LPS preconditioning redirects TLR signaling following stroke: TRIF-IRF3 plays a seminal role in mediating tolerance to ischemic injury

Crosstalk between Inflammation and the BBB in Stroke

The blood-brain barrier (BBB), which is located at the interface between the central nervous system (CNS) and the circulatory system, is instrumental in establishing and maintaining the microenvironmental homeostasis of the CNS. BBB disruption following stroke promotes inflammation by enabling leukocytes, T cells and other immune cells to migrate via both the paracellular and transcellular routes across the BBB and to infiltrate the CNS parenchyma. Leukocytes promote the removal of necrotic tissues and neuronal recovery, but they also aggravate BBB injury and exacerbate stroke outcomes, especially after late reperfusion. Moreover, the swelling of astrocyte endfeet is thought to contribute to the ‘no-reflow’ phenomenon observed after cerebral ischemia, that is, blood flow cannot return to capillaries after recanalization of large blood vessels. Pericyte recruitment and subsequent coverage of endothelial cells (ECs) alleviate BBB disruption, which causes the transmigration of inflammatory cells across the BBB to be a dynamic process. Furthermore, interneurons and perivascular microglia also make contacts with ECs, astrocytes and pericytes to establish the neurovascular unit. BBB-derived factors after cerebral ischemia triggered microglial activation. During the later stage of injury, microglia remain associated with brain ECs and contribute to repair mechanisms, including postinjury angiogenesis, by acquiring a protective phenotype, which possibly occurs through the release of microglia-derived soluble factors. Taken together, we reviewed dynamic and bidirectional crosstalk between inflammation and the BBB during stroke and revealed targeted interventions based on the crosstalk between inflammation and the BBB, which will provide novel insights for developing new therapeutic strategies.

Trained Innate Immunity by Repeated Low-Dose Lipopolysaccharide Injections Displays Long-Term Neuroprotective Effects

Disrupted immune response is an important feature of many neurodegenerative conditions, including sepsis-associated cognitive impairment. Accumulating evidence has demonstrated that immune memory occurs in microglia, which has a significant impact on pathological hallmarks of neurological diseases. However, it remains unclear whether immune memory can cause subsequent alterations in the brain immune response and affect neurobehavioral outcomes in sepsis survivors. In the present study, mice received daily intraperitoneal injection of low-dose lipopolysaccharide (LPS, 0.1 mg/kg) for three consecutive days to induce immune memory (immune tolerance) and then were subjected to sham operation or cecal ligation and puncture (CLP) 9 months later, followed by a battery of neurobehavioral and biochemical studies. Here, we showed that repeated low-dose LPS injection-induced immune memory protected mice from sepsis-induced cognitive and affective impairments, which were accompanied by significantly decreased brain proinflammatory cytokines and immune response. In conclusion, our study suggests that modulation of brain immune responses by repeated LPS injections confers neuroprotective effects by preventing overactivated immune response in response to subsequent septic insult.

Review Cerebral Ischemic Tolerance and Preconditioning: Methods, Mechanisms, Clinical Applications, and Challenges

Stroke is one of the leading causes of morbidity and mortality worldwide, and it is increasing in prevalence. The limited therapeutic window and potential severe side effects prevent the widespread clinical application of the venous injection of thrombolytic tissue plasminogen activator and thrombectomy, which are regarded as the only approved treatments for acute ischemic stroke. Triggered by various types of mild stressors or stimuli, ischemic preconditioning (IPreC) induces adaptive endogenous tolerance to ischemia/reperfusion (I/R) injury by activating a multitude cascade of biomolecules, for example, proteins, enzymes, receptors, transcription factors, and others, which eventually lead to transcriptional regulation and epigenetic and genomic reprogramming. During the past 30 years, IPreC has been widely studied to confirm its neuroprotection against subsequent I/R injury, mainly including local ischemic preconditioning (LIPreC), remote ischemic preconditioning (RIPreC), and cross preconditioning. Although LIPreC has a strong neuroprotective effect, the clinical application of IPreC for subsequent cerebral ischemia is difficult. There are two main reasons for the above result: Cerebral ischemia is unpredictable, and LIPreC is also capable of inducing unexpected injury with only minor differences to durations or intensity. RIPreC and pharmacological preconditioning, an easy-to-use and non-invasive therapy, can be performed in a variety of clinical settings and appear to be more suitable for the clinical management of ischemic stroke. Hoping to advance our understanding of IPreC, this review mainly focuses on recent advances in IPreC in stroke management, its challenges, and the potential study directions.

Neuroprotection by Neurotropin through Crosstalk of Neurotrophic and Innate Immune Receptors in PC12 Cells

Infected or damaged tissues release multiple “alert” molecules such as alarmins and damage-associated molecular patterns (DAMPs) that are recognized by innate immune receptors, and induce tissue inflammation, regeneration, and repair. Recently, an extract from inflamed rabbit skin inoculated with vaccinia virus (Neurotropin^®, NTP) was found to induce infarct tolerance in mice receiving permanent ischemic attack to the middle cerebral artery. Likewise, we report herein that NTP prevented the neurite retraction in PC12 cells by nerve growth factor (NGF) deprivation. This effect was accompanied by interaction of Fyn with high-affinity NGF receptor TrkA. Sucrose density gradient subcellular fractionation of NTP-treated cells showed heretofore unidentified membrane fractions with a high-buoyant density containing Trk, B subunit of cholera toxin-bound ganglioside, flotillin-1 and Fyn. Additionally, these new membrane fractions also contained Toll-like receptor 4 (TLR4). Inhibition of TLR4 function by TAK-242 prevented the formation of these unidentified membrane fractions and suppressed neuroprotection by NTP. These observations indicate that NTP controls TrkA-mediated signaling through the formation of clusters of new membrane microdomains, thus providing a platform for crosstalk between neurotrophic and innate immune receptors. Neuroprotective mechanisms through the interaction with innate immune systems may provide novel mechanism for neuroprotection.

