Lipopolysaccharide preconditioning induces robust protection against brain injury resulting from deep hypothermic circulatory arrest

Qilong capsule alleviates ponatinib-induced ischemic stroke in a zebrafish model by regulating coagulation, inflammation and apoptosis

ETHOPHARMACOLOGICAL RELEVANCE

Qilong capsule (QLC) is a compound traditional Chinese medicine commonly used to treat ischemic stroke (IS). QLC is made of eight kinds of medicinal materials. It has two kinds of monarch medicine and six kinds of minister medicine. However, the pharmacodynamic mechanism of QLC is still unknown.

AIM OF THE STUDY

The aim of this paper was to evaluate the pharmacology mechanism of QLC against ischemic stroke through coagulation, inflammation and apoptosis.

MATERIALS AND METHODS

We used an existing zebrafish model to explore the protective mechanism of QLC on IS. We treated normally-developing zebrafish larvae with QLC and ponatinib 2 days post fertilization (dpf), and used the area of cerebral vascular thrombosis, red blood cell staining intensity, and brain cell apoptosis to quantitate QLC efficacy against IS. Evaluation of brain inflammation in zebrafish by observing macrophage aggregation and migration. In addition, we also explored the effect of QLC on zebrafish angiogenesis. Quantitative polymerase chain reaction (qPCR) was used to detect changes in the expression of genes involved in coagulation, inflammation, vascular endothelium, and apoptosis.

RESULTS

We found that QLC reduced the area affected by ponatinib-induced cerebral vascular embolism, erythrocyte staining intensity, and the number of apoptotic brain cells. For inflammation, QLC can improve the aggregation and migration of macrophages. QLC can significantly promote the formation of blood vessels in zebrafish. qPCR showed that QLC inhibited the expression of genes related to coagulation, inflammation, and apoptosis.

CONCLUSION

Qilong capsule had a significant protective efficacy in ponatinib-induced IS.

Effects of intraperitoneal injection of lipopolysaccharide-induced peripheral inflammation on dopamine neuron damage in rat midbrain

Introduction

Current studies have documented neuroinflammation is implicated in Parkinson's disease. Recently, growing evidence indicated peripheral inflammation plays an important role in regulation of neuroinflammation and thus conferring protection against dopamine (DA) neuronal damage. However, the underlying mechanisms are not clearly illuminated.

Methods

The effects of intraperitoneal injection of LPS (LPS_[i.p.])‐induced peripheral inflammation on substantia nigra (SN) injection of LPS (LPS_[SN])‐elicited DA neuronal damage in rat midbrain were investigated. Rats were intraperitoneally injected with LPS (0.5 mg/kg) daily for 4 consecutive days and then given single injection of LPS (8 μg) into SN with an interval of 0 (LPS_(i.p.) 0 day ± LPS_(SN)), 30 (LPS_(i.p.) 30 days ± LPS_(SN)), and 90 (LPS_(i.p.) 90 days ± LPS_(SN)) days after LPS_(i.p.) administration.

Results

LPS_(i.p.) increased the levels of inflammatory factors in peripheral blood in (LPS_(i.p.) 0 day ± LPS_(SN)). Importantly, in (LPS_(i.p.) 0 day ± LPS_(SN)) and (LPS_(i.p.) 30 days ± LPS_(SN)), LPS_(i.p.) attenuated LPS_(SN)‐induced DA neuronal loss in SN. Besides, LPS_(i.p.) reduced LPS_(SN)‐induced microglia and astrocytes activation in SN. Furtherly, LPS_(i.p.) reduced pro‐inflammatory M1 microglia markers mRNA levels and increased anti‐inflammatory M2 microglia markers mRNA levels. In addition, the increased T‐cell marker expression and the decreased M1 microglia marker expression and more DA neuronal survival were discerned at the same area of rat midbrain in LPS_(SN)‐induced DA neuronal damage 30 days after LPS_(i.p.) application.

Conclusion

This study suggested LPS_(i.p.)‐induced peripheral inflammation might cause T cells to infiltrate the brain to regulate microglia‐mediated neuroinflammation, thereby protecting DA neurons.

Strain-specific pre-existing immunity: A key to understanding the role of chronic Toxoplasma infection in cognition and Alzheimer's diseases?

Toxoplasma exposure can elicit cellular and humoral immune responses. In the case of chronic Toxoplasma infection, these immune responses are long-lasting. Some studies suggest that pre-existing immunity from Toxoplasma infection can shape immune responses and resistance to other pathogens and brain insults later in life. Much evidence has been generated suggesting Toxoplasma infection may contribute to cognitive impairment in the elderly. However, there have also been studies that disagree with the conclusion. Toxoplasma has many strain types, with virulence being the most notable difference. There is also considerable variation in the outcomes following Toxoplasma exposure ranging from resolved to persistent infection. Therefore, the brain microenvironment, particularly cellular constituents, differs based on the infecting strain (virulent versus hypovirulent) and infection stage (resolved versus persistent). Such difference might play a critical role in determining the outcome of the host on subsequent challengings to the brain. The ability of Toxoplasma strains to set up distinct stages for neurodegenerative pathology through varying degrees of virulence provides unique experimental tools for characterizing these pathways.

