Transcriptional regulation of beta-secretase by p25/cdk5 leads to enhanced amyloidogenic processing

BACE1 in Alzheimer's disease

Targeting BACE1 (β-site APP cleaving enzyme 1 or β-secretase) is the focus of Alzheimer's disease (AD) research because this aspartyl protease is involved in the abnormal production of β amyloid plaques (Aβ), the hallmark of its pathophysiology. Evidence suggests that there is a strong connection between AD and BACE1. As such, strategies to inhibit Aβ formation in the brain should prove beneficial for AD treatment. Aβ, the product of the large type1 trans-membrane protein amyloid precursor protein (APP), is produced in a two-step proteolytic process initiated by BACE1 (β-secretase) and followed by γ-secretase. Due to its apparent rate limiting function, BACE1 appears to be a prime target to prevent Aβ generation in AD. Following its discovery, the BACE1 has been cloned, its structure solved, novel physiologic substrates discovered and numerous inhibitors developed. This review focuses on elucidating the role of BACE1 to facilitate drug development in the treatment of AD.

Deregulated Cdk5 triggers aberrant activation of cell cycle kinases and phosphatases inducing neuronal death

Aberrant activation of cell cycle proteins is believed to play a critical role in Alzheimer's disease (AD) pathogenesis; although, the molecular mechanisms leading to their activation in diseased neurons remain elusive. The goal of this study was to investigate the mechanistic link between Cdk5 deregulation and cell cycle re-activation in β-amyloid(1-42) (Aβ(1-42))-induced neurotoxicity. Using a chemical genetic approach, we identified Cdc25A, Cdc25B and Cdc25C as direct Cdk5 substrates in mouse brain lysates. We show that deregulated Cdk5 directly phosphorylates Cdc25A, Cdc25B and Cdc25C at multiple sites, which not only increases their phosphatase activities but also facilitates their release from 14-3-3 inhibitory binding. Cdc25A, Cdc25B and Cdc25C in turn activate Cdk1, Cdk2 and Cdk4 kinases causing neuronal death. Selective inhibition of Cdk5 abrogates Cdc25 and Cdk activations in Aβ(1-42)-treated neurons. Similarly, phosphorylation-resistant mutants of Cdc25 isoforms at Cdk5 sites are defective in activating Cdk1, Cdk2 and Cdk4 in Aβ(1-42)-treated primary cortical neurons, emphasizing a major role of Cdk5 in the activation of Cdc25 isoforms and Cdks in AD pathogenesis. These results were further confirmed in human AD clinical samples, which had higher Cdc25A, Cdc25B and Cdc25C activities that were coincident with increased Cdk5 activity, as compared to age-matched controls. Inhibition of Cdk5 confers the highest neuroprotection against Aβ(1-42) toxicity, whereas inhibition of Cdc25 isoforms was partially neuroprotective, further emphasizing a decisive role of Cdk5 deregulation in cell-cycle-driven AD neuronal death.

Epidemiology of Alzheimer disease

The global prevalence of dementia has been estimated to be as high as 24 million, and is predicted to double every 20 years until at least 2040. As the population worldwide continues to age, the number of individuals at risk will also increase, particularly among the very old. Alzheimer disease is the leading cause of dementia beginning with impaired memory. The neuropathological hallmarks of Alzheimer disease include diffuse and neuritic extracellular amyloid plaques in brain that are frequently surrounded by dystrophic neurites and intraneuronal neurofibrillary tangles. The etiology of Alzheimer disease remains unclear, but it is likely to be the result of both genetic and environmental factors. In this review we discuss the prevalence and incidence rates, the established environmental risk factors, and the protective factors, and briefly review genetic variants predisposing to disease.

Concerted perturbation observed in a hub network in Alzheimer's disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disease involving the alteration of gene expression at the whole genome level. Genome-wide transcriptional profiling of AD has been conducted by many groups on several relevant brain regions. However, identifying the most critical dys-regulated genes has been challenging. In this work, we addressed this issue by deriving critical genes from perturbed subnetworks. Using a recent microarray dataset on six brain regions, we applied a heaviest induced subgraph algorithm with a modular scoring function to reveal the significantly perturbed subnetwork in each brain region. These perturbed subnetworks were found to be significantly overlapped with each other. Furthermore, the hub genes from these perturbed subnetworks formed a connected hub network consisting of 136 genes. Comparison between AD and several related diseases demonstrated that the hub network was robustly and specifically perturbed in AD. In addition, strong correlation between the expression level of these hub genes and indicators of AD severity suggested that this hub network can partially reflect AD progression. More importantly, this hub network reflected the adaptation of neurons to the AD-specific microenvironment through a variety of adjustments, including reduction of neuronal and synaptic activities and alteration of survival signaling. Therefore, it is potentially useful for the development of biomarkers and network medicine for AD.

Consequences of inhibiting amyloid precursor protein processing enzymes on synaptic function and plasticity

Alzheimer's disease (AD) is a neurodegenerative disease, one of whose major pathological hallmarks is the accumulation of amyloid plaques comprised of aggregated β-amyloid (Aβ) peptides. It is now recognized that soluble Aβ oligomers may lead to synaptic dysfunctions early in AD pathology preceding plaque deposition. Aβ is produced by a sequential cleavage of amyloid precursor protein (APP) by the activity of β- and γ-secretases, which have been identified as major candidate therapeutic targets of AD. This paper focuses on how Aβ alters synaptic function and the functional consequences of inhibiting the activity of the two secretases responsible for Aβ generation. Abnormalities in synaptic function resulting from the absence or inhibition of the Aβ-producing enzymes suggest that Aβ itself may have normal physiological functions which are disrupted by abnormal accumulation of Aβ during AD pathology. This interpretation suggests that AD therapeutics targeting the β- and γ-secretases should be developed to restore normal levels of Aβ or combined with measures to circumvent the associated synaptic dysfunction(s) in order to have minimal impact on normal synaptic function.

Calpain inhibitor A-705253 mitigates Alzheimer's disease-like pathology and cognitive decline in aged 3xTgAD mice

Calpains are cysteine proteinases that selectively cleave proteins in response to calcium signals. Exacerbated activation of calpain has been implicated as a major component in the signaling cascade that leads to β-amyloid (Aβ) production and tau hyperphosphorylation in Alzheimer's disease (AD). In this study, we analyzed the potential therapeutic efficacy of inhibiting the activation of calpain by a novel calpain inhibitor in aged 3xTgAD mice with well-established cognitive impairment, plaques, and tangles. The administration of a novel inhibitor of calpain, A-705253, attenuated cognitive impairment and synaptic dysfunction in a dose-dependent manner in 3xTgAD mice. Inhibition of calpain lowered Aβ(40) and Aβ(42) levels in both detergent-soluble and detergent-insoluble fractions and also reduced the total number and size of thioflavin S-positive fibrillar Aβ deposits. Mechanistically, these effects were, in part, explained by a down-regulation of β-secretase 1 (BACE1) and an up-regulation of ATP-binding cassette transporter A1 (ABCA1) expression, which, in turn, contributed to reduced production and increased clearance of Aβ, respectively. Moreover, A-705253 decreased the activation of cyclin-dependent kinase 5 (CDK5) and thereby diminished the hyperphosphorylation of tau. Finally, blockage of calpain activation reduced the astrocytic and microglial responses associated with AD-like pathological characteristics in aged 3xTgAD mice. Our data provide relevant functional and molecular insights into the beneficial therapeutic effects of inhibiting calpain activation for the management of AD.

