MRI monitoring of focal cerebral ischemia in peroxisome proliferator-activated receptor (PPAR)-deficient mice

PPARα Inhibits Astrocyte Inflammation Activation by Restoring Autophagic Flux after Transient Brain Ischemia

Astrocyte inflammation activation is an important cause that hinders the recovery of motor function after cerebral ischemia. However, its molecular mechanism has not yet been clearly clarified. The peroxisome proliferator-activated receptor α (PPARα) is a ligand-activated nuclear transcriptional factor. This study aims to further clarify the role of PPARα in astrocyte inflammation activation after cerebral ischemia and to explore the underlying mechanism. Astrocyte activation was induced in an in vivo model by transient middle cerebral artery occlusion (tMCAO) in mice. The in vitro model was induced by an oxygen-glucose deprivation/reoxygenation (OGD/R) in a primary culture of mouse astrocyte. PPARα-deficient mice were used to observe the effects of PPARα on astrocyte activation and autophagic flux. Our results showed that PPARα was mainly expressed in activated astrocytes during the chronic phase of brain ischemia and PPARα dysfunction promoted astrocyte inflammatory activation. After cerebral ischemia, the expressions of LC3-II/I and p62 both increased. Autophagic vesicle accumulation was observed by electron microscopy in astrocytes, and the block of autophagic flux was indicated by an mRFP-GFP-LC3 adenovirus infection assay. A PPARα deficit aggravated the autophagic flux block, while PPARα activation preserved the lysosome function and restored autophagic flux in astrocytes after OGD/R. The autophagic flux blocker bafilomycin A1 and chloroquine antagonized the effect of the PPARα agonist on astrocyte activation inhibition. This study identifies a potentially novel function of PPARα in astrocyte autophagic flux and suggests a therapeutic target for the prevention and treatment of chronic brain ischemic injury.

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.

New PAR1 Agonist Peptide Demonstrates Protective Action in a Mouse Model of Photothrombosis-Induced Brain Ischemia

Protease-activated receptors (PARs) are involved not only in hemostasis but also in the development of ischemic brain injury. In the present work, we examined in vivo effects of a new peptide (AP9) composing Asn⁴⁷-Phen⁵⁵ of PAR1 “tethered ligand” generated by activated protein C. We chose a mouse model of photothrombosis (PT)-induced ischemia to assess AP9 effects in vivo. To reveal the molecular mechanism of AP9 action, mice lacking β-arrestin-2 were used. AP9 was injected intravenously once 10 min before PT at doses of 0.2, 2, or 20 mg/kg, or twice, that is, 10 min before and 1 h after PT at a dose of 20 mg/kg. Lesion volume was measured by magnetic resonance imaging and staining of brain sections with tetrazolium salt. Neurologic deficit was estimated using the cylinder and the grid-walk tests. Blood–brain barrier (BBB) disruption was assessed by Evans blue dye extraction. Eosin-hematoxylin staining and immunohistochemical staining were applied to evaluate the number of undamaged neurons and activated glial cells in the penumbra. A single administration of AP9 (20 mg/kg), as well as its two injections (20 mg/kg), decreased brain lesion volume. A double administration of AP9 also reduced BBB disruption and neurological deficit in mice. We did not observe the protective effect of AP9 in mice lacking β-arrestin-2 after PT. Thus, we demonstrated for the first time protective properties of a PAR1 agonist peptide, AP9, in vivo. β-Arrestin-2 was required for the protective action of AP9 in PT-induced brain ischemia.

Neurovascular protection by peroxisome proliferator-activated receptor α in ischemic stroke

Ischemic stroke is a leading cause of death and disability worldwide. Currently, the only pharmacological therapy for ischemic stroke is thrombolysis with tissue plasminogen activator that has a narrow therapeutic window and increases the risk of intracerebral hemorrhage. New pharmacological treatments for ischemic stroke are desperately needed, but no neuroprotective drugs have successfully made it through clinical trials. Beneficial effects of peroxisome proliferator-activated receptor alpha (PPARα) activation on vascular integrity and function have been reported, and PPARα agonists have clinically been used for many years to manage cardiovascular disease. Thus, PPARα has gained interest in recent years as a target for neurovascular disease such as ischemic stroke. Accumulating preclinical evidence suggests that PPARα activation modulates several pathophysiological hallmarks of stroke such as oxidative stress, blood-brain barrier (BBB) dysfunction, and neuroinflammation to improve functional recovery. Therefore, this review summarizes the various actions PPARα exerts in neurovascular health and disease and the potential of employing exogenous PPARα agonists for future pharmacological treatment of ischemic stroke.

Methods for detection of brain injury after photothrombosis-induced ischemia in mice: Characteristics and new aspects of their application

BACKGROUND

Photothrombosis is a minimally invasive method for induction of cortical ischemia. However, different ways of applying some methods to assess photothrombosis-induced damage need to be developed.

