Fang Cai

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%

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.

Increased NF-κB signalling up-regulates BACE1 expression and its therapeutic potential in Alzheimer's disease.

Elevated levels of β-site APP cleaving enzyme 1 (BACE1) were found in the brain of some sporadic Alzheimers disease (AD) patients; however, the underlying mechanism is unknown. BACE1 cleaves β-amyloid precursor protein (APP) to generate amyloid β protein (Aβ), a central component of neuritic plaques in AD brains. Nuclear factor-kappa B (NF-κB) signalling plays an important role in gene regulation and is implicated in inflammation, oxidative stress and apoptosis. In this report we found that both BACE1 and NF-κB p65 levels were significantly increased in the brains of AD patients. Two functional NF-κB-binding elements were identified in the human BACE1 promoter region. We found that NF-κB p65 expression resulted in increased BACE1 promoter activity and BACE1 transcription, while disruption of NF-κB p65 decreased BACE1 gene expression in p65 knockout (RelA-knockout) cells. In addition, NF-κB p65 expression leads to up-regulated β-secretase cleavage and Aβ production, while non-steroidal anti-inflammatory drugs (NSAIDs) inhibited BACE1 transcriptional activation induced by strong NF-κB activator tumour necrosis factor-alpha (TNF-α). Taken together, our results clearly demonstrate that NF-κB signalling facilitates BACE1 gene expression and APP processing, and increased BACE1 expression mediated by NF-κB signalling in the brain could be one of the novel molecular mechanisms underlying the development of AD in some sporadic cases. Furthermore, NSAIDs could block the inflammation-induced BACE1 transcription and Aβ production. Our study suggests that inhibition of NF-κB-mediated BACE1 expression may be a valuable drug target for AD therapy.

Control of BACE1 degradation and APP processing by ubiquitin carboxyl-terminal hydrolase L1.

J. Neurochem. (2012) 120, 1129–1138.

Overexpression of ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) delays Alzheimer's progression in vivo

Deposition of amyloid β protein (Aβ) to form neuritic plaques in the brain is the pathological hallmark of Alzheimers disease (AD). Aβ is produced by β- and γ-cleavages of amyloid β precursor protein (APP). Ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) is a de-ubiquitinating enzyme that cleaves ubiquitin at its carboxyl terminal. Dysfunction of UCHL1 has been reported in neurodegenerative diseases. However, whether UCHL1 affects Aβ production and AD progression remains unknown. Here we report that UCHL1 interacts with APP and regulates Aβ production. UCHL1 increases free ubiquitin level and accelerates the lysosomal degradation of APP by promoting its ubiquitination. Furthermore, we demonstrate that overexpression of UCHL1 by intracranial injection of UCHL1-expressing rAAV reduces Aβ production, inhibits neuritic plaque formation and improves memory deficits in AD transgenic model mice. Our study suggests that UCHL1 may delay Alzheimers progression by regulating APP degradation in a long-term fashion, and that overexpression of UCHL1 may be a safe and effective disease-modifying strategy to treat AD.

Degradation of nicastrin involves both proteasome and lysosome

The glycoprotein nicastrin (NCT) is an essential component of the γ‐secretase complex, a high molecular weight complex which also contains the presenilin proteins, Aph‐1 and Pen‐2. The γ‐secretase complex is not only involved in APP processing but also in the processing of an increasing number of other type I integral membrane proteins. As the largest subunit of the γ‐secretase complex, NCT plays a crucial role in its activation. Considerable information exists on the distribution, structure and function of NCT; however, little is known of its proteolysis. The present study is aimed at exploring the molecular mechanism of NCT degradation. We found that either proteasomal or lysosomal inhibition can significantly increase the levels of both endogenous and exogenous NCT in various cell lines, and the effect of these inhibitions on NCT was time‐ and dose‐dependent. Immunofluorescent microscopic analysis revealed that NCT accumulates in the ER and Golgi apparatus after proteasomal inhibition, while lysosomal inhibition leads to the accumulation of NCT in the lysosomal apparatus. Co‐immunoprecipitation can pull down both NCT and ubiquitin. Taken together, our results demonstrate that NCT degradation involves both the proteasome and the lysosome.

Biological function of Presenilin and its role in AD pathogenesis

Presenilins (PSs) are the catalytic core of γ-secretase complex. However, the mechanism of FAD-associated PS mutations in AD pathogenesis still remains elusive. Here we review the general biology and mechanism of γ-secretase and focus on the catalytic components – presenilins and their biological functions and contributions to the AD pathogenesis. The functions of presenilins are divided into γ-secretase dependent and γ-secretase independent ones. The γ-secretase dependent functions of presenilins are exemplified by the sequential cleavages in the processing of APP and Notch; the γ-secretase independent functions of presenilins include stabilizing β-catenin in Wnt signaling pathway, regulating calcium homeostasis and their interaction with synaptic transmission.

Amyloid-β protein (Aβ) Glu11 is the major β-secretase site of β-site amyloid-β precursor protein-cleaving enzyme 1(BACE1), and shifting the cleavage site to Aβ Asp1 contributes to Alzheimer pathogenesis

Cleavage of amyloid‐β precursor protein (APP) at the Asp1 β‐secretase site of the amyloid‐β protein (Aβ) domain by β‐site Aβ precursor protein‐cleaving enzyme 1 (BACE1) is required for the generation of Aβ, a central component of neuritic plaques in the Alzheimers disease (AD) brain. In this study, we found that Aβ Glu11 is the major β‐secretase site for cleavage of APP by BACE1 to generate soluble secreted APP (sAPPβ)606 and the C‐terminal membrane‐bound fragment (CTF)β product C89. Cleavage of C89 by γ‐secretase resulted in truncated Aβ generation in a non‐amyloidogenic pathway. A familial AD‐associated Swedish APP mutation adjacent to Aβ Asp1 shifted the major APP β‐secretase cleavage site from Aβ Glu11 to Asp1, resulting in significant increases in sAPPβ596 and CTFβ C99 generation and the C99/89 ratio, in turn leading to increased Aβ production in cultured cells in vitro and transgenic AD model mouse brains in vivo. Furthermore, increased BACE1 expression facilitated APP being processed by the β‐secretase processing pathway rather than the α‐secretase pathway, leading to more Aβ production. Our results suggest that potentiating BACE1 cleavage of APP at both the Asp1 and Glu11 sites, or shifting the cleavage from the Glu11 site to the Asp1 site, could result in increased Aβ production and facilitate neuritic plaque formation. Our study provides new insights into how alteration of BACE1 expression and β‐secretase cleavage site selection could contribute to Alzheimer pathogenesis and the pharmaceutical potential of modulating BACE1 expression and its cleavage site selection.