Low-dose lipopolysaccharide as an immune regulator for homeostasis maintenance in the central nervous system through transformation to neuroprotective microglia

Microglia, which are tissue-resident macrophages in the brain, play a central role in the brain innate immunity and contribute to the maintenance of brain homeostasis. Lipopolysaccharide is a component of the outer membrane of gram-negative bacteria, and activates immune cells including microglia via Toll-like receptor 4 signaling. Lipopolysaccharide is generally known as an endotoxin, as administration of high-dose lipopolysaccharide induces potent systemic inflammation. Also, it has long been recognized that lipopolysaccharide exacerbates neuroinflammation. In contrast, our study revealed that oral administration of lipopolysaccharide ameliorates Alzheimer's disease pathology and suggested that neuroprotective microglia are involved in this phenomenon. Additionally, other recent studies have accumulated evidence demonstrating that controlled immune training with low-dose lipopolysaccharide prevents neuronal damage by transforming the microglia into a neuroprotective phenotype. Therefore, lipopolysaccharide may not a mere inflammatory inducer, but an immunomodulator that can lead to neuroprotective effects in the brain. In this review, we summarized current studies regarding neuroprotective microglia transformed by immune training with lipopolysaccharide. We state that microglia transformed by lipopolysaccharide preconditioning cannot simply be characterized by their general suppression of proinflammatory mediators and general promotion of anti-inflammatory mediators, but instead must be described by their complex profile comprising various molecules related to inflammatory regulation, phagocytosis, neuroprotection, anti-apoptosis, and antioxidation. In addition, microglial transformation seems to depend on the dose of lipopolysaccharide used during immune training. Immune training of neuroprotective microglia using low-dose lipopolysaccharide, especially through oral lipopolysaccharide administration, may represent an innovative prevention or treatment for neurological diseases; however more vigorous studies are still required to properly modulate these treatments.

Whole Body Periodic Acceleration (pGz) as a non-invasive preconditioning strategy for pediatric cardiac surgery

We hypothesized that pGz has cardio and neuroprotective effects due to upregulation of pathways which include eNOS, anti-apoptotic, and anti-inflammatory pathways. We analyze protein expression of these pathways in the brain of neonatal piglets, as well as report on the myocardial function after Deep Hypothermic Circulatory Arrest (DHCA) and pGz preconditioning. Animal data affirms both a cardio and neuroprotective role for pGz. These findings suggest that pGz can be a simple, non-invasive cardio and neuroprotective strategy preconditioning strategy in children requiring surgical intervention.

Peripheral immune tolerance alleviates the intracranial lipopolysaccharide injection-induced neuroinflammation and protects the dopaminergic neurons from neuroinflammation-related neurotoxicity

BACKGROUND

Neuroinflammation plays a critical role in the onset and development of neurodegeneration disorders such as Parkinson's disease. The immune activities of the central nervous system are profoundly affected by peripheral immune activities. Immune tolerance refers to the unresponsiveness of the immune system to continuous or repeated stimulation to avoid excessive inflammation and unnecessary by-stander injury in the face of continuous antigen threat. It has been proved that the immune tolerance could suppress the development of various peripheral inflammation-related diseases. However, the role of immune tolerance in neuroinflammation and neurodegenerative diseases was not clear.

METHODS

Rats were injected with repeated low-dose lipopolysaccharide (LPS, 0.3 mg/kg) intraperitoneally for 4 days to induce peripheral immune tolerance. Neuroinflammation was produced using intracranial LPS (15 μg) injection. Inflammation cytokines were measured using enzyme-linked immunosorbent assay (ELISA) and quantitative real-time polymerase chain reaction (qRT-PCR). Microglial activation were measured using immunostaining of Iba-1 and ED-1. Dopaminergic neuronal damage was evaluated using immunochemistry staining and stereological counting of TH-positive neurons. Behavioral impairment was evaluated using amphetamine-induced rotational behavioral assessment.

RESULTS

Compared with the non-immune tolerated animals, pre-treatment of peripheral immune tolerance significantly decreased the production of inflammatory cytokines, suppressed the microglial activation, and increased the number of dopaminergic neuronal survival in the substantia nigra.

CONCLUSIONS

Our results indicated that peripheral immune tolerance attenuated neuroinflammation and inhibited neuroinflammation-induced dopaminergic neuronal death.

Pathogenic mechanisms following ischemic stroke

Stroke is the second most common cause of death and the leading cause of disability worldwide. Brain injury following stroke results from a complex series of pathophysiological events including excitotoxicity, oxidative and nitrative stress, inflammation, and apoptosis. Moreover, there is a mechanistic link between brain ischemia, innate and adaptive immune cells, intracranial atherosclerosis, and also the gut microbiota in modifying the cerebral responses to ischemic insult. There are very few treatments for stroke injuries, partly owing to an incomplete understanding of the diverse cellular and molecular changes that occur following ischemic stroke and that are responsible for neuronal death. Experimental discoveries have begun to define the cellular and molecular mechanisms involved in stroke injury, leading to the development of numerous agents that target various injury pathways. In the present article, we review the underlying pathophysiology of ischemic stroke and reveal the intertwined pathways that are promising therapeutic targets.

