Tau hyperphosphorylation and increased BACE1 and RAGE levels in the cortex of PPARβ/δ-null mice

Melatonin Enhanced Microglia M2 Polarization in Rat Model of Neuro-inflammation Via Regulating ER Stress/PPARδ/SIRT1 Signaling Axis

Neuro-inflammation involves distinct alterations of microglial phenotypes, containing nocuous pro-inflammatory M1-phenotype and neuroprotective anti-inflammatory M-phenotype. Currently, there is no effective treatment for modulating such alterations. M1/M2 marker of primary microglia influenced by Melatonin were detected via qPCR. Functional activities were explored by western blotting, luciferase activity, EMSA, and ChIP assay. Structure interaction was assessed by molecular docking and LIGPLOT analysis. ER-stress detection was examined by ultrastructure TEM, calapin activity, and ERSE assay. The functional neurobehavioral evaluations were used for investigation of Melatonin on the neuroinflammation in vivo. Melatonin had targeted on Peroxisome Proliferator Activated Receptor Delta (PPARδ) activity, boosted LPS-stimulated alterations in polarization from the M1 to the M2 phenotype, and thereby inhibited NFκB-IKKβ activation in primary microglia. The PPARδ agonist L-165,041 or over-expression of PPARδ plasmid (ov-PPARδ) showed similar results. Molecular docking screening, dynamic simulation approaches, and biological studies of Melatonin showed that the activated site was located at PPARδ (phospho-Thr256-PPARδ). Activated microglia had lowered PPARδ activity as well as the downstream SIRT1 formation via enhancing ER-stress. Melatonin, PPARδ agonist and ov-PPARδ all effectively reversed the above-mentioned effects. Melatonin blocked ER-stress by regulating calapin activity and expression in LPS-activated microglia. Additionally, Melatonin or L-165,041 ameliorated the neurobehavioral deficits in LPS-aggravated neuroinflammatory mice through blocking microglia activities, and also promoted phenotype changes to M2-predominant microglia. Melatonin suppressed neuro-inflammation in vitro and in vivo by tuning microglial activation through the ER-stress-dependent PPARδ/SIRT1 signaling cascade. This treatment strategy is an encouraging pharmacological approach for the remedy of neuro-inflammation associated disorders.

A tau fragment links depressive-like behaviors and cognitive declines in Alzheimer's disease mouse models through attenuating mitochondrial function

Introduction

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease characterized by extracellular senile plaques including amyloid-β peptides and intracellular neurofibrillary tangles consisting of abnormal Tau. Depression is one of the most common neuropsychiatric symptoms in AD, and clinical evidence demonstrates that depressive symptoms accelerate the cognitive deficit of AD patients. However, the underlying molecular mechanisms of depressive symptoms present in the process of AD remain unclear.

Methods

Depressive-like behaviors and cognitive decline in hTau mice were induced by chronic restraint stress (CRS). Computational prediction and molecular experiments supported that an asparagine endopeptidase (AEP)-derived Tau fragment, Tau N368 interacts with peroxisome proliferator-activated receptor delta (PPAR-δ). Further behavioral studies investigated the role of Tau N368-PPAR-δ interaction in depressive-like behaviors and cognitive declines of AD models exposed to CRS.

Results

We found that mitochondrial dysfunction was positively associated with depressive-like behaviors and cognitive deficits in hTau mice. Chronic stress increased Tau N368 and promoted the interaction of Tau N368 with PPAR-δ, repressing PPAR-δ-mediated transactivation in the hippocampus of mice. Then we predicted and identified the binding sites of PPAR-δ. Finally, inhibition of AEP, clearance of Tau N368 and pharmacological activation of PPAR-δ effectively alleviated CRS-induced depressive-like behaviors and cognitive decline in mice.

Conclusion

These results demonstrate that Tau N368 in the hippocampus impairs mitochondrial function by suppressing PPAR-δ, facilitating the occurrence of depressive-like behaviors and cognitive decline. Therefore, our findings may provide new mechanistic insight in the pathophysiology of depression-like phenotype in mouse models of Alzheimer's disease.

The Contribution of Hippocampal All-Trans Retinoic Acid (ATRA) Deficiency to Alzheimer's Disease: A Narrative Overview of ATRA-Dependent Gene Expression in Post-Mortem Hippocampal Tissue

There is accumulating evidence that vitamin A (VA) deficiency contributes to the pathogenesis and progression of Alzheimer's disease (AD). All-trans retinoic acid (ATRA), a metabolite of VA in the brain, serves distinct roles in the human hippocampus. Agonists of retinoic acid receptors (RAR), including ATRA, promote activation of the non-amyloidogenic pathway by enhancing expression of α-secretases, providing a mechanistic basis for delaying/preventing amyloid beta (Aβ) toxicity. However, whether ATRA is actually deficient in the hippocampi of patients with AD is not clear. Here, using a publicly available human transcriptomic dataset, we evaluated the extent to which ATRA-sensitive genes are dysregulated in hippocampal tissue from post-mortem AD brains, relative to age-matched controls. Consistent with ATRA deficiency, we found significant dysregulation of many ATRA-sensitive genes and significant upregulation of RAR co-repressors, supporting the idea of transcriptional repression of ATRA-mediated signaling. Consistent with oxidative stress and neuroinflammation, Nrf2 and NfkB transcripts were upregulated, respectively. Interestingly, transcriptional targets of Nrf2 were not upregulated, accompanied by upregulation of several histone deacetylases. Overall, our investigation of ATRA-sensitive genes in the human hippocampus bolsters the scientific premise of ATRA depletion in AD and that epigenetic factors should be considered and addressed as part of VA supplementation.

