Roberta Borghi

Oxidative stress increases expression and activity of BACE in NT2 neurons.

Recently an aspartyl protease with beta-secretase activity called BACE was identified. In the present paper we showed that BACE is modulated by the oxidative stress product 4-hydroxynonenal (HNE). Exposure of NT(2) neurons to the two classical pro-oxidant stimuli ascorbate/FeSO(4) and H(2)O(2)/FeSO(4) resulted in a significant generation of HNE, which is temporally followed by an increased production of BACE protein levels. HNE mediated BACE induction is accompanied by a proportional elevation of carboxy-terminal fragments of amyloid precursor protein. Moreover, the direct relationship between BACE induction and lipid peroxidation products was strongly confirmed by the protection exerted by a short pretreatment with alpha-tocopherol, the most important antioxidant known to prevent the formation of aldehydic end-products of lipid peroxidation, including HNE. Our results support the hypothesis that oxidative stress and A beta production are strictly interrelated events and suggest that inhibition of BACE may have a therapeutic effect synergic with antioxidant compounds.

Full length α-synuclein is present in cerebrospinal fluid from Parkinson's disease and normal subjects

Several clues suggest that α-synuclein, a presynaptic protein, plays a central role in the pathogenesis of idiopathic Parkinsons disease (PD). To search a peripheral marker of PD, we analyzed presence and amount of α-synuclein in CSF from 12 PD patients and 10 neurologically normal subjects. The protein was extracted from CSF samples through immunoprecipitation and immunoblotting with different specific anti-α-synuclein antibodies. We identified a 19 kDa band that corresponds to monomeric α-synuclein, given its comigration with homologue human recombinant peptide as well as with the protein extracted from cerebral cortex of normal subjects. The amount of CSF 19 kDa α-synuclein did not significantly vary in PD and normal cases. These findings have two implications: (a) full length α-synuclein is released by neurons in the extracellular space; (b) α-synuclein does not appear a peripheral marker of PD pathology.

Oxidative stress activates a positive feedback between the γ‐ and β‐secretase cleavages of the β‐amyloid precursor protein

Sequential cleavages of the β‐amyloid precursor protein cleaving enzyme 1 (BACE1) by β‐secretase and γ‐secretase generate the amyloid β‐peptides, believed to be responsible of synaptic dysfunction and neuronal cell death in Alzheimer’s disease (AD). Levels of BACE1 are increased in vulnerable regions of the AD brain, but the underlying mechanism is unknown. Here we show that oxidative stress (OS) stimulates BACE1 expression by a mechanism requiring γ‐secretase activity involving the c‐jun N‐terminal kinase (JNK)/c‐jun pathway. BACE1 levels are increased in response to OS in normal cells, but not in cells lacking presenilins or amyloid precursor protein. Moreover, BACE1 is induced in association with OS in the brains of mice subjected to cerebral ischaemia/reperfusion. The OS‐induced BACE1 expression correlates with an activation of JNK and c‐jun, but is absent in cultured cells or mice lacking JNK. Our findings suggest a mechanism by which OS induces BACE1 transcription, thereby promoting production of pathological levels of amyloid β in AD.

β‐Site APP cleaving enzyme up‐regulation induced by 4‐hydroxynonenal is mediated by stress‐activated protein kinases pathways

