Debora Monteleone

Amyloid-β Production: Major Link Between Oxidative Stress and BACE1

Sequential endoproteolytic cleavages operated by the γ-secretase and the β-secretase (BACE1) on the β-amyloid precursor protein result in the production of the β-amyloid (Aβ) species, with two C-terminal variants, at residue 40 or at residue 42. Accumulation in brain tissue of aggregates of Aβ42 is the major pathogenetic event in Alzheimer’s disease (AD). The causes of Aβ accumulation in the common sporadic form of AD are not completely understood, but they are likely to include oxidative stress (OS). Data reviewed here shed light on how Aβ generation, oxidative stress, and secretase functions are intimately related in sporadic AD. According to our hypothesis, in sporadic AD, OS resulted from several cellular insults such as aging, hypoxia, hyperglycemia, and hypercholesterolemia—that are well-known risk factors for AD development—can determine a primary induction of γ-secretase and BACE1. The loop proceeds with the generation of Aβ42 and its signaling to BACE1 transcription.

Aβ1-42-mediated down-regulation of Uch-L1 is dependent on NF-κB activation and impaired BACE1 lysosomal degradation.

Amyloid‐β 1‐42 accumulation is the major pathogenetic event in Alzheimer’s disease (AD), believed to be responsible for synaptic dysfunction and neuronal cell death. However, the physiologic activity of Aβ peptides remains elusive: Aβ might not only play a toxic role, but also act as a functional signaling intermediate. We recently reported that Aβ1‐42 promotes BACE1 transcription through the activation of the JNK‐c‐jun pathway. Here, we show that the Aβ1‐42‐mediated increase in BACE1 expression is accompanied by a decrease in ubiquitin C‐terminal hydrolase L1 (Uch‐L1) expression and activity in different cellular models such as neuroblastoma SH‐SY5Y as well as NT2 neuronal cells. We also found that the increase in BACE1 and the decrease in Uch‐L1 are related events and depend on NF‐κB pathway; thus, Aβ1‐42 is able to activate NF‐κB pathway and the pretreatment with a pharmacological inhibitor, able to block the nuclear translocation of the transactivating unit p65, almost completely prevents both the decrease in Uch‐L1 and the increase in BACE1 expression. In addition, the decrease in Uch‐L1 activity interferes with the lysosomal degradation of BACE1, as demonstrated by the decrease in Cathepsin D activity and the partial accumulation of BACE1 in lysosomes after Aβ1‐42 treatment as well after Uch‐L1 inhibition. In support of the in vitro data, we observed low protein levels of Uch‐L1 associated with high protein levels of BACE1 in sporadic AD brains. Our data suggest that Uch‐L1 could be an attractive target for the development of new therapeutic approaches for AD.

Up-regulation of β-amyloidogenesis in neuron-like human cells by both 24- and 27-hydroxycholesterol: Protective effect of N-acetyl-cysteine

An abnormal accumulation of cholesterol oxidation products in the brain of patients with Alzheimers disease (AD) would further link an impaired cholesterol metabolism in the pathogenesis of the disease. The first evidence stemming from the content of oxysterols in autopsy samples from AD and normal brains points to an increase in both 27‐hydroxycholesterol (27‐OH) and 24‐hydroxycholesterol (24‐OH) in the frontal cortex of AD brains, with a trend that appears related to the disease severity. The challenge of differentiated SK‐N‐BE human neuroblastoma cells with patho‐physiologically relevant amounts of 27‐OH and 24‐OH showed that both oxysterols induce a net synthesis of Aβ1‐42 by up‐regulating expression levels of amyloid precursor protein and β‐secretase, as well as the β‐secretase activity. Interestingly, cell pretreatment with N‐acetyl‐cysteine (NAC) fully prevented the enhancement of β‐amyloidogenesis induced by the two oxysterols. The reported findings link an impaired cholesterol oxidative metabolism to an excessive β‐amyloidogenesis and point to NAC as an efficient inhibitor of oxysterols‐induced Aβ toxic peptide accumulation in the brain.

Aβ1-42 monomers or oligomers have different effects on autophagy and apoptosis

The role of autophagy and its relationship with apoptosis in Alzheimer disease (AD) pathogenesis is poorly understood. Disruption of autophagy leads to buildup of incompletely digested substrates, amyloid-β (Aβ) peptide accumulation in vacuoles and cell death. Aβ, in turn, has been found to affect autophagy. Thus, Aβ might be part of a loop in which it is both the substrate of altered autophagy and its cause. Given the relevance of different soluble forms of Aβ1-42 in AD, we have investigated whether monomers and oligomers of the peptide have a differential role in causing altered autophagy and cell death. Using differentiated SK-N-BE neuroblastoma cells, we found that monomers hamper the formation of the autophagic BCL2-BECN1/Beclin 1 complex and activate the MAPK8/JNK1-MAPK9/JNK2 pathway phosphorylating BCL2. Monomers also inhibit apoptosis and allow autophagy with intracellular accumulation of autophagosomes and elevation of levels of BECN1 and LC3-II, resulting in an inhibition of substrate degradation due to an inhibitory action on lysosomal activity. Oligomers, in turn, favor the formation of the BCL2-BECN1 complex favoring apoptosis. In addition, they cause a less profound increase in BECN1 and LC3-II levels than monomers without affecting the autophagic flux. Thus, data presented in this work show a link for autophagy and apoptosis with monomers and oligomers, respectively. These studies are likely to help the design of novel disease modifying therapies.