Function and therapeutic value of astrocytes in diabetic cognitive impairment

Diabetic cognitive impairment (DCI) is a complex complication of diabetes in the central nervous system, and its pathological mechanism is still being explored. Astrocytes are abundant glial cells in central nervous system that perform diverse functions in health and disease. Accumulating excellent research has identified astrocyte dysfunction in many neurodegenerative diseases (such as Alzheimer's disease, aging and Parkinson's disease), and summarized and discussed its pathological mechanisms and potential therapeutic value. However, the contribution of astrocytes to DCI has been largely overlooked. In this review, we first systematically summarized the effects and mechanisms of diabetes on brain astrocytes, and found that the diabetic environment (such as hyperglycemia, advanced glycation end products and cerebral insulin resistance) mediated brain reactive astrogliosis, which was specifically reflected in the changes of cell morphology and the remodeling of signature molecules. Secondly, we emphasized the contribution and potential targets of reactive astrogliosis to DCI, and found that reactive astrogliosis-induced increased blood-brain barrier permeability, glymphatic system dysfunction, neuroinflammation, abnormal cell communication and cholesterol metabolism dysregulation worsened cognitive function. In addition, we summarized effective strategies for treating DCI by targeting astrocytes. Finally, we discuss the application of new techniques in astrocytes, including single-cell transcriptome, in situ sequencing, and prospected new functions, new subsets and new targets of astrocytes in DCI.

High glucose inhibits neural differentiation by excessive autophagy <em>via</em> peroxisome proliferator-activated receptor gamma

The high prevalence of prediabetes and diabetes globally has led to the widespread occurrence of severe complications, such as diabetic neuropathy, which is a result of chronic hyperglycemia. Studies have demonstrated that maternal diabetes can lead to neural tube defects by suppressing neurogenesis during neuroepithelium development. While aberrant autophagy has been associated with abnormal neuronal differentiation, the mechanism by which high glucose suppresses neural differentiation in stem cells remains unclear. Therefore, we developed a neuronal cell differentiation model of retinoic acid induced P19 cells to investigate the impact of high glucose on neuronal differentiation in vitro. Our findings indicate that high glucose (HG) hinders neuronal differentiation and triggers excessive. Furthermore, HG treatment significantly reduces the expression of markers for neurons (Tuj1) and glia (GFAP), while enhancing autophagic activity mediated by peroxisome proliferator-activated receptor gamma (PPARγ). By manipulating PPARγ activity through pharmacological approaches and genetically knocking it down using shRNA, we discovered that altering PPARγ activity affects the differentiation of neural stem cells exposed to HG. Our study reveals that PPARγ acts as a downstream mediator in high glucose-suppressed neural stem cell differentiation and that refining autophagic activity via PPARγ at an appropriate level could improve neuronal differentiation efficiency. Our data provide novel insights and potential therapeutic targets for the clinical management of gestational diabetes mellitus.

Peroxisomal Proliferator-Activated Receptor β/δ Deficiency Induces Cognitive Alterations

Peroxisome proliferator-activated receptor β/δ (PPARβ/δ), the most PPAR abundant isotype in the central nervous system, is involved in microglial homeostasis and metabolism, whose disturbances have been demonstrated to play a key role in memory impairment. Although PPARβ/δ function is well-established in metabolism, its contribution to neuronal and specifically memory process is underexplored. Therefore, the aim of the study is to determine the role of PPARβ/δ in the neuropathological pathways involved in memory impairment and as to whether a risk factor implicated in memory loss such as obesity modulates neuropathological markers. To carry out this study, 6-month-old total knock-out for the Ppard gene male mice with C57BL/6X129/SV background (PPARβ/δ-/-) and wild-type (WT) littermates with the same genetic background were used. Animals were fed, after the weaning (at 21 days old), and throughout their growth, either conventional chow (CT) or a palmitic acid-enriched diet (HFD). Thus, four groups were defined: WT CT, WT HFD, PPARβ/δ-/- CT, and PPARβ/δ-/- HFD. Before sacrifice, novel object recognition test (NORT) and glucose and insulin tolerance tests were performed. After that, animals were sacrificed by intracardiac perfusion or cervical dislocation. Different techniques, such as GolgiStain kit or immunofluorescence, were used to evaluate the role of PPARβ/δ in memory dysfunction. Our results showed a decrease in dendritic spine density and synaptic markers in PPARβ/δ-/- mice, which were corroborated in the NORT. Likewise, our study demonstrated that the lack of PPARβ/δ receptor enhances gliosis in the hippocampus, contributing to astrocyte and microglial activation and to the increase in neuroinflammatory biomarkers. Additionally, alterations in the hippocampal insulin receptor pathway were found. Interestingly, while some of the disturbances caused by the lack of PPARβ/δ were not affected by feeding the HFD, others were exacerbated or required the combination of both factors. Taken together, the loss of PPARβ/δ-/- affects neuronal and synaptic structure, contributing to memory dysfunction, and they also present this receptor as a possible new target for the treatment of memory impairment.