Mitochondrial Complex I Is an Essential Player in LPS-Induced Preconditioning in Differentiated PC12 Cells

Preconditioning (PC) as a protective strategy against noxious insults can decline cell death and apoptosis. It has been approved that mitochondria play a key role in PC mechanism. The critical role of complex I (CI) in oxidative phosphorylation machinery and intracellular ROS production, particularly in the brain, accentuates its possible role in PC-induced neuroprotection. Here, differentiated PC12 cells were preconditioned with ultra-low dose LPS (ULD, 3 μg/mL) prior to exposure to high concentration of LPS (HD, 750 μg/mL). Our results showed that HD LPS treatment reduces cell viability and CI activity, and intensifies expression of cleaved caspase 3 compared to the control group. Intriguingly, PC induction resulted in enhancement of cell viability and CI activity and reduction of caspase3 cleavage compared to HD LPS group. In order to explore the role of CI in PC, we combined the ULD LPS with rotenone, a CI inhibitor. Following rotenone administration, cell viability significantly reduced while caspase3 cleavage increased compared to PC induction group. Taken together, cell survival and reduction of apoptosis followed by PC can be at least partially attributed to the preservation of mitochondrial CI function.

Neuroprotective Effects of Trilobatin, a Novel Naturally Occurring Sirt3 Agonist from Lithocarpus polystachyus Rehd., Mitigate Cerebral Ischemia/Reperfusion Injury: Involvement of TLR4/NF-κB and Nrf2/Keap-1 Signaling

Aims: Neuroinflammation and oxidative stress are deemed the prime causes of brain injury after cerebral ischemia/reperfusion (I/R). Since the silent mating-type information regulation 2 homologue 3 (Sirt3) pathway plays an imperative role in protecting against neuroinflammation and oxidative stress, it has been verified as a target to treat ischemia stroke. Therefore, we attempted to seek novel Sirt3 agonist and explore its underlying mechanism for stroke treatment both in vivo and in vitro. Results: Trilobatin (TLB) not only dramatically suppressed neuroinflammation and oxidative stress injury after middle cerebral artery occlusion in rats, but also effectively mitigated oxygen and glucose deprivation/reoxygenation injury in primary cultured astrocytes. These beneficial effects, along with the reduced proinflammatory cytokines via suppressing Toll-like receptor 4 (TLR4) signaling pathway, lessened oxidative injury via activating nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathways, in keeping with the findings in vivo. Intriguingly, the TLB-mediated neuroprotection on cerebral I/R injury was modulated by reciprocity between TLR4-mediated neuroinflammatory responses and Nrf2 antioxidant responses as evidenced by molecular docking and silencing TLR4 and Nrf2, respectively. Most importantly, TLB not only directly bonded to Sirt3 but also increased Sirt3 expression and activity, indicating that Sirt3 might be a promising therapeutic target of TLB. Innovation: TLB is a naturally occurring Sirt3 agonist with potent neuroprotective effects via regulation of TLR4/nuclear factor-kappa B and Nrf2/Kelch-like ECH-associated protein 1 (Keap-1) signaling pathways both in vivo and in vitro. Conclusion: Our findings indicate that TLB protects against cerebral I/R-induced neuroinflammation and oxidative injury through the regulation of neuroinflammatory and oxidative responses via TLR4, Nrf2, and Sirt3, suggesting that TLB might be a promising Sirt3 agonist against ischemic stroke.

Inflammation Induced by Natural Neuronal Death and LPS Regulates Neural Progenitor Cell Proliferation in the Healthy Adult Brain

Inflammation is typically considered a negative response to injury or insult; however, recent advances demonstrate that inflammatory cells regulate development, plasticity, and homeostasis through anticytotoxic, progenerative responses. Here, we extend analyses of neuroinflammation to natural neurodegenerative and homeostatic states by exploiting seasonal plasticity in cytoarchitecture of the avian telencephalic song control nucleus, high vocal center [HVC (proper name)], in the songbird Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). We report that local injection of the endotoxin lipopolysaccharide into HVC of birds in both breeding (high circulating testosterone level) and nonbreeding (low circulating testosterone level) conditions increased neural progenitor cell proliferation in the nearby but distinct ventricular zone. Additionally, we found that oral administration of the anti-inflammatory drug minocycline during seasonal regression of HVC reduced microglia activation in HVC and prevented the normal proliferative response in the ventricular zone to apoptosis in HVC. Our results suggest that local neuroinflammation positively regulates neural progenitor cell proliferation and, in turn, contributes to the previously described repatterning of HVC cytoarchitecture following seasonally induced neuronal loss.

Ethanol extracts from Ilex pubescens promotes cerebral ischemic tolerance via modulation of TLR4-MyD88/TRIF signaling pathway in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Pubescent Holly Root is the dry root of Ilex pubescens Hook. et Arn. It is clinically using in the treatment for stroke and coronary artery disease. It remains unclear whether the ethanol extracts of Ilex pubescens(IPEE) treatment can promote cerebral ischemic tolerance (CIT) and exert endogenous neuroprotective effects and thus to alleviate the nerve injury caused by the subsequent persistent cerebral ischemic attacks.

AIM OF THE STUDY

To investigate the effects of IPEE on CIT and its underlying molecular mechanisms.