Preclinical Development of a Prophylactic Neuroprotective Therapy for the Preventive Treatment of Anticipated Ischemia-Reperfusion Injury

Ischemia-reperfusion brain injury can be iatrogenically induced secondary to life-saving procedures. Prophylactic treatment of these patients offers a promising prevention for lifelong complications. We postulate that a cytosine-guanine (CpG) oligodeoxynucleotide (ODN) can provide robust antecedent protection against cerebral ischemic injury with minimal release of pro-inflammatory cytokines, making it an ideal candidate for further clinical development. Mouse and nonhuman primate (NHP) models of cerebral ischemic injury were used to test whether an A-type CpG ODN, which induces minimal systemic inflammatory cytokine responses, can provide prophylactic protection. Extent of injury in the mouse was measured by histological staining of live tissue. In the NHP, injury was assessed 2 and 7 days post-occlusion from T2-weighted magnetic resonance images and neurological and motor deficits were cataloged daily. Plasma cytokine levels were measured using species-specific Luminex assays. Prophylactic administration of an A-type CpG ODN provided robust protection against cerebral ischemic injury in the mouse with minimal systemic inflammation. Rhesus macaques treated with D192935, a mixture of human optimized A-type CpG ODNs, had smaller infarcts and demonstrated significantly less neurological and motor deficits following ischemic injury. Our findings demonstrate the translational potential of D192935 as a prophylactic treatment for patients at risk of cerebral ischemic injury.

Neuregulin 1 Attenuates Neuronal Apoptosis Induced by Deep Hypothermic Circulatory Arrest Through ErbB4 Signaling in Rats

Mounting evidence suggests that neurological injury occurs after deep hypothermic circulatory arrest (DHCA), a protocol widely used in surgery for congenital heart diseases and aortic repair. Neuregulin 1 (NRG1), a neurotrophic factor highly expressed in the central nervous system, is crucial for neuronal survival. However, whether NRG1 is protective against apoptosis induced by DHCA is still unclear, as are the putative mechanisms involved. In this study, exogenous human NRG1 pretreatment (2.5 and 3.75 ng/kg, intracarotid injection) significantly inhibited neuronal apoptosis in DHCA-treated male rats, and notably, endogenous NRG1 expression was also increased. Bcl-2, as well as phosphorylated phosphatidylinositol-3-kinase, Akt, and cAMP-response element binding protein, were all increased, resulting in phosphorylation and subsequent activation of the ErbB4 receptor. Finally, expression of the apoptosis-related protein cleaved-caspase-3 was decreased, resulting in the inhibition of neuronal apoptosis induced by DHCA. Thus, our data indicate that NRG1 treatment inhibited DHCA-induced neuronal apoptosis by activating ErbB4 signaling, providing a potential therapeutic pathway for the prevention of neurological injury induced by DHCA.

Role of toll-like receptor 4 in melatonin-induced cardioprotection

Melatonin protects the heart against myocardial ischemia/reperfusion injury via the activation of the survivor activating factor enhancement (SAFE) pathway which involves tumor necrosis factor alpha (TNFα) and the signal transducer and activator of transcription 3 (STAT3). Toll-like receptor 4 (TLR4) plays a crucial role in myocardial ischemia/reperfusion injury and activates TNFα. In this study, we investigated whether melatonin may target TLR4 to activate the SAFE pathway. Isolated hearts from rats or mice were subjected to ischemia/reperfusion injury. Melatonin (75 ng/L) and/or TAK242 (a specific inhibitor of TLR4 signaling, 500 nm) were administered to the rat hearts before the induction of ischemia. Pre-ischemic myocardial STAT3 was evaluated by Western blotting. Lipopolysaccharide (LPS, a stimulator of TLR4) was administered to wild type, TNFα receptor 2 knockout or cardiomyocyte-specific STAT3-deficient mice (2.8 mg/kg, i.p) 45 min before the heart isolation. Myocardial infarct size was measured as an endpoint. Compared to the control, administration of melatonin reduced myocardial infarct size (34.7 ± 2.8% versus 62.6 ± 2.7%, P < 0.01). This protective effect was abolished in the presence of TAK242 (49.2 ± 6.5%). Melatonin administered alone increased the pre-ischemic activation of mitochondrial STAT3, and this effect was attenuated with TAK242. Furthermore, stimulation of TLR4 with LPS pretreatment to mice reduced myocardial infarct size of the hearts isolated from wild-type animals but failed to protect the hearts isolated from TNFα receptor 2-knockout mice or cardiomyocyte-specific STAT3-deficient mice (P < 0.001). Taken together, these data suggest that cardioprotection induced by melatonin is mediated by TLR4 to activate the SAFE pathway.