NEW METHODS

We applied the tongue protrusion test and H&E staining of brain sections to detect ischemic damage after photothrombosis. Evaluation of the local status of the BBB using Evans blue dye was proposed. We also assessed the sensitivity of the grid-walk test. Moreover, we examined the interchangeability of MRI and TTC staining to measure lesion volume.

RESULTS

We evaluated ischemic outcomes at 24 h after photothrombosis in mice. The tongue protrusion test did not reveal impairments of the neurological status whereas the grid-walk test showed the high sensitivity. Using histological techniques, we determined the reduction in the number of neurons with normal morphology in the penumbra. 3D reconstruction of the brain, which reflected Evans blue dye distribution in the nervous tissue, revealed BBB disruption in areas remote from the ischemic core. We also showed the strong correlation between damage volumes assessed by MRI and TTC staining.

COMPARISON WITH EXISTING METHODS

The present work demonstrates the efficacy of the classical histological approach and TTC staining that are more affordable than MRI and immunohistochemical methods. Detection of 3D distribution of Evans blue dye in the brain in contrast to its total extraction reveals BBB damage in details.

CONCLUSIONS

We proposed the simple methods for describing the severity of brain ischemia at the cellular and whole organism levels without significant labor and financial expenditures.

Hippocampal Genetic Knockdown of PPARδ Causes Depression-Like Behaviors and Neurogenesis Suppression

BACKGROUND

Although depression is the leading cause of disability worldwide, its pathophysiology is poorly understood. Our previous study showed that hippocampal peroxisome proliferator-activated receptor δ (PPARδ) overexpression displays antidepressive effect and enhances hippocampal neurogenesis during chronic stress. Herein, we further extended our curiosity to investigate whether downregulating PPARδ could cause depressive-like behaviors through downregulation of neurogenesis.

METHODS

Stereotaxic injection of lentiviral vector, expressing short hairpin RNA complementary to the coding exon of PPARδ, was done into the bilateral dentate gyri of the hippocampus, and the depression-like behaviors were observed in mice. Additionally, hippocampal neurogenesis, brain-derived neurotrophic factor and cAMP response element-binding protein were measured both in vivo and in vitro.

RESULTS

Hippocampal PPARδ knockdown caused depressive-like behaviors and significantly decreased neurogenesis, neuronal differentiation, levels of mature brain-derived neurotrophic factor and phosphorylated cAMP response element-binding protein in the hippocampus. In vitro study further confirmed that PPARδ knockdown could inhibit proliferation and differentiation of neural stem cells. Furthermore, these effects were mimicked by repeated systemic administration of a PPARδ antagonist, GSK0660 (1 or 3 mg/kg i.p. for 21 d).

CONCLUSIONS

These findings suggest that downregulation of hippocampal PPARδ is associated with depressive behaviors in mice through an inhibitory effect on cAMP response element-binding protein/brain-derived neurotrophic factor-mediated adult neurogenesis in the hippocampus, providing new insights into the pathogenesis of depression.

Integration of phospholipid-complex nanocarrier assembly with endogenous N-oleoylethanolamine for efficient stroke therapy

Background

Leading to more and more deaths and disabilities, stroke has become a serious threat to human health. What’s more, few effective drugs are available in clinic till now.

Results

In this research, we prepared a novel neuroprotective nanoformation (OEA–SPC NPs) via the combination of the nanoparticle drug delivery system with the endogenous N-oleoylethanolamine (OEA). By forming hydrogen bond between OEA and the carrier—soybean phosphatidylcholine (SPC), the form of OEA was turned into amorphus state when loading to the nanoparticles, which greatly improved its bioavailability. Then the following systematic experiments revealed the efficient neuroprotective effect of OEA–SPC NPs in vivo. Compared with the MCAO group, the cerebral infarct volume was reduced by 81.1%, and the edema degree by 78.4% via the oral administration of OEA–SPC NPs. And the neurological deficit scores illustrated that the MCAO rats treated with OEA–SPC NPs exhibited significantly less neurological dysfunction. The Morris water maze test indicated that the spatial learning and memory of cerebral ischemia model rats were almost recovered to the normal level. Besides, the OEA–SPC NPs could inhibit the inflammation of reperfusion to a very slight level.

Conclusions

These results suggest that the OEA–SPC NPs have a great chance to be a potential anti-stroke formation for clinic application and actually bring hope to thousands of stroke patients.

Electronic supplementary material

The online version of this article (10.1186/s12951-019-0442-x) contains supplementary material, which is available to authorized users.

