Alzheimer disease (AD) is a neurodegenerative disease which is characterized by the presence of extracellular senile plaques mainly composed of amyloid-beta peptide (Abeta), intracellular neurofibrillary tangles, and selective synaptic and neuronal loss. AD brains revealed elevated levels of oxidative stress markers which have been implicated in Abeta-induced toxicity. In the present work we addressed the hypothesis that oxidative stress occurs early in the development of AD and evaluated the extension of the oxidative stress and the levels of antioxidants in an in vivo model of AD, the triple-transgenic mouse, which develops plaques, tangles, and cognitive impairments and thus mimics AD progression in humans. We have shown that in this model, levels of antioxidants, namely, reduced glutathione and vitamin E, are decreased and the extent of lipid peroxidation is increased. We have also observed increased activity of the antioxidant enzymes glutathione peroxidase and superoxide dismutase. These alterations are evident during the Abeta oligomerization period, before the appearance of Abeta plaques and neurofibrillary tangles, supporting the view that oxidative stress occurs early in the development of the disease.

In this study, we analyzed whether ER Ca2+ release, induced by amyloid-beta (Abeta) and prion (PrP) peptides activates the mitochondrial-mediated apoptotic pathway. In cortical neurons, addition of the synthetic Abeta1-40 or PrP106-126 peptides depletes ER Ca2+ content, leading to cytosolic Ca2+ overload. The Ca2+ released through ryanodine (RyR) and inositol 1,4,5-trisphosphate (IP3R) receptors was shown to be involved in the loss of mitochondrial membrane potential, Bax translocation to mitochondria and apoptotic death. Our data further demonstrate that Ca2+ released from the ER leads to the depletion of endogenous GSH levels and accumulation of reactive oxygen species, which were also involved in the depolarization of the mitochondrial membrane. These results illustrate that the early Abeta- and PrP -induced perturbation of ER Ca2+ homeostasis affects mitochondrial function, activating the mitochondrial-mediated apoptotic pathway and help to clarify the mechanism implicated in neuronal death that occurs in AD and PrD.

Prion (PrP) and amyloid-β (Aβ) peptides are involved in the neuronal loss that occurs in Prion disorders (PrD) and Alzheimer's disease (AD), respectively, partially due to Ca 2+ dysregulation. Besides, the endoplasmic reticulum (ER) stress has an active role in the neurotoxic mechanisms that lead to these pathologies. Here, we analyzed whether the ER-mediated apoptotic pathway is involved in the toxic effect of synthetic PrP and Aβ peptides. In PrP106-126-and Aβ1-40-treated cortical neurons, the release of Ca 2+ through ER ryanodine (RyR) and inositol 1,4,5-trisphosphate (IP 3 R) receptors induces ER stress and leads to increased cytosolic Ca 2+ and reactive oxygen species (ROS) levels and subsequently to apoptotic death involving mitochondrial cytochrome c release and caspases activation. These results demonstrate that the early PrP-and Aβ-induced perturbation of ER Ca 2+ homeostasis is a death message that leads to neuronal loss, suggesting that the regulation of ER Ca 2+ levels may be a potential therapeutical target for PrD and AD.

The nature of the toxic form of amyloid-beta peptide (Abeta) involved in early Alzheimer's disease (AD) pathology and whether it is the fibrillar or the oligomeric peptide that is the most deleterious to neurons remain controversial. This work aimed to compare the neurotoxicity of different amyloid-beta peptide 1-42 (Abeta1-42) assemblies, using fresh and aged samples enriched in oligomeric and fibrillar species, respectively, and also isolated oligomers and fibrils. The results obtained with fresh and aged Abeta1-42 preparations suggested that oligomeric species are more toxic to cortical neurons in culture than fibrillar forms, which was confirmed by using isolated oligomers and fibrils. In order to further elucidate the mechanisms involved in soluble Abeta toxicity, the involvement of endoplasmic reticulum (ER) calcium (Ca(2+)) release in oligomer-induced apoptosis was evaluated. We observed that oligomeric Abeta1-42 depletes ER Ca(2+) levels leading to intracellular Ca(2+) dyshomeostasis involving phospholipase C activation. Moreover, in the presence of dantrolene, an inhibitor of ER Ca(2+) release through ryanodine receptors, the oligomer-induced apoptosis was prevented demonstrating the involvement of ER Ca(2+) release.