Rational modulation of the innate immune system for neuroprotection in ischemic stroke

The innate immune system plays a dualistic role in the evolution of ischemic brain damage and has also been implicated in ischemic tolerance produced by different conditioning stimuli. Early after ischemia, perivascular astrocytes release cytokines and activate metalloproteases (MMPs) that contribute to blood–brain barrier (BBB) disruption and vasogenic oedema; whereas at later stages, they provide extracellular glutamate uptake, BBB regeneration and neurotrophic factors release. Similarly, early activation of microglia contributes to ischemic brain injury via the production of inflammatory cytokines, including tumor necrosis factor (TNF) and interleukin (IL)-1, reactive oxygen and nitrogen species and proteases. Nevertheless, microglia also contributes to the resolution of inflammation, by releasing IL-10 and tumor growth factor (TGF)-β, and to the late reparative processes by phagocytic activity and growth factors production. Indeed, after ischemia, microglia/macrophages differentiate toward several phenotypes: the M1 pro-inflammatory phenotype is classically activated via toll-like receptors or interferon-γ, whereas M2 phenotypes are alternatively activated by regulatory mediators, such as ILs 4, 10, 13, or TGF-β. Thus, immune cells exert a dualistic role on the evolution of ischemic brain damage, since the classic phenotypes promote injury, whereas alternatively activated M2 macrophages or N2 neutrophils prompt tissue remodeling and repair. Moreover, a subdued activation of the immune system has been involved in ischemic tolerance, since different preconditioning stimuli act via modulation of inflammatory mediators, including toll-like receptors and cytokine signaling pathways. This further underscores that the immuno-modulatory approach for the treatment of ischemic stroke should be aimed at blocking the detrimental effects, while promoting the beneficial responses of the immune reaction.

Function and mechanism of toll-like receptors in cerebral ischemic tolerance: from preconditioning to treatment

Increasing evidence suggests that toll-like receptors (TLRs) play an important role in cerebral ischemia-reperfusion injury. The endogenous ligands released from ischemic neurons activate the TLR signaling pathway, resulting in the production of a large number of inflammatory cytokines, thereby causing secondary inflammation damage following cerebral ischemia. However, the preconditioning for minor cerebral ischemia or the preconditioning with TLR ligands can reduce cerebral ischemic injury by regulating the TLR signaling pathway following ischemia in brain tissue (mainly, the inhibition of the TLR4/NF-κB signaling pathway and the enhancement of the interferon regulatory factor-dependent signaling), resulting in TLR ischemic tolerance. Additionally, recent studies found that postconditioning with TLR ligands after cerebral ischemia can also reduce ischemic damage through the regulation of the TLR signaling pathway, showing a significant therapeutic effect against cerebral ischemia. These studies suggest that the ischemic tolerance mediated by TLRs can serve as an important target for the prevention and treatment of cerebral ischemia. On the basis of describing the function and mechanism of TLRs in mediating cerebral ischemic damage, this review focuses on the mechanisms of cerebral ischemic tolerance induced by the preconditioning and postconditioning of TLRs and discusses the clinical application of TLRs for ischemic tolerance.

Immune mechanisms in cerebral ischemic tolerance

Stressor-induced tolerance is a central mechanism in the response of bacteria, plants, and animals to potentially harmful environmental challenges. This response is characterized by immediate changes in cellular metabolism and by the delayed transcriptional activation or inhibition of genetic programs that are not generally stressor specific (cross-tolerance). These programs are aimed at countering the deleterious effects of the stressor. While induction of this response (preconditioning) can be established at the cellular level, activation of systemic networks is essential for the protection to occur throughout the organs of the body. This is best signified by the phenomenon of remote ischemic preconditioning, whereby application of ischemic stress to one tissue or organ induces ischemic tolerance (IT) in remote organs through humoral, cellular and neural signaling. The immune system is an essential component in cerebral IT acting simultaneously both as mediator and target. This dichotomy is based on the fact that activation of inflammatory pathways is necessary to establish IT and that IT can be, in part, attributed to a subdued immune activation after index ischemia. Here we describe the components of the immune system required for induction of IT and review the mechanisms by which a reprogrammed immune response contributes to the neuroprotection observed after preconditioning. Learning how local and systemic immune factors participate in endogenous neuroprotection could lead to the development of new stroke therapies.

Preconditioning provides neuroprotection in models of CNS disease: paradigms and clinical significance

Preconditioning is a phenomenon in which brief episodes of a sublethal insult induce robust protection against subsequent lethal injuries. Preconditioning has been observed in multiple organisms and can occur in the brain as well as other tissues. Extensive animal studies suggest that the brain can be preconditioned to resist acute injuries, such as ischemic stroke, neonatal hypoxia/ischemia, surgical brain injury, trauma, and agents that are used in models of neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease. Effective preconditioning stimuli are numerous and diverse, ranging from transient ischemia, hypoxia, hyperbaric oxygen, hypothermia and hyperthermia, to exposure to neurotoxins and pharmacological agents. The phenomenon of "cross-tolerance," in which a sublethal stress protects against a different type of injury, suggests that different preconditioning stimuli may confer protection against a wide range of injuries. Research conducted over the past few decades indicates that brain preconditioning is complex, involving multiple effectors such as metabolic inhibition, activation of extra- and intracellular defense mechanisms, a shift in the neuronal excitatory/inhibitory balance, and reduction in inflammatory sequelae. An improved understanding of brain preconditioning should help us identify innovative therapeutic strategies that prevent or at least reduce neuronal damage in susceptible patients. In this review, we focus on the experimental evidence of preconditioning in the brain and systematically survey the models used to develop paradigms for neuroprotection, and then discuss the clinical potential of brain preconditioning.

