurofibrillary tangles. To study the interaction between Abeta and tau and their effect on synaptic function, we derived a triple-transgenic model (3xTg-AD) harboring PS1(M146V), APP(Swe), and tau(P301L) transgenes. Rather than crossing independent lines, we microinjected two transgenes into single-cell embryos from homozygous PS1(M146V) knockin mice, generating mice with the same genetic background. 3xTg-AD mice progressively develop plaques and tangles. Synaptic dysfunction, including LTP deficits, manifests in an age-related manner, but before plaque and tangle pathology. Deficits in long-term synaptic plasticity correlate with the accumulation of intraneuronal Abeta. These studies suggest a novel pathogenic role for intraneuronal Abeta with regards to synaptic plasticity. The recapitulation of salient features of AD in these mice clarifies the relationships between Abeta, synaptic dysfunction, and tangles and provides a valuable model for evaluating potential AD therapeutics as the impact on both lesions can be assessed.

PMR1/SPCA Ca2+ pumps and the role of the Golgi apparatus as a Ca2+ store

Besides the well-known sarco/endoplasmic-reticulum Ca(2+)-transport ATPases (SERCA), animal cells contain a much less characterized P-type Ca(2+)-transport ATPase: the PMR1/SPCA Ca(2+)/Mn(2+)-transport ATPase. SPCA is mainly targeted to the Golgi apparatus. Phylogenetic analysis indicates that it might be more closely related to a putative ancestral Ca(2+) pump than SERCA. SPCA supplies the Golgi apparatus, and possibly other more distal compartments of the secretory pathway, with the Ca(2+) and Mn(2+) necessary for the production and processing of secretory proteins. In the lactating mammary gland, SPCA appears to be the primary pump responsible for supplementing the milk with high (60-100 mM) Ca(2+). It could also play a role in detoxification of cells overloaded with Mn(2+). Mutations in the human gene encoding the SPCA pump ( ATP2C1) result in Hailey-Hailey disease, a keratinocyte disorder characterized by incomplete cell adhesion. Recent observations show that the Golgi apparatus can function as a Ca(2+) store, which can be involved in setting up cytosolic Ca(2+) oscillations.