Weihong Song

Dopamine-dependent neurotoxicity of alpha-synuclein: a mechanism for selective neurodegeneration in Parkinson disease.

The mechanism by which dopaminergic neurons are selectively lost in Parkinson disease (PD) is unknown. Here we show that accumulation of α-synuclein in cultured human dopaminergic neurons results in apoptosis that requires endogenous dopamine production and is mediated by reactive oxygen species. In contrast, α-synuclein is not toxic in non-dopaminergic human cortical neurons, but rather exhibits neuroprotective activity. Dopamine-dependent neurotoxicity is mediated by 54–83-kD soluble protein complexes that contain α-synuclein and 14-3-3 protein, which are elevated selectively in the substantia nigra in PD. Thus, accumulation of soluble α-synuclein protein complexes can render endogenous dopamine toxic, suggesting a potential mechanism for the selectivity of neuronal loss in PD.

Presenilins are required for γ -secretase cleavage of β -APP and transmembrane cleavage of Notch-1

Presenilins are required for γ -secretase cleavage of β -APP and transmembrane cleavage of Notch-1

Hypoxia facilitates Alzheimer's disease pathogenesis by up-regulating BACE1 gene expression

The molecular mechanism underlying the pathogenesis of the majority of cases of sporadic Alzheimers disease (AD) is unknown. A history of stroke was found to be associated with development of some AD cases, especially in the presence of vascular risk factors. Reduced cerebral perfusion is a common vascular component among AD risk factors, and hypoxia is a direct consequence of hypoperfusion. Previously we showed that expression of the β-site β-amyloid precursor protein (APP) cleavage enzyme 1 (BACE1) gene BACE1 is tightly controlled at both the transcriptional and translational levels and that increased BACE1 maturation contributes to the AD pathogenesis in Downs syndrome. Here we have identified a functional hypoxia-responsive element in the BACE1 gene promoter. Hypoxia up-regulated β-secretase cleavage of APP and amyloid-β protein (Aβ) production by increasing BACE1 gene transcription and expression both in vitro and in vivo. Hypoxia treatment markedly increased Aβ deposition and neuritic plaque formation and potentiated the memory deficit in Swedish mutant APP transgenic mice. Taken together, our results clearly demonstrate that hypoxia can facilitate AD pathogenesis, and they provide a molecular mechanism linking vascular factors to AD. Our study suggests that interventions to improve cerebral perfusion may benefit AD patients.

Valproic acid inhibits Aβ production, neuritic plaque formation, and behavioral deficits in Alzheimer's disease mouse models

Neuritic plaques in the brains are one of the pathological hallmarks of Alzheimers disease (AD). Amyloid β-protein (Aβ), the central component of neuritic plaques, is derived from β-amyloid precursor protein (APP) after β- and γ-secretase cleavage. The molecular mechanism underlying the pathogenesis of AD is not yet well defined, and there has been no effective treatment for AD. Valproic acid (VPA) is one of the most widely used anticonvulsant and mood-stabilizing agents for treating epilepsy and bipolar disorder. We found that VPA decreased Aβ production by inhibiting GSK-3β–mediated γ-secretase cleavage of APP both in vitro and in vivo. VPA treatment significantly reduced neuritic plaque formation and improved memory deficits in transgenic AD model mice. We also found that early application of VPA was important for alleviating memory deficits of AD model mice. Our study suggests that VPA may be beneficial in the prevention and treatment of AD.

Valproic acid inhibits Ab production, neuritic plaque formation, and behavioral deficits in Alzheimer's disease mouse models

Qing et al. 2008. J. Exp. Med. doi:10.1084/jem.20081588 [OpenUrl][1][Abstract/FREE Full Text][2] [1]: {openurl}?query=rft_id%253Dinfo%253Adoi%252F10.1084%252Fjem.20081588%26rft_id%253Dinfo%253Apmid%252F18955571%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%

Transcriptional Regulation of BACE1, the β-Amyloid Precursor Protein β-Secretase, by Sp1

ABSTRACT Proteolytic processing of the β-amyloid precursor protein (APP) at the β site is essential to generate Aβ. BACE1, the major β-secretase involved in cleaving APP, has been identified as a type 1 membrane-associated aspartyl protease. We have cloned a 2.1-kb fragment upstream of the human BACE1 gene and identified key regions necessary for promoter activity. BACE1 gene expression is controlled by a TATA-less promoter. The region of bp −619 to +46 is the minimal promoter to control the transcription of the BACE1 gene. Several putative cis-acting elements, such as a GC box, HSF-1, a PU box, AP1, AP2, and lymphokine response element, are found in the 5′ flanking region of the BACE1 gene. Transcriptional activation and gel shift assays demonstrated that the BACE1 promoter contains a functional Sp1 response element, and overexpression of the transcription factor Sp1 potentiates BACE gene expression and APP processing to generate Aβ. Furthermore, Sp1 knockout reduced BACE1 expression. These results suggest that BACE1 gene expression is tightly regulated at the transcriptional level and that the transcription factor Sp1 plays an important role in regulation of BACE1 to process APP generating Aβ in Alzheimers disease.

