Transcriptional regulation of APH-1A and increased gamma-secretase cleavage of APP and Notch by HIF-1 and hypoxia

Natural polyphenols against neurodegenerative disorders: potentials and pitfalls

Within the last years, a rapidly growing number of polyphenolic compounds with neuroprotective effects have been described. Many efforts have been made to explore the mechanisms behind the neuroprotective action of polyphenols. However, many pathways and mechanisms considered for mediating these effects are rather general than specific. Moreover, despite the beneficial effects of polyphenols in experimental treatment of neurodegeneration, little has been achieved in bringing them into routine clinical applications. In this review, we have summarized the protective effects of polyphenols against neurodegeneration, and we have also discussed some of the barricades in translating these biochemical compounds, into relevant therapeutics for neurodegenerative diseases.

HIG1, a novel regulator of mitochondrial γ-secretase, maintains normal mitochondrial function

The γ-secretase complex (which contains presenilins, nicastrin, anterior pharynx defective-1, and presenilin enhancer-2) cleaves type I transmembrane proteins, including Notch and amyloid precursor protein. Dysregulated γ-secretase activity has been implicated in the pathogenesis of Alzheimer's disease, stroke, atherosclerosis, and cancer. Tight regulation of γ-secretase activity is required for normal physiology. Here, we isolated HIG1 (hypoxia inducible gene 1, domain member 1A) from a functional screen of γ-secretase inhibitory genes. HIG1 was highly expressed in the brain. Interestingly, HIG1 was localized to the mitochondria and was directly bound to γ-secretase components on the mitochondrial membrane in SK-N-SH neuroblastoma cells. Overexpresssion of HIG1 attenuated hypoxia-induced γ-secretase activation on the mitochondrial membrane and the accumulation of intracellular amyloid β. This accumulation was accompanied by hypoxia-induced mitochondrial dysfunction. The latter half domain of HIG1 was required for binding to the γ-secretase complex and suppression of γ-secretase activity. Moreover, depletion of HIG1 increased γ-secretase activation and enhanced hypoxia-induced mitochondrial dysfunction. In summary, HIG1 is a novel modulator of the mitochondrial γ-secretase complex, and may play a role in the maintenance of normal mitochondrial function.

Oxygen-dependent cleavage of the p75 neurotrophin receptor triggers stabilization of HIF-1α

Homeostatic control of oxygen availability allows cells to survive oxygen deprivation. Although the transcription factor hypoxia-inducible factor 1α (HIF-1α) is the main regulator of the hypoxic response, the upstream mechanisms required for its stabilization remain elusive. Here, we show that p75 neurotrophin receptor (p75(NTR)) undergoes hypoxia-induced γ-secretase-dependent cleavage to provide a positive feed-forward mechanism required for oxygen-dependent HIF-1α stabilization. The intracellular domain of p75(NTR) directly interacts with the evolutionarily conserved zinc finger domains of the E3 RING ubiquitin ligase Siah2 (seven in absentia homolog 2), which regulates HIF-1α degradation. p75(NTR) stabilizes Siah2 by decreasing its auto-ubiquitination. Genetic loss of p75(NTR) dramatically decreases Siah2 abundance, HIF-1α stabilization, and induction of HIF-1α target genes in hypoxia. p75(NTR-/-) mice show reduced HIF-1α stabilization, vascular endothelial growth factor (VEGF) expression, and neoangiogenesis after retinal hypoxia. Thus, hypoxia-induced intramembrane proteolysis of p75(NTR) constitutes an apical oxygen-dependent mechanism to control the magnitude of the hypoxic response.

Regulation of β-amyloid level in the brain of rats with cerebrovascular hypoperfusion

Cerebrovascular hypoperfusion occurs prior to clinical symptoms of Alzheimer's disease (AD) and represents the most accurate indicator predicting whether an individual develops AD in a future time. In order to explore the contribution of cerebrovascular hypoperfusion to AD, cerebrovascular hypoperfusion induced by bilateral carotid occlusion surgery in adult rats was used to investigate its impacts on spatial memory, amyloid-β protein (Aβ) production and clearance in the brain. The progressive spatial memory deficits were observed through Morris water maze test of the rats with cerebrovascular hypoperfusion induced by occlusion surgery. The memory deficits were accompanied with the increase of brain Aβ associated with Aβ overproduction due to the increased expression of β-amyloid precursor protein (APP) and enhanced activities of amyloid precursor protein cleavage enzymes such as β- and γ-secretases. Western blot and immunohistochemisty studies further revealed that cerebrovascular hypoperfusion could induce abnormal expression of β-amyloid receptor proteins including the receptor for advanced glycation end products (RAGE) and low-density lipoprotein receptor-related protein-1 (LRP-1), and result in a shift of immunoreactivity between neurons and vasculatures. Taken together, our results suggested that chronic cerebrovascular hypoperfusion could cause memory impairment and Aβ accumulation in brain associated with increased generation and impaired clearance of Aβ. Cerebrovascular hypoperfusion plays an important role in the pathogenesis and development of AD.

