The class B scavenger receptor CD36 mediates free radical production and tissue injury in cerebral ischemia

CD36 in the periphery and brain synergizes in stroke injury in hyperlipidemia

OBJECTIVE

Hyperlipidemia exacerbates ischemic stroke outcome and increases CD36 expression in the postischemic brain as well as in peripheral monocytes/macrophages. By exchanging bone marrow-derived cells between CD36-expressing and CD36-deficient mice, this study investigates the contribution of peripheral CD36 in comparison with that of brain CD36 to stroke pathology in hyperlipidemia.

METHODS

Following bone marrow transplantation, mice were fed a high-fat diet for 11 weeks and then subjected to ischemic stroke. Stroke outcome, expression of brain CD36, monocyte chemoattractant protein-1 (MCP-1), CCR2, and plasma MCP-1 levels were determined at 3 days postischemia. CD36 and CCR2 expression were also determined in splenocytes incubated with serum obtained from CD36-expressing or CD36-deficient mice.

RESULTS

Infiltrating immune cells from the periphery are the major source of CD36 in the postischemic brain and contribute to stroke-induced brain injury. This CD36 effect was dependent on the modulation of MCP-1 and CCR2 expression in peripheral immune cells as well as CD36-expressing cells in the host brain.

INTERPRETATION

This study demonstrates that CD36 expressed in the periphery and brain synergize in ischemic brain injury through regulation of the MCP-1/CCR2 chemokine axis in hyperlipidemic conditions.

CD36 is involved in astrocyte activation and astroglial scar formation

Inflammation is an essential component for glial scar formation. However, the upstream mediator(s) that triggers the process has not been identified. Previously, we showed that the expression of CD36, an inflammatory mediator, occurs in a subset of astcotyes in the peri-infarct area where the glial scar forms. This study investigates a role for CD36 in astrocyte activation and glial scar formation in stroke. We observed that the expression of CD36 and glial fibrillary acidic protein (GFAP) coincided in control and injured astrocytes and in the brain. Furthermore, GFAP expression was attenuated in CD36 small interfering RNA transfected astrocytes or in the brain of CD36 knockout (KO) mice, suggesting its involvement in GFAP expression. Using an in-vitro model of wound healing, we found that CD36 deficiency attenuated the proliferation of astrocytes and delayed closure of the wound gap. Furthermore, stroke-induced GFAP expression and scar formation were significantly attenuated in the CD36 KO mice compared with wild type. These findings identify CD36 as a novel mediator for injury-induced astrogliosis and scar formation. Targeting CD36 may serve as a potential strategy to reduce glial scar formation in stroke.

Transglutaminase inhibition protects against oxidative stress-induced neuronal death downstream of pathological ERK activation

Molecular deletion of transglutaminase 2 (TG2) has been shown to improve function and survival in a host of neurological conditions including stroke, Huntington's disease, and Parkinson's disease. However, unifying schemes by which these cross-linking or polyaminating enzymes participate broadly in neuronal death have yet to be presented. Unexpectedly, we found that in addition to TG2, TG1 gene expression level is significantly induced following stroke in vivo or due to oxidative stress in vitro. Forced expression of TG1 or TG2 proteins is sufficient to induce neuronal death in Rattus norvegicus cortical neurons in vitro. Accordingly, molecular deletion of TG2 alone is insufficient to protect Mus musculus neurons from oxidative death. By contrast, structurally diverse inhibitors used at concentrations that inhibit TG1 and TG2 simultaneously are neuroprotective. These small molecules inhibit increases in neuronal transamidating activity induced by oxidative stress; they also protect neurons downstream of pathological ERK activation when added well after the onset of the death stimulus. Together, these studies suggest that multiple TG isoforms, not only TG2, participate in oxidative stress-induced cell death signaling; and that isoform nonselective inhibitors of TG will be most efficacious in combating oxidative death in neurological disorders.

Reperfusion rather than ischemia drives the formation of ubiquitin aggregates after middle cerebral artery occlusion

BACKGROUND AND PURPOSE

Cerebral ischemia leads to accumulation of ubiquitinated protein aggregates. However, the factors triggering ubiquitination and their impact on the outcome of cerebral ischemia remain poorly understood. Here we investigate the relationship between ubiquitin aggregation and duration of ischemia/reperfusion, infarct volume, and proteasomal activity in a mouse model of focal ischemia.

METHODS

Free ubiquitin and ubiquitin aggregate levels were examined by Western blotting in the mouse neocortex and striatum after different periods of ischemia/reperfusion and permanent ischemia induced by middle cerebral artery occlusion. Infarct volumes were measured in thionin-stained brain sections. Proteasome activity was studied by fluorometric peptidase activity assay.

RESULTS

Following transient ischemia, ubiquitin aggregates were detected in the ipsilateral neocortex and, to a lesser extent, striatum only after induction of reperfusion. In permanent ischemia, no ubiquitin aggregates were found. Shorter ischemic periods producing no or minimal tissue damage (10-15 minutes) resulted in ubiquitin aggregate levels similar to those produced by ischemia resulting in substantial infarction (30 minutes). Proteasomal impairment was greatest in ischemia without reperfusion, in which no ubiquitin aggregates were detected.

CONCLUSIONS

The data demonstrate that reperfusion rather than ischemia leads to the appearance of ubiquitinated aggregates, which form in the absence of major tissue damage and are not correlated with decreased proteasomal peptidase activity. Ubiquitin aggregates may form in potentially viable brain tissue, which may be later recruited into infarction by factors independent of ubiquitination.

Salvianolic acid B inhibits macrophage uptake of modified low density lipoprotein (mLDL) in a scavenger receptor CD36-dependent manner

CD36, a class B scavenger receptor, has been implicated in the pathogenesis of a host of vascular inflammatory diseases. Through a high-throughput screening (HTS) assay for CD36 antagonist, we previously identified salvianolic acid B (SAB), a hydrophilic component derived from the herb Danshen, as a potential candidate. Danshen, the dried roots of Salvia miltiorrhiza, has been widely used in China for the prevention and treatment of atherosclerosis-related disorders. Previous studies showed that SAB acted as an anti-oxidant by preventing lipid peroxidation and oxidized LDL (oxLDL) formation. The present study was to investigate the specificity and efficacy of SAB in the inhibition of CD36-mediated lipid uptake. SAB reduced modified LDL (mLDL) uptake in a dose-dependent manner in phorbol-12-myristate-13-acetate (PMA)-stimulated THP-1 and RAW 264.7 cells. In the CD36 silenced THP-1 cells, SAB had no effect in reducing mLDL uptake, whereas its overexpression in CHO cells reinstates the effect, indicating a specific involvement of SAB in antagonizing the CD36's function. Surface plasmon resonance (SPR) analysis revealed a direct binding of SAB to CD36 with a high affinity (K(D) = 3.74 μM), confirming physical interactions of SAB with the receptor. Additionally, SAB reduced oxLDL-induced CD36 gene expression in the cultured cell lines and primary macrophages. In ApoE KO mice fed a high fat diet, SAB reduced CD36 gene expression and lipid uptake in macrophages, showing its ability to antagonize CD36 pathways in vivo. These results demonstrate that SAB is an effective CD36 antagonist and suggest SAB as a potential anti-atherosclerotic agent.