Pharmacologic preconditioning: translating the promise

A transient, ischemia-resistant phenotype known as "ischemic tolerance" can be established in brain in a rapid or delayed fashion by a preceding noninjurious "preconditioning" stimulus. Initial preclinical studies of this phenomenon relied primarily on brief periods of ischemia or hypoxia as preconditioning stimuli, but it was later realized that many other stressors, including pharmacologic ones, are also effective. This review highlights the surprisingly wide variety of drugs now known to promote ischemic tolerance, documented and to some extent mechanistically characterized in preclinical animal models of stroke. Although considerably more experimentation is needed to thoroughly validate the ability of any currently identified preconditioning agent to protect ischemic brain, the fact that some of these drugs are already clinically approved for other indications implies that the growing enthusiasm for translational success in the field of pharmacologic preconditioning may be well justified.

TLR-3 receptor activation protects the very immature brain from ischemic injury

Background

We have shown that preconditioning by lipopolysaccharide (LPS) will result in 90% reduction in ischemic brain damage in P7 rats. This robust LPS neuroprotection was not observed in P3 or P5 pups (corresponding to human premature infant). LPS is a known Toll-like receptor 4 (TLR-4) ligand. We hypothesized that TLRs other than TLR-4 may mediate preconditioning against cerebral ischemic injury in the developing brain.

Methods

TLR-2, TLR-3, TLR-4, and TLR-9 expression was detected in brain sections from P3, P5, and P7 rats by immuno-staining. In subsequent experiments, P5 rats were randomly assigned to TLR-3 specific agonist, poly I:C, or saline treated group. At 48 h after the injections, hypoxic-ischemic (HI) injury was induced by unilateral carotid artery ligation followed by hypoxia for 65 min. Brains were removed 1 week after HI injury and infarct volumes were compared in H&E stained sections between the two groups.

Results

TLR-2 and TLR-3 were highly expressed in brains of P3 and P5 but not in P7 rats. The number of TLR-4 positive cells was lower in P3 and P5 compared to P7 brains (P <0.05). TLR-3 was predominately expressed in P5 pups (P <0.05). There was no significant difference in TLR-9 expression in the three age groups. There was a significant reduction in infarct volume (P = 0.01) in poly I:C compared to saline pre-treated P5 pups. Pre-treatment with poly I:C downregulated NF-κB and upregulated IRF3 expression in P5 rat ischemic brains. Pre-treatment with poly I:C did not offer neuroprotection in P7 rat brains.

Conclusion

TLRs expression and function is developmentally determined. Poly I:C-induced preconditioning against ischemic injury may be mediated by modulation of TLR-3 signaling pathways. This is the first study to show that TLR-3 is expressed in the immature brain and mediates preconditioning against ischemic injury.

Therapeutic role of toll-like receptor modification in cardiovascular dysfunction

Toll-like receptors (TLR) are key pattern recognition receptors in the innate immune system. The TLR-mediated immune response against pathogens is usually protective however inappropriate TLR activation may lead to excessive tissue damage. It is well recognised that TLRs respond to a variety of endogenous as well as exogenous ligands. By responding to endogenous ligands that are exposed during cellular damage, TLRs have been implicated in a range of pathological conditions associated with cardiovascular dysfunction. Increasing knowledge on the mechanisms involved in TLR signalling has encouraged the exploration of therapeutic pharmacological modulation of TLR activation in conditions such as atherosclerosis, ischaemic heart disease, heart failure and ischaemic reperfusion injury. The aim of this review is to explore the translational potentials of TLR modification in cardiovascular dysfunction, where these agents have been studied.

Toll-like receptor 7 preconditioning induces robust neuroprotection against stroke by a novel type I interferon-mediated mechanism

BACKGROUND AND PURPOSE

Systemic administration of Toll-like receptor (TLR) 4 and TLR9 agonists before cerebral ischemia have been shown to reduce ischemic injury by reprogramming the response of the brain to stroke. Our goal was to explore the mechanism of TLR-induced neuroprotection by determining whether a TLR7 agonist also protects against stroke injury.

METHODS

C57Bl/6, TNF(-/-), interferon (IFN) regulatory factor 7(-/-), or type I IFN receptor (IFNAR)(-/-) mice were subcutaneously administered the TLR7 agonist Gardiquimod (GDQ) 72 hours before middle cerebral artery occlusion. Infarct volume and functional outcome were determined after reperfusion. Plasma cytokine responses and induction of mRNA for IFN-related genes in the brain were measured. IFNAR(-/-) mice also were treated with the TLR4 agonist (lipopolysaccharide) or the TLR9 agonist before middle cerebral artery occlusion and infarct volumes measured.

RESULTS

The results show that GDQ reduces infarct volume as well as functional deficits in mice. GDQ pretreatment provided robust neuroprotection in TNF(-/-) mice, indicating that TNF was not essential. GDQ induced a significant increase in plasma IFNα levels and both IRF7(-/-) and IFNAR(-/-) mice failed to be protected, implicating a role for IFN signaling in TLR7-mediated protection.

