Transcriptional regulation of β-secretase-1 by 12/15-lipoxygenase results in enhanced amyloidogenesis and cognitive impairments

Eicosanoid signaling in neuroinflammation associated with Alzheimer's disease

Alzheimer's disease (AD) is a prevalent neurodegenerative condition affecting a substantial portion of the global population. It is marked by a complex interplay of factors, including the accumulation of amyloid plaques and tau tangles within the brain, leading to neuroinflammation and neuronal damage. Recent studies have underscored the role of free lipids and their derivatives in the initiation and progression of AD. Eicosanoids, metabolites of polyunsaturated fatty acids like arachidonic acid (AA), emerge as key players in this scenario. Remarkably, eicosanoids can either promote or inhibit the development of AD, and this multifaceted role is determined by how eicosanoid signaling influences the immune responses within the brain. However, the precise molecular mechanisms dictating the dual role of eicosanoids in AD remain elusive. In this comprehensive review, we explore the intricate involvement of eicosanoids in neuronal function and dysfunction. Furthermore, we assess the therapeutic potential of targeting eicosanoid signaling pathways as a viable strategy for mitigating or halting the progression of AD.

Neurochemical Research of LOXBlock-1 and ZnSO4 against Neurodegenerative Damage Induced by Amyloid Beta(1-42)

Synaptosomes offer an intriguing ex vivo model system for investigating the molecular mechanisms of neurodegenerative processes. Lipoxygenases significantly affect the course of neurodegenerative diseases. Homeostasis of trace elements such as zinc is necessary for the continuity of brain functions. In this study, we purpose to determine whether LOXBlock-1, a 12/15 lipoxygenase inhibitor, and zinc sulfate (ZnSO4) provide any biochemical protection during neurodegenerative damage in synaptosomes induced by amyloid beta 1-42 (Aβ1-42). In this study, animals (30 Wistar Albino male rats 30) were divided into 5 groups (6 animals in each group): Control, 10µM Aβ1-42, 10µM Aβ1-42+25mM LOXBlock-1, 10µM Aβ1-42+10µM ZnSO4, and 10µM Aβ1-42+25mM LOXBlock-1+10µM ZnSO4. Synaptosomes were isolated from the rat cerebral cortex. Following, 8-hydroxy-2-deoxyguanosine (8-OHdG) levels, malondialdehyde (MDA) levels, adenosine deaminase (ADA) levels, reduced-glutathione (GSH) levels, neuronal nitric oxide synthase (nNOS) levels, acetylcholinesterase (AChE) activity, catalase (CAT) activity, and 8-OHdG levels in synaptosomes were detected according to the ELISA method. ADA and AChE expression and protein levels were analyzed. MDA, nNOS, AChE, and 8-OHdG levels in synaptosomes treated with Aβ1-42 resulted in an increase, while there was a decrease in ADA, GSH, and CAT levels (p<0.001 vs. control). Conversely, LOXBlock-1 and ZnSO4 treatments in synaptosomes treated with Aβ1-42 decreased MDA, nNOS, AChE, and 8-OHdG levels, while ADA, GSH, and CAT levels increased. Moreover, the most effective improvement was seen in the co-treatment group of LOXBlock-1 and ZnSO4. Our data showed that LOXBlock-1 and ZnSO4 co-treatment may protect against Aβ1-42 exposure in rat brain synaptosomes.

The Emerging Role of COX-2, 15-LOX and PPARγ in Metabolic Diseases and Cancer: An Introduction to Novel Multi-target Directed Ligands (MTDLs)

Emerging evidence supports an intertwining framework for the involvement of different inflammatory pathways in a common pathological background for a number of disorders. Of importance are pathways involving arachidonic acid metabolism by cyclooxygenase-2 (COX-2) and 15-lipoxygenase (15-LOX). Both enzyme activities and their products are implicated in a range of pathophysiological processes encompassing metabolic impairment leading to adipose inflammation and the subsequent vascular and neurological disorders, in addition to various pro- and antitumorigenic effects. A further layer of complexity is encountered by the disparate, and often reciprocal, modulatory effect COX-2 and 15-LOX activities and metabolites exert on each other or on other cellular targets, the most prominent of which is peroxisome proliferator-activated receptor gamma (PPARγ). Thus, effective therapeutic intervention with such multifaceted disorders requires the simultaneous modulation of more than one target. Here, we describe the role of COX-2, 15-LOX, and PPARγ in cancer and complications of metabolic disorders, highlight the value of designing multi-target directed ligands (MTDLs) modifying their activity, and summarizing the available literature regarding the rationale and feasibility of design and synthesis of these ligands together with their known biological effects. We speculate on the potential impact of MTDLs in these disorders as well as emphasize the need for structured future effort to translate these early results facilitating the adoption of these, and similar, molecules in clinical research.