Microglial physiopathology: how to explain the dual role of microglia after acute neural disorders?

Microglia are the resident macrophages of the central nervous system (CNS). In physiological conditions, resting microglia maintain tissue integrity by scanning the entire CNS parenchyma through stochastic and complex movements of their long processes to identify minor tissue alterations. In pathological conditions, over-activated microglia contribute to neuronal damage by releasing harmful substances, including inflammatory cytokines, reactive oxygen species, and proteinases, but they can provide tissue repair by releasing anti-inflammatory cytokines and neurotrophic factors. The reasons for this apparent paradox are unknown. In this paper, we first review the physiological role as well as both detrimental and beneficial actions of microglial during acute CNS disorders. Further, we discuss the possible reasons for this microglial dual role following CNS insults, considering that the final microglial phenotype is a direct consequence of both noxious and beneficial stimuli released into the extracellular space during the pathological insult. The nature of these micro-glial ligands is unknown, but we hypothesize that harmful and beneficial stimuli may be preferentially located at specific anatomical niches along the pathological environment triggering both beneficial and deleterious actions of these glial cells. According to this notion, there are no natural populations of detrimental microglia, but is the pathological environment that determines the final microglial phenotype.

Multiple sclerosis is not a disease of the immune system

Multiple sclerosis is a complex neurodegenerative disease, thought to arise through autoimmunity against antigens of the central nervous system. The autoimmunity hypothesis fails to explain why genetic and environmental risk factors linked to the disease in one population tend to be unimportant in other populations. Despite great advances in documenting the cell and molecular mechanisms underlying MS pathophysiology, the autoimmunity framework has also been unable to develop a comprehensive explanation of the etiology of the disease. I propose a new framework for understanding MS as a dysfunction of the metabolism of lipids. Specifically, the homeostasis of lipid metabolism collapses during acute-phase inflammatory response triggered by a pathogen, trauma, or stress, starting a feedback loop of increased oxidative stress, inflammatory response, and proliferation of cytoxic foam cells that cross the blood brain barrier and both catabolize myelin and prevent remyelination. Understanding MS as a chronic metabolic disorder illuminates four aspects of disease onset and progression: 1) its pathophysiology; 2) genetic susceptibility; 3) environmental and pathogen triggers; and 4) the skewed sex ratio of patients. It also suggests new avenues for treatment.

Negative regulation of glial engulfment activity by Draper terminates glial responses to axon injury

Neuronal injury elicits potent cellular responses from glia, but molecular pathways modulating glial activation, phagocytic function, and termination of reactive responses remain poorly defined. Here we show that positive or negative regulation of glial reponses to axon injury are molecularly encoded by unique isoforms of the Drosophila engulfment receptor Draper. Draper-I promotes engulfment of axonal debris through an immunoreceptor tyrosine-based activation motif (ITAM). In contrast, Draper-II, an alternative splice variant, potently inhibits glial engulfment function. Draper-II suppresses Draper-I signaling through a novel immunoreceptor tyrosine-based inhibitory motif (ITIM)-like domain and the tyrosine phosphatase Corkscrew (Csw). Intriguingly, loss of Draper-II/Csw signaling prolongs expression of glial engulfment genes after axotomy and reduces the ability of glia to respond to secondary axotomy. Our work highlights a novel role for Draper-II in inhibiting glial responses to neurodegeneration, and indicates a balance of opposing Draper-I/-II signaling events is essential to maintain glial sensitivity to brain injury.

CD36 participates in PrP(106-126)-induced activation of microglia

Microglial activation is a characteristic feature of the pathogenesis of prion diseases. The molecular mechanisms that underlie prion-induced microglial activation are not very well understood. In the present study, we investigated the role of the class B scavenger receptor CD36 in microglial activation induced by neurotoxic prion protein (PrP) fragment 106-126 (PrP(106-126)). We first examined the time course of CD36 mRNA expression upon exposure to PrP(106-126) in BV2 microglia. We then analyzed different parameters of microglial activation in PrP(106-126)-treated cells in the presence or not of anti-CD36 monoclonal antibody (mAb). The cells were first incubated for 1 h with CD36 monoclonal antibody to block the CD36 receptor, and were then treated with neurotoxic prion peptides PrP(106-126). The results showed that PrP(106-126) treatment led to a rapid yet transitory increase in the mRNA expression of CD36, upregulated mRNA and protein levels of proinflammatory cytokines (IL-1β, IL-6 and TNF-α), increased iNOS expression and nitric oxide (NO) production, stimulated the activation of NF-κB and caspase-1, and elevated Fyn activity. The blockade of CD36 had no effect on PrP(106-126)-stimulated NF-κB activation and TNF-α protein release, abrogated the PrP(106-126)-induced iNOS stimulation, downregulated IL-1β and IL-6 expression at both mRNA and protein levels as well as TNF-α mRNA expression, decreased NO production and Fyn phosphorylation, reduced caspase-1 cleavage induced by moderate PrP(106-126)-treatment, but had no effect on caspase-1 activation after treatment with a high concentration of PrP(106-126). Together, these results suggest that CD36 is involved in PrP(106-126)-induced microglial activation and that the participation of CD36 in the interaction between PrP(106-126) and microglia may be mediated by Src tyrosine kinases. Our findings provide new insights into the mechanisms underlying the activation of microglia by neurotoxic prion peptides and open perspectives for new therapeutic strategies for prion diseases by modulation of CD36 signaling.

Microglial cells contribute to endogenous brain defenses after acute neonatal focal stroke

Macrophages are viewed as amplifiers of ischemic brain injury, but the origin of injury-producing macrophages is poorly defined. The role of resident brain macrophages-microglial cells-in stroke remains controversial. To determine whether microglial cells exert injurious effects after neonatal focal stroke, we selectively depleted these cells with intracerebral injection of liposome-encapsulated clodronate before transient middle cerebral artery occlusion in postnatal day 7 rats. Phagocytosis of apoptotic neurons by activated microglia was poor in animals with unmanipulated microglia, and depletion of these cells did not increase the number of apoptotic neurons. Lack of microglia increased the brain levels of several cytokines and chemokines already elevated by ischemia-reperfusion, and also increased the severity and volume of injury, suggesting that microglial cells contribute to endogenous protection during the subacute injury phase. Then, to determine whether accumulation of reactive oxygen species in microglia adversely affects phagocytosis of dying neurons and contributes to injury, we delivered reduced glutathione (GSH) into microglia, again using liposomes. Remarkably, pharmacologically increased intracellular GSH concentrations in microglia induced superoxide accumulation in lipid rafts in these cells, further increased the brain levels of macrophage chemoattractants, and exacerbated injury. Together, these data show that microglia are part of the endogenous defense mechanisms and that, while antioxidants can protect the injured neonatal brain, high levels of reducing equivalents in activated microglia, GSH, trigger superoxide production, favor the reorganization of lipids, amplify local inflammation and exacerbate injury.