Phosphorylation of p35 and p39 by Cdk5 determines the subcellular location of the holokinase in a phosphorylation-site-specific manner

Cdk5 is a member of the cyclin-dependent kinase (Cdk) family, which is activated by neuronal activators p35 or p39. Cdk5 regulates a variety of neuronal activities including migration, synaptic activity and neuronal death. p35 and p39 impart cytoplasmic membrane association of p35-Cdk5 and p39-Cdk5, respectively, through their myristoylation, but it is not clearly understood how the cellular localization is related to different functions. We investigated the role of Cdk5 activity in the subcellular localization of p35-Cdk5 and p39-Cdk5. Cdk5 activity affected the localization of p35-Cdk5 and p39-Cdk5 through phosphorylation of p35 or p39. Using unphosphorylated or phosphomimetic mutants of p35 and p39, we found that phosphorylation at Ser8, common to p35 and p39, by Cdk5 regulated the cytoplasmic localization and perinuclear accumulation of unphosphorylated S8A mutants, and whole cytoplasmic distribution of phosphomimetic S8E mutants. Cdk5 activity was necessary to retain Cdk5-activator complexes in the cytoplasm. Nevertheless, small but distinct amounts of p35 and p39 were detected in the nucleus. In particular, nuclear p35 and p39 were increased when the Cdk5 activity was inhibited. p39 had a greater propensity to accumulate in the nucleus than p35, and phosphorylation at Thr84, specific to p39, regulated the potential nuclear localization activity of the Lys cluster in p39. These results suggest that the subcellular localization of the Cdk5-activator complexes is determined by its kinase activity, and also implicate a role for p39-Cdk5 in the nucleus.

Obesity, leptin, and Alzheimer's disease

Obesity has various deleterious effects on health largely associated with metabolic abnormalities including abnormal glucose and lipid homeostasis that are associated with vascular injury and known cardiac, renal, and cerebrovascular complications. Advanced age is also associated with increased adiposity, decreased lean mass, and increased risk for obesity-related diseases. Although many of these obesity- and age-related disease processes have long been subsumed to be secondary to metabolic or vascular dysfunction, increasing evidence indicates that obesity also modulates nonvascular diseases such as Alzheimer's disease (AD) dementia. The link between peripheral obesity and neurodegeneration will be explored, using adipokines and AD as a template. After an introduction to the neuropathology of AD, the relationship between body weight, obesity, and dementia will be reviewed. Then, population-based and experimental studies that address whether leptin modulates brain health and mitigates AD pathways will be explored. These studies will serve as a framework for understanding the role of adipokines in brain health.

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 Alzheimer's 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.

Oxidative stress increases BACE1 protein levels through activation of the PKR-eIF2α pathway

Beta-site APP cleaving enzyme 1 (BACE1) is the rate limiting enzyme for accumulation of amyloid β (Aβ)-peptide in the brain in Alzheimer's disease (AD). Oxidative stress (OS) that leads to metabolic dysfunction and apoptosis of neurons in AD enhances BACE1 expression and activity. The activation of c-jun N-terminal kinase (JNK) pathway was proposed to explain the BACE1 mRNA increase under OS. However, little is known about the translational control of BACE1 in OS. Recently, a post-transcriptional increase of BACE1 level controlled by phosphorylation of eIF2α (eukaryotic translation initiation factor-2α) have been described after energy deprivation. PKR (double-stranded RNA dependant protein kinase) is a pro-apoptotic kinase that phosphorylates eIF2α and modulates JNK activation in various cellular stresses. We investigated the relations between PKR, eIF2α and BACE1 in AD brains in APP/PS1 knock-in mice and in hydrogen peroxide-induced OS in human neuroblastoma (SH-SY5Y) cell cultures. Immunoblotting results showed that activated PKR (pPKR) and activated eIF2α (peIF2α) and BACE1 levels are increased in AD cortices and BACE1 correlate with phosphorylated eIF2α levels. BACE1 protein levels are increased in response to OS in SH-SY5Y cells and specific inhibitions of PKR-eIF2α attenuate BACE1 protein levels in this model. Our findings provide a new translational regulation of BACE1, under the control of PKR in OS, where eIF2α phosphorylation regulates BACE1 protein expression.

Cdk5 protein inhibition and Aβ42 increase BACE1 protein level in primary neurons by a post-transcriptional mechanism: implications of CDK5 as a therapeutic target for Alzheimer disease

The β-secretase enzyme BACE1 initiates production of the amyloid-β (Aβ) peptide that comprises plaques in Alzheimer disease (AD) brain. BACE1 levels are increased in AD, potentially accelerating Aβ generation, but the mechanisms of BACE1 elevation are not fully understood. Cdk5/p25 has been implicated in neurodegeneration and BACE1 regulation, suggesting therapeutic Cdk5 inhibition for AD. In addition, caspase 3 has been implicated in BACE1 elevation. Here, we show that the Cdk5 level and p25:p35 ratio were elevated and correlated with BACE1 level in brains of AD patients and 5XFAD transgenic mice. Mouse primary cortical neurons treated with Aβ42 oligomers had increased BACE1 level and p25:p35 ratio. Surprisingly, the Aβ42-induced BACE1 elevation was not blocked by Cdk5 inhibitors CP68130 and roscovitine, and instead the BACE1 level was increased greater than with Aβ42 treatment alone. Moreover, Cdk5 inhibitors alone elevated BACE1 in a time- and dose-dependent manner that coincided with increased caspase 3 cleavage and decreased Cdk5 level. Caspase 3 inhibitor benzyloxycarbonyl-VAD failed to prevent the Aβ42-induced BACE1 increase. Further experiments suggested that the Aβ42-induced BACE1 elevation was the result of a post-transcriptional mechanism. We conclude that Aβ42 may increase the BACE1 level independently of either Cdk5 or caspase 3 and that Cdk5 inhibition for AD may cause BACE1 elevation, a potentially negative therapeutic outcome.