Oral administration of a novel lipophilic PPARδ agonist is not neuroprotective after rodent cerebral ischemia

Peroxisome proliferator-activated receptors are regulators of inflammatory signaling. This has fostered hope that PPAR agonists might have neuroprotective potential. We hypothesized that PPARδ activation by the novel orally administered lipophilic PPARδ agonist SAR145 may improve short- and long-term outcome after focal brain ischemia. We induced ischemia by transient filamentous middle cerebral artery occlusion (MCAo) in 227 C57BL/6 mice and administered SAR145 in varying doses and time windows post-injury. Outcome was assessed by three functional tests and histologically determining ischemic lesion sizes. In a second experiment, we tested SAR145 treatment in 40 PPARδ-knockout mice using the same procedures. Three independent groups treated with 10 mg/kg bodyweight SAR145 directly after filament removal showed a mean reduction in lesion sizes of 18 ± 10% compared to vehicle-treated groups. We did not observe a consistent improvement in the long-term functional outcome by SAR145-treatment. PPARδ-knockout mice showed a significantly higher mortality after MCAo. As expected, we did not find a reduction of lesion size by SAR145-treatment in PPARδ-knockout mice. In summary, we found no evidence of a long-term neuroprotective effect of post-injury SAR145 treatment in cerebral ischemia. However, PPARδ appears to play a pathophysiologic role in acute infarct development and overall mortality after brain ischemia.

Deficiency in the voltage-gated proton channel Hv1 increases M2 polarization of microglia and attenuates brain damage from photothrombotic ischemic stroke

Microglia become activated during cerebral ischemia and exert pro-inflammatory or anti-inflammatory role dependent of microglial polarization. NADPH oxidase (NOX)-dependent reactive oxygen species (ROS) production in microglia plays an important role in neuronal damage after ischemic stroke. Recently, NOX and ROS are consistently reported to participate in the microglial activation and polarization; NOX2 inhibition or suppression of ROS production are shown to shift the microglial polarization from M1 toward M2 state after stroke. The voltage-gated proton channel, Hv1, is selectively expressed in microglia and is required for NOX-dependent ROS generation in the brain. However, the effect of Hv1 proton channel on microglial M1/M2 polarization state after cerebral ischemia remains unknown. In this study, we investigated the role of microglial Hv1 proton channel in modulating microglial M1/M2 polarization during the pathogenesis of ischemic cerebral injury using a mouse model of photothrombosis. Following photothrombotic ischemic stroke, wild-type mice presented obvious brain infarct, neuronal damage, and impaired motor coordination. However, mice lacking Hv1 (Hv1(-/-)) were partially protected from brain damage and motor deficits compared to wild-type mice. These rescued phenotypes in Hv1(-/-) mice in ischemic stroke is accompanied by reduced ROS production, shifted the microglial polarization from M1 to M2 state. Hv1 deficiency was also found to shift the M1/M2 polarization in primary cultured microglia. Our study suggests that the microglial Hv1 proton channel is a unique target for modulation of microglial M1/M2 polarization in the pathogenesis of ischemic stroke. The voltage-gated proton channel, Hv1, is selectively expressed in microglia and is required for NOX-dependent generation of reactive oxygen species (ROS) in the brain. ROS participate in microglial activation and polarization. However, the effect of Hv1 on microglial M1/M2 polarization state after cerebral ischemia remains unknown. Hv1 deficiency was found to shift the microglial polarization from M1 to M2 state in ischemic stroke accompanied by reduced ROS production. Our study suggests that the microglial Hv1 proton channel is a unique target for modulation of microglial M1/M2 polarization in the pathogenesis of ischemic stroke.

PPARδ agonist GW0742 ameliorates Aβ1-42-induced hippocampal neurotoxicity in mice

Amyloid-β deposition is thought to be associated with memory deficits, neuroinflammation, apoptotic responses, and progressive neuronal death manifested in Alzheimer's disease. Peroxisome proliferator-activated receptor δ (PPARδ) is a transcription factor with potent anti-inflammatory effect. In the current study, the effect of GW0742, a selective PPARδ agonist, on Aβ1-42-induced neurotoxicity was investigated in the hippocampus of mice. Intra-hippocampal infusion of aggregated Aβ1-42 oligomer (410pmol/mouse) remarkably damaged learning and memory in the Morris water maze (MWM) and Y-maze tests, accompanied by decreased expression of PPARδ in the hippocampus as confirmed by Western blot. Intra-hippocampal infusion of GW0742 (1.06 mM/mouse) significantly improved Aβ1-42-induced memory deficits in mice, reversed Aβ1-42-induced hippocampal PPARδ down-regulation and repressed Aβ1-42-triggered neuroinflammatory and apoptotic responses, indicated by decreased nuclear NF-κB p65, TNF-α, IL-1β as well as a decrease in cleaved caspase-3 and increased ratio of Bcl-2/Bax in the hippocampus. These results suggest that PPARδ activation ameliorates Aβ1-42-induced hippocampal neurotoxicity, and it might play a crucial role in Alzheimer's disease.

Hippocampal PPARδ Overexpression or Activation Represses Stress-Induced Depressive Behaviors and Enhances Neurogenesis

BACKGROUND

Emerging data have demonstrated that peroxisome proliferator-activated receptor δ (PPARδ) activation confers a potentially neuroprotective role in some neurodegenerative diseases. However, whether PPARδ is involved in depression is unknown.