Toll-like receptors: new players in myocardial ischemia/reperfusion injury

Innate immune and inflammatory responses have been implicated in myocardial ischemia/reperfusion (I/R) injury. However, the mechanisms by which innate immunity and inflammatory response are involved in myocardial I/R have not been elucidated completely. Recent studies highlight the role of Toll-like receptors (TLRs) in the induction of innate immune and inflammatory responses. Growing evidence has demonstrated that TLRs play a critical role in myocardial I/R injury. Specifically, deficiency of TLR4 protects the myocardium from ischemic injury, whereas modulation of TLR2 induces cardioprotection against ischemic insult. Importantly, cardioprotection induced by modulation of TLRs involves activation of the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway, suggesting that there is a crosstalk between TLRs and PI3K/Akt signaling pathways. In addition, TLRs also associate with other coreceptors, such as macrophage scavenger receptors in the recognition of their ligands. TLRs are also involved in the induction of angiogenesis, modulation of stem cell function, and expression of microRNA, which are currently important topic areas in myocardial I/R. Understanding how TLRs contribute to myocardial I/R injury could provide basic scientific knowledge for the development of new therapeutic approaches for the treatment and management of patients with heart attack.

Microglial cells contribute to endogenous brain defenses after acute neonatal focal stroke

Macrophages are viewed as amplifiers of ischemic brain injury, but the origin of injury-producing macrophages is poorly defined. The role of resident brain macrophages-microglial cells-in stroke remains controversial. To determine whether microglial cells exert injurious effects after neonatal focal stroke, we selectively depleted these cells with intracerebral injection of liposome-encapsulated clodronate before transient middle cerebral artery occlusion in postnatal day 7 rats. Phagocytosis of apoptotic neurons by activated microglia was poor in animals with unmanipulated microglia, and depletion of these cells did not increase the number of apoptotic neurons. Lack of microglia increased the brain levels of several cytokines and chemokines already elevated by ischemia-reperfusion, and also increased the severity and volume of injury, suggesting that microglial cells contribute to endogenous protection during the subacute injury phase. Then, to determine whether accumulation of reactive oxygen species in microglia adversely affects phagocytosis of dying neurons and contributes to injury, we delivered reduced glutathione (GSH) into microglia, again using liposomes. Remarkably, pharmacologically increased intracellular GSH concentrations in microglia induced superoxide accumulation in lipid rafts in these cells, further increased the brain levels of macrophage chemoattractants, and exacerbated injury. Together, these data show that microglia are part of the endogenous defense mechanisms and that, while antioxidants can protect the injured neonatal brain, high levels of reducing equivalents in activated microglia, GSH, trigger superoxide production, favor the reorganization of lipids, amplify local inflammation and exacerbate injury.

Phosphatidylinositol-3-kinase gamma plays a central role in blood-brain barrier dysfunction in acute experimental stroke

BACKGROUND AND PURPOSE

Phosphoinositide 3-kinase (PI3K)-γ is linked to inflammation and oxidative stress. This study was conducted to investigate the role of the PI3Kγ in the blood-brain barrier dysfunction and brain damage induced by focal cerebral ischemia/reperfusion.

METHODS

Wild-type and PI3Kγ knockout mice were subjected to middle cerebral artery occlusion (60 minutes) followed by reperfusion. Evans blue leakage, brain edema, infarct volumes, and neurological deficits were examined. Oxidative stress, neutrophil infiltration, and matrix metallopeptidase-9 were assessed. Activation of nuclear factor-κB and expression of proinflammatory and pro-oxidative genes were studied.

RESULTS

PI3Kγ deficiency significantly reduced blood-brain barrier permeability and brain edema formation, which were time-dependently correlated with preventing the degradation of the tight junction protein, claudin-5, and the basal lamina protein, collagen IV, and the phosphorylation of myosin light chain in brain microvessels. PI3Kγ deficiency suppressed ischemia/reperfusion-induced nuclear factor-κB p65 (Ser536) phosphorylation and the expression of the pro-oxidant enzyme NADPH oxidase (Nox1, Nox2, and Nox4) and proinflammatory adhesion molecules (E- and P-selectin, intercellular adhesion molecule-1) at different time points. These molecular changes were associated with significant inhibition of oxidative stress (superoxide production and malondialdehyde content), neutrophil infiltration, and matrix metallopeptidase-9 expression/activity in PI3Kγ knockout mice. Eventually, PI3Kγ deficiency significantly reduced infarct volumes and neurological scores at 24 hours after ischemia/reperfusion.

CONCLUSIONS

Our results provide the first direct demonstration that PI3Kγ plays a significant role in ischemia/reperfusion-induced blood-brain barrier disruption and brain damage. Future studies need to explore PI3Kγ as a potential target for stroke therapy.

Pathogenic role of scavenger receptor CD36 in the metabolic syndrome and diabetes

Obesity is increasing at epidemic proportions in the United States and is a major contributor to the development of both metabolic syndrome (glucose intolerance, dyslipidemia, hypertension) and atherosclerotic cardiovascular disease. A wide body of evidence has linked systemic low-grade inflammation as underlying obesity and insulin-resistant states via monocyte/macrophage activation. Transgenic deletion of scavenger receptor type B CD36 in rodents has suggested a pivotal role for CD36 in mediating inflammation, insulin resistance, and atherogenesis through transport of fatty acids and uptake of oxidized lipids, respectively. CD36 signaling pathways involving c-Jun N-terminal kinase (JNK) activation and Toll-like receptors have been implicated in the induction of insulin resistance. This review will focus on the pathogenic role of CD36 receptors in metabolic syndrome and type 2 diabetes.

Genetic variant of BDNF (Val66Met) polymorphism attenuates stroke-induced angiogenic responses by enhancing anti-angiogenic mediator CD36 expression

Brain-derived neurotrophic factor (BDNF) has been shown to be necessary and sufficient for post-stroke recovery in rodents. From these observations, we and others have hypothesized that a common single nucleotide polymorphism (SNP) in the pro-domain of bdnf that leads to a methionine (Met) substitution for valine (Val) at codon 66 (Val66Met) will affect stroke outcome. Here we investigate the effect of the BDNF genetic variant on ischemic outcome by using mice with a genetic knock-in of the human BDNF variant in both alleles (BDNF(Met/Met)). Compared with wild-type mice, BDNF(Met/Met) mice exhibited reduced CNS BDNF levels without a discernable effect on infarct size. Diminished BDNF levels in BDNF(Met/Met) mice were associated with greater deficits in post-stroke locomotor functions. Additionally, the BDNF(Met/Met) mice showed reduced angiogenesis and elevated expression of thrombospondin-1 (TSP-1) and its receptor CD36, anti-angiogenic factors. To assess the functional role of CD36 in antagonizing angiogenic response in Met homozygosity at the BDNF locus, we crossed BDNF(Met/Met) mice with CD36 knock-out mice. The double-mutant mice rescued the angiogenic deficit associated with the BDNF(Met/Met) mice without alterations in BDNF levels, indicating that the behavioral deficit in BDNF(Met/Met) mice after stroke is partly related to an unfavorable balance in pro-angiogenic BDNF and anti-angiogenic TSP-1/CD36. The results suggest that CD36 inhibition may be a viable strategy to enhance angiogenesis and possible recovery in human stroke victims who are Met homozygotes at codon 66 of the BDNF locus.