CONCLUSIONS

Our studies provide the first evidence that TLR7 preconditioning can mediate neuroprotection against ischemic injury. Moreover, we show that the mechanism of protection is unique from other TLR preconditioning ligands in that it is independent of TNF and dependent on IFNAR.

Toll-like receptors in cerebral ischemic inflammatory injury

Cerebral ischemia triggers acute inflammation, which has been associated with an increase in brain damage. The mechanisms that regulate the inflammatory response after cerebral ischemia are multifaceted. An important component of this response is the activation of the innate immune system. However, details of the role of the innate immune system within the complex array of mechanisms in cerebral ischemia remain unclear. There have been recent great strides in our understanding of the innate immune system, particularly in regard to the signaling mechanisms of Toll-like receptors (TLRs), whose primary role is the initial activation of immune cell responses. So far, few studies have examined the role of TLRs in cerebral ischemia. However, work with experimental models of ischemia suggests that TLRs are involved in the enhancement of cell damage following ischemia, and their absence is associated with lower infarct volumes. It may be possible that therapeutic targets could be designed to modulate activities of the innate immune system that would attenuate cerebral brain damage. Ischemic tolerance is a protective mechanism induced by a variety of preconditioning stimuli. Interpreting the molecular mechanism of ischemic tolerance will open investigative avenues into the treatment of cerebral ischemia. In this review, we discuss the critical role of TLRs in mediating cerebral ischemic injury. We also summarize evidence demonstrating that cerebral preconditioning downregulates pro-inflammatory TLR signaling, thus reducing the inflammation that exacerbates ischemic brain injury.

Multiple preconditioning paradigms converge on interferon regulatory factor-dependent signaling to promote tolerance to ischemic brain injury

Ischemic tolerance can be induced by numerous preconditioning stimuli, including various Toll-like receptor (TLR) ligands. We have shown previously that systemic administration of the TLR4 ligand LPS or the TLR9 ligand unmethylated CpG oligodeoxynucleotide before transient brain ischemia in mice confers substantial protection against ischemic damage. To elucidate the molecular mechanisms of preconditioning, we compared brain genomic profiles in response to preconditioning with these TLR ligands and with preconditioning via exposure to brief ischemia. We found that exposure to the TLR ligands and brief ischemia induced genomic changes in the brain characteristic of a TLR pathway-mediated response. Interestingly, all three preconditioning stimuli resulted in a reprogrammed response to stroke injury that converged on a shared subset of 13 genes not evident in the genomic profile from brains that were subjected to stroke without prior preconditioning. Analysis of the promoter region of these shared genes showed sequences required for interferon regulatory factor (IRF)-mediated transcription. The importance of this IRF gene network was tested using mice deficient in IRF3 or IRF7. Our data show that both transcription factors are required for TLR-mediated preconditioning and neuroprotection. These studies are the first to discover a convergent mechanism of neuroprotection induced by preconditioning--one that potentially results in reprogramming of the TLR-mediated response to stroke and requires the presence of IRF3 and IRF7.

Lipopolysaccharide-induced preconditioning against ischemic injury is associated with changes in toll-like receptor 4 expression in the rat developing brain

Lipopolysaccharide (LPS) preconditioning reduces ischemic injury in adult brain by activating Toll-like receptor 4 (TLR-4). We sought to investigate the effect of brain maturity on the efficacy of LPS preconditioning against hypoxic-ischemic (HI) injury in the developing rat brain. Rat pups at the specified age were randomly assigned to LPS-treated (0.1 mg/kg) or saline-treated groups. HI injury was induced 48 h later by occluding the right common carotid artery followed by transient hypoxia. Brains were removed 1 wk after HI injury, and infarct volumes were compared between the two groups. TLR-4 expression was also compared among different ages. We found that LPS treated P7, P9, and P14 rat pups had significantly smaller infarct volume compared with saline-treated pups (p = 0.006, 0.03, and 0.01, respectively). This significant reduction in infarct volume was not observed in P3 and P5 rats. TLR-4 expression was significantly higher in older rats compared with P3 and P5 rats (p < 0.01). These findings indicate that LPS-induced preconditioning is a robust neuroprotective phenomenon in the ischemic developing brain that is age dependent. Pattern of TLR-4 expression is also affected by brain maturity and likely to be responsible for differences in the efficacy of LPS preconditioning.

Toll-like receptors in inflammation of the central nervous system

Toll-like receptors (TLRs) belong to pattern-recognition receptor family that could recognize exogenous pathogen-associated molecular patterns and endogenous damage-associated molecular patterns. TLRs play pivotal roles in innate and adaptive immune responses. In this review we summarize the ligands and signal transduction pathways of TLRs and highlight recent progress of the involvement of TLRs in neuroinflammation related disorders, including cerebral ischemia/stroke, brain trauma and hemorrhage, pathogen infection and autoimmune diseases, and explore the potential of TLR signaling as therapeutic targets against these disorders.