METHODS

In this study, PPARδ was firstly investigated in the chronic mild stress (CMS) and learned helplessness (LH) models of depression. The changes in depressive behaviors and hippocampal neurogenesis were investigated after PPARδ overexpression by microinfusion of the lentiviral vector, containing the coding sequence of mouse PPARδ (LV-PPARδ), into the bilateral dentate gyri of the hippocampus or PPARδ activation by repeated systemic administration of PPARδ agonist GW0742 (5 or 10mg/kg.d, i.p., for 21 d).

RESULTS

We found that both CMS and LH resulted in a significant decrease in the PPARδ expression in the hippocampi of mice, and this change was reversed by treatment with the antidepressant fluoxetine. PPARδ overexpression and PPARδ activation each suppressed the CMS- and LH-induced depressive-like behavior and produced an antidepressive effect. In vivo or in vitro studies also showed that both overexpression and activation of PPARδ enhanced proliferation or differentiation of neural stem cells in the hippocampi of mice.

CONCLUSIONS

These results suggest that hippocampal PPARδ upregulation represses stress-induced depressive behaviors, accompanied by enhancement of neurogenesis.

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.

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.

PPAR-alpha and PPAR-beta expression changes in the hippocampus of rats undergoing global cerebral ischemia/reperfusion due to PPAR-gamma status

Background

Peroxisome proliferator-activated receptors (PPARs, including alpha, beta and gamma subtypes) and their agonists have a protective role in treatment of central nervous system (CNS) diseases. The present study was designed to investigate the expression changes of PPAR-alpha, -beta, -gamma and NF-kappa B in the hippocampus of rats with global cerebral ischemia/reperfusion injury (GCIRI) after treatment with agonists or antagonists of PPAR-gamma.

Methods

A rat GCIRI model was established by occlusion of bilateral common carotid arteries and cervical vena retransfusion. GW9662 (5 μg), a selective PPAR- gamma antagonist, was intraventricularly injected at 0.5 h before GCIR; Rosiglitazone (0.8, 2.4 and 7.2 mg/kg), a selective PPAR- gamma agonist, was injected intraperitoneally at 1 h before GCIRI. The expression changes of PPAR-alpha, -beta and -gamma at mRNA and protein levels were detected by RT-PCR and western blotting. The changes of spatial learning and memory (SLM) functions were assessed by using a Morris water maze; the pathohistological changes of hippocampal neurons were evaluated by hematoxylin-eosin (HE) staining; the contents of IL-1, IL-6, IL-10 and TNF-alpha, and the NF- kappa B expression were measured by enzyme-linked immunosorbent assay (ELISA) and immunohistochemical staining. The superoxide dismutase (SOD) activity and malondialdehyde (MDA) content were also detected.

Results

The SLM function and hippocampal neurons were significantly impaired after the occurrence of GCIRI. The MDA, IL-1, IL-6, IL-10, TNF-alpha content and expression of PPARs increased significantly, but the SOD activity and NF-kappa B expression were weakened in the hippocampus. Rosiglitazone treatment significantly protected rats from SLM function impairment and neuron death, and resulted in higher expressions of SOD activity and NF-kappa B, but lower contents of MDA and inflammatory factors. After treatment with rosiglitazone or GW9662, no significant change in PPAR-alpha or -beta expression was detected.

Conclusions

Rosiglitazone, a PPAR-gamma agonist, plays a protective role in hippocampal neuron damage of GCIRI rats by inhibiting the oxidative stress response and inflammation. The activation or antagonism of PPAR-gamma did not affect the expression of PPAR-alpha or -beta, indicating that the three subtypes of PPARs act in independent pathways in the CNS.

Histological, cellular and behavioral assessments of stroke outcomes after photothrombosis-induced ischemia in adult mice

BACKGROUND

Following the onset of focal ischemic stroke, the brain experiences a series of alterations including infarct evolvement, cellular proliferation in the penumbra, and behavioral deficits. However, systematic study on the temporal and spatial dependence of these alterations has not been provided.

RESULTS

Using multiple approaches, we assessed stroke outcomes by measuring brain injury, dynamic cellular and glial proliferation, and functional deficits at different times up to two weeks after photothrombosis (PT)-induced ischemic stroke in adult mice. Results from magnetic resonance imaging (MRI) and Nissl staining showed a maximal infarction, and brain edema and swelling 1-3 days after PT. The rate of Bromodeoxyuridine (Brdu)-labeled proliferating cell generation is spatiotemporal dependent in the penumbra, with the highest rate in post ischemic days 3-4, and higher rate of proliferation in the region immediate to the ischemic core than in the distant region. Similar time-dependent generation of proliferating GFAP+ astrocytes and Iba1+ microglia/macrophage were observed in the penumbra. Using behavioral tests, we showed that PT resulted in the largest functional deficits during post ischemic days 2-4.

CONCLUSION

Our study demonstrated that first a few days is a critical period that causes brain expansion, cellular proliferation and behavioral deficits in photothrombosis-induced ischemic model, and proliferating astrocytes only have a small contribution to the pools of proliferating cells and reactive astrocytes.