The scavenger receptor CD36 contributes to the neurotoxicity of bone marrow-derived monocytes through peroxynitrite production

CD36, a class B scavenger receptor present in microglia, endothelium and leukocytes, plays a key role in ischemic brain injury by promoting the expression of inflammatory genes and production of reactive oxygen species (ROS). However, it is not known whether ischemic brain damage is mediated by CD36 activation in resident brain cells, i.e., microglia, or by blood-borne cells that infiltrate the brain. To address this question, we studied oxygen-glucose deprivation (OGD) in hippocampal slice cultures, a model of ischemic injury that does not involve cells extrinsic to the brain. We found that CD36 gene knockout does not afford protection of hippocampal slices to OGD-induced cytotoxicity. In contrast, immunoactivated bone marrow-derived monocytes-macrophages (BMM) from wild type (WT) mice trigger hippocampal damage when incubated with brain slices via a mechanism that is prevented in CD36-/- BMM. The neurotoxic activity of CD36+/+ BMM was attributed to reactive oxygen species (ROS) since it was concomitant with increased ROS production and could be prevented by treatment with a selective ROS scavenger, MnTBAP, or a peroxynitrite decomposition catalyst, FeTPPS. Importantly, ROS production and accumulation 3-nitrotyrosine in hippocampal proteins (a hallmark of peroxynitrite production) was significantly dampened in immunoactivated CD36-/- BMM, whereas production of NO-derived metabolites (nitrite and nitrate) was unaltered. We conclude that CD36 signaling may not contribute to injury induced by OGD in the brain itself but is involved in the neurotoxicity mediated by activated BMM. These findings are consistent with the hypothesis that CD36 in infiltrating inflammatory cells drives peroxynitrite-mediated ischemic brain damage. Accordingly, targeting CD36 in the vascular compartment may protect against neurotoxicity in the ischemic brain.

Modeling focal cerebral ischemia in vivo

Ischemic stroke is among the leading causes of mortality and long-term disability in the western world. Despite enormous research activities in the last decades, current therapeutic options for acute stroke patients are still very limited. Reliable and realistic in vivo animal models represent sine qua non for -successful translation from bench to bedside. To date, several animal models of focal and global cerebral ischemia have been developed to mimic the clinical situation in humans as accurately as possible. This chapter focuses on models of focal cerebral ischemia, in particular on the most commonly used model: the intraluminal filament model of middle cerebral artery occlusion. The main objective is to provide a detailed instruction manual for researchers interested in learning this technique.

A role for spleen monocytes in post-ischemic brain inflammation and injury

Although infiltration of peripheral monocytes/macrophages is implicated in stroke pathology, in vivo data regarding the deployment of monocytes and their mobilization to the infarct area is scarce. Recent literature showed that mouse monocytes exhibit two distinct populations that represent pro-inflammatory (Ly-6Chi/CCR2+) and anti-inflammatory (Ly-6Clow/CCR2-) subsets and that spleen is a major source for monocyte deployment upon injury. By reducing post-ischemic infection with antibacterial moxifloxacin (MFX) treatment, the present study investigates the effect of the treatment on Ly-6C and CCR2 expression in the spleen following ischemia and the extent to which the effect is associated with attenuation of post-ischemic inflammation and injury. Mice subjected to a middle cerebral artery occlusion (MCAO) showed a significant reduction in their spleen weights compared to sham animals. Compared to vehicle controls, splenocytes obtained from daily MFX-treated mice 7 days after ischemia exhibited significantly reduced mean Ly-6C expression within pro-inflammatory subsets, whereas the distribution of pro- and anti-inflammatory subsets was not different between the treatment groups. Additionally, MFX treatment significantly reduced CCR2 expression in the spleen tissue and in the post-ischemic brain and attenuated infarct size. The study suggests a potential contributing role of spleen monocytes in post-ischemic inflammation and injury. The influence of peripheral inflammatory status on the primary injury in the CNS further implies that the attenuation of post-stroke infection may be beneficial in mitigating stroke-induced brain injury.

Scavenger receptor class-A has a central role in cerebral ischemia-reperfusion injury

The innate immune response is involved in the pathophysiology of cerebral ischemia-reperfusion (I/R) injury. Recent evidence suggests that scavenger receptors have a role in the induction of innate immunity. In this study, we examined the role of scavenger receptor A (SR-A) in focal cerebral I/R injury. Both SR-A(-/-) mice (n=10) and age-matched wild-type (WT) mice (n=9) were subjected to focal cerebral ischemia (60 minutes), followed by reperfusion (for 24 hours). Infarct size was determined by TTC (triphenyltetrazolium chloride) staining. The morphology of neurons in the brain sections was examined by Nissl's staining. Activation of intracellular signaling was analyzed by western blot. Cerebral infarct size in SR-A(-/-) mice was significantly reduced by 63.9% compared with WT mice after cerebral I/R. In SR-A(-/-) mice, there was less neuronal damage in the hippocampus compared with WT mice. Levels of FasL, Fas, FADD, caspase-3 activity, and terminal deoynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate-biotin nick end labeling-positive apoptotic cells were significantly increased in WT mice after cerebral I/R, but not in SR-A(-/-) mice. Cerebral I/R increased nuclear factor-κB activation in WT mice, but not in SR-A(-/-) mice. These data suggest that SR-A has a central role in cerebral I/R injury and that suppression of SR-A may be a useful approach for ameliorating brain injury in stroke patients.

NF-κB and innate immunity in ischemic stroke

Acute cerebral ischemia elicits an innate immune response that leads to a cascade of events that culminates in necrotic death of neurons and injury to their supportive structures in the neurovascular unit. Indeed, clinical studies have shown a close relationship between elevated levels of inflammatory markers and the risk for ischemic stroke. However, the signaling pathways that link these events are not well understood. A central regulator of inflammatory response is the transcription factor, nuclear factor-kappa B (NF-κB). The activation of NF-κB is required for the transcriptional induction of many proinflammatory mediators involved in innate immunity, such as cellular adhesion molecules, cytokines, and growth factors. Therefore, factors that modulate the activity of NF-κB could potentially regulate inflammatory processes in ischemic stroke. Here, we review the relationship between NF-κB and ischemic stroke, its role in the neurovascular unit, and discuss some animal models that suggest that this relationship is causal.

CD36 participates in a signaling pathway that regulates ROS formation in murine VSMCs

CD36 is a membrane glycoprotein expressed on platelets, monocytes, macrophages, and several other cell types that was recently demonstrated to be involved in platelet activation in response to oxidized phospholipids, including oxidized LDL. Although the role of CD36 in other vascular cells has not been well defined, previous studies have demonstrated that cd36-knockout (cd36-/-) mice have prolonged thrombosis times after vascular injury, which can be protective in the state of hyperlipidemia. Here, we found significantly less ROS in the vessel walls of cd36-/- mice compared with WT after chemically induced arterial injury, suggesting that CD36 may contribute to ROS generation in the VSMCs themselves. Gene expression analysis revealed that the antioxidant enzymes peroxiredoxin-2 (Prdx2) and heme oxygenase-1 were upregulated in cd36-/- VSMCs. Molecular dissection of the pathway in isolated mouse VSMCs revealed CD36 ligand-dependent induction of Fyn phosphorylation, with subsequent phosphorylation and degradation of the redox-sensitive transcription factor Nrf2. Chromatin immunoprecipitation experiments further showed that Nrf2 directly occupied the Prdx2 promoter. The importance of this pathway was evidenced by increased ROS generation in prdx2-/- mice and decreased thrombosis times in both prdx2-/- and nrf2-/- mice after vascular injury. These data suggest that CD36-mediated downregulation of antioxidant systems in VSMCs may contribute to its prothrombotic, proinflammatory, and atherogenic effects.