MATERIALS AND METHODS

Adult male Wistar rats were used in the present study. The bilateral common carotid arteries were blocked for 10 min followed a subsequent reperfusion to create the cerebral ischemic preconditioning (CIP); After 3 days post CIP, rats were subjected to middle cerebral artery occlusion/reperfusion (MCAO/R)-injury. Rats were continuously fed with IPEE for 5 days throughout the experiment period at the dose of 100 mg/kg and 200 mg/kg, respectively. Then, the brain infarct volume, histopathology, neurological deficits, and the gene/protein expression related with the TLR4-MyD88/TRIF signaling pathway were evaluated after 24 h of MCAO/R experiment.

RESULTS

IPEE pretreatment significantly reduced the cerebral infarct volume, the neurological deficit scores, and the plasma level of neuron specific enolase (NSE) at the dose of 100 mg/kg. Meanwhile, IPEE pretreatment significantly decreased the levels of inflammatory cytokines including TNF-α, IL-6, MCP-1, MIP-1α and RANTES, while it increased the levels of anti-inflammatory cytokines, such as IL-10 and TGF-β, when compared with the group with CIP treatment alone. Moreover, the effect of IPEE treatment on CIT was in a dose-dependent manner, showing as a better effect in the group pretreated with IPEE with the dose of 100 mg/kg than that in group pretreated with IPEE with the dose of 200 mg/kg. In addition, IPEE pretreatment significantly inhibited the expressions of MyD88 mRNA and the protein expression of COX-2 and NF-κBp65, while it strengthened the expressions of TRIF mRNA and protein. The effects of IPEE pretreatment on the expression of these genes were better than that in the group treated with CIP alone.

CONCLUSIONS

The present study demonstrates that IPEE pretreatment can enhance cerebral ischemic tolerance with a underlying mechanism involved in the toll-like receptor 4 (TLR4) signaling pathway through inhibiting the production of proteins or cytokines in the downstream of MyD88 and activating TRIF dependent anti-inflammatory pathways.

Dose-Dependent Microglial and Astrocytic Responses Associated With Post-ischemic Neuroprotection After Lipopolysaccharide-Induced Sepsis-Like State in Mice

In contrast to lipopolysaccharide (LPS)-induced preconditioning, which has repeatedly been examined in the past, the effects of post-ischemic LPS-induced sepsis, although clinically considerably more important, have not systemically been studied. We exposed mice to transient intraluminal middle cerebral artery occlusion (MCAO) and examined the effects of intraperitoneal LPS (0.1 or 1 mg/kg) which was administered 24 h post-ischemia. Post-ischemic glial reactivity, neuronal survival and neurological outcome were differently modulated by the higher and the lower LPS dose. Although both doses promoted neuronal survival after 72 h, the underlying mechanisms were not similar. Mice receiving 1 mg/kg LPS exhibited transient hypothermia at 1 and 3 hours post sepsis (hps), followed by reduced focal neurological deficits at 24, 48 and 72 hps. The lower dose (0.1 mg/kg) did not induce hypothermia, but reduced microglia/macrophage activation with the appearance of an anti-inflammatory CD206 positive cell phenotype in the brain parenchyma. Together, our results indicate a novel, dose-dependent modulation of microglial cells that is intricately involved in brain protection.

Resveratrol inhibits LPS-induced inflammation through suppressing the signaling cascades of TLR4-NF-κB/MAPKs/IRF3

Resveratrol (Res) is a natural compound that possesses anti-inflammatory properties. However, the protective molecular mechanisms of Res against lipopolysaccharide (LPS)-induced inflammation have not been fully studied. In the present study, RAW264.7 cells were stimulated with LPS in the presence or absence of Res, and the subsequent modifications to the LPS-induced signaling pathways caused by Res treatment were examined. It was identified that Res decreased the mRNA levels of Toll-like receptor 4 (TLR4), myeloid differentiation primary response protein MyD88, TIR domain-containing adapter molecule 2, which suggested that Res may inhibit the activation of the TLR4 signaling pathway. It suppressed the expression levels of total and phosphorylated TLR4, NF-κB inhibitor, p38 mitogen-activated protein kinase (MAPK), c-Jun N-terminal kinase, extracellular signal-regulated kinase 1/2 and interferon (IFN) regulatory factor 3 (IRF3) proteins. Following treatment with Res or specific inhibitors, the production of pro-inflammatory mediators including tumor necrosis factor-α, interleukin (IL)-6, IL-8 and IFN-β were decreased and the expression of anti-inflammatory mediator IL-10 was increased. These results suggested that Res may inhibit the signaling cascades of NF-κB, MAPKs and IRF3, which modulate pro-inflammatory cytokines. In conclusion, Res exhibited a therapeutic effect on LPS-induced inflammation through suppression of the TLR4-NF-κB/MAPKs/IRF3 signaling cascades.

The role of microglia in ischemic preconditioning

Ischemic preconditioning (IPC) is an experimental phenomenon in which a brief ischemic stimulus confers protection against a subsequent prolonged ischemic event. Initially thought to be due to mechanistic changes in neurons, our understanding of IPC has evolved to encompass a global reprogramming of the Central Nervous System (CNS) after transient ischemia/reperfusion that requires innate immune signaling pathways including Toll-like receptors (TLRs) and Type I interferons. Microglia are the CNS resident neuroimmune cells that express these key innate immune receptors. Studies suggest that microglia are required for IPC-mediated neuronal and axonal protection. Multiple paradigms targeting TLRs have converged on a distinctive Type I interferon response in microglia that is critical for preconditioning-mediated protection against ischemia. These pathways can be targeted through administration of TLR agonists, cytokines including interferon-β, and pharmaceutical agents that induce preconditioning through cross-tolerance mechanisms. Transcriptomic analyses and single cell RNA studies point to specific gene expression signatures in microglia that functionally shift these mutable cells to an immunomodulatory or protective phenotype. Although there are technological challenges and gaps in knowledge to overcome, the targeting of specific molecular signaling pathways in microglia is a promising direction for development of novel and effective pharmacotherapies for stroke. Studies on preconditioning in animal models, including nonhuman primates, show promise as prophylactic preconditioning treatments for selected at risk patient populations. In addition, our growing understanding of the mechanisms of IPC-mediated protection is identifying novel cellular and molecular targets for therapeutic interventions that could apply broadly to both acute stroke and chronic vascular cognitive impairment patients.