Downregulation of autophagy by 12/15Lipoxygenase worsens the phenotype of an Alzheimer's disease mouse model with plaques, tangles, and memory impairments

Among the different initiating events in Alzheimer's disease (AD) pathogenesis, oxidative stress and neuroinflammation are some of the most iimportant. In the central nervous system, the 12/15Lipoxygenase (12/15LO) enzyme is the source of potent pro-oxidants and inflammatory lipid mediators. Previous works showed that this pathway is up-regulated in AD brains and that its pharmacological targeting modulates the phenotype of transgenic mouse models of the disease. Here we investigate the effect of brain 12/15LO gene delivery on the AD-like phenotype of a mouse model with plaques, tangles and behavioral deficits, the 3xTg mice. Compared with controls, mice over-expressing 12/15LO manifested an exacerbation of spatial learning and memory impairments, which was associated with significant increase in Aβ formation and deposition, and accumulation of hyper-phosphorylated insoluble tau secondary to a down-regulation of autophagy. In addition, the same mice manifested a worsening of neuroinflammation and synaptic pathology. Taken together our study supports the hypothesis that the 12/15LO enzymatic pathway by impairing neuronal autophagy plays a functional role in exacerbating AD-related neuropathologies and cognitive impairments. It provides further critical preclinical evidence to justify developing and testing new and selective 12/15LO inhibitors for AD treatment.

Autophagy Dysfunction in Alzheimer's Disease: Mechanistic Insights and New Therapeutic Opportunities

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive memory loss due to aberrant accumulation of misfolded proteins inside and outside neurons and glial cells, leading to a loss of cellular protein homeostasis. Today, no therapy is available to block or slow down AD progression, and the mechanisms of the disease are not fully understood. Autophagy is an intracellular degradation pathway crucial to maintaining cellular homeostasis by clearing damaged organelles, pathogens, and unwanted protein aggregates. In recent years, autophagy dysfunction has gained considerable attention in AD and other neurodegenerative diseases because it has been linked to the accumulation of misfolded proteins that ultimately causes neuronal death in many of these disorders. Interestingly, autophagy-activating compounds have also shown some promising results in both clinical trials and preclinical studies. This review aims at summarizing the current knowledge on autophagy dysfunction in the context of AD pathophysiology, providing recent mechanistic insights on AD-mediated autophagic flux disruption and highlighting potential and novel therapeutic opportunities that target this system for AD therapy.

Fair-Weather Friends: Evidence of Lipoxin Dysregulation in Neurodegeneration

Lipoxins (LXs) are autacoids, specialized proresolving lipid mediators (SPMs) acting locally in a paracrine or autocrine fashion. They belong to a complex superfamily of dietary small polyunsaturated fatty acid (PUFA)-metabolites, which direct potent cellular responses to resolve inflammation and restore tissue homeostasis. Together, these SPM activities have been intensely studied in systemic inflammation and acute injury or infection, but less is known about LX signaling and activities in the central nervous system. LXs are derived from arachidonic acid, an omega-6 PUFA. In addition to well-established roles in systemic inflammation resolution, they have increasingly become implicated in regulating neuroinflammatory and neurodegenerative processes. In particular, chronic inflammation plays a central role in Alzheimer's disease (AD) etiology, and dysregulated LX production and activities have been reported in a variety of AD rodent models and clinical tissue samples, yet with complex and sometimes conflicting results. In addition, reduced LX production following retinal injury has been reported recently by the authors, and an intriguing direct neuronal activity promoting survival and homeostasis in retinal and cortical neurons is demonstrated. Here, the authors review and clarify this growing literature and suggest new research directions to further elaborate the role of lipoxins in neurodegeneration.

The effects of omega-3 fatty acid deficiency during development on oxidative fatty acid degradation during maturity in a mouse model of Alzheimer's disease

Metabolic conditions during brain development may have long-term consequences on brain metabolism, thereby influencing the risk of neurodegenerative disease in later life. To ascertain the long-term consequences of omega-3 (ω3) fatty acid deficiency during brain development on oxidative fatty acid degradation in the brain and the development of Alzheimer-like pathology, wild-type (WT) female mice were fed diets that were either replete or deficient in ω3 fatty acids for 5 weeks. These females were then mated with hemizygous 5xFAD male transgenic (TG) mouse models of Alzheimer's disease, and the progeny were continued on diets that were either ω3-replete or ω3-deficient. When the progeny were 6 months of age, they received radiolabeled arachidonic acid (ARA) by intracerebroventricular injection. Five days after these injections, the brains were harvested and oxidative degradation of the radiolabeled ARA was characterized. Among the progeny of female mice on an ω3-replete diet, TG progeny had lower PSD-95 expression and higher oxidative ARA degradation than WT progeny. Progeny on an ω3-deficient diet, however, had no significant differences in PSD-95 expression between TG and WT mice, or in the extent of ARA degradation. In TG mice, an ω3-deficient diet reduced oxidative ARA degradation to a greater extent than in WT mice. The reductions in oxidative ARA degradation occurred even if the progeny of female mice on an ω3-deficient diet resumed an ω3-replete diet immediately on weaning. These results demonstrate that dietary ω3 fatty acid deficiency during development can cause long-term changes in the expression of a synaptic marker and long-term reductions in the rate of ARA degradation in the WT brain, which are not completely alleviated by an ω3-replete diet after weaning. The elimination of differences between TG and WT mice by an ω3-deficient diet suggests that mechanisms regulating PSD-95 expression and the oxidative degradation of ARA are related and that the timing of dietary ω3 intake during development may influence Alzheimer's disease-related pathological changes later in life.