Brain dendritic cells in ischemic stroke: time course, activation state, and origin

The immune response to stroke is comprised of inflammatory and regulatory processes. One cell type involved in both innate and adaptive immunity is the dendritic cell (DC). A DC population residing in the healthy brain (bDC) was identified using a transgenic mouse expressing enhanced yellow fluorescent protein (EYFP) under the promoter for the DC marker, CD11c (CD11c/EYFP Tg). To determine if bDC are involved in the immune response to cerebral ischemia, transient (40 min) middle cerebral artery occlusion (MCAO) followed by 6, 24, or 72 h reperfusion was conducted in CD11c/EYFP Tg mice. Our results demonstrated that DC accumulated in the ischemic hemisphere at 24 h post-MCAO-reperfusion, particularly in the border region of the infarct where T lymphocytes accrued. To distinguish resident bDC from the infiltrating peripheral DC, radiation chimeras [1. wild type (WT) hosts restored with CD11c/EYFP Tg bone marrow (BM) or 2. CD11c/EYFP Tg hosts restored with WT BM] were generated and examined by immunocytochemistry. These data confirmed that DC populating the core of the infarct at 72 h were of peripheral origin, whereas those in the border region were comprised primarily of resident bDC. The brain resident (CD45 intermediate) cells of CD11c/EYFP Tg mice were analyzed by flow cytometry. Compared to microglia, bDC displayed increased major histocompatibility class II (MHC II) and co-stimulatory molecules following MCAO-reperfusion. High levels of MHC II and the co-stimulatory molecule CD80 on bDC at 72 h corresponded to peak lymphocyte infiltration, and suggested a functional interaction between these two immune cell populations.

Key role of CD36 in Toll-like receptor 2 signaling in cerebral ischemia

BACKGROUND AND PURPOSE

Toll-like receptors (TLRs) and the scavenger receptor CD36 are key molecular sensors for the innate immune response to invading pathogens. However, these receptors may also recognize endogenous "danger signals" generated during brain injury, such as cerebral ischemia, and trigger a maladaptive inflammatory reaction. Indeed, CD36 and TLR2 and 4 are involved in the inflammation and related tissue damage caused by brain ischemia. Because CD36 may act as a coreceptor for TLR2 heterodimers (TLR2/1 or TLR2/6), we tested whether such interaction plays a role in ischemic brain injury.

METHODS

The TLR activators FSL-1 (TLR2/6), Pam3 (TLR2/1), or lipopolysaccharide (TLR4) were injected intracerebroventricularly into wild-type or CD36-null mice, and inflammatory gene expression was assessed in the brain. The effect of TLR activators on the infarct produced by transient middle cerebral artery occlusion was also studied.

RESULTS

The inflammatory response induced by TLR2/1 activation, but not TLR2/6 or TLR4 activation, was suppressed in CD36-null mice. Similarly, TLR2/1 activation failed to increase infarct volume in CD36-null mice, whereas TLR2/6 or TLR4 activation exacerbated postischemic inflammation and increased infarct volume. In contrast, the systemic inflammatory response evoked by TLR2/6 activation, but not by TLR2/1 activation, was suppressed in CD36-null mice.

CONCLUSIONS

In the brain, TLR2/1 signaling requires CD36. The cooperative signaling of TLR2/1 and CD36 is a critical factor in the inflammatory response and tissue damage evoked by cerebral ischemia. Thus, suppression of CD36-TLR2/1 signaling could be a valuable approach to minimize postischemic inflammation and the attendant brain injury.

Type 2 scavenger receptor CD36 in platelet activation: the role of hyperlipemia and oxidative stress

Platelet hyper-reactivity and a systemic prothrombotic state are associated with atherosclerosis and other inflammatory conditions. CD36, a member of the Type 2 scavenger receptor family, is a multiligand pattern recognition receptor that recognizes specific oxidized phospholipids, molecules expressed on microbial pathogens, apoptotic cells, and cell-derived microparticles. Recent studies have demonstrated that CD36 binding to oxidized LDL or microparticles activates a specific signaling pathway that induces platelet activation. This pathway is activated in vivo in the setting of hyperlipidemia and oxidant stress. Genetic deletion of CD36 protects mice from pathological thrombosis associated with hyperlipidemia without any apparent effect on normal hemostasis. Targeting CD36 or its signaling pathway could potentially lead to the development of novel antithrombotic therapies for patients with atheroinflammatory disorders.

Hypoxia up-regulates CD36 expression and function via hypoxia-inducible factor-1- and phosphatidylinositol 3-kinase-dependent mechanisms

Neovascular and degenerative diseases of the eye are leading causes of impaired vision and blindness in the world. Hypoxia or reduced oxygen tension is considered central to the pathogenesis of these disorders. Although the CD36 scavenger receptor features prominently in ocular homeostasis and pathology, little is known regarding its modulation by hypoxia. Herein we investigated the role and regulation of CD36 by hypoxia and by the major hypoxia effector, hypoxia-inducible factor (HIF)-1. In vivo, hypoxia markedly induced CD36 mRNA in corneal and retinal tissue. Subsequent experiments on human retinal pigment epithelial cells revealed that hypoxia time-dependently increased CD36 mRNA, protein, and surface expression; these responses were reliant upon reactive oxygen species production. As an important novel finding, we demonstrate that hypoxic stimulation of CD36 is mediated by HIF-1; HIF-1alpha down-regulation abolished CD36 induction by both hypoxia and cobalt chloride. Sequence analysis of the human CD36 promoter region revealed a functional HIF-1 binding site. A luciferase reporter construct containing this promoter fragment was activated by hypoxia, whereas mutation at the HIF-1 consensus site decreased promoter activation. Specific binding of HIF-1 to this putative site in hypoxic cells was detected by a chromatin immunoprecipitation assay. Interestingly, inhibition of the phosphatidylinositol 3-kinase pathway blocked the hypoxia-dependent induction of CD36 expression and promoter activity. Functional ramifications of CD36 hypoxic accumulation were evinced by CD36-dependent increases in scavenging and anti-angiogenic activities. Together, our findings indicate a novel mechanism by which hypoxia induces CD36 expression via activation of HIF-1 and the phosphatidylinositol 3-kinase pathway.