Toll-like receptor 4: A promising therapeutic target for pneumonia caused by Gram-negative bacteria

Gram‐negative bacteria (GNB) emerge as important pathogens causing pulmonary infection, which can develop into sepsis due to bacterial resistance to antibiotics. GNB pneumonia poses a huge social and economic burden all over the world. During GNB infection in the lung, Toll‐like receptor 4 (TLR4) can form a complex with MD2 and CD14 after recognizing lipopolysaccharide of GNB, initiate the MyD88‐ and TRIF‐dependent signalling pathways and stimulate host non‐specific immune response. In this review, we summarize recent progress in our understanding of the role of TLR4 in GNB pneumonia. The latest experimental results, especially in TLR4 knockout animals, suggest a promising potential of targeting TLR4 signalling pathway for the treatment of GNB pneumonia. Furthermore, we highlight the benefits of Traditional Chinese Medicine as novel candidates for the therapy of GNB pneumonia due to the modulation of TLR4 signalling pathway. Finally, we discuss the promise and challenge in the development of TLR4‐based drugs for GNB pneumonia.

Oridonin Ameliorates Lipopolysaccharide-Induced Endometritis in Mice via Inhibition of the TLR-4/NF-κBpathway

Endometritis is a health threat to both humans and animals and poses a huge economic burden. Oridonin (Ori) is a natural diterpenoid isolated from the traditional Chinese herb Rabdosiarubescens (R. rubescens) and has multiple health-promoting effects, including antioxidant, anti-inflammatory, and antitumor effects. There is little evidence showing that Ori can effectively treat endometritis, and the relevant mechanisms need to be further clarified. In this study, we investigated the effects of Ori on LPS-induced endometritis in vivo. Additionally, we examined the effects of Ori on LPS-stimulated mouse endometrial epithelial cells (mEECs). The results showed that Ori treatment significantly alleviated LPS-induced endometritis and reduced the activity of myeloperoxidase. ELISA and qPCR results indicated that Ori dose-dependently decreased the expression of TNF-α, IL-1β, and IL-6 both in tissues and in mEECs. In addition, Ori was found to inhibit LPS-induced TLR4/NF-κB signaling pathway activation. These results suggest that Ori effectively attenuates LPS-induced endometritis by inhibiting the TLR4/NF-κB signaling pathway and that Ori might be an effective drug for the prevention and treatment of LPS-induced endometritis.

[Collateral circulation and Toll-like receptor 4 levels in patients with acute cerebral infarction after intravenous thrombolysis]

OBJECTIVE

To investigate the relationship between Toll-like receptor 4 (TLR4) and collateral circulation in patients with acute cerebral infarction (AIS) after thrombolytic therapy.

METHODS

This retrospective, observational cohort study was conducted among 65 patients with AIS receiving thrombolytic therapy, who were divided according to findings by computed tomographic angiography (CTA) into good collateral circulation (group A, n = 34) and poor collateral circulation (group B, n = 31). Serum samples were collected from all the patients and the levels of TLR4 were measured with ELISA.

RESULTS

The patients in group A had significantly better outcomes than those in group B. The NIHSS scores at 24 h and 30 days after thrombolytic therapy, mRS scores at 90 days and serum TLR4 levels were significantly lower in group A than in group B (P < 0.05); the percentages of patients with symptomatic intracerebral hemorrhage were comparable between the two groups. The serum levels of TLR4 were negatively correlated with the rMLC score (P < 0.05). Multivariate logistic regression analysis showed that a high level of TLR4 was associated with a poor collateral circulation after thrombolysis.

CONCLUSIONS

Good collateral circulation can increase the benefit of intravenous thrombolysis in patients with ACI, and the level of TLR4 is a predictive factor for the compensation of collateral circulation following ACI.

Identification of key transcription factors associated with cerebral ischemia‑reperfusion injury based on gene‑set enrichment analysis

Cerebral ischemia‑reperfusion injury (CIRI) usually causes detrimental complications following reperfusion therapy in stroke patients. The present study systematically investigated the regulatory mechanism involved in the pathogenesis of CIRI using gene set enrichment analysis of the transient middle cerebral artery occlusion mouse stroke model. The results revealed a total of 13 CIRI‑related transcription factors (TFs), including CCAAT enhancer binding protein b (Cebpb), Cebpa, early growth response‑1, Fos, Rela, Jund, signal transduction and activator of transcription 5a/b, transformation related protein 53, GLI family zinc finger 2 (Gli2), Sp3, TF AP‑2 α (Tfap2a) and spleen focus forming virus proviral integration oncogene (Spi1). To the best of our knowledge, five TFs (Cebpa, Gli2, Sp3, Tfap2a and Spi1) were the first to be reported associated with CIRI in the present study. The five novel CIRI‑related TFs were mainly associated with pathways of inflammation and responses to reperfusion, including the tumor necrosis factor signaling pathway (Gli2, Spi1 and Tfap2a, P=0.0035, 0.0035 and 0.048, respectively), interleuking‑17 signaling pathway (Cebpa, Gli2, Sp3, Spi1 and Tfap2a, P=0.019, 0.047, 0.019, 0.035 and 0.005, respectively) and fluid shear stress and atherosclerosis (Gli2, Sp3, Spi1 and Tfap2a, P=0.047, 0.046, 0.013 and 0.003, respectively). These results may improve understanding of the potential molecular mechanism underlying the pathogenesis of CIRI at the genome‑wide level.