IL-10 deficiency exacerbates the brain inflammatory response to permanent ischemia without preventing resolution of the lesion

Stroke induces inflammation that can aggravate brain damage. This work examines whether interleukin-10 (IL-10) deficiency exacerbates inflammation and worsens the outcome of permanent middle cerebral artery occlusion (pMCAO). Expression of IL-10 and IL-10 receptor (IL-10R) increased after ischemia. From day 4, reactive astrocytes showed strong IL-10R immunoreactivity. Interleukin-10 knockout (IL-10 KO) mice kept in conventional housing showed more mortality after pMCAO than the wild type (WT). This effect was associated with the presence of signs of colitis in the IL-10 KO mice, suggesting that ongoing systemic inflammation was a confounding factor. In a pathogen-free environment, IL-10 deficiency slightly increased infarct volume and neurologic deficits. Induction of proinflammatory molecules in the IL-10 KO brain was similar to that in the WT 6 hours after ischemia, but was higher at day 4, while differences decreased at day 7. Deficiency of IL-10 promoted the presence of more mature phagocytic cells in the ischemic tissue, and enhanced the expression of M2 markers and the T-cell inhibitory molecule CTLA-4. These findings agree with a role of IL-10 in attenuating local inflammatory reactions, but do not support an essential function of IL-10 in lesion resolution. Upregulation of alternative immunosuppressive molecules after brain ischemia can compensate, at least in part, the absence of IL-10.

Peroxisome proliferator-activated receptor (PPAR)β/δ, a possible nexus of PPARα- and PPARγ-dependent molecular pathways in neurodegenerative diseases: Review and novel hypotheses

Peroxisome proliferator-activated receptors (PPARα, -β/δ and -γ) are lipid-activated transcription factors. Synthetic PPARα and PPARγ ligands have neuroprotective properties. Recently, PPARβ/δ activation emerged as the focus of a novel approach for the treatment of a wide range of neurodegenerative diseases. To fill the gap of knowledge about the role of PPARβ/δ in brain, new hypotheses about PPARβ/δ involvement in neuropathological processes are requested. In this paper, we describe a novel hypothesis, claiming the existence of tight interactions between the three PPAR isotypes, which we designate the "PPAR triad". We propose that PPARβ/δ has a central control of the PPAR triad. The majority of studies analyze the regulation only by one of the PPAR isotypes. A few reports describe the mutual regulation of expression levels of all three PPAR isotypes by PPAR agonists. Analysis of these studies where pairwise interactions of PPARs were described allows us to support the existence of the PPAR triad with central role for PPARβ/δ. In the present review, we propose the hypothesis that in a wide range of brain disorders, PPARβ/δ plays a central role between PPARα and PPARγ. Finally, we prove the advantages of the PPAR triad concept by describing hypotheses of PPARβ/δ involvement in the regulation of myelination, glutamate-induced neurotoxicity, and signaling pathways of reactive oxygen species/NO/Ca(2+).

Peroxisome proliferator-activated receptor δ agonist, HPP593, prevents renal necrosis under chronic ischemia

The Goldblatt’s 2 kidney 1 clip (2K1C) rat animal model of renovascular hypertension is characterized by ischemic nephropathy of the clipped kidney. 2K1C rats were treated with a specific peroxisome proliferator-activated receptor δ (PPARδ) agonist, HPP593. Clipped kidneys from untreated rats developed tubular and glomerular necrosis and massive interstitial, periglomerular and perivascular fibrosis. HPP593 kidneys did not exhibit any histochemical features of necrosis; fibrotic lesions were present only in perivascular areas. Necrosis in the untreated clipped kidneys was associated with an increased oxidative stress, up regulation and mitochondrial translocation of the pro-death protein BNIP3 specifically in tubules. In the kidneys of HPP593-treated rats oxidative stress was attenuated and BNIP3 protein decreased notably in the mitochondrial fraction when compared to untreated animals. In untreated clipped kidneys, mitochondria were dysfunctional as revealed by perturbations in the levels of MCAD, COXIV, TFAM, and Parkin proteins and AMPK activation, while in HPP593-treated rats these proteins remained at the physiological levels. Nuclear amounts of oxidative stress-responsive proteins, NRF1 and NRF2 were below physiological levels in treated kidneys. Mitochondrial biogenesis and autophagy were inhibited similarly in both treated and untreated 2K1C kidneys as indicated by a decrease in PGC1-α and deficiency of the autophagy-essential proteins LC3-II and ATG5. However, HPP593 treatment resulted in increased accumulation of p62 protein, an autophagic substrate and an enhancer of NRF2 activity. Therefore, inhibition of BNIP3 activation by the preservation of mitochondrial function and control of oxidative stress by PPARδ is the most likely mechanism to account for the prevention of necrotic death in the kidney under conditions of persistent ischemia.