Production of superoxide anions by keratinocytes initiates P. acnes-induced inflammation of the skin

Acne vulgaris is a chronic inflammatory disorder of the sebaceous follicles. Propionibacterium acnes (P. acnes), a gram-positive anareobic bacterium, plays a critical role in the development of these inflammatory lesions. This study aimed at determining whether reactive oxygen species (ROS) are produced by keratinocytes upon P. acnes infection, dissecting the mechanism of this production, and investigating how this phenomenon integrates in the general inflammatory response induced by P. acnes. In our hands, ROS, and especially superoxide anions (O₂^•−), were rapidly produced by keratinocytes upon stimulation by P. acnes surface proteins. In P. acnes-stimulated keratinocytes, O₂^•− was produced by NAD(P)H oxidase through activation of the scavenger receptor CD36. O₂^•− was dismuted by superoxide dismutase to form hydrogen peroxide which was further detoxified into water by the GSH/GPx system. In addition, P. acnes-induced O₂^•− abrogated P. acnes growth and was involved in keratinocyte lysis through the combination of O₂^•− with nitric oxide to form peroxynitrites. Finally, retinoic acid derivates, the most efficient anti-acneic drugs, prevent O₂^•− production, IL-8 release and keratinocyte apoptosis, suggesting the relevance of this pathway in humans.

Acne vulgaris is a chronic inflammatory disorder of the sebaceous follicles. It is the most common skin disease, affecting up to 80% of individuals at some point between the ages of 11 and 30 years. Propionibacterium acnes (P. acnes) plays a role in the development of inflammatory acne lesions, but whether it causes inflammation by itself or through indirect mechanisms is not clear yet. Therefore, by exposing epidermal cells to P. acnes in vitro, we tested whether reactive oxygen species (ROS) production (oxidative burst) was involved in the inflammatory process. We found that one particular ROS, superoxide anion, was generated by epidermal cells following P. acnes stimulation. This phenomenon is associated with the production of a soluble pro inflammatory molecule, IL-8, and epidermal cell death. The abrogation of P. acnes-induced oxidative burst by the most commonly used and most efficient treatments of acne suggests that superoxide anions produced by epidermal cells are critical in the development of acne inflammatory lesions.

CD36, a scavenger receptor involved in immunity, metabolism, angiogenesis, and behavior

CD36 is a membrane glycoprotein present on platelets, mononuclear phagocytes, adipocytes, hepatocytes, myocytes, and some epithelia. On microvascular endothelial cells, CD36 is a receptor for thrombospondin-1 and related proteins and functions as a negative regulator of angiogenesis. On phagocytes, through its functions as a scavenger receptor recognizing specific oxidized phospholipids and lipoproteins, CD36 participates in internalization of apoptotic cells, certain bacterial and fungal pathogens, and modified low-density lipoproteins, thus contributing to inflammatory responses and atherothrombotic diseases. CD36 also binds long-chain fatty acids and facilitates their transport into cells, thus participating in muscle lipid utilization, adipose energy storage, and gut fat absorption and possibly contributing to the pathogenesis of metabolic disorders, such as diabetes and obesity. On sensory cells, CD36 is involved in insect pheromone signaling and rodent fatty food preference. The signaling pathways downstream of CD36 involve ligand-dependent recruitment and activation of nonreceptor tyrosine kinases, specific mitogen-activated protein kinases, and the Vav family of guanine nucleotide exchange factors; modulation of focal adhesion constituents; and generation of intracellular reactive oxygen species. CD36 in many cells is localized in specialized cholesterol-rich membrane microdomains and may also interact with other membrane receptors, such as tetraspanins and integrins. Identification of the precise CD36 signaling pathways in specific cells elicited in response to specific ligands may yield novel targets for drug development.

Advanced glycation endproducts and their receptor RAGE in Alzheimer's disease

Alzheimer's disease (AD) is the most common dementing disorder of late life. Although there might be various different triggering events in the early stages of the disease, they seem to converge on a few characteristic final pathways in the late stages, characterized by inflammation and neurodegeneration. In this review, we revisit the hypothesis that advanced glycation endproducts (AGEs) and their receptor RAGE may play an important role in disease pathogenesis. Accumulation of AGEs in cells and tissues is a normal feature of aging, but is accelerated in AD. In AD, AGEs can be detected in pathological deposits such as amyloid plaques and neurofibrillary tangles. AGEs explain many of the neuropathological and biochemical features of AD such as extensive protein crosslinking, glial induction of oxidative stress and neuronal cell death. Oxidative stress and AGEs initiate a positive feedback loop, where normal age-related changes develop into a pathophysiological cascade. RAGE and its decoy receptor soluble RAGE, may contribute to or protect against AD pathogenesis by influencing transport of β-amyloid into the brain or by manipulating inflammatory mechanisms. Targeted pharmacological interventions using AGE-inhibitors, RAGE-antagonists, RAGE-antibodies, soluble RAGE or RAGE signalling inhibitors such as membrane-permeable antioxidants may be promising therapeutic strategies to slow down the progression of AD.

CD36: a multi-modal target for acute stroke therapy

A role for CD36 in the pathogenesis of atherosclerosis, inflammation and lipid metabolism has been well-documented. However, little is known about the role of CD36 in cerebral ischemia. The intent of this review is to develop the concept that CD36, whose functions have been implicated in other pathological events, is a prototypic inflammatory receptor that contributes to the pathogenesis of cerebral ischemia. The importance of CD36 as a treatment target is indicated by the fact that many treatment strategies that are effective in experimental models of stroke exhibit little or no efficacy in clinical trials. The failure of clinical trials may be due to the use of animal models of stroke that do not reflect traditional risk factors for stroke in humans. The discussion will be focused on two risk factors, hyperlipidemia and diabetes, that modulate CD36 responses. Blocking the expression and function of CD36 by pharmacological or genetic means will provide insight not only toward identifying CD36 as a novel molecular target but also for developing effective therapeutic strategies to treat stroke victims. More importantly, coupling clinically relevant conditions with CD36-mediated ischemic injury may provide an appropriate animal model paradigm and develop a scientific understanding that could lead to clinical translational studies involving human subjects.

Mutant alpha-synuclein overexpression mediates early proinflammatory activity

Microglia provide immune surveillance for the brain through both the removal of cellular debris and protection against infection by microorganisms and "foreign" molecules. Upon activation, microglia display an altered morphology and increased expression of proinflammatory molecules. Increased numbers of activated microglia have been identified in a number of neurodegenerative diseases including Parkinson's disease (PD). What remains to be determined is whether activated microglia result from ongoing cell death or are involved in disease initiation and progression. To address this question we utilized a transgenic mouse model that expresses a mutated form of a key protein involved in Parkinson's disease, alpha-synuclein. Herein, we report an increase in activated microglia and proinflammatory molecules in 1-month-old transgenic mice well before cell death occurs in this model. Frank microglial activation is resolved by 6 months of age while a subset of proinflammatory molecules remain elevated for 12 months. Both tyrosine hydroxylase mRNA expression and alpha-synuclein protein are decreased in the striatum of older animals evidence of dystrophic neuritic projections. To determine whether mutated alpha-synuclein could directly activate microglia primary microglia-enriched cell cultures were treated with exogenous mutated alpha-synuclein. The data reveal an increase in activated microglia and proinflammatory molecules due to direct interaction with mutated alpha-synuclein. Together, these data demonstrate that mutated alpha-synuclein mediates a proinflammatory response in microglia and this activity may participate in PD pathogenesis.