A comprehensive review on RAGE-facilitated pathological pathways connecting Alzheimer’s disease, diabetes mellitus, and cardiovascular diseases

Background

Alzheimer’s disease (AD), cardiovascular disease (CVD), and diabetes are some of the most common causes of morbidity and mortality among the aging populations and cause a heavy burden on the worldwide healthcare system. In this review, we briefly highlighted cellular inflammation-based pathways of diabetes mellitus and CVD through receptor for advanced glycation end products AGEs or RAGE leading to Alzheimer’s disease and interrelation between these vascular and metabolic disorders. The articles were retrieved from Google Scholar, Science Direct, and PubMed databases using the following terms: Alzheimer’s; AGEs; RAGE; RAGE in Alzheimer’s; AGEs in Alzheimer’s; RAGE in diabetes; RAGE related pathways of CVD; RAGE in hypertension; RAGE and RAS system; RAGE and oxidative stress.

Main body of the abstract

AD is a neurodegenerative disease characterized by cognitive dysfunction and neuronal cell death. Vascular complications like hypertension, coronary artery disease, and atherosclerosis as well as metabolic syndromes like obesity and diabetes are related to the pathophysiology of AD. RAGE plays significant role in the onset and progression of AD. Amyloid plaques and neurofibrillary tangles (NFT) are two main markers of AD that regulates via RAGE and other RAGE/ligands interactions which also induces oxidative stress and a cascade of other cellular inflammation pathways leading to AD. Though AD and diabetes are two different disorders but may be inter-linked by AGEs and RAGE. In long-term hyperglycemia, upregulated AGEs interacts with RAGE and produces reactive oxygen species which induces further inflammation and vascular complications. Aging, hypercholesterolemia, atherosclerosis, hypertension, obesity, and inflammation are some of the main risk factors for both diabetes and dementia. Chronic hypertension and coronary artery disease disrupt the functions of the blood-brain barrier and are responsible for the accumulation of senile plaques and NFTs.

Short conclusion

RAGE plays a role in the etiology of Aβ and tau hyperphosphorylation, both of which contribute to cognitive impairment. So far, targeting RAGE may provide a potential sight to develop therapies against some metabolic disorders.

Neurochemical changes underlying cognitive impairment in olfactory bulbectomized rats and the impact of the mGlu5-positive allosteric modulator CDPPB

The olfactory bulbectomized (OBX) rat model is a well-established model of depression in which antidepressant drugs reverse deficits in the passive avoidance test 14 days after administration. Recently, the olfactory bulbectomized rat model has been proposed to be a model of Alzheimer's disease (AD), and the available data indicate similarities between the changes that typically occur in AD and those observed in OBX animals. In the present study, the occurrence of neurochemical impairments related to AD were investigated 8 months after OB ablation. The expression of the nitric oxide synthases eNOS and nNOS, receptor for advanced glycation endproducts (RAGEs) and dimethylarginine dimethylaminohydrolase (DDAH1) in the prefrontal cortices (PFCs), hippocampi and striata of olfactory bulbectomized and sham-operated rats was evaluated. Subsequently, the impact of the administration of a positive allosteric modulator of the mGlu₅ receptor, CDPPB (14 days, 2.5 or 5 mg/kg), on OBX-related changes was assessed. OB ablation induced typical deficits in passive avoidance. Significant aberrations in the expression of both isoforms of NOS were observed in the hippocampus and striatum, and the expression of DDAH1 was increased in the PFCs of OBX animals. CDPPB at a dose of 5 mg/kg ameliorated cognitive impairment in the passive avoidance test and partially reversed the changes in eNOS and nNOS expression induced by the lesion. The results of this study confirm that some of the neurochemical changes observed in OBX animals may resemble those associated with AD pathology and that activation of the mGlu₅ receptor may partially counteract these pathological alterations.

Therapeutic potential of mangiferin in the treatment of various neuropsychiatric and neurodegenerative disorders

Xanthones are important chemical class of bioactive products that confers therapeutic benefits. Of several xanthones, mangiferin is known to be distributed widely across several fruits, vegetables and medicinal plants. Mangiferin has been shown to exert neuroprotective effects in both in-vitro and in-vivo models. Mangiferin attenuates cerebral infarction, cerebral edema, lipid peroxidation (MDA), neuronal damage, etc. Mangiferin further potentiate levels of endogenous antioxidants to confer protection against the oxidative stress inside the neurons. Mangiferin is involved in the regulation of various signaling pathways that influences the production and levels of proinflammatory cytokines in brain. Mangiferin cosunteracted the neurotoxic effect of amyloid-beta, MPTP, rotenone, 6-OHDA etc and confer protection to neurons. These evidence suggested that the mangiferin may be a potential therapeutic strategy for the treatment of various neurological disorders. The present review demonstrated the pharmacodynamics-pharmacokinetics of mangiferin and neurotherapeutic potential in several neurological disorders with underlying mechanisms.