7,8-dihydroxyflavone, a small-molecule TrkB agonist, reverses memory deficits and BACE1 elevation in a mouse model of Alzheimer's disease

Increasing evidence suggests that reductions in brain-derived neurotrophic factor (BDNF) and its receptor tyrosine receptor kinase B (TrkB) may have a role in the pathogenesis of Alzheimer's disease (AD). However, the efficacy and safety profile of BDNF therapy (eg, gene delivery) remains to be established toward clinical trials. Here, we evaluated the effects of 7,8-dihydroxyflavone (7,8-DHF), a recently identified small-molecule TrkB agonist that can pass the blood-brain barrier, in the 5XFAD transgenic mouse model of AD. 5XFAD mice at 12-15 months of age and non-transgenic littermate controls received systemic administration of 7,8-DHF (5 mg/kg, i.p.) once daily for 10 consecutive days. We found that 7,8-DHF rescued memory deficits of 5XFAD mice in the spontaneous alternation Y-maze task. 5XFAD mice showed impairments in the hippocampal BDNF-TrkB pathway, as evidenced by significant reductions in BDNF, TrkB receptors, and phosphorylated TrkB. 7,8-DHF restored deficient TrkB signaling in 5XFAD mice without affecting endogenous BDNF levels. Meanwhile, 5XFAD mice exhibited elevations in the β-secretase enzyme (BACE1) that initiates amyloid-β (Aβ) generation, as observed in sporadic AD. Interestingly, 7,8-DHF blocked BACE1 elevations and lowered levels of the β-secretase-cleaved C-terminal fragment of amyloid precursor protein (C99), Aβ40, and Aβ42 in 5XFAD mouse brains. Furthermore, BACE1 expression was decreased by 7,8-DHF in wild-type mice, suggesting that BDNF-TrkB signaling is also important for downregulating baseline levels of BACE1. Together, our findings indicate that TrkB activation with systemic 7,8-DHF can ameliorate AD-associated memory deficits, which may be, at least in part, attributable to reductions in BACE1 expression and β-amyloidogenesis.

Synaptic deficits are rescued in the p25/Cdk5 model of neurodegeneration by the reduction of β-secretase (BACE1)

Alzheimer's disease (AD) is the most common cause of dementia, and is characterized by memory loss and cognitive decline, as well as amyloid β (Aβ) accumulation, and progressive neurodegeneration. Cdk5 is a proline-directed serine/threonine kinase whose activation by the p25 protein has been implicated in a number of neurodegenerative disorders. The CK-p25 inducible mouse model exhibits progressive neuronal death, elevated Aβ, reduced synaptic plasticity, and impaired learning following p25 overexpression in forebrain neurons. Levels of Aβ, as well as the APP processing enzyme, β-secretase (BACE1), are also increased in CK-p25 mice. It is unknown what role increased Aβ plays in the cognitive and neurodegenerative phenotype of the CK-p25 mouse. In the current work, we restored Aβ levels in the CK-p25 mouse to those of wild-type mice via the partial genetic deletion of BACE1, allowing us to examine the Aβ-independent phenotype of this mouse model. We show that, in the CK-p25 mouse, normalization of Aβ levels led to a rescue of synaptic and cognitive deficits. Conversely, neuronal loss was not ameliorated. Our findings indicate that increases in p25/Cdk5 activity may mediate cognitive and synaptic impairment via an Aβ-dependent pathway in the CK-p25 mouse. These findings explore the impact of targeting Aβ production in a mouse model of neurodegeneration and cognitive impairment, and how this may translate into therapeutic approaches for sporadic AD.

Cdk5: a multifaceted kinase in neurodegenerative diseases

Since the identification of cyclin-dependent kinase-5 (Cdk5) as a tau kinase and member of the Cdk family almost 20 years ago, deregulation of Cdk5 activity has been linked to an array of neurodegenerative diseases. As knowledge on the etiopathological mechanisms of these diseases evolved through the years, Cdk5 has also been implicated in additional cellular events that are affected under these pathological conditions. From the role of Cdk5 in the regulation of synaptic functions to its involvement in autophagy deregulation, significant insights have been obtained regarding the role of Cdk5 as a key regulator of neurodegeneration. Here, we summarize recent findings on the involvement of Cdk5 in the pathophysiological mechanisms underlying various neurodegenerative diseases.

DNA methylation of the 5'-untranslated region at +298 and +351 represses BACE1 expression in mouse BV-2 microglial cells

BACE1, which cleaves the amyloid precursor protein, is the rate-limiting enzyme for β-amyloid peptide production, leading to the pathogenesis of Alzheimer's disease (AD). A high plasma level of homocysteine, acting as a potent methyltransferase inhibitor, is assumed to be a risk factor for AD onset. Using the demethylating drug 5-aza-2'-deoxycytidine (5-Aza), we tested whether and how BACE1 expression is regulated in mouse BV-2 microglial cells. 5-Aza increased both BACE1 mRNA and protein levels in a dose-dependent manner. Bisulfite-sequencing analysis revealed that two CpG sites at positions +298 and +351 in the 5'-untranslated region (5'-UTR) of the BACE1 gene were specifically demethylated in BV-2 cells treated with 5-Aza. In silico analysis showed that the +351 site is the STAT3/CTCF-binding site; the function of the +298 site has not been identified. To assess whether these two CpG sites play an important role in 5-Aza-induced transcriptional activation of BACE1, we constructed a BACE1 gene promoter including the 5'-UTR (-1136 to +500) fused to a CpG-free luciferase gene (pCpGL-BACE1) and its mutant pCpGL-BACE1-AA, which has substituted CG dinucleotides at the two CpG sites of pCpGL-BACE1 to AA. Promoter analysis showed a significant decrease (∼30%) in the activity of pCpGL-BACE1-AA compared with that of pCpGL-BACE1. Furthermore, in vitro methylation of these two reporter constructs showed a complete silencing of their promoter activities. Our data demonstrate that BACE1 gene expression is regulated by DNA methylation of at least two CpG sites at positions +298 and +351 in the 5'-UTR in BV-2 microglial cells.

Regulation of β-site APP-cleaving enzyme 1 gene expression and its role in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder leading to dementia. Neuritic plaques are the hallmark neuropathology in AD brains. Proteolytic processing of amyloid-β precursor protein at the β site by beta-site amyloid-β precursor protein-cleaving enzyme 1 (BACE1) is essential to generate Aβ, a central component of the neuritic plaques. BACE1 is increased in some sporadic AD brains, and dysregulation of BACE1 gene expression plays an important role in AD pathogenesis. This review will focus on the regulation of BACE1 gene expression at the transcriptional, post-transcriptional, translation initiation, translational and post-translational levels, and its role in AD pathogenesis. Further studies on BACE1 gene expression regulation will greatly contribute to our understanding of AD pathogenesis and reveal potential novel approaches for AD prevention and drug development.

The potential for estrogens in preventing Alzheimer's disease and vascular dementia

Estrogens are the best-studied class of drugs for potential use in the prevention of Alzheimer's disease (AD). These steroids have been shown to be potent neuroprotectants both in vitro and in vivo, and to exert effects that are consistent with their potential use in prevention of AD. These include the prevention of the processing of amyloid precursor protein (APP) into beta-amyloid (Aß), the reduction in tau hyperphosphorylation, and the elimination of catastrophic attempts at neuronal mitosis. Further, epidemiological data support the efficacy of early postmenopausal use of estrogens for the delay or prevention of AD. Collectively, this evidence supports the further development of estrogen-like compounds for prevention of AD. Several approaches to enhance brain specificity of estrogen action are now underway in an attempt to reduce the side effects of chronic estrogen therapy in AD.