Effect of genetic polymorphism +294T/C in peroxisome proliferator-activated receptor delta on the risk of ischemic stroke in a Tunisian population

PPARδ +294T/C polymorphism was investigated in diabetics, in normolipidemic healthy controls, in dyslipidemic and nondyslipidemic coronary artery disease patients but never in ischemic stroke patients. The aim of this study was to explore, for the first time, the relationship between the genetic polymorphism of PPARδ and the risk of ischemic stroke among patients with diabetes. The study group consisted of 196 patients with ischemic stroke and 192 controls. Plasma concentrations of total cholesterol, triglycerides, low-, and high-density lipoprotein did not differ significantly between subjects carrying the TT genotype and those carrying the CC/TC genotype in both ischemic stroke patients (with or without diabetes) and control groups. The +294C allele (CC + CT genotypes) as compared with TT genotypes was found to be higher in total ischemic stroke patients than in controls. On the other hand, no interaction between diabetes and PPAR +294T/C polymorphism on the risk of ischemic stroke was found (p = 0.089). The PPARδ +294T/C polymorphism was associated with the risk of ischemic stroke in Tunisian subjects. This polymorphism has no influence on plasma lipoprotein concentrations and body mass index either in healthy subjects or in ischemic stroke patients with or without diabetes both in males and females.

Expression Pattern of Peroxisome Proliferator-Activated Receptors in Rat Hippocampus following Cerebral Ischemia and Reperfusion Injury

The present study was designed to investigate the pattern of time-dependent expression of peroxisome proliferator-activated receptors (PPARα, β, and γ) after global cerebral ischemia and reperfusion (I/R) damage in the rat hippocampus. Male Sprague Dawley (SD) rats were subjected to global cerebral I/R. The rat hippocampi were isolated to detect the expression of PPARs mRNA and protein levels at 30 min–30 d after I/R by RT-PCR and Western blot analysis, respectively. The expression levels of PPARs mRNA and protein in the rat hippocampus significantly increased and peaked at 24 h for PPARα and γ (at 48 h for PPARβ) after I/R, then gradually decreased, and finally approached control levels on d 30. The present results suggest that global cerebral I/R can cause obvious increases of hippocampal PPARs mRNA and protein expression within 15 d after I/R. These findings may help to guide the experimental and clinical therapeutic use of PPARs agonists against brain injury.

Modulation of Preactivation of PPAR-β on Memory and Learning Dysfunction and Inflammatory Response in the Hippocampus in Rats Exposed to Global Cerebral Ischemia/Reperfusion

The aim of this study is to investigate the neuroprotective effects and relevant mechanism of GW0742, an agonist of PPAR-β, after global cerebral ischemia-reperfusion injury (GCIRI) in rats. The rats showed memory and cognitive impairment and cytomorphological change in the hippocampus neurons following GCIRI. These effects were significantly improved by pretreatment with GW0742 in the dose-dependent manner. The expressions of IL-1β, IL-6, and TNF-α were increased after GCIRI, while the increases in these proinflammatory cytokines by GCIRI were inhibited by GW0742 pretreatment. Similarly, GW0742 pretreatment also improved the GCIRI-induced decrease in the expression of IL-10, which can act as an inhibitory cytokine to reduce cerebral ischemic injury. For another, NF-κB p65 expression was significantly increased in hippocampal neurons with apparent nuclear translocation after global cerebral IRI, and these phenomena were also largely attenuated by GW0742 pretreatment. Moreover, the mRNA and protein expressions of PPAR-β were significantly decreased in GCIRI + GW0742 groups when compared with those in GCIRI group. Our data suggests that the PPAR-β agonist GW0742 can exert significant neuroprotective effect against GCIRI in rats via PPAR-β activation and its anti-inflammation effect mediated by the inhibition of expression and activation of NF-κB in the hippocampus.

Orally administered oleoylethanolamide protects mice from focal cerebral ischemic injury by activating peroxisome proliferator-activated receptor α

Oleoylethanolamide (OEA) is a high-affinity agonist of peroxisome proliferator-activated receptor α (PPARα) which may act as an endogenous neuroprotective factor. However, it is not clear whether orally administered OEA is effective against ischemic brain injury. In our study, transient focal cerebral ischemia was induced by middle cerebral artery occlusion for 90 min followed by reperfusion. To evaluate its preventive effects, OEA (10, 20 or 40 mg/kg, ig) was administered for 3 days before ischemia. To evaluate its therapeutic effects, OEA (40 mg/kg, ig) was administered at 0.5 or 1h before reperfusion or at 0 or 1h after reperfusion. In some experiments, the PPARα antagonist MK886 (10mg/kg, ig) was administered 0.5h before OEA. Neurological deficit score, infarct volume and brain edema degree were determined at 24h after reperfusion. Blood-brain barrier (BBB) disruption was evaluated by Evans blue (EB) leakage at 6h after reperfusion. Real-time RT-PCR and western blot were performed to detect PPARα mRNA and protein expression. Oral OEA pretreatment improved neurological dysfunction reduced infarct volume and alleviated brain edema in a dose-dependent manner; the most effective dose was 40 mg/kg. The therapeutic time is within 1h after reperfusion. OEA also increased PPARα mRNA and protein expression in the ischemic brain. The PPARα antagonist MK886 abolished the protective effects of OEA. In conclusion, our results indicate that orally administered OEA protects against acute cerebral ischemic injury in mice, at least in part by activating PPARα.