Studies with in-vitro-cultured neurons treated with amyloid-beta (A beta) peptides demonstrated neuronal loss by apoptosis that is due, at least in part, to the perturbation of intracellular Ca(2+) homeostasis. In addition, it was shown that an endoplasmic reticulum (ER)-specific apoptotic pathway mediated by caspase-12, which is activated upon the perturbation of ER Ca(2+) homeostasis, may contribute to A beta toxicity. To elucidate the involvement of deregulation of ER Ca(2+) homeostasis in neuronal death induced by A beta peptides, we have performed a comparative study using the synthetic peptides A beta(25-35) or A beta(1-40) and thapsigargin, a selective inhibitor of Ca(2+) uptake into the ER. Incubation of cortical neurons with thapsigargin (2.5 microM) increased the intracellular Ca(2+) levels and activated caspase-3, leading to a significant increase in the number of apoptotic cells. Similarly, upon incubation of cortical cultures with the A beta peptides (A beta(25-35), 25 microM; A beta(1-40), 0.5 microM), we observed a significant increase in [Ca(2+)](i), in caspase-3-like activity, and in number of neurons exhibiting apoptotic morphology. The role of ER Ca(2+) release through ryanodine receptors (RyR) or inositol 1,4,5-trisphosphate receptors (IP(3)R) in A beta neurotoxicity has been also investigated. Dantrolene and xestospongin C, inhibitors of ER Ca(2+) release through RyR or IP(3)R, were able to prevent the increase in [Ca(2+)](i) and the activation of caspase-3 and to protect partially against apoptosis induced by treatment with A beta(25-35) or A beta(1-40). In conclusion, our results demonstrate that the release of Ca(2+) from the ER, mediated by both RyR and IP(3)R, is involved in A beta toxicity and can contribute, together with the activation of other intracellular neurotoxic mechanisms, to A beta-induced neuronal death. This study suggests that A beta accumulation may have a key role in the pathogenesis of AD as a result of deregulation of ER Ca(2+) homeostasis.

No abstract

Alzheimer's disease (AD) is a severe neurodegenerative disorder and the most common type of dementia in the elderly. The clinical symptoms of AD include a progressive loss of memory and impairment of cognitive functions interfering with daily life activities. The main neuropathological features consist in extracellular amyloid-β (Aβ) plaque deposition and intracellular Neurofibrillary tangles (NFTs) of hyperphosphorylated Tau. Understanding the pathophysiological mechanisms that underlie neurodegeneration in AD is essential for rational design of neuroprotective agents able to prevent disease progression. According to the “Amyloid Cascade Hypothesis” the critical molecular event in the pathogenesis of AD is the accumulation of Aβ neurotoxic oligomers. Since the proteolytic processing of Amyloid Precursor Protein (APP) by β-secretase (beta-site APP cleaving enzyme 1, BACE1) is the rate-limiting step in the production of Aβ, this enzyme is considered a major therapeutic target and BACE1 inhibitors have the potential to be disease-modifying drugs for AD treatment. Therefore, intensive efforts to discover and develop inhibitors that can reach the brain and effectively inhibit BACE1 have been pursued by several groups worldwide. The aim of this review is to highlight the progress in the discovery of potent and selective small molecule BACE1 inhibitors over the past decade.

Oxidative stress and endoplasmic reticulum (ER) stress have been associated with Alzheimer's disease (AD) progression. In this study we analyzed whether oxidative stress involving changes in Nrf2 and ER stress may constitute early events in AD pathogenesis by using human peripheral blood cells and an AD transgenic mouse model at different disease stages. Increased oxidative stress and increased phosphorylated Nrf2 (p(Ser40)Nrf2) were observed in human peripheral blood mononuclear cells (PBMCs) isolated from individuals with mild cognitive impairment (MCI). Moreover, we observed impaired ER Ca2+ homeostasis and increased ER stress markers in PBMCs from MCI individuals and mild AD patients. Evidence of early oxidative stress defense mechanisms in AD was substantiated by increased p(Ser40)Nrf2 in 3month-old 3xTg-AD male mice PBMCs, and also with increased nuclear Nrf2 levels in brain cortex. However, SOD1 protein levels were decreased in human MCI PBMCs and in 3xTg-AD mice brain cortex; the latter further correlated with reduced SOD1 mRNA levels. Increased ER stress was also detected in the brain cortex of young female and old male 3xTg-AD mice. We demonstrate oxidative stress and early Nrf2 activation in AD human and mouse models, which fails to regulate some of its targets, leading to repressed expression of antioxidant defenses (e.g., SOD-1), and extending to ER stress. Results suggest markers of prodromal AD linked to oxidative stress associated with Nrf2 activation and ER stress that may be followed in human peripheral blood mononuclear cells.