Transcriptomic immaturity inducible by neural hyperexcitation is shared by multiple neuropsychiatric disorders

Biomarkers are needed to improve the diagnosis of neuropsychiatric disorders, which are often associated to excitatory/inhibitory imbalances in neural transmission and abnormal maturation. Here, we characterized different disease conditions by mapping changes in the expression patterns of maturation-related genes whose expression was altered by experimental neural hyperexcitation in published studies. This analysis revealed two gene expression patterns: decreases in maturity markers and increases in immaturity markers. These two groups of genes were characterized by the over-representation of genes related to synaptic function and chromosomal modification, respectively. Using these two groups in a transdiagnostic analysis of 87 disease datasets for eight neuropsychiatric disorders and 12 datasets from corresponding animal models, we found that transcriptomic pseudoimmaturity inducible by neural hyperexcitation is shared by multiple neuropsychiatric disorders, such as schizophrenia, Alzheimer disorders, and amyotrophic lateral sclerosis. Our results indicate that this endophenotype serves as a basis for the transdiagnostic characterization of these disorders.

Tomoyuki Murano et al. showed that neural hyperexcitation increases the expression of immaturity related genes. These changes in gene expression are shared among different neuropsychiatric and neurological conditions, hinting at their potential role as biomarkers.

Prestimulation of Microglia Through TLR4 Pathway Promotes Interferon Beta Expression in a Rat Model of Alzheimer's Disease

Soluble amyloid beta (Aβ) oligomers are the most common forms of Aβ in the early stage of Alzheimer's disease (AD). They are highly toxic to the neurons but their capability to activate microglia remains controversial. Microglia develop two distinct phenotypes, classic (M1) and alternative (M2). Tuning of microglia to the alternative (anti-inflammatory) state is of major interest in treatment of neuroinflammatory disease. This study aimed to assess tuning the microglia to produce interferon beta (IFN-β) as an anti-inflammatory cytokine through TLR4 pathway in a rat model of AD. Microglial BV-2 cells were treated with 1 μg/ml lipopolysaccharides (LPS), Monophosphoryl lipid A (MPL), or vehicles for 24 h, and then incubated with Aβ oligomer. After 24 h, cell pellets were harvested and TIR-domain-containing adapter-inducing interferon-β (TRIF), interferon regulatory factor 3 (IRF3), and IFN-β levels were measured. The ligands/vehicle were microinjected into the right ventricle of male Wistar rats every 3 days. Two weeks later, an osmotic pump filled with oligomeric Aβ/vehicle was implanted in the left ventricle. After 2 weeks, TRIF, IRF3, and IFN-β levels were measured in the hippocampal tissue. TNF-α and IFN-β levels were assessed in the hippocampus using immunohistochemistry. The oligomeric Aβ did not change TRIF, IRF3, and IFN-β levels in both cell culture and hippocampal tissue. However, pretreatment with LPS or MPL increased the level of these proteins. BV-2 cells morphologically express M1 state in presence of higher dose of Aβ oligomer (10 μM). Pretreatment with LPS or MPL decreased the TNF-α and increased the number of IFN-β positive cells in the hippocampus of Aβ-treated rats. In conclusion, pretreatment with low dose TLR4 agonists could induce microglia to produce neuroprotective cytokines including IFN-β which may be considered as a potential strategy to combat neuronal degeneration in AD.

Positive and negative conditioning in the neonatal brain

Brain injury in the perinatal period occurs in many clinical settings, e.g. hypoxic-ischemic encephalopathy (HIE) in term infants, neonatal stroke, encephalopathy of prematurity, and infections. These insults often result in life-long disabilities including cerebral palsy, cognitive deficits, visual dysfunction, hearing impairments, and epilepsy. However, the success of clinical implementation of a broad array of potential neuroprotective strategies tested experimentally has been limited with the exception of therapeutic hypothermia (TH) used within hours of birth in term human babies with mild to moderate HIE. There is an extensive search for adjuvant therapeutic approaches to enhance the outcomes. One strategy is to modify susceptibility in the developing CNS by means of preconditioning or postconditioning using sublethal stress. The pre-clinical and clinical literature has shown that CNS immaturity at the time of ischemic insult plays a central role in the response to injury. Thus, better understanding of the molecular regulation of the endogenous vulnerability of the immature brain is needed. Further, the use of sublethal stressors of different origin may help shed light on mechanistic similarities and distinctions beween conditioning strategies. In this review we discuss the mechanisms of protection that are achieved by an interplay of changes on the systemic level and brain level, and via changes of intracellular and mitochondrial signaling. We also discuss the barriers to improving our understanding of how brain immaturity and the type of insult-hypoxic, ischemic or inflammatory-affect the efficacy of conditioning efforts in the immature brain.

Potential Medications or Compounds Acting on Toll-like Receptors in Cerebral Ischemia

Background:

Toll-like receptors play an integral role in the process of inflammatory response after ischemic in-jury. The therapeutic potential acting on TLRs is worth of evaluations. The aim of this review was to introduce readers some potential medications or compounds which could alleviate the ischemic damage via TLRs.

Methods:

Research articles online on TLRs were reviewed. Categorizations were listed according to the follows, methods acting on TLRs directly, modulations of MyD88 or TRIF signaling pathway, and the ischemic tolerance induced by the pre-conditioning or postconditioning with TLR ligands or minor cerebral ischemia via acting on TLRs.