Proof of concept: pharmacological preconditioning with a Toll-like receptor agonist protects against cerebrovascular injury in a primate model of stroke

Cerebral ischemic injury is a significant portion of the burden of disease in developed countries; rates of mortality are high and the costs associated with morbidity are enormous. Recent therapeutic approaches have aimed at mitigating the extent of damage and/or promoting repair once injury has occurred. Often, patients at high risk of ischemic injury can be identified in advance and targeted for antecedent neuroprotective therapy. Agents that stimulate the innate pattern recognition receptor, Toll-like receptor 9, have been shown to induce tolerance (precondition) to ischemic brain injury in a mouse model of stroke. Here, we demonstrate for the first time that pharmacological preconditioning against cerebrovascular ischemic injury is also possible in a nonhuman primate model of stroke in the rhesus macaque. The model of stroke used is a minimally invasive transient vascular occlusion, resulting in brain damage that is primarily localized to the cortex and as such, represents a model with substantial clinical relevance. Finally, K-type (also referred to as B-type) cytosine-guanine-rich DNA oligonucleotides, the class of agents employed in this study, are currently in use in human clinical trials, underscoring the feasibility of this treatment in patients at risk of cerebral ischemia.

Fluoro jade-C staining in the assessment of brain injury after deep hypothermia circulatory arrest

OBJECTIVE

To evaluate the efficacy of Fluoro Jade-C staining (FJC) in the assessment of brain injury after deep hypothermia circulatory arrest (DHCA).

METHODS

Six healthy adult miniature male pigs underwent DHCA, the rectal temperature was down to 18°C, circulation was stopped , circulatory arrest was maintained for 60 minutes. On postoperative day 1, perfusion-fixation was performed on brain tissue. Cerebral cortex, hippocampus, cerebellum were taken for sampling. FJC, hematoxylin-eosin staining (HE), nissl staining (NISSL), terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) were performed to detect the histological and pathological changes. Histological scores of all slices were ranked. Comparison between the FJC and other techniques was done by analysis of variance (ANOVA) according to histological scores.

RESULTS

All animals survived the operation. On the cerebral cortex, in comparison of FJC between HE, NISSL and TUNEL, the p value was 0.90, 0.40, 0.16 respectively (p>0.05). On the hippocampus, the comparison of FJC with HE, NISSL and TUNEL had a p value of 0.12, 0.23, 0.62 respectively (p>0.05). On the cerebellum, in comparing FJC with HE, NISSL and TUNEL, the p value was 0.96, 0.77, 0.96 respectively (p>0.05). On representative regions, the results of FJC were in accordance with that of TUNEL, NISSL and HE. Furthermore, ascertainment of brain injury is easier with FJC.

CONCLUSION

FJC is a reliable and convenient method to assess brain injury after DHCA.

Toll-like receptor 4 in CNS pathologies

The responses of the brain to infection, ischemia and trauma share remarkable similarities. These and other conditions of the CNS coordinate an innate immune response marked by activation of microglia, the macrophage-like cells of the nervous system. An important contributor to microglial activation is toll-like receptor 4, a pathogen-associated molecular pattern receptor known to initiate an inflammatory cascade in response to various CNS stimuli. The present review traces new efforts to characterize and control toll-like receptor 4 in inflammatory etiologies of the nervous system.

The TRIF-dependent signaling pathway is not required for acute cerebral ischemia/reperfusion injury in mice

TIR domain-containing adaptor protein (TRIF) is an adaptor protein in Toll-like receptor (TLR) signaling pathways. Activation of TRIF leads to the activation of interferon regulatory factor 3 (IRF3) and nuclear factor kappa B (NF-kappaB). While studies have shown that TLRs are implicated in cerebral ischemia/reperfusion (I/R) injury and in neuroprotection against ischemia afforded by preconditioning, little is known about TRIF's role in the pathological process following cerebral I/R. The present study investigated the role that TRIF may play in acute cerebral I/R injury. In a mouse model of cerebral I/R induced by transient middle cerebral artery occlusion, we examined the activation of NF-kappaB and IRF3 signaling in ischemic cerebral tissue using ELISA and Western blots. Neurological function and cerebral infarct size were also evaluated 24h after cerebral I/R. NF-kappaB activity and phosphorylation of the inhibitor of kappa B (IkappaBalpha) increased in ischemic brains, but IRF3, inhibitor of kappaB kinase complex-epsilon (IKKepsilon), and TANK-binding kinase1 (TBK1) were not activated after cerebral I/R in wild-type (WT) mice. Interestingly, TRIF deficit did not inhibit NF-kappaB activity or p-IkappaBalpha induced by cerebral I/R. Moreover, although cerebral I/R induced neurological and functional impairments and brain infarction in WT mice, the deficits were not improved and brain infarct size was not reduced in TRIF knockout mice compared to WT mice. Our results demonstrate that the TRIF-dependent signaling pathway is not required for the activation of NF-kappaB signaling and brain injury after acute cerebral I/R.