Peroxisome proliferator-activated receptor gamma (PPAR-γ) and neurodegenerative disorders

As the growth of the aging population continues to accelerate globally, increased prevalence of neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and stroke, has generated substantial public concern. Unfortunately, despite of discoveries of common factors underlying these diseases, few drugs are available to effectively treat these diseases. Peroxisome proliferator-activated receptor gamma (PPAR-γ) is a ligand-activated transcriptional factor that belongs to the nuclear hormone receptor superfamily. PPAR-γ has been shown to influence the expression or activity of a large number of genes in a variety of signaling networks, including regulation of insulin sensitivity, glucose homeostasis, fatty acid oxidation, immune responses, redox balance, cardiovascular integrity, and cell fates. Recent epidemiological, preclinical animal, and clinical studies also show that PPAR-γ agonists can lower the incidence of a number of neurological disorders, despite of multiple etiological factors involved in the development of these disorders. In this manuscript, we review current knowledge on mechanisms underlying the beneficial effect of PPAR-γ in different neurodegenerative diseases, in particular, AD, PD, and stroke, and attempt to analyze common and overlapping features among these diseases. Our investigation unveiled information suggesting the ability for PPAR-γ to inhibit NF-κB-mediated inflammatory signaling at multiple sites, and conclude that PPAR-γ agonists represent a novel class of drugs for treating neuroinflammatory diseases.

Neuroprotective Mechanisms of PPARδ: Modulation of Oxidative Stress and Inflammatory Processes

Peroxisome proliferator-activated receptors (PPARα, δ, and γ) are ligand-activated transcription factors that regulate a wide range of cellular processes, including inflammation, proliferation, differentiation, metabolism, and energy homeostasis. All three PPAR subtypes have been identified in the central nervous system (CNS) of rodents. While PPARα and PPARγ are expressed in more restricted areas of the CNS, PPARδ is ubiquitously expressed and is the predominant subtype. Although data regarding PPARδ are limited, studies have demonstrated that administration of PPARδ agonists confers neuroprotection following various acute and chronic injuries to the CNS, such as stroke, multiple sclerosis, and Alzheimer's disease. The antioxidant and anti-inflammatory properties of PPARδ agonists are thought to underly their neuroprotective efficacy. This review will focus on the putative neuroprotective benefits of therapeutically targeting PPARδ in the CNS, and specifically, highlight the antioxidant and anti-inflammatory functions of PPARδ agonists.

Peroxisome proliferator-activated receptor and age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of new blindness in the western world and is becoming more of a socio-medical problem as the proportion of the aged population increases. There are multiple efforts underway to better understand this disease process. AMD involves the abnormal retinal pigment epithelium (RPE), drusen formation, photoreceptor atrophy, and choroidal neovascularization. Peroxisome proliferator-activated receptors (PPARs) play an important role in lipid degeneration, immune regulation, regulation of reactive oxygen species (ROSs), as well as regulation of vascular endothelial growth factor (VEGF), matrix metalloproteinase-9 (MMP-9), and docosahexaenoic acid (DHA). These molecules have all been implicated in the pathogenesis of AMD. In addition, PPAR gamma is expressed in RPE, an essential cell in photoreceptor regeneration and vision maintenance. This review summarizes the interactions between PPAR, AMD-related molecules, and AMD-related disease processes.

PPAR Regulation of Inflammatory Signaling in CNS Diseases

Central nervous system (CNS) is an immune privileged site, nevertheless inflammation associates with many CNS diseases. Peroxisome proliferator-activated receptors (PPARs) are a family of nuclear hormone receptors that regulate immune and inflammatory responses. Specific ligands for PPARα, γ, and δ isoforms have proven effective in the animal models of multiple sclerosis (MS), Alzheimer's disease, Parkinson's disease, and trauma/stroke, suggesting their use in the treatment of neuroinflammatory diseases. The activation of NF-κB and Jak-Stat signaling pathways and secretion of inflammatory cytokines are critical in the pathogenesis of CNS diseases. Interestingly, PPAR agonists mitigate CNS disease by modulating inflammatory signaling network in immune cells. In this manuscript, we review the current knowledge on how PPARs regulate neuroinflammatory signaling networks in CNS diseases.

In vivo MRI assessment of permanent middle cerebral artery occlusion by electrocoagulation: pitfalls of procedure

Permanent middle cerebral artery (MCA) occlusion (pMCAO) by electrocoagulation is a commonly used model but with potential traumatic lesions. Early MRI monitoring may assess pMCAO for non-specific brain damage. The surgical steps of pMCAO were evaluated for traumatic cerebral injury in 22 Swiss mice using diffusion and T2-weighted MRI (7T) performed within 1 h and 24 h after surgery. Temporal muscle cauterization without MCA occlusion produced an early T2 hyperintensity mimicking an infarct. No lesion was visible after temporal muscle incision or craniotomy. Early MRI monitoring is useful to identify non-specific brain injury that could hamper neuroprotective drugs assessment.