Analysis of eicosanoid oxidation products in Alzheimer brain by LC-MS with uniformly 13C-labeled internal standards

The quantitative analysis of polyunsaturated fatty acyl (PUFA) chain oxidation products in tissue samples by mass spectrometry is hindered by the lack of durable internal standards for the large number of possible products. To address this problem in a study of oxidative PUFA degradation in Alzheimer's disease (AD) brain, uniformly 13C-labeled arachidonic acid (ARA) was produced biosynthetically, and allowed to oxidize under controlled conditions into a mixture of U-13C-labeled ARA oxidation products. The components of this mixture were characterized with respect to their partitioning behavior during lipid extraction, their durability during saponification, trends in mouse brain tissue concentrations during post mortem intervals, and their overall suitability as internal standards for multiple-reaction monitoring tandem mass spectrometry. This mixture has now been used as a set of internal standards to determine the relative abundance of ARA and 54 non-stereospecific oxidation products in milligram samples of brain tissue. Many of these oxidation products were recovered from both healthy mouse and healthy human brain, although some of them were unique to each source, and some have not heretofore been described. The list of oxidation products detected in AD brain tissue was the same as in healthy human brain, although simple hydroxy-eicosanoids were significantly increased in AD brain. while more complex oxidation products were not. These results are consistent with an increased level of chemically-mediated oxidative ARA degradation in Alzheimer's disease. However, they also point to the existence of processes that selectively produce or eliminate specific oxidation products, and those processes may account for some of the inconsistencies in previously reported results.

Therapeutic Potential of Baicalein in Alzheimer's Disease and Parkinson's Disease

Alzheimer's disease and Parkinson's disease are the two most common, progressive central neurodegenerative diseases affecting the population over the age of 60 years. Apart from treatments that temporarily improve symptoms, there is no medicine currently available to inhibit or reverse the progression of Alzheimer's disease and Parkinson's disease. In traditional Chinese medicine, the root of Scutellaria baicalensis Georgi is a classic compatible component in the decoction of herbal medicine used for treating central nervous system diseases. Modern pharmacokinetic studies have confirmed that baicalein (5,6,7-trihydroxyflavone) is a major bioactive flavone constituent root of S. baicalensis Georgi. Studies showed that baicalein possesses a range of key pharmacological properties, such as reducing oxidative stress, anti-inflammatory properties, inhibiting aggregation of disease-specific amyloid proteins, inhibiting excitotoxicity, stimulating neurogenesis and differentiation action, and anti-apoptosis effects. Based on these properties, baicalein shows therapeutic potential for Alzheimer's disease and Parkinson's disease. In this review, we summarize the pharmacological protective actions of baicalein that make it suitable for the treatment of Alzheimer's disease and Parkinson's disease, and discuss the potential mechanisms underlying the effects.

Five lipoxygenase hypomethylation mediates the homocysteine effect on Alzheimer's phenotype

Environmental and genetic risk factors are implicated in the pathogenesis of Alzheimer’s disease (AD). However, how they interact and influence its pathogenesis remains to be investigated. High level of homocysteine (Hcy) is an AD risk factor and associates with an up-regulation of the ALOX5 gene. In the current paper we investigated whether this activation is responsible for the Hcy effect on the AD phenotype and the mechanisms involved. Triple transgenic mice were randomized to receive regular chow diet, a diet deficient in folate and B vitamins (Diet), which results in high Hcy, or the Diet plus zileuton, a specific ALOX5 inhibitor, for 7 months. Compared with controls, Diet-fed mice had a significant increase in Hcy levels, memory and learning deficits, up-regulation of the ALOX5 pathway, increased Aβ levels, tau phosphorylation, and synaptic pathology, which were absent in mice treated with zileuton. In vivo and vitro studies demonstrated that the mechanism responsible was the hypomethylation of the ALOX5 promoter. Our findings demonstrate that the up-regulation of the ALOX5 is responsible for the Hcy-dependent worsening of the AD phenotype in a relevant mouse model of the disease. The discovery of this previously unknown cross-talk between these two pathways could afford novel therapeutic opportunities for treating or halting AD.