Induction of Parkinson disease-related proteins in motor neurons after transient spinal cord ischemia in rabbits

The mechanism of spinal cord injury has been thought to be related to the vulnerability of spinal motor neuron cells against ischemia. However, the mechanisms of such vulnerability are not fully understood. We investigated a possible mechanism of neuronal death by immunohistochemical analysis for DJ-1, PINK1, and alpha-Synuclein. We used a 15-min rabbit spinal cord ischemia model, with use of a balloon catheter. Western blot analysis for DJ-1, PINK1, and alpha-Synuclein; temporal profiles of DJ-1, PINK1, and alpha-Synuclein immunoreactivity; and double-label fluorescence immunocytochemical studies were performed. Western blot analysis revealed scarce immunoreactivity for DJ-1, PINK1, and alpha-Synuclein in the sham-operated spinal cords. However, they became apparent at 8 h after transient ischemia, which returned to the baseline level at 1 day. Double-label fluorescence immunocytochemical study revealed that both DJ-1 and PINK1, and DJ-1 and alpha-Synuclein were positive at 8 h of reperfusion in the same motor neurons, which eventually die. The induction of DJ-1 and PINK1 proteins in the motor neurons at the early stage of reperfusion may indicate oxidative stress, and the induction of alpha-Synuclein may be implicated in the programmed cell death change after transient spinal cord ischemia.

The role of the scavenger receptor CD36 in regulating mononuclear phagocyte trafficking to atherosclerotic lesions and vascular inflammation

AIMS

CD36 has been shown to associate with non-receptor Src kinases to activate mitogen-activated protein kinases and trigger cytoskeletal remodelling, important events in foam cell formation and macrophage migration. Yet, its role in regulating circulating mononuclear phagocyte trafficking to atherosclerotic lesions has not been investigated. The aim of the present study was to investigate the role of CD36 in modulating the recruitment of mononuclear phagocytes to the arterial wall and the associated vascular inflammation, using both pharmacological and genetic approaches.

METHODS AND RESULTS

Apolipoprotein E-deficient (apoE(-/-)) mice fed a high-fat, high-cholesterol diet were treated daily with a CD36 ligand, EP 80317 (300 microg/kg), or 0.9% NaCl for 6 or 12 weeks. Forty-eight hours before sacrifice, mice were injected iv with (111)Indium-labelled macrophages. A 65% (P < 0.001) reduction of labelled macrophage accumulation at aortic lesions was observed in EP 80317-treated mice, mainly at the level of the aortic arch and iliac arteries, correlating with a 43% reduction of atherosclerotic lesion areas. This was associated with reduced phosphorylation of the focal adhesion kinase Pyk2 following stimulation with oxidized phospholipid in a Src kinase- and CD36-dependent manner. At the vascular level, EP 80317 treatment reduced the expression of pro-inflammatory proteins, including NADPH oxidase, inducible nitric oxide synthase, vascular endothelial cell adhesion molecule-1, and CCL2 chemokine. Plasma IL-6 levels were also reduced by 40% (P < 0.05). In contrast, none of these proteins was modulated in EP 80317-treated apoE/CD36 double knockout (apoE(-/-)/CD36(-/-)) mice.

CONCLUSION

Our results support a role for CD36 signalling in the regulation of mononuclear phagocyte trafficking to atherosclerotic-prone sites and in the associated vascular wall inflammation.

CD36 regulates oxidative stress and inflammation in hypercholesterolemic CKD

Scavenger receptors play a central role in atherosclerosis by processing oxidized lipoproteins and mediating their cellular effects. Recent studies suggested that the atherogenic state correlates with progression of chronic kidney disease (CKD); therefore, scavenger receptors are candidate mediators of renal fibrogenesis. Here, we investigated the role of CD36, a class B scavenger receptor, in a hypercholesterolemic model of CKD. We placed CD36-deficient mice and wild-type male mice on a high-fat Western diet for 7 to 8 wk and then performed either sham or unilateral ureteral obstruction surgery. CD36-deficient mice developed significantly less fibrosis compared with wild-type mice at days 3, 7, and 14 after obstruction. Compared with wild-type mice, CD36-deficient mice had significantly more interstitial macrophages at 7 d but not at 14 d. CD36-deficient mice exhibited reduced levels of activated NF-kappaB and oxidative stress (assessed by measuring fatty acid-derived hydroxyoctadecadienoic acid and protein carbonyl content) and decreased accumulation of interstitial myofibroblasts compared with wild-type mice. These data suggest that CD36 is a key modulator of proinflammatory and oxidative pathways that promote fibrogenesis in CKD.

The HMGB1 receptor RAGE mediates ischemic brain damage

In ischemic stroke, the necrotic core is surrounded by a zone of inflammation, in which delayed cell death aggravates the initial insult. Here, we provide evidence that the receptor for advanced glycation end products (RAGE) functions as a sensor of necrotic cell death and contributes to inflammation and ischemic brain damage. The RAGE ligand high mobility group box 1 (HMGB1) was elevated in serum of stroke patients and was released from ischemic brain tissue in a mouse model of cerebral ischemia. A neutralizing anti-HMGB1 antibody and HMGB1 box A, an antagonist of HMGB1 at the receptor RAGE, ameliorated ischemic brain damage. Interestingly, genetic RAGE deficiency and the decoy receptor soluble RAGE reduced the infarct size. In vitro, expression of RAGE in (micro)glial cells mediated the toxic effect of HMGB1. Addition of macrophages to neural cultures further enhanced the toxic effect of HMGB1. To test whether immigrant macrophages in the ischemic brain mediate the RAGE effect, we generated chimeric mice by transplanting RAGE(-/-) bone marrow to wild-type mice. RAGE deficiency in bone marrow-derived cells significantly reduced the infarct size. Thus, HMGB1-RAGE signaling links necrosis with macrophage activation and may provide a target for anti-inflammatory therapy in stroke.

The neuroprotective effect of prostaglandin E2 EP1 receptor inhibition has a wide therapeutic window, is sustained in time and is not sexually dimorphic

We investigated the preclinical characteristics of the neuroprotective effect of the prostaglandin E2 type 1 receptor (EP1) antagonist SC51089 in models of focal cerebral ischemia produced by occlusion of the mouse middle cerebral artery (MCA). We found that systemic administration of SC51089 (5 to 20 microg/kg; i.p.) reduces the brain injury produced by transient (-50%+/-8%; n=12; P<0.05) or permanent (-39%+/-7%; n=12; P<0.05) MCA occlusion. SC51089 was effective even when administered up to 12 h after ischemia. The protective effect was observed both in male and female mice and was sustained for at least 2 weeks after induction of ischemia. The reduction in injury volume was associated with an improvement in neurological function assessed by the Bederson deficit score, the hanging wire test and the corner test. The data provide proof of principle that EP1 receptor inhibition is a potentially valuable strategy for neuroprotection that deserves further preclinical investigation for therapeutic application in human stroke.