BACE2 expression increases in human neurodegenerative disease

β-Secretase, the rate-limiting enzymatic activity in the production of the amyloid-β (Aβ) peptide, is a major target of Alzheimer's disease (AD) therapeutics. There are two forms of the enzyme: β-site Aβ precursor protein cleaving enzyme (BACE) 1 and BACE2. Although BACE1 increases in late-stage AD, little is known about BACE2. We conducted a detailed examination of BACE2 in patients with preclinical to late-stage AD, including amnestic mild cognitive impairment, and age-matched controls, cases of frontotemporal dementia, and Down's syndrome. BACE2 protein and enzymatic activity increased as early as preclinical AD and were found in neurons and astrocytes. Although the levels of total BACE2 mRNA were unchanged, the mRNA for BACE2 splice form C (missing exon 7) increased in parallel with BACE2 protein and activity. BACE1 and BACE2 were strongly correlated with each other at all levels, suggesting that their regulatory mechanisms may be largely shared. BACE2 was also elevated in frontotemporal dementia but not in Down's syndrome, even in patients with substantial Aβ deposition. Thus, expression of both forms of β-secretase are linked and may play a combined role in human neurologic disease. A better understanding of the normal functions of BACE1 and BACE2, and how these change in different disease states, is essential for the future development of AD therapeutics.

Inhibitory effect of a tyrosine-fructose Maillard reaction product, 2,4-bis(p-hydroxyphenyl)-2-butenal on amyloid-β generation and inflammatory reactions via inhibition of NF-κB and STAT3 activation in cultured astrocytes and microglial BV-2 cells

BACKGROUND

Amyloidogenesis is linked to neuroinflammation. The tyrosine-fructose Maillard reaction product, 2,4-bis(p-hydroxyphenyl)-2-butenal, possesses anti-inflammatory properties in cultured macrophages, and in an arthritis animal model. Because astrocytes and microglia are responsible for amyloidogenesis and inflammatory reactions in the brain, we investigated the anti-inflammatory and anti-amyloidogenic effects of 2,4-bis(p-hydroxyphenyl)-2-butenal in lipopolysaccharide (LPS)-stimulated astrocytes and microglial BV-2 cells.

METHODS

Cultured astrocytes and microglial BV-2 cells were treated with LPS (1 μg/ml) for 24 h, in the presence (1, 2, 5 μM) or absence of 2,4-bis(p-hydroxyphenyl)-2-butenal, and harvested. We performed molecular biological analyses to determine the levels of inflammatory and amyloid-related proteins and molecules, cytokines, Aβ, and secretases activity. Nuclear factor-kappa B (NF-κB) DNA binding activity was determined using gel mobility shift assays.

RESULTS

We found that 2,4-bis(p-hydroxyphenyl)-2-butenal (1, 2, 5 μM) suppresses the expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) as well as the production of nitric oxide (NO), reactive oxygen species (ROS), tumor necrosis factor-α (TNF-α), and interleukin-1β (IL-1β) in LPS (1 μg/ml)-stimulated astrocytes and microglial BV-2 cells. Further, 2,4-bis(p-hydroxyphenyl)-2-butenal inhibited the transcriptional and DNA binding activity of NF-κB--a transcription factor that regulates genes involved in neuroinflammation and amyloidogenesis via inhibition of IκB degradation as well as nuclear translocation of p50 and p65. Consistent with the inhibitory effect on inflammatory reactions, 2,4-bis(p-hydroxyphenyl)-2-butenal inhibited LPS-elevated Aβ42 levels through attenuation of β- and γ-secretase activities. Moreover, studies using signal transducer and activator of transcription 3 (STAT3) siRNA and a pharmacological inhibitor showed that 2,4-bis(p-hydroxyphenyl)-2-butenal inhibits LPS-induced activation of STAT3.

CONCLUSIONS

These results indicate that 2,4-bis(p-hydroxyphenyl)-2-butenal inhibits neuroinflammatory reactions and amyloidogenesis through inhibition of NF-κB and STAT3 activation, and suggest that 2,4-bis(p-hydroxyphenyl)-2-butenal may be useful for the treatment of neuroinflammatory diseases like Alzheimer's disease.

From understanding synaptic plasticity to the development of cognitive enhancers

Accumulating evidence reveals that synaptic dysfunction precedes neuronal loss in neurodegenerative diseases such as Alzheimer's disease. Intriguingly, synaptic abnormality is also implicated in a myriad of psychiatric disorders including depression. In particular, alterations in spine density and morphology have been associated with aberrant synaptic activity in these diseased brains. Understanding the molecular mechanisms underlying the regulation of spine morphogenesis, synaptic function and plasticity under physiological and pathological conditions will therefore provide critical insights for the development of potential therapeutic agents against these diseases. Here we summarize existing knowledge on some of the molecular players in synaptic plasticity, and highlight how these findings from basic neuroscientific research aid in the identification of novel drug leads for the development of therapeutics.

Selective neuronal vulnerability in neurodegenerative diseases: from stressor thresholds to degeneration

Neurodegenerative diseases selectively target subpopulations of neurons, leading to the progressive failure of defined brain systems, but the basis of such selective neuronal vulnerability has remained elusive. Here, we discuss how a stressor-threshold model of how particular neurons and circuits are selectively vulnerable to disease may underly the etiology of familial and sporadic forms of diseases such as Alzheimer's, Parkinson's, Huntington's, and ALS. According to this model, the intrinsic vulnerabilities of neuronal subpopulations to stressors and specific disease-related misfolding proteins determine neuronal morbidity. Neurodegenerative diseases then involve specific combinations of genetic predispositions and environmental stressors, triggering increasing age-related stress and proteostasis dysfunction in affected vulnerable neurons. Damage to vasculature, immune system, and local glial cells mediates environmental stress, which could drive disease at all stages.

Silencing of CDK5 as potential therapy for Alzheimer's disease

Neurodegeneration is one of the greatest public health challenges for the 21st century. Among neurodegenerative diseases, Alzheimer's disease (AD) is the most prevalent and best characterized. Nevertheless, despite the large investment in AD research, currently there is no effective therapeutic option. In the present review, we highlight a novel alternative, which takes advantage of the biotechnological outbreak deployed by the discovery of the RNA interference-based gene silencing mechanism, and its application as a tool for neurodegeneration treatment. Here, we highlight cyclin-dependent kinase 5 (CDK5) as a key candidate target for therapeutic gene silencing. Unlike other members of the cyclin-dependent kinase family, CDK5 does not seem to play a crucial role in cell cycle regulation. By contrast, CDK5 participates in multiple functions during nervous system development and has been established as a key mediator of Tau hyperphosphorylation and neurofibrillary pathology, thus serving as an optimal candidate for targeted therapy in the adult nervous system. We propose that the use of RNA interference for CDK5 silencing presents an attractive and specific therapeutic alternative for AD and perhaps against other tauopathies.

CDK5 is essential for soluble amyloid β-induced degradation of GKAP and remodeling of the synaptic actin cytoskeleton

The early stages of Alzheimer's disease are marked by synaptic dysfunction and loss. This process results from the disassembly and degradation of synaptic components, in particular of scaffolding proteins that compose the post-synaptic density (PSD), namely PSD95, Homer and Shank. Here we investigated in rat frontal cortex dissociated culture the mechanisms involved in the downregulation of GKAP (SAPAP1), which links the PSD95 complex to the Shank complex and cytoskeletal structures within the PSD. We show that Aβ causes the rapid loss of GKAP from synapses through a pathway that critically requires cdk5 activity, and is set in motion by NMDAR activity and Ca(2+) influx. We show that GKAP is a direct substrate of cdk5 and that its phosphorylation results in polyubiquitination and proteasomal degradation of GKAP and remodeling (collapse) of the synaptic actin cytoskeleton; the latter effect is abolished in neurons expressing GKAP mutants that are resistant to phosphorylation by cdk5. Given that cdk5 also regulates degradation of PSD95, these results underscore the central position of cdk5 in mediating Aβ-induced PSD disassembly and synapse loss.