4‐Hydroxynonenal (HNE), an aldehydic product of lipid peroxidation, up‐regulates expression of the β‐site APP cleaving enzyme (BACE‐1), an aspartyl protease responsible for the β‐secretase cleavage of amyloid precursor protein (AβPP), and results in increased levels of amyloid β (Aβ) peptide. The mechanisms underlying this remain unclear but are of fundamental importance because prevention of BACE‐1 up‐regulation is viewed as an important therapeutic strategy. In this study, we exposed NT2 neurons to a range of HNE concentrations (0.5–5 µm) that elicited an up‐regulation of BACE‐1 expression, a significant increase in intracellular and secreted levels of Aβ peptides as well as apoptosis involving poly‐ADP ribose polymerase cleavage and activation of caspase 3. To delineate the molecular events involved in HNE‐mediated BACE‐1 activation, we investigated the involvement of stress‐activated protein kinases (SAPK), signal transducers and activators of transcription (STAT) and serine–threonine kinase B/phosphatidylinositol phosphate 3 kinase (Akt/PtdIns3K). Using specific pharmacological inhibitors, our results show that activation of c‐Jun N‐terminal kinases and p38MAPK., but not STAT or Akt/PtdIns3K, pathways mediate the HNE‐dependent up‐regulation of BACE‐1 expression. Therefore, HNE, an oxidative stress mediator detected in vivo in the brains of Alzheimers disease patients, may play a pathogenetic role in Alzheimers disease by selectively activating SAPK pathways and BACE‐1 that regulate the proteolytic processing of AβPP.

17A, a novel non-coding RNA, regulates GABA B alternative splicing and signaling in response to inflammatory stimuli and in Alzheimer disease

Alternative splicing is a central component of human brain complexity; nonetheless, its regulatory mechanisms are still largely unclear. In this work, we describe a novel non-coding (nc) RNA (named 17A) RNA polymerase (pol) III-dependent embedded in the human G-protein-coupled receptor 51 gene (GPR51, GABA B2 receptor). The stable expression of 17A in SHSY5Y neuroblastoma cells induces the synthesis of an alternative splicing isoform that abolish GABA B2 intracellular signaling (i.e., inhibition of cAMP accumulation and activation of K(+) channels). Indeed, 17A is expressed in human brain, and we report that it is upregulated in cerebral tissues derived from Alzheimer disease patients. We demonstrate that 17A expression in neuroblastoma cells enhances the secretion of amyloid β peptide (Aβ) and the Aβ x-42/Αβ x-40 peptide ratio and that its synthesis is induced in response to inflammatory stimuli. These data correlate, for the first time, the activity of a novel pol III-dependent ncRNA to alternative splicing events and, possibly, to neurodegeneration induced by abnormal GABA B function. We anticipate that further analysis of pol III-dependent regulation of alternative splicing will disclose novel regulatory pathways associated to brain physiology and/or pathology.

Plasma levels of amyloid β-protein 42 are increased in women with mild cognitive impairment

Background: Accumulation in the brain of small aggregates of amyloid β-protein 42 (Aβ42) is the major pathogenic event of Alzheimer disease (AD). In familial early-onset AD this event is likely the result of Aβ42 overproduction; in the most common sporadic late-onset form of the disease the mechanisms of Aβ42 accumulation are unknown. Methods: To address this issue the authors analyzed plasma levels of Aβ42 in 88 elderly patients with amnestic mild cognitive impairment (MCI), chosen as paradigm of preclinical sporadic AD. Results: The authors found a significant increase of Aβ42 plasma levels in women with MCI, in comparison to the affected men and 72 cognitively normal age-matched subjects. The levels were independent of variables in education, apolipoprotein E genotype, cholesterol, and creatinine plasma concentrations, as well as hemoglobin content. Conclusions: The elevation of Aβ42 plasma levels in women with MCI may represent a biologic explanation for the sex-dependent increased incidence of late-onset AD in women identified by epidemiologic studies.

Neprylisin decreases uniformly in Alzheimer's disease and in normal aging.

The proteolysis of β‐amyloid (Aβ) requires neprylisin, an enzyme that has been shown as reduced in Alzheimers disease (AD). We investigated whether a decrease in neprilysin levels contributes to the accumulation of amyloid deposits not only in AD but also in the normal aging. We analyzed neprilysin mRNA and protein levels in cerebral cortex from 10 cognitively normal elderly subjects with amyloid plaques (NA), 10 cases of AD, and 10 control cases free of amyloid plaques. We found a significant decrease in neprilysin mRNA levels in both AD and NA compared to control cases. Thereby, the defect of neprilysin appears to correlate with Aβ deposition but not with degeneration and dementia.