The -980C/G polymorphism in APH-1A promoter confers risk of Alzheimer's disease

We previously described an association between Alzheimer's disease (AD) and a single-nucleotide polymorphism -980C/G (rs3754048) in the promoter of the anterior pharynx-defective-1a (APH-1A) gene. Here, we examine the potential of this -980C/G polymorphism to affect APH-1A transcription and confer a risk of AD. We validated the presence of APH-1A promoter polymorphism -980C/G in other two Chinese cohort sets (450 AD and 450 controls). Subsequently, we measured APH-1A mRNA and protein levels and γ-secretase activity in C or G allele carriers. Finally, we examined the polymorphism's transcriptional function using a dual-luciferase reporter assay and also tracked transcription factor binding to the variant promoter sequence with electrophoretic mobility shift assays (EMSAs). We found that the APH-1A levels and γ-secretase activity were higher in individuals carrying allele G. The G allele increased APH-1A transcriptional activity significantly in both N2A cells and HEK293 cells. The EMSA revealed an increased binding of the transcription factor Yin Yang 1 (YY1) to allele G. Overexpression of YY1 resulted in an activation of the APH-1A promoter (2.7-fold). Specific YY1 siRNA led to decreases in APH-1A promoter activity and mRNA and protein levels. Our data indicate that the APH-1A promoter polymorphism -980C/G might alter the binding ability of YY1 transcription factor, resulting in an increased level of APH-1A and γ-secretase activity. These factors further facilitated β-amyloid (Aβ) 42 generation and ultimately modified patients' susceptibility to AD. The involvement of transcription factor YY1 might be a novel mechanism for the development of AD.

Amyloid beta resistance in nerve cell lines is mediated by the Warburg effect

Amyloid beta (Aβ) peptide accumulation in the brains of patients with Alzheimer's disease (AD) is closely associated with increased nerve cell death. However, many cells survive and it is important to understand the mechanisms involved in this survival response. Recent studies have shown that an anti-apoptotic mechanism in cancer cells is mediated by aerobic glycolysis, also known as the Warburg effect. One of the major regulators of aerobic glycolysis is pyruvate dehydrogenase kinase (PDK), an enzyme which represses mitochondrial respiration and forces the cell to rely heavily on glycolysis, even in the presence of oxygen. Recent neuroimaging studies have shown that the spatial distribution of aerobic glycolysis in the brains of AD patients strongly correlates with Aβ deposition. Interestingly, clonal nerve cell lines selected for resistance to Aβ exhibit increased glycolysis as a result of activation of the transcription factor hypoxia inducible factor 1. Here we show that Aβ resistant nerve cell lines upregulate Warburg effect enzymes in a manner reminiscent of cancer cells. In particular, Aβ resistant nerve cell lines showed elevated PDK1 expression in addition to an increase in lactate dehydrogenase A (LDHA) activity and lactate production when compared to control cells. In addition, mitochondrial derived reactive oxygen species (ROS) were markedly diminished in resistant but not sensitive cells. Chemically or genetically inhibiting LDHA or PDK1 re-sensitized resistant cells to Aβ toxicity. These findings suggest that the Warburg effect may contribute to apoptotic-resistance mechanisms in the surviving neurons of the AD brain. Loss of the adaptive advantage afforded by aerobic glycolysis may exacerbate the pathophysiological processes associated with AD.

Oxygen, a Key Factor Regulating Cell Behavior during Neurogenesis and Cerebral Diseases

Oxygen is vital to maintain the normal functions of almost all the organs, especially for brain which is one of the heaviest oxygen consumers in the body. The important roles of oxygen on the brain are not only reflected in the development, but also showed in the pathological processes of many cerebral diseases. In the current review, we summarized the oxygen levels in brain tissues tested by real-time measurements during the embryonic and adult neurogenesis, the cerebral diseases, or in the hyperbaric/hypobaric oxygen environment. Oxygen concentration is low in fetal brain (0.076-7.6 mmHg) and in adult brain (11.4-53.2 mmHg), decreased during stroke, and increased in hyperbaric oxygen environment. In addition, we reviewed the effects of oxygen tensions on the behaviors of neural stem cells (NSCs) in vitro cultures at different oxygen concentration (15.2-152 mmHg) and in vivo niche during different pathological states and in hyperbaric/hypobaric oxygen environment. Moderate hypoxia (22.8-76 mmHg) can promote the proliferation of NSCs and enhance the differentiation of NSCs into the TH-positive neurons. Next, we briefly presented the oxygen-sensitive molecular mechanisms regulating NSCs proliferation and differentiation recently found including the Notch, Bone morphogenetic protein and Wnt pathways. Finally, the future perspectives about the roles of oxygen on brain and NSCs were given.

The overexpression of AP-4 as a prognostic indicator for gastric carcinoma

As a transcription factor belonging to the basic helix-loop-helix leucine-zipper subgroup, AP-4 can control target gene expression by altering cell signal transduction, and regulate growth, development, and cell apoptosis. Under pathological circumstances, it is involved in tumorigenesis. Herein, immunohistochemistry and real-time PCR were used to detect the transcription factor AP-4 expression in gastric cancer, and these data were examined for correlation with histology, pTNM stage, and prognosis. The AP-4 expression rate was 83.67% in a total of 98 gastric cancer tissues, which was significantly higher than 40.91% in non-neoplastic tissues; AP-4 mRNA relative expression shows a significant difference between gastric cancer and normal tissues, and AP-4 expression has a significantly positive correlation with the depth of tumor invasion (P < 0.0001), degree of tumor differentiation (P = 0.0058), lymph node metastasis (P = 0.0255), and pTNM stage (P = 0.001). Survival analysis showed that AP-4-positive patients' median survival time (12.60 months) was significantly shorter than that (41.40 months) of AP-4-negative patients. AP-4 expression in gastric cancer is associated with clinicopathological parameters of gastric cancer, such as differentiation, lymph node metastasis, depth of invasion (P = 0.0010), and pTNM stage. What's more, AP-4 overexpression indicated a worse prognosis for patients. So AP-4 may be a molecular marker to predict the progression and prognosis of the tumor.