Recent Insights on the Role of PPAR-β/δ in Neuroinflammation and Neurodegeneration, and Its Potential Target for Therapy

Peroxisome proliferator-activated receptor (PPAR) β/δ belongs to the family of hormone and lipid-activated nuclear receptors, which are involved in metabolism of long-chain fatty acids, cholesterol, and sphingolipids. Similar to PPAR-α and PPAR-γ, PPAR-β/δ also acts as a transcription factor activated by dietary lipids and endogenous ligands, such as long-chain saturated and polyunsaturated fatty acids, and selected lipid metabolic products, such as eicosanoids, leukotrienes, lipoxins, and hydroxyeicosatetraenoic acids. Together with other PPARs, PPAR-β/δ displays transcriptional activity through interaction with retinoid X receptor (RXR). In general, PPARs have been shown to regulate cell differentiation, proliferation, and development and significantly modulate glucose, lipid metabolism, mitochondrial function, and biogenesis. PPAR-β/δ appears to play a special role in inflammatory processes and due to its proangiogenic and anti-/pro-carcinogenic properties, this receptor has been considered as a therapeutic target for treating metabolic syndrome, dyslipidemia, carcinogenesis, and diabetes. Until now, most studies were carried out in the peripheral organs, and despite of its presence in brain cells and in different brain regions, its role in neurodegeneration and neuroinflammation remains poorly understood. This review is intended to describe recent insights on the impact of PPAR-β/δ and its novel agonists on neuroinflammation and neurodegenerative disorders, including Alzheimer’s and Parkinson’s, Huntington’s diseases, multiple sclerosis, stroke, and traumatic injury. An important goal is to obtain new insights to better understand the dietary and pharmacological regulations of PPAR-β/δ and to find promising therapeutic strategies that could mitigate these neurological disorders.

Diabetes as a risk factor for Alzheimer's disease in the Middle East and its shared pathological mediators

The incidence of Alzheimer's disease (AD) has risen exponentially worldwide over the past decade. A growing body of research indicates that AD is linked to diabetes mellitus (DM) and suggests that impaired insulin signaling acts as a crucial risk factor in determining the progression of this devastating disease. Many studies suggest people with diabetes, especially type 2 diabetes, are at higher risk of eventually developing Alzheimer's dementia or other dementias. Despite nationwide efforts to increase awareness, the prevalence of Diabetes Mellitus (DM) has risen significantly in the Middle East and North African (MENA) region which might be due to rapid urbanization, lifestyle changes, lack of physical activity and rise in obesity. Growing body of evidence indicates that DM and AD are linked because both conditions involve impaired glucose homeostasis and altered brain function. Current theories and hypothesis clearly implicate that defective insulin signaling in the brain contributes to synaptic dysfunction and cognitive deficits in AD. In the periphery, low-grade chronic inflammation leads to insulin resistance followed by tissue deterioration. Thus insulin resistance acts as a bridge between DM and AD. There is pressing need to understand on how DM increases the risk of AD as well as the underlying mechanisms, due to the projected increase in age related disorders. Here we aim to review the incidence of AD and DM in the Middle East and the possible link between insulin signaling and ApoE carrier status on Aβ aggregation, tau hyperphosphorylation, inflammation, oxidative stress and mitochondrial dysfunction in AD. We also critically reviewed mutation studies in Arab population which might influence DM induced AD. In addition, recent clinical trials and animal studies conducted to evaluate the efficiency of anti-diabetic drugs have been reviewed.

Current and future clinical utilities of Parkinson's disease and dementia biomarkers: can they help us conquer the disease?

Introduction: Biomarkers for Parkinson's disease and Alzheimer's disease are essential, not only for disease detection, but also provide insight into potential disease relationships leading to better detection and therapy. As metabolic disease is known to increase neurodegeneration risk, such mechanisms may reveal such novel targets for PD and AD. Moreover, metabolic disease, including insulin resistance, offer novel biomarker and therapeutic targets for neurodegeneration, including glucagon-like-peptide-1, dipeptidyl peptidase-4 and adiponectin. Areas covered: The authors reviewed PubMed-listed research articles, including ours, on a number of putative PD, AD and neurodegenerative disease targets of interest, focusing on the relevance of metabolic syndrome and insulin resistance mechanisms, especially type II diabetes, to PD and AD. We highlighted various issues surrounding the current state of knowledge and propose avenues for future development. Expert opinion: Biomarkers for PD and AD are indispensable for disease diagnosis, prognostication and tracking disease severity, especially for clinical therapy trials. Although no validated PD biomarkers exist, their potential utility has generated tremendous interest. Combining insulin-resistance biomarkers with other core biomarkers or using them to predict non-motor symptoms of PD may be clinically useful. Collectively, although still unclear, potential biomarkers and therapies can aid in shedding new light on novel aspects of both PD and AD.

Activation of peroxisome proliferator-activated receptor delta suppresses BACE1 expression by up-regulating SOCS1 in a JAK2/STAT1-dependent manner

Neuronal expression of beta-secretase 1 (BACE1) has been implicated in the progression of Alzheimer's disease. However, the mechanisms that regulate BACE1 expression are unclear. Here, we show that peroxisome proliferator-activated receptor delta (PPARδ) decreases BACE1 expression by up-regulating suppressor of cytokine signaling 1 (SOCS1) in SH-SY5Y neuroblastoma cells. The activation of PPARδ by GW501516, a specific PPARδ agonist, inhibited expression of BACE1. This effect was abrogated by shRNA-mediated knockdown of PPARδ and by treatment with the PPARδ antagonist GSK0660, indicating that PPARδ is involved in GW501516-mediated suppression of BACE1 expression. On the other hand, GW501516-activated PPARδ induced expression of SOCS1, which is a negative regulator of cytokine signal transduction, at the transcriptional level by binding to a PPAR response element in its promoter. This GW501516-mediated induction of SOCS1 expression led to down-regulation of BACE1 expression via inactivation of signal transducer and activator of transcription 1. GW501516-activated PPARδ suppressed the generation of neurotoxic amyloid beta (Aβ) in accordance with the decrease in BACE1 expression. Taken together, these results indicate that PPARδ attenuates BACE1 expression via SOCS1-mediated inhibition of signal transducer and activator of transcription 1 signaling, thereby suppressing BACE1-associated generation of neurotoxic Aβ.

