Xiulian Sun

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%

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.

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.

Distinct transcriptional regulation and function of the human BACE2 and BACE1 genes

Amyloid β protein (Aβ) is the principal component of neuritic plaques in Alzheimers disease (AD). Aβ is derived from β amyloid precursor protein (APP) by β‐ and γ‐secretases. Beta‐site APP cleaving enzyme 1 (BACE1) has been identified as the major β‐secretase. BACE2 is the homolog of BACE1. The BACE2 gene is on chromosome 21 and has been implicated in the pathogenesis of AD. However, the function of BACE2 in Aβ generation is controversial. Some studies have shown that BACE2 cleaved APP at the β‐site whereas other studies showed it cleaved around the α‐secretase site. To elucidate the involvement of BACE2 in AD pathogenesis, we compared BACE2 and BACE1 gene regulation and their functions in Aβ generation. We cloned and functionally characterized the human BACE2 promoter. The BACE2 gene is controlled by a TATA‐less promoter. Though Sp1 can regulate both BACE1 and BACE2 genes, comparative sequence analysis and transcription factor prediction showed little similarity between the two promoters. BACE1 increased APP cleavage at the β‐site and Aβ production whereas BACE2 did not. Overexpression of BACE2 significantly increased sAPP levels in conditioned media but markedly reduced Aβ production. Knockdown of BACE2 resulted in increased APP C83. Our data indicate that despite being homologous in amino acid sequence, BACE2 and BACE1 have distinct functions and transcriptional regulation. BACE2 is not a β‐secretase, but processes APP within the Aβ domain at a site downstream of the α‐secretase cleavage site. Our data argue against BACE2 being involved in the formation of neuritic plaques in AD.—Sun, X., Wang, Y., Qing, H., Christensen, M. A., Liu, Y., Zhou, W., Tong, Y., Xiao, C., Huang, Y., Zhang, S., Liu, X., Song, W. Distinct transcriptional regulation and function of the human BACE2 and BACE1 genes. FASEB J. 19, 739–749 (2005)

Degradation of regulator of calcineurin 1 (RCAN1) is mediated by both chaperone-mediated autophagy and ubiquitin proteasome pathways

Regulator of calcineurin 1 (RCAN1), a gene identified from the critical region of Down syndrome, has been implied in pathogenesis of Alzheimers disease (AD). RCAN1 expression was shown to be increased in AD brains;however, the mechanism of RCAN1 gene regulation is not well defined. The present study was designed to investigate the molecular mechanism of RCAN1 protein degradation. In addition to being degraded through the ubiquitin proteasome pathway, we found that lysosomal inhibition markedly increased RCAN1 protein expression in a time‐ and dosage‐dependent manner. Inhibition of macroautophagy reduced RCAN1 expression, indicating that RCAN1 degradation is not through a macroautophagy pathway. However, disruption of chaperone‐mediated autophagy (CMA) increased RCAN1 expression. Two CMA recognition motifs were indentified in RCAN1 protein to mediate its degradation through a CMA‐lysosome pathway. A promoter assay further demonstrated that inhibition of RCAN1 degradation in cells reduced calcineurin‐NFAT activity. Dysfunctions of ubiquitin‐proteasome and autophagy‐lysosome pathways have been implicated in neurodegenerative diseases. Therefore, elucidation of RCAN1 degradation by a ubiquitin proteasome pathway and CMA‐lysosome pathway in the present study may greatly advance our understanding of AD pathogenesis.—Liu, H., Wang, P., Song, W., Sun, X. Degradation of regulator of calcineurin 1 (RCAN1) is mediated by both chaperone‐mediated autophagy and ubiquitin proteasome pathways. FASEB J. 23, 3383–3392 (2009). www.fasebj.org

Regulator of calcineurin 1 (RCAN1) facilitates neuronal apoptosis through caspase 3 activation

Individuals with Down syndrome (DS) will inevitably develop Alzheimer disease (AD) neuropathology sometime after middle age, which may be attributable to genes triplicated in individuals with DS. The characteristics of AD neuropathology include neuritic plaques, neurofibrillary tangles, and neuronal loss in various brain regions. The mechanism underlying neurodegeneration in AD and DS remains elusive. Regulator of calcineurin 1 (RCAN1) has been implicated in the pathogenesis of DS. Our data show that RCAN1 expression is elevated in the cortex of DS and AD patients. RCAN1 expression can be activated by the stress hormone dexamethasone. A functional glucocorticoid response element was identified in the RCAN1 isoform 1 (RCAN1-1) promoter region, which is able to mediate the up-regulation of RCAN1 expression. Here we show that overexpression of RCAN1-1 in primary neurons activates caspase-9 and caspase-3 and subsequently induces neuronal apoptosis. Furthermore, we found that the neurotoxicity of RCAN1-1 is inhibited by knock-out of caspase-3 in caspase-3−/− neurons. Our study provides a novel mechanism by which RCAN1 functions as a mediator of stress- and Aβ-induced neuronal death, and overexpression of RCAN1 due to an extra copy of the RCAN1 gene on chromosome 21 contributes to AD pathogenesis in DS.

Increased BACE1 maturation contributes to the pathogenesis of Alzheimer's disease in Down syndrome

Almost all Down syndrome (DS) patients develop characteristic Alzheimers disease (AD) neuropathology, including neuritic plaques and neurofibrillary tangles, after middle age. The mechanism underlying AD neuropathology in DS has been unknown. Aβ is the central component of neuritic plaques and is generated from APP by cleavage by the β‐ and γ‐secretases. Here we show that β‐secretase activity is markedly elevated in DS. The ratio of mature to immature forms of BACE1 is altered in DS. DS has significantly higher levels of mature BACE1 proteins in Golgi than normal controls. Time‐lapse live image analysis showed that BACE1 proteins were predominantly immobile in Golgi in DS cells, while they underwent normal trafficking in controls. Thus, overproduction of Aβ in DS is caused by abnormal BACE1 protein trafficking and maturation. Our results provide a novel molecular mechanism by which AD develops in DS and support the therapeutic potential of inhibiting BACE1 in AD and DS.—Sun, X., Tong, Y., Qing, H., Chen, C‐H., Song, W. Increased BACE1 maturation contributes to Alzheimers disease pathogenesis in Down syndrome. FASEB J. 20, 1361–1368 (2006)

RCAN1 Overexpression Exacerbates Calcium Overloading-Induced Neuronal Apoptosis

Down Syndrome (DS) patients develop characteristic Alzheimers Disease (AD) neuropathology after their middle age. Prominent neuronal loss has been observed in the cortical regions of AD brains. However, the underlying mechanism leading to this neuronal loss in both DS and AD remains to be elucidated. Calcium overloading and oxidative stress have been implicated in AD pathogenesis. Two major isoforms of regulator of calcineurin 1 (RCAN1), RCAN1.1 and RCAN1.4, are detected in human brains. In this report we defined the transcriptional regulation of RCAN1.1 and RCAN1.4 by two alternative promoters. Calcium overloading upregulated RCAN1.4 expression by activating RCAN1.4 promoter through calcineurin-NFAT signaling pathway, thus forming a negative feedback loop in isoform 4 regulation. Furthermore, RCAN1.4 overexpression exacerbated calcium overloading-induced neuronal apoptosis, which was mediated by caspase-3 apoptotic pathway. Our results suggest that downregulating RCAN1.4 expression in neurons could be beneficial to AD patients.