Homocysteine modulates 5-lipoxygenase expression level via DNA methylation

Elevated levels of homocysteinemia (Hcy), a risk factor for late-onset Alzheimer's disease (AD), have been associated with changes in cell methylation. Alzheimer's disease is characterized by an upregulation of the 5-lipoxygenase (5LO), whose promoter is regulated by methylation. However, whether Hcy activates 5LO enzymatic pathway by influencing the methylation status of its promoter remains unknown. Brains from mice with high Hcy were assessed for the 5LO pathway and neuronal cells exposed to Hcy implemented to study the mechanism(s) regulating 5LO expression levels and the effect on amyloid β formation. Diet- and genetically induced high Hcy resulted in 5LO protein and mRNA upregulation, which was associated with a significant increase of the S-adenosylhomocysteine (SAH)/S-adenosylmethionine ratio, and reduced DNA methyltrasferases and hypomethylation of 5-lipoxygenase DNA. In vitro studies confirmed these results and demonstrated that the mechanism involved in the Hcy-dependent 5LO activation and amyloid β formation is DNA hypomethylation secondary to the elevated levels of SAH. Taken together these findings represent the first demonstration that Hcy directly influences 5LO expression levels and establish a previously unknown cross talk between these two pathways, which is highly relevant for AD pathogenesis. The discovery of such a novel link not only provides new mechanistic insights in the neurobiology of Hcy, but most importantly new therapeutic opportunities for the individuals bearing this risk factor for the disease.

Linking deregulation of non-coding RNA to the core pathophysiology of Alzheimer's disease: An integrative review

The human genome encodes a vast repertoire of protein non-coding RNAs (ncRNA), some specific to the brain. MicroRNAs, which interfere with the translation of target mRNAs, are of particular interest since their deregulation has been implicated in neurodegenerative disorders like Alzheimer's disease (AD). However, it remains challenging to link the complex body of observations on miRNAs and AD into a coherent framework. Using extensive graphical support, this article discusses how a diverse panoply of miRNAs convergently and divergently impact (and are impacted by) core pathophysiological processes underlying AD: neuroinflammation and oxidative stress; aberrant generation of β-amyloid-42 (Aβ42); anomalies in the production, cleavage and post-translational marking of Tau; impaired clearance of Aβ42 and Tau; perturbation of axonal organisation; disruption of synaptic plasticity; endoplasmic reticulum stress and the unfolded protein response; mitochondrial dysfunction; aberrant induction of cell cycle re-entry; and apoptotic loss of neurons. Intriguingly, some classes of miRNA provoke these cellular anomalies, whereas others act in a counter-regulatory, protective mode. Moreover, changes in levels of certain species of miRNA are a consequence of the above-mentioned anomalies. In addition to miRNAs, circular RNAs, piRNAs, long non-coding RNAs and other types of ncRNA are being increasingly implicated in AD. Overall, a complex mesh of deregulated and multi-tasking ncRNAs reciprocally interacts with core pathophysiological mechanisms underlying AD. Alterations in ncRNAs can be detected in CSF and the circulation as well as the brain and are showing promise as biomarkers, with the ultimate goal clinical exploitation as targets for novel modes of symptomatic and course-altering therapy.

Mice deficient in L-12/15 lipoxygenase show increased vulnerability to 3-nitropropionic acid neurotoxicity

Considerable evidence supports a contributory role for leukocyte-type 12/15 Lipoxygenase (L-12/15 LO) in mediating hippocampal and cortical neuronal injury in models of Alzheimer's disease and stroke. Whether L-12/15 LO contributes to neuronal injury in a model of Huntington's disease (HD) has yet to be determined. HD is characterized by marked striatal neuronal loss, which can be mimicked in humans and animals by inhibition of mitochondrial complex II using 3-Nitropropionic acid (3-NP). Herein, we compared histological and behavioral outcomes between mice that were wild-type or null for L-12/15 LO following systemic injection of 3NP. We found that mice deficient in L-12/15 LO had a higher incidence of striatal lesions coincident with an increase in morbidity as compared to their wild-type littermate controls. This could not be explained by differential metabolism of 3-NP as striatal succinate dehydrogenase activity was inhibited to the same extent in both genotypes. The present results show that deleting L-12/15 LO is detrimental to the striatum in the setting of chronic, systemic 3-NP exposure and are consistent with the overall conclusion that region-specific effects may determine the ultimate outcome of L-12/15 LO activation in the setting of brain injury.

12/15-Lipoxygenase Inhibition Reverses Cognitive Impairment, Brain Amyloidosis, and Tau Pathology by Stimulating Autophagy in Aged Triple Transgenic Mice

BACKGROUND

The 12/15-lipoxygenase (12/15-LO) enzyme is upregulated in the brains of patients with Alzheimer's disease (AD), and its expression levels influence the onset of the AD-like phenotype in mouse models. However, whether targeting this pathway after the neuropathology and behavioral impairments have been established remains to be investigated.

METHODS

Triple transgenic (3xTg) mice received either PD146176-a selective and specific pharmacological inhibitor of 12/15-LO-or placebo starting at 12 months of age for 12 weeks. They were then assessed for the effect of the treatment on neuropathologies and behavioral impairments.

RESULTS

At the end of the study, mice in the control group showed a worsening of memory and learning abilities, whereas mice receiving PD146176 were undistinguishable from wild-type mice. The same group also had significantly lower amyloid beta levels and deposition, less tau neuropathology, increased synaptic integrity, and autophagy activation. Ex vivo and in vitro genetic and pharmacological studies found that the mechanism involved in these effects was the activation of neuronal autophagy.

CONCLUSIONS

Our findings provide new insights into the disease-modifying action of 12/15-LO pharmacological inhibition and establish it as a viable therapeutic approach for patients with AD.