Systemic lipopolysaccharide protects the brain from ischemic injury by reprogramming the response of the brain to stroke: a critical role for IRF3

Lipopolysaccharide (LPS) preconditioning provides neuroprotection against subsequent cerebral ischemic injury through activation of its receptor, Toll-like receptor 4 (TLR4). Paradoxically, TLR activation by endogenous ligands after ischemia worsens stroke damage. Here, we define a novel, protective role for TLRs after ischemia in the context of LPS preconditioning. Microarray analysis of brains collected 24 h after stroke revealed a unique set of upregulated genes in LPS-pretreated animals. Promoter analysis of the unique gene set identified an overrepresentation of type I interferon (IFN)-associated transcriptional regulatory elements. This finding suggested the presence of type I IFNs or interferon regulatory factors (IRFs), which upregulate interferon-stimulated genes. Upregulation of IFNbeta was confirmed by real-time reverse transcription-PCR. Direct administration of IFNbeta intracerebroventricularly at the time of stroke was sufficient for neuroprotection. TLR4 can induce both IFNbeta and interferon-stimulated genes through its adapter molecule Toll/interleukin receptor domain-containing adaptor-inducing IFNbeta (TRIF) and the IRF3 transcription factor. We show in oxygen glucose deprivation of cortical neurons, an in vitro model of stroke, that activation of TRIF after stroke reduces neuronal death. Furthermore, mice lacking IRF3 were not protected by LPS preconditioning in our in vivo model. Our studies constitute the first demonstration of the neuroprotective capacity of TRIF/IRF3 signaling and suggest that interferon-stimulated genes, whether induced by IFNbeta or by enhanced TLR signaling to IRF3, are a potent means of protecting the brain against ischemic damage.

Toll-like receptor signaling in endogenous neuroprotection and stroke

Stroke and other cerebral vascular diseases are a leading cause of morbidity and mortality in the United States. Despite intensive research to identify interventions that lessen cerebrovascular injury, no major therapies exist. Development of stroke prophylaxis involves an understanding of the mechanisms of damage following cerebral ischemia, and elucidation of the endogenous mechanisms that combat further brain injury. Toll-like receptors (TLRs) are critical components of the innate immune system that have been shown recently to mediate ischemic injury. Paradoxically, TLR ligands administered systemically induce a state of tolerance to subsequent ischemic injury. Herein we suggest that stimulation of TLRs prior to ischemia reprograms TLR signaling that occurs following ischemic injury. Such reprogramming leads to suppressed expression of pro-inflammatory molecules and enhanced expression of numerous anti-inflammatory mediators that collectively confer robust neuroprotection. Our findings indicate that numerous preconditioning stimuli lead to TLR activation, an event that occurs prior to ischemia and ultimately leads to TLR reprogramming. Thus genomic reprogramming of TLR signaling may be a unifying principle of tolerance to cerebral ischemia.

Mechanisms of ischemic brain damage

In the United States stroke is the third leading cause of death and the leading cause of disability. Brain injury following stroke results from the complex interplay of multiple pathways including excitotoxicity, acidotoxicity, ionic imbalance, peri-infarct depolarization, oxidative and nitrative stress, inflammation and apoptosis. There are very few treatments for stroke and the development of new treatments requires a comprehensive understanding of the diverse mechanisms of ischemic brain damage that are responsible for neuronal death. Here, we discuss the underlying pathophysiology of this devastating disease and reveal the intertwined pathways that are the target of therapeutic intervention.

Pediatric stroke: current developments

PURPOSE OF REVIEW

Pediatric arterial ischemic stroke is relatively rare but carries a considerable impact and high risk of recurrence--especially in patients with sickle cell disease and various forms of vasculopathy. We will discuss risk factors, vascular physiology, and primary/secondary/rehabilitative therapies. Goals for future investigation and treatment are suggested.

RECENT FINDINGS

Risk factors include chronic disease, vasculopathy, acute illness, cardiac disease, head and neck trauma, infection, and prothrombic disorders. Research has begun to implicate genetic risk factors--initially in sickle cell disease and more recently in prothrombotic disorders, moyamoya, and nitric oxide regulation. The vascular physiology of pediatric arterial ischemic stroke, especially sickle cell disease stroke, is currently undergoing study in animal models and in humans. No primary prevention therapy for pediatric arterial ischemic stroke is known. Various primary and secondary prevention therapies are used, however, in at-risk sickle cell disease patients. Aspirin, coumadin, and heparin are often initiated as secondary prevention therapies for non-sickle cell disease-associated arterial ischemic stroke, but no studies have assessed efficacy.

SUMMARY

Pediatric arterial ischemic stroke is under-recognized and under-studied. Investigation into the hemodynamic aspects of arterial ischemic stroke, although best studied thus far in sickle cell disease, has been neglected. It is likely that enhanced study of hemodynamics and autoregulation will elucidate both new prevention opportunities and novel treatments.

Toll-like receptors: novel pharmacological targets for the treatment of neurological diseases

Toll-like receptors (TLRs) are a family of evolutionarily conserved molecules that directly detect pathogen invasion or tissue damage and initiate a biological response. TLRs can signal through two primary intracellular pathways and as such can induce either immuno-stimulatory or immuno-modulatory molecules. Both sides of this twin-edged sword are being examined for their therapeutic potential in combating neurological disease. The immuno-stimulatory properties of TLRs are being used to generate tumor-specific immune responses to CNS tumors while the immuno-modulatory properties are being used to suppress damaging inflammatory responses to stroke. Recently, a third component of TLR signaling has begun to emerge--that of direct neuroprotection. Hence, the TLRs offer novel targets for the treatment of neurological disease.