The increased activity of BACE1 correlates with oxidative stress in Alzheimer's disease.

We evaluated expression, protein levels and activity of the Beta-site cleaving enzyme (BACE1) as well as the amount of products of lipid peroxidation in frontal cortex of three groups of cases: sporadic Alzheimers disease (AD); control subjects (CTR); cognitively normal subjects with abundant amyloid plaques (NA). We found a significant increase of BACE1 activity and products of lipid peroxidation in brain tissue of AD cases, with normal gene expression, and non-significant elevation of protein levels. CTR and NA samples showed similar levels of BACE1 activity and oxidative products. BACE1 activity and the amount of oxidative products were significantly correlated in all cases.Moreover, both BACE1 activity and the level of 4-hydroxynonenal were correlated with the amount of Beta-amyloid pyroglutamated 3-42, the more toxic Beta-amyloid peptide that is characteristic of AD. These findings suggest that BACE1 activity reflects the type of ABeta species, rather than the Beta-amyloid plaques load. Hence, the increase of BACE1 activity occurring in sporadic AD is likely the effect, rather the cause, of ABeta accumulation and oxidative stress.

JNK and ERK1/2 pathways have a dual opposite effect on the expression of BACE1

The activity of beta-secretase (BACE1), the endo-protease essential for the production of amyloid beta (Abeta) peptides, is increased in brain of late-onset sporadic Alzheimers disease (AD), and oxidative stress is the potential cause of this event. Oxidative stress up-regulates the expression and the activity of BACE1 in cellular and animal models, through a mechanism that involves the increase of gamma-secretase cleavage on APP and the activation of c-jun N-terminal kinase/activator protein 1 (JNK/AP1) pathway. We further characterized the cellular pathways that control BACE1 expression under oxidative stress. We investigated the involvement of extracellular signal regulated MAP kinase (ERK1/2) pathway in the regulation of BACE1 expression, since it has been recently shown that ERK1/2 is an endogenous regulator of the gamma-secretase activity. We found that ERK1/2 pathway negatively modulates BACE1 expression and activity. Moreover, we observed that conditions that abrogate the gamma-secretase activity favor the activation of signalling pathways that promote cell survival, such as ERK1/2 and the serine/threonine kinase Akt/protein kinase B (Akt). These data suggest that the positive or negative cellular responses to oxidative stress parallel the activities of the beta- and the gamma-secretase. ERK1/2 and JNK pathways are involved in this bipartite response, which can lead to neurodegeneration or neuroprotection depending on the cellular and environmental conditions or cooperation with other signalling pathways such as Akt cascade.

Mutant presenilin 1 increases the expression and activity of BACE1

Mutations of the presenilin 1 (PS1) gene are the most common cause of early onset familial Alzheimer disease (FAD). PS1 mutations alter the activity of the γ-secretase on the β-amyloid precursor protein (APP), leading to selective overproduction of β-amyloid (Aβ) 42 peptides, the species that forms oligomers that may exert toxic effects on neurons. Here we show that PS1 mutations, expressed both transiently and stably, in non-neuronal and neuronal cell lines increase the expression and the activity of the β-secretase (BACE1), the rate-limiting step of Aβ production. Also, BACE1 expression and activity are elevated in brains of PS1 mutant knock-in mice compared with wild type littermates as well as in cerebral cortex of FAD cases bearing various PS1 mutations compared with in sporadic AD cases and controls. The up-regulation of BACE1 by PS1 mutations requires the γ-secretase cleavage of APP and is proportional to the amount of secreted Aβ42. Aβ42, and not AICD (APP intracellular domain), is indeed the APP derivative that mediates the overexpression of BACE1. The effect of PS1 mutations on BACE1 may contribute to determine the wide clinical and pathological phenotype of early onset FAD.