Experimental selection of hypoxia-tolerant Drosophila melanogaster

Through long-term laboratory selection (over 200 generations), we have generated Drosophila melanogaster populations that tolerate severe, normally lethal, levels of hypoxia. Because of initial experiments suspecting genetic mechanisms underlying this adaptation, we compared the genomes of the hypoxia-selected flies with those of controls using deep resequencing. By applying unique computing and analytical methods we identified a number of DNA regions under selection, mostly on the X chromosome. Several of the hypoxia-selected regions contained genes encoding or regulating the Notch pathway. In addition, previous expression profiling revealed an activation of the Notch pathway in the hypoxia-selected flies. We confirmed the contribution of Notch activation to hypoxia tolerance using a specific γ-secretase inhibitor, N-[N-(3,5-Difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT), which significantly reduced adult survival and life span in the hypoxia-selected flies. We also demonstrated that flies with loss-of-function Notch mutations or RNAi-mediated Notch knockdown had a significant reduction in hypoxia tolerance, but those with a gain-of-function had a dramatic opposite effect. Using the UAS-Gal4 system, we also showed that specific overexpression of the Notch intracellular domain in glial cells was critical for conferring hypoxia tolerance. Unique analytical tools and genetic and bioinformatic strategies allowed us to discover that Notch activation plays a major role in this hypoxia tolerance in Drosophila melanogaster.

Hypoxic regulation of glycosylation via the N-acetylglucosamine cycle

Glucose is an energy substrate, as well as the primary source of nucleotide sugars, which are utilized as donor substrates in protein glycosylation. Appropriate glycosylation is necessary to maintain the stability of protein, and is also important in the localization and trafficking of proteins. The dysregulation of glycosylation results in the development of a variety of disorders, such as cancer, diabetes mellitus and emphysema. Glycosylation is kinetically regulated by dynamically changing the portfolio of glycosyltransferases, nucleotide sugars, and nucleotide sugar transporters, which together form a part of what is currently referred to as the ”Glycan cycle”. An excess or a deficiency in the expression of glycosyltransferases has been shown to alter the glycosylation pattern, which subsequently leads to the onset, progression and exacerbation of a number of diseases. Furthermore, alterations in intracellular nucleotide sugar levels can also modulate glycosylation patterns. It is observed that pathological hypoxic microenvironments frequently occur in solid cancers and inflammatory foci. Hypoxic conditions dramatically change gene expression profiles, by activating hypoxia-inducible factor-1, which mediates adaptive cellular responses. Hypoxia-induced glycosyltransferases and nucleotide sugar transporters have been shown to modulate glycosylation patterns that are part of the mechanism associated with cancer metastasis. Hypoxia-inducible factor-1 also induces the expression of glucose transporters and various types of glycolytic enzymes, leading to shifts in glucose metabolic patterns. This fact strongly suggests that hypoxic conditions are an important factor in modulating various nucleotide sugar biosynthetic pathways. This review discusses some of the current thinking of how hypoxia alters glucose metabolic fluxes that can modulate cellular glycosylation patterns and consequently modify cellular functions, particularly from the standpoint of the N-acetylglucosamine cycle, a part of the ”Glycan cycle”.

Localization and trafficking of endogenous anterior pharynx-defective 1, a component of Alzheimer's disease related gamma-secretase

Anterior pharynx-defective 1 (Aph-1) is a multi-spanning membrane protein and an integral component of the high molecular weight gamma-secretase complex that also contains presenilin, nicastrin, and Pen-2. In order to clarify the existence of an endogenous fragment of Aph-1 and dissect the localization and processing of endogenous Aph-1 proteins, we examined cell lines and primary cell cultures with our own carboxyl terminal-specific antibodies for Aph-1aL. Fractionation and immunofluorescence studies indicated that the endogenous full-length Aph-1aL isoform localizes primarily to the endoplasmic reticulum as well as Golgi intermediate compartment, but small amount of it was detected at Golgi apparatus where most of its carboxyl terminal domain fragment existed. In primary neuronal and glial cultures, Aph-1aL was present in the neurites and glial cell processes. Endogenous Aph-1a and its proteolytic fragment have unique properties for cleavage control that may have implications for gamma-secretase regulation and intracellular distribution.

Salidroside attenuates hypoxia-induced abnormal processing of amyloid precursor protein by decreasing BACE1 expression in SH-SY5Y cells

Hypoxia which is mainly mediated by hypoxia-inducible factor 1 (HIF-1), can greatly contribute to the occurrence of Alzheimer's disease (AD) by increasing beta-site APP cleaving enzyme (BACE1) gene expression, protein level and beta-secretase activity, resulting in a significant generation of amyloid-beta (Abeta). Salidroside has been reported to have great neuroprotective effects. The aim of this study was to investigate the effects of salidroside on hypoxia-induced abnormal processing of the amyloid precursor protein (APP) in SH-SY5Y cells and its possible mechanism. Western blot analysis showed that 200muM of salidroside pretreatment significantly decreased BACE1 protein level and promoted the secretion of sAPPalpha in hypoxic condition. Salidroside had no effect on the level of APP, ADAM10 and ADAM17. ELISA analysis revealed that salidroside was able to inhibit the increase of beta-secretase activity and Abeta generation induced by hypoxia, with no effect on gamma-secretase activity. Notably, under hypoxia condition, mRNA of BACE1 and protein level of HIF-1alpha were decreased by salidroside pretreatment. These results demonstrated for the first time that salidroside was able to attenuate abnormal processing of amyloid precursor protein induced by hypoxia in SH-SY5Y cells, providing a new insight into prevention and treatment of Alzheimer's disease.