The retromer complex system in a transgenic mouse model of AD: influence of age

Deficiencies of the retrograde transport mediated by the retromer complex have been described in Alzheimer's disease (AD). Genetic manipulation of retromer modulates brain amyloidosis in Tg2576 mice. However, whether the complex is altered during the development of the AD-like phenotype remains unknown. In this study we assayed the expression levels of the vacuolar sorting protein 35 (VPS35), VPS26, VPS29, and its cargo proteins, cation independent mannose 6-phosphate receptor, sortilin-related receptor in brains of Tg2576 and controls at the ages of 3, 8, and 14 months. While cortex showed an age-dependent decrease in all but VPS29, levels of the same proteins in the cerebellum were unchanged at any age. Neuronal cells expressing human amyloid beta precursor protein Swedish mutant had also reduced retromer complex levels. However, incubation with a pharmacological chaperone dose-dependently restored these levels together with a reduction in amyloid beta. Our study is the first to show that in a transgenic mouse model of AD the changes in the expression levels of the retromer complex are age and region dependent, and that the complex is a viable therapeutic target since its deficiency can be restored pharmacologically by a retromer chaperone.

Early Growth Response 1 (Egr-1) Is a Transcriptional Activator of β-Secretase 1 (BACE-1) in the Brain

Accumulation of amyloid-β peptide (Aβ) in the brain is regarded as central to Alzheimer's disease (AD) pathogenesis. Aβ is generated by a sequential cleavage of amyloid precursor protein (APP) by β-secretase 1 (BACE-1) followed by γ-secretase. BACE-1 cleavage of APP is the committed step in Aβ synthesis. Understanding the mechanism by which BACE-1 is activated leading to Aβ synthesis in the brain can provide better understanding of AD pathology and help to develop novel therapies. In this study, we found that the levels of Aβ and BACE-1 are significantly reduced in the brains of mice lacking transcription factor early growth response 1 (Egr-1) when compared with the WT. We demonstrate that in COS-7 cells, Egr-1 binds to the BACE-1 promoter and activates BACE-1 transcription. In rat hippocampal primary neurons, overexpression of Egr-1 induces BACE-1 expression, activates BACE-1, promotes amyloidogenic APP processing, and enhances Aβ synthesis. In mouse hippocampal primary neurons, knockdown of BACE-1 almost completely blocks Egr-1-induced amyloidogenic APP processing and Aβ synthesis. Our data indicate that Egr-1 promotes Aβ synthesis via transcriptional activation of BACE-1 and suggest that Egr-1 plays role in activation of BACE-1 and acceleration of Aβ synthesis in AD brain. Egr-1 is a potential therapeutic target for AD.

The flavonoid baicalein rescues synaptic plasticity and memory deficits in a mouse model of Alzheimer's disease

Increasing evidence suggests that disruptions of synaptic functions correlate with the severity of cognitive deficit in Alzheimer's disease (AD). Our previous study demonstrated that baicalein enhances long-term potentiation (LTP) in acute rat hippocampal slices and improves hippocampus-dependent contextual fear conditioning in rats. Given that baicalein possess various biological activities, especially its effects on synaptic plasticity and cognitive function, we examined the effect of baicalein on synaptic function both in vitro and in vivo in AD model. The effect of baicalein on Aβ42 oligomer impaired LTP was investigated by electrophysiological methods. Baicalein was administered orally via drinking water to the APP/PS1 mice and sex- and age-matched wild-type mice. Treatment started at 5 months of age and mice were assessed for cognition and AD-like pathology at 7-month-old. Cognition was analyzed by Morris water maze test, fear conditioning test, and novel object recognition test. Changes in hippocampal 12/15 Lipoxygenase (12/15LO) and glycogen synthase kinase 3β (GSK3β) activity, Aβ production, tau phosphorylation, synaptic plasticity, and dendritic spine density were evaluated. Baicalein prevented Aβ-induced impairments in hippocampal LTP through activation of serine threonine Kinase (Akt) phosphorylation. Long-term oral administration of baicalein inhibited 12/15LO and GSK3β activity, reduced β-secretase enzyme (BACE1), decreased the concentration of total Aβ, and prevented phosphorylation of tau in APP/PS1 mice. Meanwhile, baicalein restored spine number, synaptic plasticity, and memory deficits. Our results strengthen the potential of the flavonoid baicalein as a novel and promising oral bioactive therapeutic agent that prevents memory deficits in AD.