Glutathione-S-transferase P1 is a critical regulator of Cdk5 kinase activity

Cyclin dependent kinase-5 (Cdk5) activity is deregulated in Alzheimer's disease (AD) and contributes to all three hallmarks: neurotoxic β-amyloid formation, neurofibrillary tangles, and neuronal death. However, the mechanism leading to Cdk5 deregulation remains controversial. Cdk5 deregulation in AD is usually linked to the formation of p25, a proteolysis product of Cdk5 activator p35, which leads to Cdk5 mislocalization and hyperactivation. A few studies have indeed shown increased p25 levels in AD brains; however, others have refuted this observation. These contradictory findings suggest that additional factors contribute to Cdk5 deregulation. This study identified glutathione-S-transferase pi 1 (GSTP1) as a novel Cdk5 regulatory protein. We demonstrate that it is a critical determinant of Cdk5 activity in human AD brains and various cancer and neuronal cells. Increased GSTP1 levels were consistently associated with reduced Cdk5 activity. GSTP1 directly inhibits Cdk5 by dislodging p25/p35, and indirectly by eliminating oxidative stress. Cdk5 promotes and is activated by oxidative stress, thereby engaging a feedback loop which ultimately leads to cell death. Not surprisingly, GSTP1 transduction conferred a high degree of neuroprotection under neurotoxic conditions. Given the critical role of oxidative stress in AD pathogenesis, an increase in GSTP1 level may be an alternative way to modulate Cdk5 signaling, eliminate oxidative stress, and prevent neurodegeneration.

Addressing the complex etiology of Alzheimer’s disease: the role of p25/Cdk5

Alzheimer’s disease (AD) is an age-related neurodegenerative disorder characterized by the progressive loss of forebrain neurons and the deterioration of learning and memory. Therapies for AD have primarily focused upon either the inhibition of amyloid synthesis or its deposition in the brain, but clinical testing to date has not yet found an effective amelioration of cognitive symptoms. Synaptic loss closely correlates with the degree of dementia in AD patients. However, mouse AD models that target the amyloid-β pathway generally do not exhibit a profound loss of synapses, despite extensive synaptic dysfunction. The increased generation of p25, an activator of the cyclin-dependent kinase 5 (Cdk5) has been found in both human patients and mouse models of neurodegeneration. The current work reviews our knowledge, to date, on the role of p25/Cdk5 in Alzheimer’s disease, with a focus upon the interaction of amyloid-β and p25/Cdk5 in synaptic dysfunction and neuronal loss.

Vascular and metabolic dysfunction in Alzheimer's disease: a review

Alzheimer's disease (AD) is thought to start years or decades prior to clinical diagnosis. Overt pathology such as protein misfolding and plaque formation occur at later stages, and factors other than amyloid misfolding contribute to the initiation of the disease. Vascular and metabolic dysfunctions are excellent candidates, as they are well-known features of AD that precede pathology or clinical dementia. While the general notion that vascular and metabolic dysfunctions contribute to the etiology of AD is becoming accepted, recent research suggests novel mechanisms by which these/such processes could possibly contribute to AD pathogenesis. Vascular dysfunction includes reduced cerebrovascular flow and cerebral amyloid angiopathy. Indeed, there appears to be an interaction between amyloid β (Aβ) and vascular pathology, where Aβ production and vascular pathology both contribute to and are affected by oxidative stress. One major player in the vascular pathology is NAD(P)H oxidase, which generates vasoactive superoxide. Metabolic dysfunction has only recently regained popularity in relation to its potential role in AD. The role of metabolic dysfunction in AD is supported by the increased epidemiological risk of AD associated with several metabolic diseases such as diabetes, dyslipidemia and hypertension, in which there is elevated oxidative damage and insulin resistance. Metabolic dysfunction is further implicated in AD as pharmacological inhibition of metabolism exacerbates pathology, and several metabolic enzymes of the glycolytic, tricarboxylic acid cycle (TCA) and oxidative phosphorylation pathways are damaged in AD. Recent studies have highlighted the role of insulin resistance, in contributing to AD. Thus, vascular and metabolic dysfunctions are key components in the AD pathology throughout the course of disease. The common denominator between vascular and metabolic dysfunction emerging from this review appears to be oxidative stress and Aβ. This review also provides a framework for evaluation of current and future therapeutics for AD.

The role of Cdk5 in cognition and neuropsychiatric and neurological pathology

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase that is ubiquitous in the nervous system and interacts with a myriad of substrates. Its modulation of synaptic plasticity and associated mechanisms of learning and memory as well as neurodegeneration and cognitive disease highlights its importance in the human brain. Cdk5 is active throughout the neuron via its kinase activity, protein-protein interactions, and nuclear associations. It regulates functions thought vital to memory and plasticity, including synaptic vesicle recycling, dendritic spine formation, neurotransmitter receptor density, and neuronal excitability. Although conditional knockout of Cdk5 improves learning and plasticity, the associated deleterious effects of increased excitability cast doubts on the therapeutic efficacy of systemic inhibitors. However, through further work on the regulation of Cdk5 and its effectors, this important molecule promises to aid in elucidating key pathways involved in learning and memory and uncover innovative therapeutic targets to treat neurodegenerative and neuropsychiatric diseases.

Decreased expression of GGA3 protein in Alzheimer's disease frontal cortex and increased co-distribution of BACE with the amyloid precursor protein

BACE initiates the amyloidogenic processing of the amyloid precursor protein (APP) that results in the production of Aβ peptides associated with Alzheimer's disease (AD). Previous studies have indicated that BACE is elevated in the frontal cortex of AD patients. Golgi-localized γ-ear containing ADP ribosylation factor-binding proteins (GGA) control the cellular trafficking of BACE and may alter its levels. To investigate a link between BACE and GGA expression in AD, frontal cortex samples from AD (N = 20) and healthy, age-matched controls (HC, N =17) were analyzed by immunoblotting. After normalization to the neuronal marker β-tubulin III, the data indicate an average two-fold increase of BACE protein (p = 0.01) and a 64% decrease of GGA3 in the AD group compared to the HC (p = 0.006). GGA1 levels were also decreased in AD, but a statistical significance was not achieved. qRT-PCR analysis of GGA3 mRNA showed no difference between AD and HC. There was a strong correlation between GGA1 and GGA3 in both AD and HC, but no correlation between BACE and GGA levels. Subcellular fractionation of AD cortex with low levels of GGA proteins showed an alteration of BACE distribution and extensive co-localization with APP. These data suggest that altered compartmentalization of BACE in AD promotes the amyloidogenic processing of APP.