Results:

There are only a few studies concerning on direct effects. Anti-TLR4 or anti-TLR2 therapies may serve as promis-ing strategies in acute events. Approaches targeting on inhibiting NF-κB signaling pathway and enhancing interferon regu-latory factor dependent signaling have attracted great interests. Not only drugs but compounds extracted from traditional Chinese medicine have been used to identify their neuroprotective effects against cerebral ischemia. In addition, many re-searchers have reported the positive therapeutic effects of preconditioning with agonists of TLR2, 3, 4, 7 and 9. Several trails have also explored the potential of postconditioning, which provide a new idea in ischemic treatments. Considering all the evidence above, many drugs and new compounds may have great potential to reduce ischemic insults.

Conclusion:

This review will focus on promising therapies which exerting neuroprotective effects against ischemic injury by acting on TLRs.

Recent advances in endotoxin tolerance

Endotoxin tolerance is defined as a reduced capacity of a cell to respond endotoxin (lipopolysaccharide, LPS) challenge after an initial encounter with endotoxin in advance. The body becomes tolerant to subsequent challenge with a lethal dose of endotoxin and cytokines release and cell/tissue damage induced by inflammatory reaction are significantly reduced in the state of endotoxin tolerance. The main characteristics of endotoxin tolerance are downregulation of inflammatory mediators such as tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β), and C-X-C motif chemokine 10 (CXCL10) and upregulation of anti-inflammatory cytokines such as IL-10 and transforming growth factor β (TGF-β). Therefore, endotoxin tolerance is often regarded as the regulatory mechanism of the host against excessive inflammation. Endotoxin tolerance is a complex pathophysiological process and involved in multiple cellular signal pathways, receptor alterations, and biological molecules. However, the exact mechanism remains elusive up to date. To better understand the underlying cellular and molecular mechanisms of endotoxin tolerance, it is crucial to investigate the comprehensive cellular signal pathways, signaling proteins, cell surface molecules, proinflammatory and anti-inflammatory cytokines, and other mediators. Endotoxin tolerance plays an important role in reducing the mortality of sepsis, endotoxin shock, and other endotoxin-related diseases. Recent reports indicated that endotoxin tolerance is also related to other diseases such as cystic fibrosis, acute coronary syndrome, liver ischemia-reperfusion injury, and cancer. The aim of this review is to discuss the recent advances in endotoxin tolerance mainly based on the cellular and molecular mechanisms by outline the current state of the knowledge of the involvement of the toll-like receptor 4 (TLR4) signaling pathways, negative regulate factor, microRNAs, apoptosis, chromatin modification, and gene reprogramming of immune cells in endotoxin tolerance. We hope to provide a new idea and scientific basis for the rational treatment of endotoxin-related diseases such as endotoxemia, sepsis, and endotoxin shock clinically.

Pivotal role of innate myeloid cells in cerebral post-ischemic sterile inflammation

Inflammatory responses play a multifaceted role in regulating both disability and recovery after ischemic brain injury. In the acute phase of ischemic stroke, resident microglia elicit rapid inflammatory responses by the ischemic milieu. After disruption of the blood-brain barrier, peripheral-derived neutrophils and mononuclear phagocytes infiltrate into the ischemic brain. These infiltrating myeloid cells are activated by the endogenous alarming molecules released from dying brain cells. Inflammation after ischemic stroke thus typically consists of sterile inflammation triggered by innate immunity, which exacerbates the pathologies of ischemic stroke and worsens neurological prognosis. Infiltrating immune cells sustain the post-ischemic inflammation for several days; after this period, however, these cells take on a repairing function, phagocytosing inflammatory mediators and cellular debris. This time-specific polarization of immune cells in the ischemic brain is a potential novel therapeutic target. In this review, we summarize the current understanding of the phase-dependent role of innate myeloid cells in ischemic stroke and discuss the cellular and molecular mechanisms of their inflammatory or repairing polarization from a therapeutic perspective.

Endogenous Protection from Ischemic Brain Injury by Preconditioned Monocytes

Exposure to low-dose lipopolysaccharide (LPS) before cerebral ischemia is neuroprotective in stroke models, a phenomenon termed preconditioning (PC). Although it is well established that LPS-PC induces central and peripheral immune responses, the cellular mechanisms modulating ischemic injury remain unclear. Here, we investigated the role of immune cells in the brain protection afforded by PC and tested whether monocytes may be reprogrammed by ex vivo LPS exposure, thus modulating inflammatory injury after cerebral ischemia in male mice. We found that systemic injection of low-dose LPS induces a Ly6C^hi monocyte response that protects the brain after transient middle cerebral artery occlusion (MCAO) in mice. Remarkably, adoptive transfer of monocytes isolated from preconditioned mice into naive mice 7 h after transient MCAO reduced brain injury. Gene expression and functional studies showed that IL-10, inducible nitric oxide synthase, and CCR2 in monocytes are essential for neuroprotection. This protective activity was elicited even if mouse or human monocytes were exposed ex vivo to LPS and then injected into male mice after stroke. Cell-tracking studies showed that protective monocytes are mobilized from the spleen and reach the brain and meninges, where they suppress postischemic inflammation and neutrophil influx into the brain parenchyma. Our findings unveil a previously unrecognized subpopulation of splenic monocytes capable of protecting the brain with an extended therapeutic window and provide the rationale for cell therapies based on the delivery of autologous or allogeneic protective monocytes in patients after ischemic stroke.SIGNIFICANCE STATEMENT Inflammation is a key component of the pathophysiology of the brain in stroke, a leading cause of death and disability with limited therapeutic options. Here, we investigate endogenous mechanisms of protection against cerebral ischemia. Using lipopolysaccharide (LPS) preconditioning (PC) as an approach to induce ischemic tolerance in mice, we found generation of neuroprotective monocytes within the spleen, from which they traffic to the brain and meninges, suppressing postischemic inflammation. Importantly, systemic LPS-PC can be mimicked by adoptive transfer of in vitro-preconditioned mouse or human monocytes at translational relevant time points after stroke. This model of neuroprotection may facilitate clinical efforts to increase the efficacy of BM mononuclear cell treatments in acute neurological diseases such as cerebral ischemia.