Activated PPARγ Abrogates Misprocessing of Amyloid Precursor Protein, Tau Missorting and Synaptotoxicity

Type 2 diabetes increases the risk for dementia, including Alzheimer’s disease (AD). Pioglitazone (Pio), a pharmacological agonist of the peroxisome proliferator-activated receptor γ (PPARγ), improves insulin sensitivity and has been suggested to have potential in the management of AD symptoms, albeit through mostly unknown mechanisms. We here investigated the potential of Pio to counter synaptic malfunction and loss, a characteristic of AD pathology and its accompanying cognitive deficits. Results from experiments on primary mouse neuronal cultures and a human neural cell line (SH-SY5Y) show that Pio treatment attenuates amyloid β (Aβ)-triggered the pathological (mis-) processing of amyloid precursor protein (APP) and inhibits Aβ-induced accumulation and hyperphosphorylation of Tau. These events are accompanied by increased glutamatergic receptor 2B subunit (GluN2B) levels that are causally linked with neuronal death. Further, Pio treatment blocks Aβ-triggered missorting of hyperphosphorylated Tau to synapses and the subsequent loss of PSD95-positive synapses. These latter effects of Pio are PPARγ-mediated since they are blocked in the presence of GW9662, a selective PPARγ inhibitor. Collectively, these data show that activated PPARγ buffer neurons against APP misprocessing, Tau hyperphosphorylation and its missorting to synapses and subsequently, synaptic loss. These first insights into the mechanisms through which PPARγ influences synaptic loss make a case for further exploration of the potential usefulness of PPARγ agonists in the prevention and treatment of synaptic pathology in AD.

The Full Spectrum of Alzheimer's Disease Is Rooted in Metabolic Derangements That Drive Type 3 Diabetes

The standard practice in neuropathology is to diagnose Alzheimer's disease (AD) based on the distribution and abundance of neurofibrillary tangles and Aβ deposits. However, other significant abnormalities including neuroinflammation, gliosis, white matter degeneration, non-Aβ microvascular disease, and insulin-related metabolic dysfunction require further study to understand how they could be targeted to more effectively remediate AD. This review addresses non-Aβ and non-pTau AD-associated pathologies, highlighting their major features, roles in neurodegeneration, and etiopathic links to deficits in brain insulin and insulin-like growth factor signaling and cognitive impairment. The discussion delineates why AD with its most characteristic clinical and pathological phenotypic profiles should be regarded as a brain form of diabetes, i.e., type 3 diabetes, and entertains the hypothesis that type 3 diabetes is just one of the categories of insulin resistance diseases that can occur independently or overlap with one or more of the others, including type 2 diabetes, metabolic syndrome, and nonalcoholic fatty liver disease.

PPARs and Energy Metabolism Adaptation during Neurogenesis and Neuronal Maturation

Peroxisome proliferator activated receptors (PPARs) are a class of ligand-activated transcription factors, belonging to the superfamily of receptors for steroid and thyroid hormones, retinoids, and vitamin D. PPARs control the expression of several genes connected with carbohydrate and lipid metabolism, and it has been demonstrated that PPARs play important roles in determining neural stem cell (NSC) fate. Lipogenesis and aerobic glycolysis support the rapid proliferation during neurogenesis, and specific roles for PPARs in the control of different phases of neurogenesis have been demonstrated. Understanding the changes in metabolism during neuronal differentiation is important in the context of stem cell research, neurodegenerative diseases, and regenerative medicine. In this review, we will discuss pivotal evidence that supports the role of PPARs in energy metabolism alterations during neuronal maturation and neurodegenerative disorders.

Changes in PPARδ expression in a rat model of stress-induced depression

Depression is a common mental disorder that has been linked to a decrease in the expression of serotonin and/or the serotonin transporter in the brain. Antidepressants that target the monoaminergic system are widely used in the clinical setting. Peroxisome proliferator-activated receptor δ (PPAR δ) overexpression or activation is thought to improve depression-like behaviours in rodents. The present study was designed to characterize the changes in PPARδ expression in the hippocampus in rats with stress-induced depression. We used an unpredictable chronic mild stress (CMS) model in rats to study the role of PPARδ in the hippocampus. Behaviour was evaluated via a forced swim test (FST), a tail suspension test (TST), and a sucrose preference test (SPT). Then, the changes in PPARδ expression and other signals were determined using Western blots. We found that PPARδ expression in the hippocampus was markedly reduced in rats with depression. Moreover, the expression of the serotonin transporter was also significantly decreased. Treatment with a PPARδ agonist enhanced the expression of PPARδ and the serotonin transporter in the hippocampus of rats with stress-induced depression. Additionally, treatment with a PPARδ agonist increased the expression of the serotonin transporter in cultured hippocampal (H19-7) cells, and this action was ablated in the absence of PPARδ, which was attenuated with shRNA. Taken together, we found that PPARδ plays an important role in the regulation of serotonin transporter expression and that chronic stress may lower PPARδ expression in the brain via apoptosis and may attenuate serotonin transporter expression, thus inducing depression in rats.

Impact of Cytokines and Chemokines on Alzheimer's Disease Neuropathological Hallmarks

BACKGROUND

Alzheimer's disease (AD) is the most common neurodegenerative disorder, neuropathologically characterized by aggregates of β-amyloid peptides, which deposit as senile plaques, and of TAU protein, which forms neurofibrillary tangles. It is now widely accepted that neuroinflammation is implicated in AD pathogenesis.