Role of mitochondrial-mediated signaling pathways in Alzheimer disease and hypoxia

Development of effective treatments for Alzheimer's disease is complicated by the poor understanding of its pathophysiology. Recent work suggests mitochondria may play a primary role in neurodegeneration, due to alterations in mitochondria turnover and that the brain is specifically susceptible, due to high energy demand. Mitochondria are the major source of cellular energy through oxidative phosphorylation and regulate intracellular calcium levels and survival pathways. Hypoxia has been implicated in several neurodegenerative diseases including Alzheimer's disease. During hypoxic events, mitochondrial complex III produces high levels of reactive oxygen species (ROS). These ROS seem to have a primary role in the regulation of the transcription factor hypoxia inducible factor 1alpha that triggers death effectors. Here we discuss the role of mitochondria in AD putting focus on the activation of hypoxia-mediated mitochondrial pathways, which could eventually lead to cell degeneration and death.

Acute hypoxia promote the phosphorylation of tau via ERK pathway

Tau phosphorylation and hypoxia are both linked to the pathology of Alzheimer's disease. To find out the possible connection between hypoxia and tau phosphorylation, we performed this study to evaluate the level of phosphorylated tau under hypoxic or normal condition. We found in our study that hypoxia promoted the phosphorylation of tau protein via ERK pathway, which suggest hypoxia might be involved in the process of tau pathology.

p53-dependent control of transactivation of the Pen2 promoter by presenilins

The senile plaques found in the brains of patients with Alzheimer's disease are mainly due to the accumulation of amyloid beta-peptides (A beta) that are liberated by gamma-secretase, a high molecular weight complex including presenilins, PEN-2, APH-1 and nicastrin. The depletion of each of these proteins disrupts the complex assembly into a functional protease. Here, we describe another level of regulation of this multimeric protease. The depletion of both presenilins drastically reduces Pen2 mRNA levels and its promoter transactivation. Furthermore, overexpression of presenilin-1 lowers Pen2 promoter transactivation, a phenotype abolished by a double mutation known to prevent presenilin-dependent gamma-secretase activity. PEN-2 expression is decreased by depletion of beta-amyloid precursor protein (APP) and increased by the APP intracellular domain (AICD). We show that AICD and APP complement for Pen2 mRNA levels in APP/APLP1-2 knockout fibroblasts. Interestingly, overexpression of presenilin-2 greatly increases Pen2 promoter transactivation. The opposite effect triggered by both presenilins was reminiscent of our previous study, which showed that these two proteins elicit antagonistic effects on p53. Therefore, we examined the contribution of p53 on Pen2 transcription. Pen2 promoter transactivation, and Pen2 mRNA and protein levels were drastically reduced in p53(-/-) fibroblasts. Furthermore, PEN-2 expression could be rescued by p53 complementation in p53- and APP-deficient cells. Interestingly, PEN-2 expression was also reduced in p53-deficient mouse brain. Overall, our study describes a p53-dependent regulation of PEN-2 expression by other members of the gamma-secretase complex, namely presenilins.

Oxidative stress mediates the pathogenic effect of different Alzheimer's disease risk factors

Alzheimer's disease (AD) is a progressive neurodegenerative disorder affecting the elderly population. Mechanistically, the major cause of the disease bases on the altered processing of the amyloid-β (Aβ) precursor protein (APP), resulting in the accumulation and aggregation of neurotoxic forms of Aβ. Aβ derives from the sequential proteolytic cleavage of the β- and γ-secretases on APP. The causes of Aβ accumulation in the common sporadic form of AD are not completely known, but they are likely to include oxidative stress (OS). OS and Aβ are linked to each other since Aβ aggregation induces OS in vivo and in vitro, and oxidant agents increase the production of Aβ. Moreover, OS produces several effects that may contribute to synaptic function and cell death in AD. We and others have shown that the expression and activity of β-secretase (named BACE1; β-site APP cleaving enzyme) is increased by oxidant agents and by lipid peroxidation product 4-hydroxynonenal and that there is a significant correlation between BACE1 activity and oxidative markers in sporadic AD. OS results from several cellular insults such as aging, hyperglycemia, hypoxic insults that are all well known risk factors for AD development. Thus, our data strengthen the hypothesis that OS is a basic common pathway of Aβ accumulation, common to different AD risk factors.

Oxygen sensing: a common crossroad in cancer and neurodegeneration

Prolyl hydroxylase domain (PHD) proteins are cellular oxygen sensors that orchestrate an adaptive response to hypoxia and oxidative stress, executed by hypoxia-inducible factors (HIFs). By increasing oxygen supply, reducing oxygen consumption, and reprogramming metabolism, the PHD/HIF pathway confers tolerance towards hypoxic and oxidative stress. This review discusses the involvement of the PHD/HIF response in two, at first sight, entirely distinct pathologies with opposite outcome, i.e. cancer leading to cellular growth and neurodegeneration resulting in cell death. However, these disorders share common mechanisms of sensing oxygen and oxidative stress. We will focus on how PHD/HIF signaling is pathogenetically implicated in metabolic and vessel alterations in these diseases and how manipulation of this pathway might offer novel treatment opportunities.