The Lipoxygenases: Their Regulation and Implication in Alzheimer's Disease

Inflammatory processes and alterations of lipid metabolism play a crucial role in Alzheimer’s disease (AD) and other neurodegenerative disorders. Polyunsaturated fatty acids (PUFA) metabolism impaired by cyclooxygenases (COX-1, COX-2), which are responsible for formation of several eicosanoids, and by lipoxygenases (LOXs) that catalyze the addition of oxygen to linolenic, arachidonic (AA), and docosahexaenoic acids (DHA) and other PUFA leading to formation of bioactive lipids, significantly affects the course of neurodegenerative diseases. Among several isoforms, 5-LOX and 12/15-LOX are especially important in neuroinflammation/neurodegeneration. These two LOXs are regulated by substrate concentration and availability, and by phosphorylation/dephosphorylation through protein kinases PKA, PKC and MAP-kinases, including ERK1/ERK2 and p38. The protein/protein interaction also is involved in the mechanism of 5-LOX regulation through FLAP protein and coactosin-like protein. Moreover, non-heme iron and calcium ions are potent regulators of LOXs. The enzyme activity significantly depends on the cell redox state and is differently regulated by various signaling pathways. 5-LOX and 12/15-LOX convert linolenic acid, AA, and DHA into several bioactive compounds e.g. hydroperoxyeicosatetraenoic acids (5-HPETE, 12S-HPETE, 15S-HPETE), which are reduced to corresponding HETE compounds. These enzymes synthesize several bioactive lipids, e.g. leucotrienes, lipoxins, hepoxilins and docosahexaenoids. 15-LOX is responsible for DHA metabolism into neuroprotectin D1 (NPD1) with significant antiapoptotic properties which is down-regulated in AD. In this review, the regulation and impact of 5-LOX and 12/15-LOX in the pathomechanism of AD is discussed. Moreover, we describe the role of several products of LOXs, which may have significant pro- or anti-inflammatory activity in AD, and the cytoprotective effects of LOX inhibitors.

Pharmacologic blockade of 12/15-lipoxygenase ameliorates memory deficits, Aβ and tau neuropathology in the triple-transgenic mice

The 12/15-lipoxygenase (12/15LO) enzyme is widely distributed within the central nervous system. Previous work showed that this protein is upregulated in Alzheimer's disease (AD), and plays an active role in the development of brain amyloidosis in amyloid beta (Aβ)-precursor protein transgenic mice (Tg2576). In the present paper, we studied the effect of its pharmacologic inhibition on the AD-like phenotype of a mouse model with plaques and tangles, the triple-transgenic mice. Compared with mice receiving placebo, the group treated with PD146176, a specific 12/15LO inhibitor, manifested a significant improvement of their memory deficits. The same animals had a significant reduction in Aβ levels and deposition, which was secondary to a decrease in the β-secretase pathway. In addition, while total tau-soluble levels were unchanged for both groups, PD146176-treated mice had a significant reduction in its phosphorylation state and insoluble fraction, which specifically associated with decrease in stress-activated protein kinase/c-Jun N-terminal kinase activity. In vitro study showed that the effect on tau and Aβ were independent from each other. These data establish a functional role for 12/15LO in the pathogenesis of the full spectrum of the AD-like phenotype and represent the successful completion of the initial step for the preclinical development of 12/15LO inhibitors as novel therapeutic agents for AD.

Expression of β-site APP-cleaving enzyme 1 in the hippocampal tissue of an insulin-resistant rat model of Alzheimer's disease

The aim of this study was to investigate the expression of β-site APP-cleaving enzyme 1 (BACE1) in the hippocampal tissue of an insulin-resistant rat model, and thereby explore the roles of BACE1 and insulin resistance (IR) in the pathogenesis of Alzheimer's disease (AD). A total of 36 male Sprague-Dawley rats, aged 2 months, were randomly divided into three groups. These were an insulin-resistant (experimental) group, a high fat control group and a blank control group. The cognitive function and behavioral changes of the rats were tested by a Morris water maze experiment. Amyloid β (Aβ) deposition was detected by an immunohistochemical method. The expression levels of BACE1 in the rat hippocampal tissues were detected by enzyme-linked immunosorbent assay, western blotting and reverse transcription-quantitative polymerase chain reaction technology. The rats in the experimental group had evident learning and memory impairment, with significantly decreased learning memory. The modeling was successful; in the experimental group, the rats exhibited IR and their glucose metabolism was significantly abnormal. However, there was no characteristic pathology of AD. The expression of BACE1 in the brain tissue of rats in the experimental group was significantly higher than that in high fat control and blank control groups (P<0.01). In conclusion, the expression of BACE1 in the brain tissue of insulin-resistant rats increased, and IR was indicated to participate in the pathogenesis of AD.

The 12/15-lipoxygenase as an emerging therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is a chronic neurodegenerative condition characterized by progressive memory loss. Mutations in genes involved in the production of amyloid-β (Aβ) are linked to the early-onset variant of AD. However, the most common form, sporadic AD, is considered to be the result of an interaction between environmental risk factors and various genes. Among them, recent work has highlighted the potential role that the 12/15-lipoxygenase (12/15LO) pathway may play in AD pathogenesis. 12/15LO is widely distributed in the central nervous system, and its levels are upregulated in patients with AD or mild cognitive impairments. Studies using animal models have implicated 12/15LO in the molecular pathology of AD, including the metabolism of Aβ and tau, synaptic integrity, and cognitive functions. We provide an overview of this pathway and its relevance to AD pathogenesis, discuss the mechanism(s) involved, and provide an assessment of how targeting 12/15LO could lead to novel AD therapeutics.