Oxidative stress potentiates BACE1 gene expression and Aβ generation

Summary.Alzheimer’s Disease (AD) is the most common neurodegenerative disorder leading to dementia and its prevalence increases with age. The pathological features of AD are characterized by the β-amyloid protein (Aβ) deposits in the core of neuritic plaques and abnormal neurofibrillary tangles in the brain of AD patients. BACE1 is the major β-secretase to cleave the β-amyloid precursor protein (APP) to generate Aβ. Oxidative stress has been shown to affect Aβ generation in the AD pathogenesis and the mechanism of such effect is unknown. In this report we generated a novel promoterless enhanced green fluorescent protein (EGFP) reporter gene cloning vector and cloned a 1.9-kb BACE1 gene promoter fragment in this vector. The BACE1 promoter fragment can efficiently activate EGFP or luciferase gene transcription. Oxidative stress induced by hydrogen peroxide resulted in significant increase in the BACE1 promoter activity. Furthermore, hydrogen peroxide treatment facilitated β-secretase activity and Aβ generation. Thus, upregulation of BACE1 transcription by oxidative stress may contribute to the pathogenesis of Alzheimer’s disease.

Control of APP processing and Aβ generation level by BACE1 enzymatic activity and transcription

Deposition of amyloid β protein (Aβ)is one of the characteristic features of Alzheimers disease (AD) neuropathology. Beta‐secretase, a β‐site APP cleaving enzyme 1 (BACE1), is essential for A β biosynthesis. Although inhibition of BACE1 is considered a valid therapeutic target for AD, the enzymatic dynamics of BACE1 in regulating APP processing and A β generation has not yet been fully defined. To examine this issue, tightly controlled inducible BACE1 gene expression was established in the neuronal cell line N2ABP1 and the non‐neuronal cell line E2BP1 using an ecdysone‐inducible system. The BACE1 protein level was increased in a time‐and dosage‐dependent manner in the inducible BACE1 stable cells by treatment with inducer ponasterone A. The generation of APP CTFβ, the β‐secretase product, increased proportionally with the level of BACE1 protein expression. However, Aβ40/42 production sharply increased to the plateau level with a relatively small increase in BACE1 expression. Although further increasing BACE1 expression increased β‐secretase activity, it had no additional effect on A β production. Furthermore, we found that BACE1 mRNA levels and BACE1 promoter activity were significantly lower than APP mRNA levels and APP promoter activity. Our data demonstrate that lower BACE transcription is responsible for the minority of APP undergoing the amyloidogenic pathway and relatively lower A β production in the normal conditions, and that a slight increase in BACE1 can induce a dramatic elevation in A β production, indicating that the increase in BACE1 can potentially increase neuritic plaque formation in the pathological condition.—Li, Y., Zhou, W., Tong, Y., He, G., Song, W. Control of APP processing and A β generation level by BACE1 enzymatic activity and transcription. FASEB J. 20, 285–292 (2006)

Inhibition of GSK3β-mediated BACE1 expression reduces Alzheimer-associated phenotypes

Deposition of amyloid β protein (Aβ) to form neuritic plaques in the brain is the pathological hallmark of Alzheimers disease (AD). Aβ is generated from sequential cleavages of the β-amyloid precursor protein (APP) by the β- and γ-secretases, and β-site APP-cleaving enzyme 1 (BACE1) is the β-secretase essential for Aβ generation. Previous studies have indicated that glycogen synthase kinase 3 (GSK3) may play a role in APP processing by modulating γ-secretase activity, thereby facilitating Aβ production. There are two highly conserved isoforms of GSK3: GSK3α and GSK3β. We now report that specific inhibition of GSK3β, but not GSK3α, reduced BACE1-mediated cleavage of APP and Aβ production by decreasing BACE1 gene transcription and expression. The regulation of BACE1 gene expression by GSK3β was dependent on NF-κB signaling. Inhibition of GSK3 signaling markedly reduced Aβ deposition and neuritic plaque formation, and rescued memory deficits in the double transgenic AD model mice. These data provide evidence for regulation of BACE1 expression and AD pathogenesis by GSK3β and that inhibition of GSK3 signaling can reduce Aβ neuropathology and alleviate memory deficits in AD model mice. Our study suggests that interventions that specifically target the β-isoform of GSK3 may be a safe and effective approach for treating AD.

Degradation of BACE by the ubiquitin-proteasome pathway

The amyloid β protein (Aβ) is derived from β‐amyloid precursor protein (APP). Cleavage of APP by β‐secretase generates a C‐terminal fragment (APPCTFβ or C99), which is subsequently cleaved by γ‐secretase to produce Aβ. BACE (or BACE1), the major β‐secretase involved in cleaving APP, has been identified as a Type 1 membrane‐associated aspartyl protease. In this study, we found that treatment with proteasome inhibitors resulted in an increase in APP C99 levels, suggesting that APP processing at the β‐secretase site may be affected by the ubiquitin‐proteasome pathway. To investigate whether the degradation of BACE is mediated by the proteasome pathway, cells stably transfected with BACE were treated with lactacystin. We found that BACE protein degradation was inhibited by lactacystin in a time‐ and dose‐dependent manner. Non‐proteasome protease inhibitors had no effect on BACE degradation. BACE protein is ubiquitinated. Furthermore, lactacystin increased APP C99 production and Aβ generation. Our data demonstrate that the degradation of BACE proteins and APP processing are regulated by the ubiquitin‐proteasome pathway.