Iron chelator-mediated alterations in gene expression: identification of novel iron-regulated molecules that are molecular targets of hypoxia-inducible factor-1 alpha and p53

Iron deficiency affects 500 million people, yet the molecular role of iron in gene expression remains poorly characterized. In addition, the alterations in global gene expression after iron chelation remain unclear and are important to assess for understanding the molecular pathology of iron deficiency and the biological effects of chelators. Considering this, we assessed the effect on whole genome gene expression of two iron chelators (desferrioxamine and 2-hydroxy-1-napthylaldehyde isonicotinoyl hydrazone) that have markedly different permeability properties. Sixteen genes were significantly regulated by both ligands, whereas a further 50 genes were significantly regulated by either compound. Apart from iron-mediated regulation of expression via hypoxia inducible factor-1 alpha, it was noteworthy that the transcription factor p53 was also involved in iron-regulated gene expression. Examining 16 genes regulated by both chelators in normal and neoplastic cells, five genes (APP, GDF15, CITED2, EGR1, and PNRC1) were significantly differentially expressed between the cell types. In view of their functions in tumor suppression, proliferation, and apoptosis, these findings are important for understanding the selective antiproliferative effects of chelators against neoplastic cells. Most of the genes identified have not been described previously to be iron-regulated and are important for understanding the molecular and cellular effects of iron depletion.

A functional mouse retroposed gene Rps23r1 reduces Alzheimer's beta-amyloid levels and tau phosphorylation

Senile plaques consisting of beta-amyloid (Abeta) and neurofibrillary tangles composed of hyperphosphorylated tau are major pathological hallmarks of Alzheimer's disease (AD). Elucidation of factors that modulate Abeta generation and tau hyperphosphorylation is crucial for AD intervention. Here, we identify a mouse gene Rps23r1 that originated through retroposition of ribosomal protein S23. We demonstrate that RPS23R1 protein reduces the levels of Abeta and tau phosphorylation by interacting with adenylate cyclases to activate cAMP/PKA and thus inhibit GSK-3 activity. The function of Rps23r1 is demonstrated in cells of various species including human, and in transgenic mice overexpressing RPS23R1. Furthermore, the AD-like pathologies of triple transgenic AD mice were improved and levels of synaptic maker proteins increased after crossing them with Rps23r1 transgenic mice. Our studies reveal a new target/pathway for regulating AD pathologies and uncover a retrogene and its role in regulating protein kinase pathways.

Contribution of hypoxia to Alzheimer's disease: is HIF-1alpha a mediator of neurodegeneration?

The mammalian brain is extremely sensitive to alterations in cellular homeostasis as a result of environmental or physiological insults. In particular, hypoxic/ischemic challenges (i.e. reduced oxygen and/or glucose delivery) cause severe and detrimental alterations in brain function and can trigger neuronal cell death within minutes. Unfortunately, as we age, oxygen delivery to cells and tissues is impaired, thereby increasing the susceptibility of neurons to damage. Thus, hypoxic (neuronal) adaptation is significantly compromised during aging. Many neurological diseases, such as stroke, Alzheimer's disease (AD), Parkinson's disease and diabetes, are characterized by hypoxia, a state that is believed to only exacerbate disease progression. However, the contribution of hypoxia and hypoxia-mediated pathways to neurodegeneration remains unclear. This review discusses current evidence on the contribution of oxygen deprivation to AD, with an emphasis on hypoxia inducible transcription factor-1 (HIF-1)-mediated pathways and the association of AD with the cytoskeleton regulator cyclin-dependent kinase 5.

Pathological role of hypoxia in Alzheimer's disease

The majority cases of Alzheimer's disease (AD) are sporadic late-onset form not being linked to APP and PS1 gene mutations. It is believed that the environmental risk factors play an important role in the onset and development of AD. Patients suffering from cerebral ischemia and stroke in which hypoxic conditions occur are much more susceptible to AD. Increasing evidence suggests that hypoxia facilitates the pathogenesis of AD through accelerating the accumulation of Abeta, increasing the hyperphosphorylation of tau, impairing the normal functions of blood-brain barrier, and promoting the degeneration of neurons. Further investigations into the relationship between hypoxia and AD may open the avenue for effective preservation and pharmacological treatments of this neurodegenerative disease.

Transcriptional regulation of the murine Presenilin-2 gene reveals similarities and differences to its human orthologue

Inherited Presenilin-2 mutations cause familial Alzheimer's disease, and its regulation may play a role in sporadic cases. The human Presenilin-2 (PSEN2) regulatory region includes two separate promoters modulated by Egr-1, a transcription factor involved in learning and memory. To enable in-vivo analysis of Presenilin-2 regulation, we characterized the murine Presenilin-2 (Psen2) promoter. We identified novel Psen2 Transcription start sites (TSSs) 10 kb upstream of previously reported sites, along with two new alternatively transcribed exons (1A, and 1BC) in the 5' untranslated region. Transcripts initiating in Exon 1A are ubiquitous, whereas exon 1BC-initiated transcripts are non-neuronal. Only the sequence surrounding exon 1A, which includes homologous sequences to the human PSEN2 promoter, harbored significant promoter activity. Sequences upstream of exon 1A and a downstream enhancer were specifically important in neuronal cells, but similar to the human promoter, the murine promoter was characteristic of a housekeeping gene, and its activity depended on Sp1 binding. Egr-1 did not bind the murine promoter. Egr-1 over-expression and down-regulation, as well as in-vivo examination of Egr-1 and Psen2 expression during fear conditioning in mice, showed that Egr-1 does not regulate the murine Psen2 promoter. Differential Psen2 regulation in human and mouse has implications for Alzheimer disease mouse models.