Calpastatin, an endogenous calpain-inhibitor protein, regulates the cleavage of the Cdk5 activator p35 to p25

Cyclin-dependent kinase 5 (Cdk5) is a Ser/Thr kinase that is activated by binding to its regulatory subunit, p35. The calpain-mediated cleavage of p35 to p25 and the resulting aberrant activity and neurotoxicity of Cdk5 have been implicated in neurological disorders, such as Alzheimer's disease. To gain further insight into the molecular mechanisms underlying the pathological function of Cdk5, we investigated the role of the calpain inhibitor protein calpastatin (CAST), in controlling the aberrant production of p25. For this purpose, brain tissue from wild-type, CAST-over-expressing (transgenic), and CAST knockout mice were analyzed. Cleavage of p35 to p25 was increased in extracts from CAST knockout mice, compared with wild-type. Conversely, generation of p25 was not detected in brain lysates from CAST-over-expressing mice. CAST expression was 5-fold higher in mouse cerebellum than cerebral cortex. Accordingly, p25 production was lower in the cerebellum than the cerebral cortex. Furthermore, the Ca(2+) -dependent degradation of p35 by proteasome was evident when calpain was inhibited. Taken together, these results suggest that CAST is a crucial regulator of calpain activity, the production of p25, and, hence, the deregulation of Cdk5. Therefore, impairment of CAST expression and its associated mechanisms may contribute to the pathogenesis of neurodegenerative disorders.

Nuclear envelope dispersion triggered by deregulated Cdk5 precedes neuronal death

Nuclear fragmentation is a common feature in many neurodegenerative diseases, including Alzheimer's disease (AD). In this study, we show that nuclear lamina dispersion is an early and irreversible trigger for cell death initiated by deregulated Cdk5, rather than a consequence of apoptosis. Cyclin-dependent kinase 5 (Cdk5) activity is significantly increased in AD and contributes to all three hallmarks: neurotoxic amyloid-β (Aβ), neurofibrillary tangles (NFT), and extensive cell death. Using Aβ and glutamate as the neurotoxic stimuli, we show that deregulated Cdk5 induces nuclear lamina dispersion by direct phosphorylation of lamin A and lamin B1 in neuronal cells and primary cortical neurons. Phosphorylation-resistant mutants of lamins confer resistance to nuclear dispersion and cell death on neurotoxic stimulation, highlighting this as a major mechanism for neuronal death. Rapid alteration of lamin localization pattern and nuclear membrane change are further supported by in vivo data using an AD mouse model. After p25 induction, the pattern of lamin localization was significantly altered, preceding neuronal death, suggesting that it is an early pathological event in p25-inducible transgenic mice. Importantly, lamin dispersion is coupled with Cdk5 nuclear localization, which is highly neurotoxic. Inhibition of nuclear dispersion rescues neuronal cells from cell death, underscoring the significance of this event to Cdk5-mediated neurotoxicity.

Characterization of nuclear sirtuins: molecular mechanisms and physiological relevance

Sirtuins are protein deacetylases/mono-ADP-ribosyltransferases found in organisms ranging from bacteria to humans. This group of enzymes relies on nicotinamide adenine dinucleotide (NAD(+)) as a cofactor linking their activity to the cellular metabolic status. Originally found in yeast, Sir2 was discovered as a silencing factor and has been shown to mediate the effects of calorie restriction on lifespan extension. In mammals seven homologs (SIRT1-7) exist which evolved to have specific biological outcomes depending on the particular cellular context, their interacting proteins, and the genomic loci to where they are actively targeted. Sirtuins biological roles are highlighted in the early lethal phenotypes observed in the deficient murine models. In this chapter, we summarize current concepts on non-metabolic functions for sirtuins, depicting this broad family from yeast to mammals.

5-lipoxygenase as an endogenous modulator of amyloid β formation in vivo

OBJECTIVE

The 5-lipoxygenase (5-LO) enzymatic pathway is widely distributed within the central nervous system, and is upregulated in Alzheimer's disease. However, the mechanism whereby it may influence the disease pathogenesis remains elusive.

METHODS

We evaluated the molecular mechanism by which 5-LO regulates amyloid β (Aβ) formation in vitro and in vivo by pharmacological and genetic approaches.

RESULTS

Here we show that 5-LO regulates the formation of Aβ by activating the cAMP-response element binding protein (CREB), which in turn increases transcription of the γ-secretase complex. Preventing CREB activation by pharmacologic inhibition or dominant negative mutants blocks the 5-LO-dependent elevation of Aβ formation and the increase of γ-secretase mRNA and protein levels. Moreover, 5-LO targeted gene disruption or its in vivo selective pharmacological inhibition results in a significant reduction of Aβ, CREB and γ-secretase levels.

INTERPRETATION

These data establish a novel functional role for 5-LO in regulating endogenous formation of Aβ levels in the central nervous system. Thus, 5-LO pharmacological inhibition may be beneficial in the treatment and prevention of Alzheimer's disease.

Amyloid-independent mechanisms in Alzheimer's disease pathogenesis

Despite the progress of the past two decades, the cause of Alzheimer's disease (AD) and effective treatments against it remain elusive. The hypothesis that amyloid-β (Aβ) peptides are the primary causative agents of AD retains significant support among researchers. Nonetheless, a growing body of evidence shows that Aβ peptides are unlikely to be the sole factor in AD etiology. Evidence that Aβ/amyloid-independent factors, including the actions of AD-related genes, also contribute significantly to AD pathogenesis was presented in a symposium at the 2010 Annual Meeting of the Society for Neuroscience. Here we summarize the studies showing how amyloid-independent mechanisms cause defective endo-lysosomal trafficking, altered intracellular signaling cascades, or impaired neurotransmitter release and contribute to synaptic dysfunction and/or neurodegeneration, leading to dementia in AD. A view of AD pathogenesis that encompasses both the amyloid-dependent and -independent mechanisms will help fill the gaps in our knowledge and reconcile the findings that cannot be explained solely by the amyloid hypothesis.

Sex- and brain region-specific acceleration of β-amyloidogenesis following behavioral stress in a mouse model of Alzheimer's disease

Background

It is hypothesized that complex interactions between multiple environmental factors and genetic factors are implicated in sporadic Alzheimer's disease (AD); however, the underlying mechanisms are poorly understood. Importantly, recent evidence reveals that expression and activity levels of the β-site APP cleaving enzyme 1 (BACE1), which initiates amyloid-β (Aβ) production, are elevated in AD brains. In this study, we investigated a molecular mechanism by which sex and stress interactions may accelerate β-amyloidogenesis and contribute to sporadic AD.

Results

We applied 5-day restraint stress (6 h/day) to the male and female 5XFAD transgenic mouse model of AD at the pre-pathological stage of disease, which showed little amyloid deposition under non-stressed control conditions. Exposure to the relatively brief behavioral stress increased levels of neurotoxic Aβ42 peptides, the β-secretase-cleaved C-terminal fragment (C99) and plaque burden in the hippocampus of female 5XFAD mice but not in that of male 5XFAD mice. In contrast, significant changes in the parameters of β-amyloidosis were not observed in the cerebral cortex of stressed male or female 5XFAD mice. We found that this sex- and brain region-specific acceleration of β-amyloidosis was accounted for by elevations in BACE1 and APP levels in response to adverse stress. Furthermore, not only BACE1 mRNA but also phosphorylation of the translation initiation factor eIF2α (a proposed mediator of the post-transcriptional upregulation of BACE1) was elevated in the hippocampus of stressed female 5XFAD mice.