Insufficient resolution response in the hippocampus of a senescence-accelerated mouse model--SAMP8

Aging is the primary risk factor for Alzheimer’s disease (AD), and it is known that inflammation is associated with both aging and AD. To resolve inflammation, biosynthesis of the specialized pro-resolving mediators (SPMs) is enhanced in a programmed and active manner. We investigated the effect of age on resolution by analyzing hippocampal tissue from 2- and 9-month-old senescence-accelerated mouse prone 8 (SAMP8), as well as age-matched senescence-accelerated mouse resistant 1 (SAMR1). Pro-inflammatory markers increased upon age in SAMP8 mice and were also higher than those in age-matched SAMR1 mice. However, neither SPMs nor their receptors were enhanced upon age in SAMP8 mice compared to age-matched SAMR1 mice. Analysis of SPM biosynthetic enzymes revealed elevated levels of leukocyte type 12-lipoxygenase (L12-LOX) and decreased 5-LOX levels upon age in SAMR1 mice, whereas they remained unchanged in SAMP8 mice. Moreover, we found partial co-localization of L12-LOX and amyloid beta (Aβ) staining, as well as correlation between L12-LOX and phosphorylated tau levels in SAMP8, but not SAMR1 mice. Thus, we conclude that the resolution response in SAMP8 mice is insufficient to counteract the increased inflammation with age, and this may have a role in the development of AD-like pathologies.

The 12-15-lipoxygenase is a modulator of Alzheimer's-related tau pathology in vivo

12/15-lipoxygenase (12-15LO) is a lipid-peroxidizing enzyme widely expressed in the central nervous system where it has been involved in the neurobiology of Alzheimer's disease (AD) because it modulates amyloid beta (Aβ) and APP processing. However, its biological effect on tau protein is unknown. We investigated the effect of 12-15LO on tau levels and metabolism in vivo and in vitro and the mechanism involved by using genetic and pharmacologic approaches. While no significant differences were observed in the levels of total tau for both groups, compared with controls, Tg2576 mice overexpressing 12-15LO had elevated levels of phosphorylated tau at two specific epitopes, Ser 202/Thr 205 and Ser 396. In vitro and in vivo studies show that 12-15LO modulates tau metabolism specifically via the cdk5 kinase pathway. Associated with these changes were biochemical markers of synaptic pathology. Finally, 12-15LO-dependent alteration of tau metabolism was independent from an effect on Aβ. Our findings reveal a novel pathway by which 12-15LO modulates endogenous tau metabolism making this protein an appealing pharmacologic target for treatment of AD and related tauopathies.

Transcriptional regulation and its misregulation in Alzheimer's disease

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder characterized by loss of memory and cognitive function. A key neuropathological event in AD is the accumulation of amyloid-β (Aβ) peptide. The production and clearance of Aβ in the brain are regulated by a large group of genes. The expression levels of these genes must be fine-tuned in the brain to keep Aβ at a balanced amount under physiological condition. Misregulation of AD genes has been found to either increase AD risk or accelerate the disease progression. In recent years, important progress has been made in uncovering the regulatory elements and transcriptional factors that guide the expression of these genes. In this review, we describe the mechanisms of transcriptional regulation for the known AD genes and the misregualtion that leads to AD susceptibility.

Pathways to neurodegeneration: mechanistic insights from GWAS in Alzheimer's disease, Parkinson's disease, and related disorders

The discovery of causative genetic mutations in affected family members has historically dominated our understanding of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal dementia (FTD), and amyotrophic lateral sclerosis (ALS). Nevertheless, most cases of neurodegenerative disease are not explained by Mendelian inheritance of known genetic variants, but instead are thought to have a complex etiology with numerous genetic and environmental factors contributing to susceptibility. Although unbiased genome-wide association studies (GWAS) have identified novel associations to neurodegenerative diseases, most of these hits explain only modest fractions of disease heritability. In addition, despite the substantial overlap of clinical and pathologic features among major neurodegenerative diseases, surprisingly few GWAS-implicated variants appear to exhibit cross-disease association. These realities suggest limitations of the focus on individual genetic variants and create challenges for the development of diagnostic and therapeutic strategies, which traditionally target an isolated molecule or mechanistic step. Recently, GWAS of complex diseases and traits have focused less on individual susceptibility variants and instead have emphasized the biological pathways and networks revealed by genetic associations. This new paradigm draws on the hypothesis that fundamental disease processes may be influenced on a personalized basis by a combination of variants - some common and others rare, some protective and others deleterious - in key genes and pathways. Here, we review and synthesize the major pathways implicated in neurodegeneration, focusing on GWAS from the most prevalent neurodegenerative disorders, AD and PD. Using literature mining, we also discover a novel regulatory network that is enriched with AD- and PD-associated genes and centered on the SP1 and AP-1 (Jun/Fos) transcription factors. Overall, this pathway- and network-driven model highlights several potential shared mechanisms in AD and PD that will inform future studies of these and other neurodegenerative disorders. These insights also suggest that biomarker and treatment strategies may require simultaneous targeting of multiple components, including some specific to disease stage, in order to assess and modulate neurodegeneration. Pathways and networks will provide ideal vehicles for integrating relevant findings from GWAS and other modalities to enhance clinical translation.