CD36/fatty acid translocase, an inflammatory mediator, is involved in hyperlipidemia-induced exacerbation in ischemic brain injury

Hyperlipidemia with accompanying increase in peripheral inflammation is a risk factor for stroke. The effect of excess lipids on stroke-induced injury and the mechanism by which lipid-mediated inflammatory responses contribute to stroke are not known. We investigated these uncertainties by subjecting normal and hyperlipidemic mice to transient middle cerebral artery occlusion, followed by measurement of stroke severity and inflammatory response. Infarct size, swelling, and lipid contents were significantly increased in the high-fat fed ApoE knock-out mice, as was the expression of the inflammatory mediators CD36 and monocyte chemoattractant protein 1 (MCP-1) in the brain and periphery. Furthermore, the hyperlipidemic mice exhibited numerous foam cells, a probable cause of increased swelling and postischemic inflammation, in the peri-infarct area. Genetic deletion of cd36 in the hyperlipidemic condition reduced proinflammatory chemokine/receptor and cytokines (MCP-1, CC chemokine receptor 2, and interleukins 1beta and 6), in the brain 6 h after ischemia. The reduced proinflammatory response also resulted in smaller ischemic injury, less swelling, and fewer foam cells at 3 d after ischemia. The results show that hyperlipidemia-induced inflammation is a negative factor for stroke outcomes and indicate that downregulating CD36 may be an effective therapeutic strategy for reducing the impact of stroke in hyperlipidemic subjects.

Acute neurodegeneration and the inflammasome: central processor for danger signals and the inflammatory response?

Activation of the inflammatory response is a crucial event in the adverse outcome of cerebral ischemia, which is promoted by proinflammatory cytokines such as interleukin (IL)-1beta. Although caspase-1 is necessary for IL-1beta processing, the 'upstream' signaling pathways were, until recently, essentially unknown. Fortunately, the inflammasome, a multiprotein complex responsible for activating caspase-1 and caspase-5, has recently been characterized. The activation of the inflammasome can result in one of several consequences such as cytokine secretion, cell death, or the development of a stress-resistant state. The significance of the inflammasome for the initiation of the inflammatory response during systemic diseases has already been shown and members of the inflammasome complex were recently found to be induced in acute brain injury. However, the specific pathophysiologic role of the inflammasome in neurodegenerative disorders still remains to be clarified. The underlying theories (e.g., danger signal theory) along with the signaling pathways that link the inflammasome to acute neurodegeneration will be discussed here. Furthermore, the stimuli that potentially activate the inflammasome in cerebral ischemia will be specified, as well as their relation to well-known pathways activating the innate immune response (e.g., Toll-like receptor signaling) and the consequences that result from their activation (beneficial versus deleterious).

Nuclear factor-kappaB activation and postischemic inflammation are suppressed in CD36-null mice after middle cerebral artery occlusion

CD36, a class-B scavenger receptor involved in multiple functions, including inflammatory signaling, may also contribute to ischemic brain injury through yet unidentified mechanisms. We investigated whether CD36 participates in the molecular events underlying the inflammatory reaction that accompanies cerebral ischemia and may contribute to the tissue damage. We found that activation of nuclear factor-kappaB, a transcription factor that coordinates postischemic gene expression, is attenuated in CD36-null mice subjected to middle cerebral artery occlusion. The infiltration of neutrophils and the glial reaction induced by cerebral ischemia were suppressed. Treatment with an inhibitor of inducible nitric oxide synthase, an enzyme that contributes to the tissue damage, reduced ischemic brain injury in wild-type mice, but not in CD36 nulls. In contrast to cerebral ischemia, the molecular and cellular inflammatory changes induced by intracerebroventricular injection of interleukin-1beta were not attenuated in CD36-null mice. The findings unveil a novel role of CD36 in early molecular events leading to nuclear factor-kappaB activation and postischemic inflammation. Inhibition of CD36 signaling may be a valuable therapeutic approach to counteract the deleterious effects of postischemic inflammation.

Absence of CD36 protects against atherosclerosis in ApoE knock-out mice with no additional protection provided by absence of scavenger receptor A I/II

AIMS

The role of scavenger receptors in atherogenesis is controversial as a result of conflicting reports and a recent hypothesis suggesting that scavenger receptor absence would enhance the pro-inflammatory, pro-atherogenic milieu. This study addresses the effect of combined absence of scavenger receptors CD36 and SRA I/II on atherosclerosis lesion development in the apolipoprotein E knock-out (apoE degrees ) model.

METHODS

We created background-related strains of apoE degrees , scavenger receptor A I/II knock-out (SRA degrees )/apoE degrees , CD36 knock-out (CD36 degrees )/apoE degrees , and CD36 degrees /SRA degrees /apoE degrees mice that were >99% C57Bl/6. Four-week-old mice were fed a Western diet for 12 weeks and were assessed for lesion burden/morphology, risk factors for atherosclerosis, inflammatory mediators, and macrophage function.

RESULTS

There was a 61 and 74% decrease in total aortic lesion area in CD36 degrees /apoE degrees males and females, respectively, compared with apoE degrees controls. The absence of SRA was protective (32% decrease in lesion) in female mice. The combined absence of CD36 and SRA provided no further protection in either gender. Macrophages from mice lacking CD36 had decreased pro-inflammatory characteristics and less migration to a pro-inflammatory stimulus. Plasma levels of cytokines/chemokines showed that CD36 degrees /apoE degrees and CD36 degrees /SRA degrees /apoE degrees mice had a less pro-inflammatory phenotype compared with apoE degrees and SRA degrees /apoE degrees mice. Oblivious mice in the apoE degrees background ruled out potential 'passenger gene' effects in the case of CD36.

CONCLUSION

These results provide new insights into the pro-atherogenic mechanisms of CD36 by implicating processes other than modified lipoprotein uptake.

Lymphocytic microparticles inhibit angiogenesis by stimulating oxidative stress and negatively regulating VEGF-induced pathways

Recent studies have demonstrated that lymphocyte-derived microparticles (LMPs) impair endothelial cell function. However, no data currently exist regarding the contribution of LMPs in the regulation of angiogenesis. In the present study, we investigated the effects of LMPs on angiogenesis in vivo and in vitro and demonstrated that LMPs strongly suppressed aortic ring microvessel sprouting and in vivo corneal neovascularization. In vitro, LMPs considerably diminished human umbilical vein endothelial cell survival and proliferation in a concentration-dependent manner. Mechanistically, the antioxidants U-74389G and U-83836E were partially protective against the antiproliferative effects of LMPs, whereas the NADPH oxidase (NOX) inhibitors apocynin and diphenyleneiodonium significantly abrogated these effects. Moreover, LMPs increased not only the expression of the NOX subunits gp91(phox), p22(phox), and p47(phox), but also the production of ROS and NOX-derived superoxide (O(2)(-)). Importantly, LMPs caused a pronounced augmentation in the protein expression of the CD36 antiangiogenic receptor while significantly downregulating the protein levels of VEGF receptor type 2 and its downstream signaling mediator, phosphorylated ERK1/2. In summary, LMPs potently suppress neovascularization in vivo and in vitro by augmenting ROS generation via NOX and interfering with the VEGF signaling pathway.