Conclusions

Our results suggest that the higher prevalence of sporadic AD in women may be attributable to the vulnerability of female brains (especially, the hippocampus) to stressful events, which alter APP processing to favor the β-amyloidogenesis through the transcriptional and translational upregulation of BACE1 combined with elevations in its substrate APP.

Regulation and role of cyclin-dependent kinase activity in neuronal survival and death

Cyclin-dependent kinase (Cdk)5 is a proline-directed Ser/Thr protein kinase that functions mainly in neurons and is activated by binding to a regulatory subunit, p35 or p39. Kinase activity is mainly determined by the amount of p35 available, which is controlled by a balance between synthesis and degradation. Kinase activity is also regulated by Cdk5 phosphorylation, but the activity of phosphorylated Cdk5 is in contrast to that of cycling Cdks. Cdk5 is a versatile protein kinase that regulates multiple neuronal activities including neuronal migration and synaptic signaling. Further, Cdk5 plays a role in both survival and death of neurons. Long-term inactivation of Cdk5 triggers cell death, and the survival activity of Cdk5 is apparent when neurons suffer from stress. In contrast, hyper-activation of Cdk5 by p25 promotes cell death, probably by reactivating cell-cycle machinery in the nucleus. The pro-death activity is suppressed by membrane association of Cdk5 via myristoylation of p35. Appropriate activity, localization, and regulation of Cdk5 may be critical for long-term survival of neurons, which is more than 80 years in the case of humans.

Phospho-eIF2α level is important for determining abilities of BACE1 reduction to rescue cholinergic neurodegeneration and memory defects in 5XFAD mice

β-Site APP-cleaving enzyme 1 (BACE1) initiates amyloid-β (Aβ) generation and thus represents a prime therapeutic target in treating Alzheimer's disease (AD). Notably, increasing evidence indicates that BACE1 levels become elevated in AD brains as disease progresses; however, it remains unclear how the BACE1 upregulation may affect efficacies of therapeutic interventions including BACE1-inhibiting approaches. Here, we crossed heterozygous BACE1 knockout mice with AD transgenic mice (5XFAD model) and compared the abilities of partial BACE1 reduction to rescue AD-like phenotypes at earlier (6-month-old) and advanced (15–18-month-old) stages of disease, which expressed normal (∼100%) and elevated (∼200%) levels of BACE1, respectively. BACE1^+/− deletion rescued memory deficits as tested by the spontaneous alternation Y-maze task in 5XFAD mice at the earlier stage and prevented their septohippocampal cholinergic deficits associated with significant neuronal loss. Importantly, BACE1^+/− deletion was no longer able to rescue memory deficits or cholinergic neurodegeneration in 5XFAD mice at the advanced stage. Moreover, BACE1^+/− deletion significantly reduced levels of Aβ42 and the β-secretase-cleaved C-terminal fragment (C99) in 6-month-old 5XFAD mouse brains, while these neurotoxic β-cleavage products dramatically elevated with age and were not affected by BACE1^+/− deletion in 15–18-month-old 5XFAD brains. Interestingly, although BACE1^+/− deletion lowered BACE1 expression by ∼50% in 5XFAD mice irrespective of age in concordance with the reduction in gene copy number, BACE1 equivalent to wild-type controls remained in BACE1^+/−·5XFAD mice at the advanced age. In accord, phosphorylation of the translation initiation factor eIF2α, an important mediator of BACE1 elevation, was dramatically increased (∼9-fold) in 15–18-month-old 5XFAD mice and remained highly upregulated (∼6-fold) in age-matched BACE1^+/−·5XFAD mice. Together, our results indicate that partial reduction of BACE1 is not sufficient to block the phospho-eIF2α-dependent BACE1 elevation during the progression of AD, thus limiting its abilities to reduce cerebral Aβ/C99 levels and rescue memory deficits and cholinergic neurodegeneration.

Calpain activation promotes BACE1 expression, amyloid precursor protein processing, and amyloid plaque formation in a transgenic mouse model of Alzheimer disease

Abnormal activation of calpain is implicated in synaptic dysfunction and participates in neuronal death in Alzheimer disease (AD) and other neurological disorders. Pharmacological inhibition of calpain has been shown to improve memory and synaptic transmission in the mouse model of AD. However, the role and mechanism of calpain in AD progression remain elusive. Here we demonstrate a role of calpain in the neuropathology in amyloid precursor protein (APP) and presenilin 1 (PS1) double-transgenic mice, an established mouse model of AD. We found that overexpression of endogenous calpain inhibitor calpastatin (CAST) under the control of the calcium/calmodulin-dependent protein kinase II promoter in APP/PS1 mice caused a remarkable decrease of amyloid plaque burdens and prevented Tau phosphorylation and the loss of synapses. Furthermore, CAST overexpression prevented the decrease in the phosphorylation of the memory-related molecules CREB and ERK in the brain of APP/PS1 mice and improved spatial learning and memory. Interestingly, treatment of cultured primary neurons with amyloid-beta (Abeta) peptides caused an increase in the level of beta-site APP-cleaving enzyme 1 (BACE1), the key enzyme responsible for APP processing and Abeta production. This effect was inhibited by CAST overexpression. Consistently, overexpression of calpain in heterologous APP expressing cells up-regulated the level of BACE1 and increased Abeta production. Finally, CAST transgene prevented the increase of BACE1 in APP/PS1 mice. Thus, calpain activation plays an important role in APP processing and plaque formation, probably by regulating the expression of BACE1.

Atherosclerosis, dementia, and Alzheimer disease in the Baltimore Longitudinal Study of Aging cohort

OBJECTIVE

Although it is now accepted that asymptomatic cerebral infarcts are an important cause of dementia in the elderly, the relationship between atherosclerosis per se and dementia is controversial. Specifically, it is unclear whether atherosclerosis can cause the neuritic plaques and neurofibrillary tangles that define Alzheimer neuropathology and whether atherosclerosis, a potentially reversible risk factor, can influence cognition independent of brain infarcts.

METHODS

We examined the relationship between systemic atherosclerosis, Alzheimer type pathology, and dementia in autopsies from 200 participants in the Baltimore Longitudinal Study of Aging, a prospective study of the effect of aging on cognition, 175 of whom had complete body autopsies.

RESULTS

Using a quantitative analysis of atherosclerosis in the aorta, heart, and intracranial vessels, we found no relationship between the degree of atherosclerosis in any of these systems and the degree of Alzheimer type brain pathology. However, we found that the presence of intracranial but not coronary or aortic atherosclerosis significantly increased the odds of dementia, independent of cerebral infarction. Given the large number of individuals with intracranial atherosclerosis in this cohort (136/200), the population attributable risk of dementia related to intracranial atherosclerosis (independent of infarction) is substantial and potentially reversible.

INTERPRETATION

Atherosclerosis of the intracranial arteries is an independent and important risk factor for dementia, suggesting potentially reversible pathways unrelated to Alzheimer pathology and stroke through which vascular changes may influence dementia risk.