Brain Lipopolysaccharide Preconditioning-Induced Gene Reprogramming Mediates a Tolerance State in Electroconvulsive Shock Model of Epilepsy

There is increasing evidence pointing toward the role of inflammatory processes in epileptic seizures, and reciprocally, prolonged seizures induce more inflammation in the brain. In this regard, effective strategies to control epilepsy resulting from neuroinflammation could be targeted. Based on the available data, preconditioning (PC) with low dose lipopolysaccharide (LPS) through the regulation of the TLR4 signaling pathway provides neuroprotection against subsequent challenge with injury in the brain. To test this, we examined the effects of a single and chronic brain LPS PC, which is expected to lead to reduction of inflammation against epileptic seizures induced by electroconvulsive shock (ECS). A total of 60 male Sprague Dawley rats were randomly assigned to five groups: control, vehicle (single and chronic), and LPS PC (single and chronic). We first recorded the data regarding the behavioral and histological changes. We further investigated the alterations of gene and protein expression of important mediators in relation to TLR4 and inflammatory signaling pathways. Interestingly, significant increased presence of NFκB inhibitors [Src homology 2-containing inositol phosphatase-1 (SHIP1) and Toll interacting protein (TOLLIP)] was observed in LPS-preconditioned animals. This result was also associated with over-expression of IRF3 activity and anti-inflammatory markers, along with down-regulation of pro-inflammatory mediators. Summarizing, the analysis revealed that PC with LPS prior to seizure induction may have a neuroprotective effect possibly by reprogramming the signaling response to injury.

Endotoxin tolerance induced by lipopolysaccharide preconditioning protects against surgery‑induced cognitive impairment in aging mice

Postoperative cognitive dysfunction (POCD) is a clinical syndrome characterized by varying degrees of cognitive functional decline in patients following major surgery. Inflammation and a dysregulated innate immune system exert broad effects in the periphery and central nervous system, yet the mechanisms underlying POCD remain poorly understood and without effective therapy. It has been reported that modulation of the dysregulated inflammatory response with low‑dose lipopolysaccharide (LPS) preconditioning, a phenomenon additionally referred to as endotoxin tolerance, has the potential to reduce neuroinflammation, blood‑brain barrier disruption, and cognitive impairment in a number of disease states. Therefore, it was hypothesized that endotoxin tolerance induced by LPS preconditioning may protect against surgery‑induced cognitive impairment in aging mice. Using a mouse model of surgery‑induced cognitive decline, the present study demonstrated that exploratory laparotomy caused a significant impairment in hippocampal‑dependent memory. Notably, one application of low‑dose LPS preconditioning at 24 h prior to surgery improved the cognitive impairment and abolished the signs of neuroinflammation in the hippocampus following surgery. However, LPS injection at 6 h or immediately prior to surgery did not confer such beneficial effects, suggesting that the effects of LPS‑induced endotoxin tolerance may depend on the time of application. In conclusion, the results of the present study suggested that low‑dose LPS preconditioning may markedly alleviate surgery‑induced neuroinflammation and cognitive impairment in aging mice, which may provide a novel approach to preventing POCD and, potentially, other forms of memory impairment.

Dioscin inhibits ischemic stroke‑induced inflammation through inhibition of the TLR4/MyD88/NF‑κB signaling pathway in a rat model

Diosgenin, as an essential natural steroidal saponin, can be extracted from numerous sources, primarily from fenugreek. It is an important raw material for the synthesis of steroid hormone drugs. It exhibits antitumor, anti‑inflammatory, antioxidation and several other significant pharmacologic actions, and is of high pharmaceutical value. In the present study, the activities and underlying mechanisms of dioscin in the inhibition of ischemic stroke in rats were investigated. Inflammatory responses wer analyzed using ELISA kits and caspase‑3 and caspase‑9 activity was analyzed using Caspase‑3 and caspase‑9 activity kits. Western blot analysis was used to measure Toll‑like receptor 4 (TLR4), myeloid differentiation factor 88 (MyD88), nuclear factor‑κB (NF‑κB), transforming growth factor‑β1 (TGF‑β1), high‑mobility group protein 1 (HMGB‑1), interleukin‑1 receptor‑associated kinase 1 (IRAK1), and tumor necrosis factor receptor‑associated factor 6 (TRAF6) protein expression. Dioscin inhibited infarct volume and neurological scores in the ischemic stroke rat model. The results demonstrated that dioscin reduced inflammatory responses, and suppressed the expression of TLR4, MyD88, NF‑κB, TGF‑β1, HMGB‑1, IRAK1, and TRAF6 in the rat ischemic stroke model. Taken together, these findings suggested that dioscin inhibited ischemic stroke‑induced inflammation through inhibition of the TLR4/MyD88/NF‑kB‑induced inflammation the rat model, which provided novel insights into the mechanisms underlying the effect of dioscin as an anti‑inflammatory candidate for the treatment of ischemic stroke in in the future.