METHOD

Indeed, inflammatory mediators, such as cytokines and chemokines (chemotactic cytokines) can impact on the Alzheimer´s amyloid precursor protein by affecting its expression levels and amyloidogenic processing and/or β -amyloid aggregation. Additionally, cytokines and chemokines can influence kinases' activities, leading to abnormal TAU phosphorylation. To date there is no cure for AD, but several therapeutic strategies have been directed to prevent neuroinflammation. Anti-inflammatory, but also anti-amyloidogenic compounds, such as flavonoids were shown to favourably modulate some pathological events associated with neurodegeneration.

CONCLUSION

This review focuses on the role of cytokines and chemokines in AD-associated pathologies, and summarizes the potential anti-inflammatory therapeutic approaches aimed at preventing or slowing down disease progression.

Insulin Resistance and Neurodegeneration: Progress Towards the Development of New Therapeutics for Alzheimer's Disease

Alzheimer's disease (AD) should be regarded as a degenerative metabolic disease caused by brain insulin resistance and deficiency, and overlapping with the molecular, biochemical, pathophysiological, and metabolic dysfunctions in diabetes mellitus, non-alcoholic fatty liver disease, and metabolic syndrome. Although most of the diagnostic and therapeutic approaches over the past several decades have focused on amyloid-beta (Aβ42) and aberrantly phosphorylated tau, which could be caused by consequences of brain insulin resistance, the broader array of pathologies including white matter atrophy with loss of myelinated fibrils and leukoaraiosis, non-Aβ42 microvascular disease, dysregulated lipid metabolism, mitochondrial dysfunction, astrocytic gliosis, neuro-inflammation, and loss of synapses vis-à-vis growth of dystrophic neurites, is not readily accounted for by Aβ42 accumulations, but could be explained by dysregulated insulin/IGF-1 signaling with attendant impairments in signal transduction and gene expression. This review covers the diverse range of brain abnormalities in AD and discusses how insulins, incretins, and insulin sensitizers could be utilized to treat at different stages of neurodegeneration.

Epigallocatechin Gallate Attenuates β-Amyloid Generation and Oxidative Stress Involvement of PPARγ in N2a/APP695 Cells

The accumulation of β-amyloid (Aβ) peptide plaques is a major pathogenic event in Alzheimer's disease (AD). Aβ is a cleaved fragment of APP via BACE1, which is the rate-limiting enzyme in APP processing and Aβ generation. Nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ) is considered to be a potential target for AD treatment, because of its potent antioxidant and inhibitory effects on Aβ production by negatively regulating BACE1. Epigallocatechin gallate (EGCG), a highly active catechin found in green tea, is known to enhance metabolic activity and cognitive ability in the mice model of AD. To investigate whether the therapeutic effect of EGCG is related to the PPARγ pathway, we analysed the alterations in the intracellular molecular expression of PPARγ after EGCG treatment in the N2a/APP695 cell line. In this study, we observed that EGCG attenuated Aβ generation in N2a/APP695 cells, such as the PPARγ agonist, pioglitazone, by suppressing the transcription and translation of BACE1 and that its effect was attenuated by the PPARγ inhibitor, GW9662. Intriguingly, EGCG significantly reinforced the activity of PPARγ by promoting its mRNA and protein expressions in N2a/APP695 cells. Moreover, EGCG also decreased the expression of pro-apoptotic proteins (Bax, caspase-3), reduced the activity of the anti-inflammatory agent NF-κB and inhibited the oxidative stress by decreasing the levels of ROS and MDA and increasing the expression of MnSOD. Co-administration of GW9662 also significantly decreased the EGCG-mediated neuroprotective effect evidenced by the increase in oxidative stress and inflammatory markers. The therapeutic efficacy of EGCG in AD may be derived from the up-regulation of PPARγ mRNA and protein expressions.

Long-term UVA eye irradiation causes decreased learning ability in mice

BACKGROUND

Long-term eye radiation with ultraviolet A (UVA) denatures the cells of the cerebral hippocampus. However, the influence on memory and learning ability in mice is not known.

METHODS

HR-1 mice were used. We irradiated the eyes or dorsal skin of the mice with UVA at a dose of 110 kJ/m(2) using an FL20SBLB-A lamp for 6, 12, and 24 months.

RESULTS

The mean escape latency in the water maze was significantly increased in the UVA-irradiated mice in comparison with that seen in the controls. In the mice in which UVA eye irradiation was performed for 24 months, the depression of memory and learning ability was remarkable. The acetylcholinesterase activity, choline acetyltransferase activity, and acetylcholine content in the brain in the UVA eye-irradiated mice were significantly less than those observed in the control mice. Furthermore, during UVA eye irradiation, the levels of β-amyloid (Aβ), γ-secretase, which produces Aβ peptide, and advanced glycation end products increased. Moreover, the effects of UVA eye irradiation increased with the duration of irradiation (or aging), and the introduction of glucose into the brain also decreased with UVA eye irradiation.

CONCLUSIONS

These results indicate that UVA eye irradiation induces a decreased memory and learning ability in mice.