Muscarinic activation attenuates abnormal processing of beta-amyloid precursor protein induced by cobalt chloride-mimetic hypoxia in retinal ganglion cells

beta-Amyloid peptide (Abeta), the major pathological factor in Alzheimer's disease, has recently been reported to be implicated in the development of glaucoma. In this study, we explored the effect of muscarinic activation on abnormal processing of beta-amyloid precursor protein (APP) induced by a risk factor hypoxia in retinal ganglion cells. Hypoxia mimetic compound cobalt chloride could increase the generation of Abeta via up-regulating the expression of APP as well as the expression of beta-secretase and gamma-secretase, whereas muscarinic receptor agonist pilocarpine could significantly attenuate this abnormal pathway, thereby resulting in a decreased amyloidogenic cleavage of APP. This finding may provide an insight into better understanding of pathophysiology for the retinal neurodegenerative disease and searching for its new modifying approach.

The up-regulation of BACE1 mediated by hypoxia and ischemic injury: role of oxidative stress and HIF1alpha

While it is well established that stroke and cerebral hypoperfusion are both significant risk factors for Alzheimer's disease, the molecular link between ischemia and amyloid precursor protein processing has only been recently established. Specifically, hypoxia significantly increases beta-site APP cleaving enzyme (BACE1) gene transcription through the over-expression of hypoxia inducible factor 1alpha, resulting in increased BACE1 secretase activity and amyloid-beta production. In this study, we significantly extend these findings both in vitro, in differentiated SK-N-BE neuroblastoma cells, and in vivo, in rats subjected to cerebral ischemia, showing that hypoxia up-regulates BACE1 expression through a biphasic mechanism. The early post-hypoxic up-regulation of BACE1 depends on the production of reactive oxygen species mediated by the sudden interruption of the mitochondrial electron transport chain, while the later expression of BACE1 is caused by hypoxia inducible factor 1alpha activation. The involvement of reactive oxygen species released by mitochondria in the BACE1 up-regulation was confirmed by the complete protection exerted by complex I inhibitors such as rotenone and diphenyl-phenylen iodonium. Moreover, the oxidative stress-mediated up-regulation of BACE1 is mediated by c-jun N terminal kinase pathway as demonstrated by the protection exerted by the silencing of c-jun N-terminal kinase isoforms 1 and 2. Our study strengthens the hypothesis that oxidative stress is a basic common mechanism of amyloid-beta accumulation.

Intracellular trafficking of presenilin 1 is regulated by beta-amyloid precursor protein and phospholipase D1

Excessive accumulation of beta-amyloid peptides in the brain is a major cause for the pathogenesis of Alzheimer disease. beta-Amyloid is derived from beta-amyloid precursor protein (APP) through sequential cleavages by beta- and gamma-secretases, whose enzymatic activities are tightly controlled by subcellular localization. Delineation of how intracellular trafficking of these secretases and APP is regulated is important for understanding Alzheimer disease pathogenesis. Although APP trafficking is regulated by multiple factors including presenilin 1 (PS1), a major component of the gamma-secretase complex, and phospholipase D1 (PLD1), a phospholipid-modifying enzyme, regulation of intracellular trafficking of PS1/gamma-secretase and beta-secretase is less clear. Here we demonstrate that APP can reciprocally regulate PS1 trafficking; APP deficiency results in faster transport of PS1 from the trans-Golgi network to the cell surface and increased steady state levels of PS1 at the cell surface, which can be reversed by restoring APP levels. Restoration of APP in APP-deficient cells also reduces steady state levels of other gamma-secretase components (nicastrin, APH-1, and PEN-2) and the cleavage of Notch by PS1/gamma-secretase that is more highly correlated with cell surface levels of PS1 than with APP overexpression levels, supporting the notion that Notch is mainly cleaved at the cell surface. In contrast, intracellular trafficking of beta-secretase (BACE1) is not regulated by APP. Moreover, we find that PLD1 also regulates PS1 trafficking and that PLD1 overexpression promotes cell surface accumulation of PS1 in an APP-independent manner. Our results clearly elucidate a physiological function of APP in regulating protein trafficking and suggest that intracellular trafficking of PS1/gamma-secretase is regulated by multiple factors, including APP and PLD1.

Novel role of CXCR2 in regulation of gamma-secretase activity

Alzheimer's disease (AD) is a progressive chronic disorder that leads to cognitive decline. Several studies have associated up-regulation of some of the chemokines and/or their receptors with altered APP processing leading to increased production of beta-amyloid protein (Abeta) and AD pathological changes. However, there is no direct evidence to date to determine whether the altered processing of APP results in up-regulation of these receptors or whether the up-regulation of the chemokine receptors causes modulated processing of APP. In the current study, we demonstrate that treatment of the chemokine receptor CXCR2 with agonists leads to enhancement of Abeta production and treatment with antagonists or immunodepletion of CXCR2's endogenous agonists leads to Abeta inhibition. Further, we found that the inhibitory effect of the antagonist of CXCR2 on Abeta40 and Abeta42 is mediated via gamma-secretase, specifically through reduction in expression of presenilin (PS), one of the gamma-secretase components. Also, in vivo chronic treatment with a CXCR2 antagonist blocked Abeta40 and Abeta42 production. Using small interfering RNAs for CXCR2, we further showed that knockdown of CXCR2 in vitro accumulates gamma-secretase substrates C99 and C83 with reduced production of both Abeta40 and Abeta42. Taken together, these findings strongly suggest for the first time that up-regulation of the CXCR2 receptor can be the driving force in increased production of Abeta. Our findings unravel new mechanisms involving the CXCR2 receptor in the pathogenesis of AD and pose it as a potential target for developing novel therapeutics for intervention in this disease. Also, we propose here a new chemical series of interest that can serve as a prototype for drug development.