Tyk2/STAT3 signaling mediates beta-amyloid-induced neuronal cell death: implications in Alzheimer's disease

One of the pathological hallmarks of Alzheimer's disease (AD) is deposition of extracellular amyloid-beta (Abeta) peptide, which is generated from the cleavage of amyloid precursor protein (APP). Accumulation of Abeta is thought to associate with the progressive neuronal death observed in AD. However, the precise signaling mechanisms underlying the action of Abeta in AD pathophysiology are not completely understood. Here, we report the involvement of the transcription factor signal transducer and activator of transcription 3 (STAT3) in mediating Abeta-induced neuronal death. We find that tyrosine phosphorylation of STAT3 is elevated in the cortex and hippocampus of APP/PS1 transgenic mice. Treatment of cultured rat neurons with Abeta or intrahippocampal injection of mice with Abeta both induces tyrosine phosphorylation of STAT3 in neurons. Importantly, reduction of either the expression or activation of STAT3 markedly attenuates Abeta-induced neuronal apoptosis, suggesting that STAT3 activation contributes to neuronal death after Abeta exposure. We further identify Tyk2 as the tyrosine kinase that acts upstream of STAT3, as Abeta-induced activation of STAT3 and caspase-3-dependent neuronal death can be inhibited in tyk2(-/-) neurons. Finally, increased tyrosine phosphorylation of STAT3 is also observed in postmortem brains of AD patients. Our observations collectively reveal a novel role of STAT3 in Abeta-induced neuronal death and suggest the potential involvement of Tyk2/STAT3 signaling in AD pathophysiology.

Neurogenesis and Alzheimer's disease: at the crossroads

While a massive and progressive neuronal loss in specific areas such as the hippocampus and cortex unequivocally underlies cognitive deterioration and memory loss in Alzheimer's disease, noteworthy alterations take place in the neurogenic microenvironments, namely, the subgranule layer of the dentate gyrus and the subventricular zone. Compromised neurogenesis presumably takes place earlier than onset of hallmark lesions or neuronal loss, and may play a role in the initiation and progression of neuropathology in Alzheimer's disease. Neurogenesis in the adult brain is thought to play a role in numerous forms and aspects of learning and memory and contribute to the plasticity of the hippocampus and olfactory system. Misregulated or impaired neurogenesis on the other hand, may compromise plasticity and neuronal function in these areas and exacerbate neuronal vulnerability. Interestingly, increasing evidence suggests that molecular players in Alzheimer's disease, including PS1, APP and its metabolites, play a role in adult neurogenesis. In addition, recent studies suggest that alterations in tau phosphorylation are pronounced in neurogenic areas, and may interfere with the potential central role of tau proteins in neuronal maturation and differentiation. On the other hand, numerous neurogenic players, such as Notch-1, ErbB4 and L1 are substrates of alpha- beta- and gamma- secretase that play a major role in Alzheimer's disease. This review will discuss current knowledge concerning alterations of neurogenesis in Alzheimer's disease with specific emphasis on the cross-talk between signaling molecules involved in both processes, and the ways by which familial Alzheimer's disease-linked dysfunction of these signaling molecules affect neurogenesis in the adult brain.

Protein aggregation diseases: pathogenicity and therapeutic perspectives

A growing number of diseases seem to be associated with inappropriate deposition of protein aggregates. Some of these diseases--such as Alzheimer's disease and systemic amyloidoses--have been recognized for a long time. However, it is now clear that ordered aggregation of pathogenic proteins does not only occur in the extracellular space, but in the cytoplasm and nucleus as well, indicating that many other diseases may also qualify as amyloidoses. The common structural and pathogenic features of these diverse protein aggregation diseases is only now being fully understood, and may provide novel opportunities for overarching therapeutic approaches such as depleting the monomeric precursor protein, inhibiting aggregation, enhancing aggregate clearance or blocking common aggregation-induced cellular toxicity pathways.

Proteases and Proteolysis in Alzheimer Disease: A Multifactorial View on the Disease Process

Alzheimer disease is characterized by the accumulation of abnormally folded protein fragments, i.e., amyloid beta peptide (Aβ) and tau that precipitate in amyloid plaques and neuronal tangles, respectively. In this review we discuss the complicated proteolytic pathways that are responsible for the generation and clearance of these fragments, and how disturbances in these pathways interact and provide a background for a novel understanding of Alzheimer disease as a multifactorial disorder. Recent insights evolve from the static view that the morphologically defined plaques and tangles are disease driving towards a more dynamic, biochemical view in which the intermediary soluble Aβ oligomers and soluble tau fragments are considered as the main mediators of neurotoxicity. The relevance of proteolytic pathways, centered on the generation and clearance of toxic Aβ, on the cleavage and nucleation of tau, and on the general proteostasis of the neurons, then becomes obvious. Blocking or stimulating these pathways provide, or have the potential to provide, interesting drug targets, which raises the hope that we will be able to provide a cure for this dreadful disorder.

Mediators of tau phosphorylation in the pathogenesis of Alzheimer's disease

The need for disease-modifying drugs for Alzheimer's disease has become increasingly important owing to escalating disease prevalence and the associated socio-economic burden. Until recently, reducing brain amyloid accumulation has been the main therapeutic focus; however, increasing evidence suggests that targeting abnormal tau phosphorylation could be beneficial. Tau is phosphorylated by several protein kinases and this is balanced by dephosphorylation by protein phosphatases. Phosphorylation at specific sites can influence the physiological functions of tau, including its role in binding to and stabilizing the neuronal cytoskeleton. aberrant phosphorylation of tau could render it susceptible to potentially pathogenic alterations, including conformational changes, proteolytic cleavage and aggregation. While strategies that reduce tau phosphorylation in transgenic models of disease have been promising, our understanding of the mechanisms through which tau becomes abnormally phosphorylated in disease is lacking.

Expression of p25, an aberrant cyclin-dependent kinase 5 activator, stimulates basal secretion in PC12 cells

Although alterations in the functions of neurotransmitter systems have been implicated in the pathology of Alzheimer's disease (AD), the mechanisms that give rise to these alterations are not well understood. The amount of p25, an aberrant cleavage product of p35 that activates cyclin-dependent kinase 5 (Cdk5), is elevated in AD brains. The role of Cdk5 in neurotransmitter release has been well established. In this study, we examined whether p25 was linked to altered neurotransmitter release in AD. Transient or stable expression of p25 significantly increased basal secretion of human growth hormone (hGH) or neurotransmitter in PC12 cells. Expression of a p25 phosphorylation-deficient mutant, T138A, inhibited basal hGH secretion relative to the p25 wild type, suggesting the involvement of Thr138 phosphorylation in secretion. The expression and activity of beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), a key protease in the generation of beta-amyloid, are increased in AD brains. Our previous studies indicated that overexpression of BACE1 enhanced basal secretion of hGH in PC12 cells. Transient coexpression of p25 and BACE1 further stimulated spontaneous basal secretion. These results indicate a novel role for p25 in the secretory pathway and suggest that elevated levels of p25 and BACE1 in AD brains may contribute to altered neurotransmitter pathology of AD through enhancing spontaneous basal secretion.