Inhibition of 12/15-lipoxygenase by baicalein induces microglia PPARβ/δ: a potential therapeutic role for CNS autoimmune disease

12/15-Lipoxygenase (12/15-LO) is an enzyme that converts polyunsaturated fatty acids into bioactive lipid derivatives. In this study, we showed that inhibition of 12/15-LO by baicalein (BA) significantly attenuated clinical severity of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). Inhibited migration of autoimmune T cells into the central nervous system (CNS) by BA treatment could be attributed to reduced activation of microglia, which was indicated by suppressed phagocytosis, and decreased production of proinflammatory cytokines and chemokines in the CNS. We further observed that inhibition of 12/15-LO with BA led to increased expression of peroxisome proliferator-activated receptor (PPAR)β/δ in microglia of EAE mice. This was confirmed in vitro in primary microglia and a microglia cell line, BV2. In addition, we demonstrated that BA did not affect 12/15-LO or 5-lipoxygenase (5-LO) expression in microglia, but significantly decreased 12/15-LO products without influencing the levels of 5-LO metabolites. Moreover, among these compounds only 12/15-LO metabolite 12-hydroxyeicosatetraenoic acid was able to reverse BA-mediated upregulation of PPARβ/δ in BV2 cells. We also showed that inhibition of microglia activation by PPARβ/δ was associated with repressed NF-κB and MAPK activities. Our findings indicate that inhibition of 12/15-LO induces PPARβ/δ, demonstrating important regulatory properties of 12/15-LO in CNS inflammation. This reveals potential therapeutic applications for MS.

Differential contribution of lipoxygenase isozymes to nigrostriatal vulnerability

The 5- and 12/15-lipoxygenase (LOX) isozymes have been implicated to contribute to disease development in CNS disorders such as Alzheimer's disease. These LOX isozymes are distinct in function, with differential effects on neuroinflammation, and the impact of the distinct isozymes in the pathogenesis of Parkinson's disease has not as yet been evaluated. To determine whether the isozymes contribute differently to nigrostriatal vulnerability, the effects of 5- and 12/15-LOX deficiency on dopaminergic tone under naïve and toxicant-challenged conditions were tested. In naïve mice deficient in 5-LOX expression, a modest but significant reduction (18.0% reduction vs. wildtype (WT)) in striatal dopamine (DA) was detected (n=6-8 per genotype). A concomitant decline in striatal tyrosine hydroxylase (TH) enzyme was also revealed in null 5-LOX vs. WT mice (26.2%); however, no changes in levels of DA or TH immunoreactivity were observed in null 12/15-LOX vs. WT mice. When challenged with the selective dopaminergic toxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), WT mice showed a marked reduction in DA (31.9%) and robust astrocytic and microglial activation as compared to saline-treated animals. In contrast, null 5-LOX littermates demonstrated no significant striatal DA depletion or astrogliosis (as noted by Western blot analyses for glial acidic fibrillary protein (GFAP) immunoreactivity). In naïve null 12/15-LOX mice, no significant change in striatal DA values was observed compared to WT, and following MPTP treatment, the transgenics revealed striatal DA reduction similar to the challenged WT mice. Taken together, these data provide the first evidence that: (i) LOX isozymes are involved in the maintenance of normal dopaminergic function in the striatum and (ii) the 5- and 12/15-LOX isozymes contribute differentially to striatal vulnerability in response to neurotoxicant challenge.

Involvement of 5-lipoxygenase activating protein in the amyloidotic phenotype of an Alzheimer's disease mouse model

BACKGROUND

The 5-lipoxygenase enzyme is widely distributed within the central nervous system and its activity is regulated by the presence and availability of another protein, called 5-lipoxygenase activating protein. While previous works have shown that 5-lipoxygenase is involved in the pathogenesis of Alzheimer's disease, no data are available on the role that 5-lipoxygenase activating protein plays in Alzheimer's disease.

METHODS

In the present paper, we studied the effect of pharmacologic inhibition of 5-lipoxygenase activating protein on the amyloidotic phenotype of Tg2576 mice.

RESULTS

Amyloid β peptide (Aβ) deposition in the brains of mice receiving MK-591, a selective and specific 5-lipoxygenase activating protein inhibitor, was significantly reduced when compared with controls. This reduction was associated with a similar decrease in brain Aβ peptides levels. MK-591 treatment did not induce any change in the steady-state levels of amyloid-β precursor protein, β-site amyloid precursor protein cleaving enzyme 1 or disintegrin and metalloproteinase domain-containing protein 10. By contrast, it resulted in a significant reduction of the γ-secretase complex, at the protein and message level. Furthermore, in vitro studies confirmed that MK-591 prevents Aβ formation by modulating γ-secretase complex levels without affecting Notch signaling.

CONCLUSIONS

These data establish a novel functional role for 5-lipoxygenase activating protein in the pathogenesis of Alzheimer's disease-like amyloidosis, and suggest that its pharmacological inhibition could provide a novel therapeutic opportunity for Alzheimer's disease.