The irreversible binding of amyloid peptide substrates to insulin-degrading enzyme: a biological perspective

Insulin-degrading enzyme (IDE) is a conserved Zn(2+)metalloendopeptidase involved in insulin degradation and in the maintenance of brain steady-state levels of amyloid beta peptide (Abeta) of Alzheimer's disease (AD). Our recent demonstration that IDE and Abeta are capable of forming a stoichiometric and extremely stable complex raises several intriguing possibilities regarding the role of this unique protein-peptide interaction in physiological and pathological conditions. These include a protective cellular function of IDE as a "dead-end chaperone" alternative to its proteolytic activity and the potential impact of the irreversible binding of Abeta to IDE upon its role as a varicella zoster virus receptor. In a pathological context, the implications for insulin signaling and its relationship to AD pathogenesis are discussed. Moreover, our findings warrant further research regarding a possible general and novel interaction between amyloidogenic peptides and other Zn(2+)metallopeptidases with an IDE-like fold and a substrate conformation-dependent recognition mechanism.

The role of oxygen availability in embryonic development and stem cell function

Low levels of oxygen (O2) occur naturally in developing embryos. Cells respond to their hypoxic microenvironment by stimulating several hypoxia-inducible factors (and other molecules that mediate O2 homeostasis), which then coordinate the development of the blood, vasculature, placenta, nervous system and other organs. Furthermore, embryonic stem and progenitor cells frequently occupy hypoxic 'niches' and low O2 regulates their differentiation. Recent work has revealed an important link between factors that are involved in regulating stem and progenitor cell behaviour and hypoxia-inducible factors, which provides a molecular framework for the hypoxic control of differentiation and cell fate. These findings have important implications for the development of therapies for tissue regeneration and disease.

Hypoxia increases Abeta generation by altering beta- and gamma-cleavage of APP

Environmental factors are significant contributors for the development of Alzheimer's disease (AD). The greatly increased incidence of AD following stroke and cerebral ischemia suggests that hypoxia is a risk factor which may accelerate AD pathogenesis by altering amyloid precursor protein (APP) processing. However, the molecular mechanisms underlying the hypoxia mediated AD pathogenesis have not been fully elucidated. In the present study we demonstrated that repeated hypoxia increased beta-amyloid (Abeta) generation and neuritic plaques formation by elevating beta-cleavage of APP in APP(swe)+PS1(A246E) transgenic mice. We also found that hypoxia enhanced the expression of APH-1a, a component of gamma-secretase complex, which in turn may lead to increase in gamma-cleavage activity. Furthermore, we demonstrated that repeated hypoxia treatment can activate macroautophagy, which may contribute to the increases in Abeta production since pretreatment with macroautophagy inhibitor 3-methyladenine significantly blocked chemical hypoxic condition-induced increase in Abeta production in SH-SY5Y cells. Taken together, our results suggest an important role of hypoxia in modulating the APP processing by facilitating both beta- and gamma-cleavage which may result in a significant increase of Abeta generation.

Signal transduction in Alzheimer disease: p21-activated kinase signaling requires C-terminal cleavage of APP at Asp664

The deficits in Alzheimer disease (AD) stem at least partly from neurotoxic beta-amyloid peptides generated from the amyloid precursor protein (APP). APP may also be cleaved intracellularly at Asp664 to yield a second neurotoxic peptide, C31. Previously, we showed that cleavage of APP at the C-terminus is required for the impairments seen in APP transgenic mice, by comparing elements of the disease in animals modeling AD, with (platelet-derived growth factor B-chain promoter-driven APP transgenic mice; PDAPP) versus without (PDAPP D664A) a functional Asp664 caspase cleavage site. However, the signaling mechanism(s) by which Asp664 contributes to these deficits remains to be elucidated. In this study, we identify a kinase protein, recently shown to bind APP at the C-terminus and to contribute to AD, whose activity is modified in PDAPP mice, but normalized in PDAPP D664A mice. Specifically, we observed a significant increase in nuclear p21-activated kinase (isoforms 1, 2, and or 3; PAK-1/2/3) activation in hippocampus of 3 month old PDAPP mice compared with non-transgenic littermates, an effect completely prevented in PDAPP D664A mice. In contrast, 13 month old PDAPP mice displayed a significant decrease in PAK-1/2/3 activity, which was once again absent in PDAPP D664A mice. Similarly, in hippocampus of early and severe AD subjects, there was a progressive and subcellular-specific reduction in active PAK-1/2/3 compared with normal controls. Interestingly, total PAK-1/2/3 protein was increased in early AD subjects, but declined in moderate AD and declined further, to significantly below that of control levels, in severe AD. These findings are compatible with previous suggestions that PAK may be involved in the pathophysiology of AD, and demonstrate that both early activation and late inactivation in the murine AD model require the cleavage of APP at Asp664.