Peroxisome Proliferator-Activated Receptor β/δ Alleviates Early Brain Injury After Subarachnoid Hemorrhage in Rats

BACKGROUND AND PURPOSE

Early brain injury is proposed to be the primary cause of the poor outcome after subarachnoid hemorrhage (SAH), which is closely related to the neural apoptosis. To date, the relationship between peroxisome proliferator-activated receptor β/δ (PPARβ/δ) and nuclear factor-κB/matrix metalloproteinase-9 (NF-κB/MMP-9) pathway, both of which are closely related to apoptotic effects, has been poorly studied in SAH. The present study was undertaken to evaluate the effects of PPARβ/δ on early brain injury and NF-κB/MMP-9 pathway after SAH in rats.

METHODS

SAH model was established by injecting nonheparinized autologous arterial blood into the prechiasmatic cistern in male Sprague-Dawley rats. Adenoviruses or small interfering RNAs were injected into the right lateral cerebral ventricle to, respectively, up- or downregulate PPARβ/δ expression before SAH. All animals were assessed with a neurological score and then killed at 24 hours after SAH surgery. The indexes of brain water content, blood-brain barrier permeability, and apoptosis were used to detect brain injury. The expression of PPARβ/δ, NF-κB, and MMP-9 were measured by immunohistochemistry, gelatin zymography, and Western Blot methods, respectively. In addition, GW0742, a specific agonist of PPARβ/δ, was used to treat SAH in rats, the effects of which were evaluated by neurological scoring and Evans blue extravasation.

RESULTS

Overexpression of PPARβ/δ by adenoviruses treatment significantly ameliorated brain injury with improvement in neurological deficits, brain edema, blood-brain barrier impairment, and neural cell apoptosis at 24 hours after SAH in rats, whereas downregulation of PPARβ/δ by small interfering RNAs administration resulted in the reverse effects of the above. The expression levels of NF-κB and MMP-9 were markedly downregulated when PPARβ/δ increased after PPARβ/δ adenovirus transfection and upregulated when PPARβ/δ decreased by PPARβ/δ small interfering RNAs treatment. Moreover, GW0742 improved neurological deficits and reduced Evans blue extravasation at 24 hours after SAH.

CONCLUSIONS

PPARβ/δ's overexpression may attenuate early brain injury after rats' SAH administration, which reduces neural apoptosis possibly through blocking NF-κB/MMP-9 pathway.

Role of RAGE in Alzheimer's Disease

Receptor for advanced glycation end products (RAGE) is a receptor of the immunoglobulin super family that plays various important roles under physiological and pathological conditions. Compelling evidence suggests that RAGE acts as both an inflammatory intermediary and a critical inducer of oxidative stress, underlying RAGE-induced Alzheimer-like pathophysiological changes that drive the process of Alzheimer's disease (AD). A critical role of RAGE in AD includes beta-amyloid (Aβ) production and accumulation, the formation of neurofibrillary tangles, failure of synaptic transmission, and neuronal degeneration. The steady-state level of Aβ depends on the balance between production and clearance. RAGE plays an important role in the Aβ clearance. RAGE acts as an important transporter via regulating influx of circulating Aβ into brain, whereas the efflux of brain-derived Aβ into the circulation via BBB is implemented by LRP1. RAGE could be an important contributor to Aβ generation via enhancing the activity of β- and/or γ-secretases and activating inflammatory response and oxidative stress. However, sRAGE-Aβ interactions could inhibit Aβ neurotoxicity and promote Aβ clearance from brain. Meanwhile, RAGE could be a promoting factor for the synaptic dysfunction and neuronal circuit dysfunction which are both the material structure of cognition, and the physiological and pathological basis of cognition. In addition, RAGE could be a trigger for the pathogenesis of Aβ and tau hyper-phosphorylation which both participate in the process of cognitive impairment. Preclinical and clinical studies have supported that RAGE inhibitors could be useful in the treatment of AD. Thus, an effective measure to inhibit RAGE may be a novel drug target in AD.

Activation of PPARδ: from computer modelling to biological effects

PPARδ is a ligand-activated receptor that dimerizes with another nuclear receptor of the retinoic acid receptor family. The dimers interact with other co-activator proteins and form active complexes that bind to PPAR response elements and promote transcription of genes involved in lipid metabolism. It appears that various natural fatty acids and their metabolites serve as endogenous activators of PPARδ; however, there is no consensus in the literature on the nature of the prime activators of the receptor. In vitro and cell-based assays of PPARδ activation by fatty acids and their derivatives often produce conflicting results. The search for synthetic and selective PPARδ agonists, which may be pharmacologically useful, is intense. Current rational modelling used to obtain such compounds relies mostly on crystal structures of synthetic PPARδ ligands with the recombinant ligand binding domain (LBD) of the receptor. Here, we introduce an original computational prediction model for ligand binding to PPARδ LBD. The model was built based on EC50 data of 16 ligands with available crystal structures and validated by calculating binding probabilities of 82 different natural and synthetic compounds from the literature. These compounds were independently tested in cell-free and cell-based assays for their capacity to bind or activate PPARδ, leading to prediction accuracy of between 70% and 93% (depending on ligand type). This new computational tool could therefore be used in the search for natural and synthetic agonists of the receptor.