Amyloid-β42 Activates the Expression of BACE1 Through the JNK Pathway

The sequential endoproteolytic cleavages operated by the γ-secretase and the β-secretase (BACE1) on the amyloid-β protein precursor (AβPP) result in the production of the amyloid-β (Aβ) species, with two C-terminal variants, at residue 40 or at residue 42. Accumulation in brain tissue of small, soluble aggregates of Aβ42 is the major pathogenic event of Alzheimers disease (AD). However, the physiologic activity of Aβ peptides is still elusive. Here, we show that expression of BACE1 is regulated by Aβ42, which augments BACE1 gene transcription through the JNK/c-jun signaling pathway. Of note, Aβ40 has much less effect on BACE1 expression. These findings unveil a positive feedback loop in which γ-secretase cleavage of AβPP releases a functionally-active peptide, Aβ42, that promotes BACE1 transcription. Thus, gene expression induced by Aβ42 may have implications in the neuronal dysfunction and degeneration that occurs in AD.

Transcriptional and post-transcriptional regulation of β-secretase.

Alzheimers disease (AD) is a devastating neurodegenerative disorder that results in loss of memory and cognitive function, eventually leading to dementia. A key neuropathological event in AD is the cerebral accumulation of senile plaques formed by aggregates of amyloid‐β‐peptides (Aβ). Aβ results from two sequential endoproteolytic cleavages operated on the amyloid‐β precursor protein (AβPP), an integral membrane protein with a single‐membrane spanning domain, a large extracellular N‐terminus and a shorter, cytoplasmic C‐terminus. First, β‐secretase (BACE1) cleaves AβPP at the N‐terminal end of the Aβ sequence to produce a secreted form of AβPP, named sAβPP, and a C‐terminal membrane‐bound 99‐aminoacid fragment (C99). Then, γ‐secretase cleaves C99 within the transmembrane domain to release the Aβ peptides of different lengths, predominantly Aβ1‐40 and Aβ1‐42.

Dual Mechanism of toxicity for extracellular injection of Tau oligomers versus monomers in human Tau mice

The mechanism of tau toxicity is still unclear. Here we report that recombinant tau oligomers and monomers, intraventricularly injected in mice with a pure human tau background, foster tau pathology through different mechanisms. Oligomeric forms of tau alter the conformation of tau in a paired helical filament-like manner. This effect occurs without tau hyperphosphorylation as well as activation of specific kinases, suggesting that oligomers of tau induce tau assembly through a nucleation effect. Monomers, in turn, induce neurodegeneration through a calpain-mediated tau cleavage that leads to accumulation of a 17 kDa neurotoxic peptide and induction of apoptotic cell death.

The Decrease of Uch-L1 Activity Is a Common Mechanism Responsible for Aβ 42 Accumulation in Alzheimer’s and Vascular Disease

Alzheimer’s disease (AD) is a multifactorial pathology causing common brain spectrum disorders in affected patients. These mixed neurological disorders not only include structural AD brain changes but also cerebrovascular lesions. The main aim of the present issue is to find the factors shared by the two pathologies. The decrease of ubiquitin C-terminal hydrolase L1 (Uch-L1), a major neuronal enzyme involved in the elimination of misfolded proteins, was observed in ischemic injury as well as in AD, but its role in the pathogenesis of AD is far to be clear. In this study we demonstrated that Uch-L1 inhibition induces BACE1 up-regulation and increases neuronal and apoptotic cell death in control as well as in transgenic AD mouse model subjected to Bengal Rose, a light-sensitive dye inducing that induces a cortical infarction through photo-activation. Under the same conditions we also found a significant activation of NF-κB. Thus, the restoration of Uch-L1 was able to completely prevent both the increase in BACE1 protein levels and the amount of cell death. Our data suggest that the Uch-L1-mediated BACE1 up-regulation could be an important mechanism responsible for Aβ peptides accumulation in vascular injury and indicate that the modulation of the activity of this enzyme could provide new therapeutic strategies in AD.

The Unexpected Role of Aβ1-42 Monomers in the Pathogenesis of Alzheimer's Disease.

Amyloid-β (Aβ) has been proposed as a biomarker and a drug target for the therapy of Alzheimer’s disease (AD). The neurotoxic entity and relevance of each conformational form of Aβ to AD pathology is still under debate; Aβ oligomers are considered the major killer form of the peptide whereas monomers have been proposed to be involved in physiological process. Here we reviewed some different effects mediated by monomers and oligomers on mechanisms involved in AD pathogenesis such as autophagy and tau aggregation. Data reported in this review demonstrate that Aβ monomers could have a major role in sustaining the pathogenesis of AD and that AD therapy should be focused not only in the removal of oligomers but also of monomers.