Involvement of γ-secretase in postnatal angiogenesis

gamma-Secretase cleaves the transmembrane domains of several integral membrane proteins involved in vasculogenesis. Here, we investigated the role of gamma-secretase in the regulation of postnatal angiogenesis using gamma-secretase inhibitors (GSI). In endothelial cell (EC), gamma-secretase activity was up-regulated under hypoxia or the treatment of vascular endothelial growth factor (VEGF). The treatment of GSI significantly attenuated growth factor-induced EC proliferation and migration as well as c-fos promoter activity in a dose-dependent manner. In vascular smooth muscle cell (VSMC), treatment of GSI significantly attenuated growth factor-induced VEGF and fibroblast growth factor-2 (FGF-2) expression. Indeed, GSI attenuated VEGF-induced tube formation and inhibited FGF-2-induced angiogenesis on matrigel in mice as quantified by FITC-lectin staining of EC. Overall, we demonstrated that gamma-secretase may be key molecule in postnatal angiogenesis which may be downstream molecule of growth factor-induced growth and migration in EC, and regulate the expression of angiogenic growth factors in VSMC.

Transcriptional Up-regulation of Inhibitory PAS Domain Protein Gene Expression by Hypoxia-inducible Factor 1 (HIF-1)

The inhibitory PAS (Per/Arnt/Sim) domain protein (IPAS), a dominant negative regulator of hypoxia-inducible transcription factors (HIFs), is potentially implicated in negative regulation of angiogenesis in such tissues as the avascular cornea of the eye. We have previously shown IPAS mRNA expression is up-regulated in hypoxic tissues, which at least in part involves hypoxia-dependent alternative splicing of the transcripts from the IPAS/HIF-3alpha locus. In the present study, we demonstrate that a hypoxia-driven transcriptional mechanism also plays a role in augmentation of IPAS gene expression. Isolation and analyses of the promoter region flanking to the first exon of IPAS gene revealed a functional hypoxia response element at position -834 to -799, whereas the sequence upstream of the HIF-3alpha first exon scarcely responded to hypoxic stimuli. A transient transfection experiment demonstrated that HIF-1alpha mediates IPAS promoter activation via the functional hypoxia response element under hypoxic conditions and that a constitutively active form of HIF-1alpha is sufficient for induction of the promoter in normoxic cells. Moreover, chromatin immunoprecipitation and electrophoretic mobility shift assays showed binding of the HIF-1 complex to the element in a hypoxia-dependent manner. Taken together, HIF-1 directly up-regulates IPAS gene expression through a mechanism distinct from RNA splicing, providing a further level of negative feedback gene regulation in adaptive responses to hypoxic/ischemic conditions.

Hypoxia-inducible factor 1alpha (HIF-1alpha)-mediated hypoxia increases BACE1 expression and beta-amyloid generation

The incidence of Alzheimer disease (AD) and vascular dementia is greatly increased following cerebral ischemia and stroke in which hypoxic conditions occur in affected brain areas. beta-Amyloid peptide (Abeta), which is derived from the beta-amyloid precursor protein (APP) by sequential proteolytic cleavages from beta-secretase (BACE1) and presenilin-1 (PS1)/gamma-secretase, is widely believed to trigger a cascade of pathological events culminating in AD and vascular dementia. However, a direct molecular link between hypoxic insults and APP processing has yet to be established. Here, we demonstrate that acute hypoxia increases the expression and the enzymatic activity of BACE1 by up-regulating the level of BACE1 mRNA, resulting in increases in the APP C-terminal fragment-beta (betaCTF) and Abeta. Hypoxia has no effect on the level of PS1, APP, and tumor necrosis factor-alpha-converting enzyme (TACE, an enzyme known to cleave APP at the alpha-secretase cleavage site). Sequence analysis, mutagenesis, and gel shift studies revealed binding of HIF-1 to the BACE1 promoter. Overexpression of HIF-1alpha increases BACE1 mRNA and protein level, whereas down-regulation of HIF-1alpha reduced the level of BACE1. Hypoxic treatment fails to further potentiate the stimulatory effect of HIF-1alpha overexpression on BACE1 expression, suggesting that hypoxic induction of BACE1 expression is primarily mediated by HIF-1alpha. Finally, we observed significant reduction in BACE1 protein levels in the hippocampus and the cortex of HIF-1alpha conditional knock-out mice. Our results demonstrate an important role for hypoxia/HIF-1alpha in modulating the amyloidogenic processing of APP and provide a molecular mechanism for increased incidence of AD following cerebral ischemic and stroke injuries.

Presenilins regulate the cellular level of the tumor suppressor PTEN

Alzheimer's Disease (AD) is characterized by amyloid plaques consisting of beta-amyloid (Abeta) peptides and neurofibrillary tangles consisting of hyperphosphorylated tau protein. Abeta is proteolytically derived from its precursor protein through cleavages by beta-secretase and gamma-secretase complex comprising presenilins (PS, PS1/PS2), nicastrin, APH-1 and PEN-2. PS1 is also known to activate the PI3K/Akt cell survival pathway in a gamma-secretase-independent manner. The tumor suppressor PTEN, which antagonizes the PI3K/Akt pathway, has increasingly been recognized to play a key role in neural functions and its level found reduced in AD brains. Here, we demonstrate that the protein level of PTEN is dramatically reduced in cultured cells and embryonic tissues deficient in PS, and in the cortical neurons of PS1/PS2 conditional double knockout mice. Restoration of PS in PS-deficient cells reverses the reduction of PTEN. Regulation of PTEN by PS is independent of the PS/gamma-secretase activity since impaired gamma-secretase by the gamma-secretase inhibitor treatment or due to nicastrin deficiency has little effect on the protein level of PTEN. Our data suggest an important role for PS in signaling pathways involving PI3K/Akt and PTEN that are crucial for physiological functions and the pathogenesis of multiple diseases.