Increased tau phosphorylation and receptor for advanced glycation endproducts (RAGE) in the brain of mice infected with Leishmania amazonensis

Leishmaniasis is a parasitosis caused by several species of the genus Leishmania, an obligate intramacrophagic parasite. Although neurologic symptoms have been observed in human cases of leishmaniasis, the manifestation of neurodegenerative processes is poorly studied. The aim of the present work was to investigate if peripheral infection of BALB/c mice with Leishmania amazonensis affects tau phosphorylation and RAGE protein content in the brain, which represent biochemical markers of neurodegenerative processes observed in diseases with a pro-inflammatory component, including Alzheimer's disease and Down syndrome. Four months after a single right hind footpad subcutaneous injection of L. amazonensis, the brain cortex of BALB/c mice was isolated. Western blot analysis indicated an increase in tau phosphorylation (Ser(396)) and RAGE immunocontent in infected animals. Brain tissue TNF-α, IL-1β, and IL-6 levels were not different from control animals; however, increased protein carbonylation, decreased IFN-γ levels and impairment in antioxidant defenses were detected. Systemic antioxidant treatment (NAC 20mg/kg, i.p.) inhibited tau phosphorylation and recovered IFN-γ levels. These data, altogether, indicate an association between impaired redox state, tau phosphorylation and RAGE up-regulation in the brain cortex of animals infected with L. amazonensis. In this context, it is possible that neurologic symptoms associated to chronic leishmaniasis are associated to disruptions in the homeostasis of CNS proteins, such as tau and RAGE, as consequence of oxidative stress. This is the first demonstration of alterations in biochemical parameters of neurodegeneration in an experimental model of Leishmania infection.

Neuroinflammation in Alzheimer's disease

Amyloid-β plaques and neurofibrillary tangles are the main neuropathological hallmarks in Alzheimer's disease (AD), the most common cause of dementia in the elderly. However, it has become increasingly apparent that neuroinflammation plays a significant role in the pathophysiology of AD. This review summarizes the current status of neuroinflammation research related to AD, focusing on the connections between neuroinflammation and some inflammation factors in AD. Among these connections, we discuss the dysfunctional blood-brain barrier and alterations in the functional responses of microglia and astrocytes in this process. In addition, we summarize and discuss the role of intracellular signaling pathways involved in inflammatory responses in astrocytes and microglia, including the mitogen-activated protein kinase pathways, nuclear factor-kappa B cascade, and peroxisome proliferator-activated receptor-gamma transcription factors. Finally, the dysregulation of the control and release of pro- and anti-inflammatory cytokines and classic AD pathology (amyloid plaques and neurofibrillary tangles) in AD is also reviewed.

Nuclear receptors in neurodegenerative diseases

Nuclear receptors have generated substantial interest in the past decade as potential therapeutic targets for the treatment of neurodegenerative disorders. Despite years of effort, effective treatments for progressive neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease and ALS remain elusive, making non-classical drug targets such as nuclear receptors an attractive alternative. A substantial literature in mouse models of disease and several clinical trials have investigated the role of nuclear receptors in various neurodegenerative disorders, most prominently AD. These studies have met with mixed results, yet the majority of studies in mouse models report positive outcomes. The mechanisms by which nuclear receptor agonists affect disease pathology remain unclear. Deciphering the complex signaling underlying nuclear receptor action in neurodegenerative diseases is essential for understanding this variability in preclinical studies, and for the successful translation of nuclear receptor agonists into clinical therapies.

Brain metabolite clearance: impact on Alzheimer's disease

Alzheimer's Disease (AD) is a complex neurodegenerative disorder often associated with aging and characterized by several critical molecular changes that take place in the brain. Among the molecular hallmarks of AD, increased levels of amyloid β-peptide (Aβ) and the subsequent Aβ-derived damage are the most well-studied factors; however, despite the large amounts of effort and resources devoted to the study of AD and AD pathophysiology, the scientific community still awaits therapeutic alternatives capable of ensuring a better outcome for AD patients. In 2012, Cramer et al. (Science 335:1503-1506 2012) astonished the scientific community by rescuing behavioral and cognitive impairments in AD mouse models via oral administration of bexarotene, a drug used to treat some types of skin cancer. Moreover, these authors demonstrated that bexarotene, a retinoid X receptor (RXR) agonist, exerts major effects on Aβ levels, mainly through increased apolipoprotein E (ApoE) expression. Apart from the valid questions addressed in Cramer's work, only a few attempts have been made to explain the effects of bexarotene. Most of these explanations have been solely based on the ability of bexarotene to reduce Aβ levels and not on the mechanisms that lead to such a reduction. Although it is well known that an imbalance in the Aβ production/excretion rate is the basis of increased Aβ levels in AD, no further explanations have been proposed to address the potential involvement of the blood-brain barrier (BBB), a critical Aβ-clearance structure, in the bexarotene-mediated effects. Moreover, no attempt has been made to explain how the different effects observed after bexarotene administration are connected to each other. Based on current information and on our own experience with nuclear receptors (NR), we offer new perspectives on the mechanisms of bexarotene action, which should help to improve our knowledge of NRs.

Impaired insulin signaling and mechanisms of memory loss

Insulin is secreted from the β-cells of the pancreas and helps maintain glucose homeostasis. Although secreted peripherally, insulin also plays a profound role in cognitive function. Increasing evidence suggests that insulin signaling in the brain is necessary to maintain health of neuronal cells, promote learning and memory, decrease oxidative stress, and ultimately increase neuronal survival. This chapter summarizes the different facets of insulin signaling necessary for learning and memory and additionally explores the association between cognitive impairment and central insulin resistance. The role of impaired insulin signaling in the advancement of cognitive dysfunction is relevant to the current debate of whether the shared pathophysiological mechanisms between diabetes and cognitive impairment implicate a direct relationship. Here, we summarize a vast amount of literature that suggests a strong association between impaired brain insulin signaling and cognitive impairment.