Fenofibrate improves cerebral blood flow after middle cerebral artery occlusion in mice

Fibrates, one group of peroxisome proliferator-activated receptor (PPAR) activators, are lipid lowering drugs. Fibrates have been shown to attenuate brain tissue injury after focal cerebral ischemia. In this study, we investigated the impact of fenofibrate on cerebral blood flow (CBF) in male wild type and PPARalpha-null mice. Animals were treated for 7 days with fenofibrate and subjected to 2 h of filamentous middle cerebral artery occlusion and reperfusion under isoflurane anesthesia. Cortical surface CBF was measured by laser speckle imaging. Regional CBF (rCBF) in nonischemic animals was measured by (14)C-iodoantipyrine autoradiography. Fenofibrate did not affect rCBF and mean arterial blood pressure in nonischemic animals. In ischemic animals, laser speckle imaging showed delayed expansions of ischemic area, which was attenuated by fenofibrate. Fenofibrate also enhanced CBF recovery after reperfusion. However, such effects of fenofibrate on CBF in the ischemic brain were not observed in PPARalpha-null mice. These findings show that fenofibrate improves CBF in the ischemic hemisphere. Moreover, fenofibrate requires PPARalpha expression for the cerebrovascular protective effects in the ischemic brain.

Neuroprotective role of mitochondrial uncoupling protein 2 in cerebral stroke

The uncoupling proteins (UCPs) are mitochondrial transporter proteins involved in proton conductance across inner mitochondrial membrane (IMM). UCP2, which is one of the members of this class of proteins, has a wide but restricted tissue distribution including brain. Its physiologic role according to emerging evidences, although still not clear, indicate that distribution of UCP2 may be related to regulation of mitochondria membrane potential (DeltaPsim), production of reactive oxygen species (ROS), preservation of calcium homeostasis, modulation of neuronal activity, and eventually inhibition of cellular damage. These factors are very important in determining the fate of neurons and damage progression in the brain during various neurodegenerative diseases including cerebral stroke. Recent evidence indicates that an increased expression and activity of UCP2 are well correlated with neuronal survival after stroke and trauma. This review briefly covers the present understanding of UCP2, which eventually may be beneficial to understand the precise role of UCP2 to develop strategy to identify its potential therapeutic application.

PPARs as new therapeutic targets for the treatment of cerebral ischemia/reperfusion injury

Stroke is a leading cause of death and long-term disability in industrialized countries. Despite advances in understanding its pathophysiology, little progress has been made in the treatment of stroke. The currently available therapies have proven to be highly unsatisfactory (except thrombolysis) and attempts are being made to identify and characterize signaling proteins which could be exploited to design novel therapeutic modalities. The peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that control lipid and glucose metabolism. PPARs regulate gene expression by binding with the retinoid X receptor (RXR) as a heterodimeric partner to specific DNA sequences, termed PPAR response elements. In addition, PPARs may modulate gene transcription also by directly interfering with other transcription factor pathways in a DNA-binding independent manner. To date, three different PPAR isoforms, designated alpha, beta/delta, and gamma, have been identified. Recently, they have been found to play an important role for the pathogenesis of various disorders of the central nervous system and accumulating data suggest that PPARs may serve as potential targets for treating ischemic stroke. Activation of all PPAR isoforms, but especially of PPARgamma, was shown to prevent post-ischemic inflammation and neuronal damage in several in vitro and in vivo models, negatively regulating the expression of genes induced by ischemia/ reperfusion (I/R). This paper reviews the evidence and recent developments relating to the potential therapeutic effects of PPAR-agonists in the treatment of cerebral I/R injury.

Peroxisome Proliferator-Activated Receptor beta/delta in the Brain: Facts and Hypothesis

peroxisome proliferator-activated receptors (PPARs) are nuclear receptors acting as lipid sensors. Besides its metabolic activity in peripheral organs, the PPAR beta/delta isotype is highly expressed in the brain and its deletion in mice induces a brain developmental defect. Nevertheless, exploration of PPARβ action in the central nervous system remains sketchy. The lipid content alteration observed in PPARβ null brains and the positive action of PPARβ agonists on oligodendrocyte differentiation, a process characterized by lipid accumulation, suggest that PPARβ acts on the fatty acids and/or cholesterol metabolisms in the brain. PPARβ could also regulate central inflammation and antioxidant mechanisms in the damaged brain. Even if not fully understood, the neuroprotective effect of PPARβ agonists highlights their potential benefit to treat various acute or chronic neurological disorders. In this perspective, we need to better understand the basic function of PPARβ in the brain. This review proposes different leads for future researches.