Role of microglia in ischemic focal stroke and recovery: focus on Toll-like receptors

Stroke is the leading cause of disability in adults. Drug treatments that target stroke-induced pathological mechanisms and promote recovery are desperately needed. In the brain, an ischemic event triggers major inflammatory responses that are mediated by the resident microglial cells. In this review, we focus on the microglia activation after ischemic brain injury as a target of immunomodulatory therapeutics. We divide the microglia-mediated events following ischemic stroke into three categories: acute, subacute, and long-term events. This division encompasses the spatial and temporal dynamics of microglia as they participate in the pathophysiological changes that contribute to the symptoms and sequela of a stroke. The importance of Toll-like receptor (TLR) signaling in the outcomes of these pathophysiological changes is highlighted. Increasing evidence shows that microglia have a complex role in stroke pathophysiology, and they mediate both detrimental and beneficial effects on stroke outcome. So far, most of the pharmacological studies in experimental models of stroke have focused on neuroprotective strategies which are impractical for clinical applications. Post-ischemic inflammation is long lasting and thus, could provide a therapeutic target for novel delayed drug treatment. However, more studies are needed to elucidate the role of microglia in the recovery process from an ischemic stroke and to evaluate the therapeutic potential of modulating post-ischemic inflammation to promote functional recovery.

TLR4 Inactivation in Myeloid Cells Accelerates Bone Healing of a Calvarial Defect Model in Mice

Supplemental Digital Content is available in the text.

Background:

Toll-like receptor 4 (TLR4) has been implicated in inflammation-induced bone destruction in various chronic bone diseases; however, its direct influence on bone healing is not well understood. The authors’ previous study showed accelerated bone healing with higher osteoclastogenesis gene expression in toll-like receptor 4 knockout mice (TLR4^-/-). This study aimed to further elucidate the underlying cellular mechanisms during fracture healing by generating a myeloid cell-specific toll-like receptor 4 knockout model (Lyz-TLR4^-/- mice).

Methods:

Calvarial defects, 1.8 mm in diameter, were created in wild-type, TLR4^-/-, and Lyz-TLR4^-/- mice. Bone healing was investigated using micro–computed tomography and histologic, histomorphometric, and immunohistochemistry analyses. Primary bone marrow–derived cells were also isolated from wild-type, TLR4^-/-, and Lyz-TLR4^-/- mice to measure their osteoclast differentiation and resorption properties.

Results:

A similar faster bone healing response, with active intramembranous bone formation, intense osteopontin staining, and more osteoblast infiltration, was observed in TLR4^-/- and Lyz-TLR4^-/- mice. Tartrate-resistant acid phosphatase staining showed more osteoclast infiltration in Lyz-TLR4^-/- mice than in wild-type mice at day 7. Primary bone marrow–derived cells isolated from TLR4^-/- and Lyz-TLR4^-/- mice presented enhanced osteoclastogenesis and resorption activity compared with those from wild-type mice. Comparable M0, M1, and M2 macrophage infiltration was found among all groups at days 1, 4, and 7.

Conclusions:

This study revealed that inactivation of toll-like receptor 4 in myeloid cells enhanced osteoclastogenesis and accelerated healing response during skull repair. Together with the role of toll-like receptor 4 in inflammation-mediated bone destruction, it suggests that toll-like receptor 4 might regulate inflammation-induced osteoclastogenesis under different clinical settings.

Toll like receptor 4 activation can be either detrimental or beneficial following mild repetitive traumatic brain injury depending on timing of activation

A history of repeated concussion has been linked to the later development of neurodegeneration, which is associated with the accumulation of hyperphosphorylated tau and the development of behavioral deficits. However, the role that exogenous factors, such as immune activation, may play in the development of neurodegeneration following repeated mild traumatic brain injury (rmTBI) has not yet been explored. To investigate, male Sprague-Dawley rats were administered three mTBIs 5days apart using the diffuse impact-acceleration model to generate ∼100G. Sham animals underwent surgery only. At 1 or 5days following the last injury rats were given the TLR4 agonist, lipopolysaccharide (LPS, 0.1mg/kg), or saline. TLR4 activation had differential effects following rmTBI depending on the timing of activation. When given at 1day post-injury, LPS acutely activated microglia, but decreased production of pro-inflammatory cytokines like IL-6. This was associated with a reduction in neuronal injury, both acutely, with a restoration of levels of myelin basic protein (MBP), and chronically, preventing a loss of both MBP and PSD-95. Furthermore, these animals did not develop behavioral deficits with no changes in locomotion, anxiety, depressive-like behavior or cognition at 3months post-injury. Conversely, when LPS was given at 5days post-injury, it was associated acutely with an increase in pro-inflammatory cytokine production, with an exacerbation of neuronal damage and increased levels of aggregated and phosphorylated tau. At 3months post-injury, there was a slight exacerbation of functional deficits, particularly in cognition and depressive-like behavior. This highlights the complexity of the immune response following rmTBI and the need to understand how a history of rmTBI interacts with environmental factors to influence the potential to develop later neurodegeneration.

The role of Toll-like receptor 4 (TLR4) in cardiac ischaemic-reperfusion injury, cardioprotection and preconditioning

Cardiac ischaemic-reperfusion injury (IRI) remains the primary cause of mortality throughout the developed world. Molecular mechanisms underlying IRI are complex and are often interlinked with each other driving a synergistic response. Toll-like receptor 4 (TLR4), an immunosurveillance receptor, is known to enhance tissue injury during IRI by enhancing the inflammatory response. The release of endogenous components during IRI bind onto TLR4 leading to the activation of multiple signalling kinases. Once this event occurs these proteins are defined as danger associated molecular patterns molecules (DAMPs) or alarmins. Examples include heat shock proteins, high mobility group box one (HMGB1) and extracellular matrix proteins, all of which are involved in IRI. However, literature in the last two decades suggests that transient stimulation of TLR4 may suppress IRI and thus improve cardiac recovery. Furthermore, it remains to be seen what role TLR4 plays during ischaemic-preconditioning where acute bouts of ischaemia, preceding a harmful bout of ischaemic-reperfusion, is cardioprotective. The other question which also needs to be considered is that if transient TLR4 signalling drives a preconditioning response then what are the ligands which drive this? Hence the second part of this review explores the possible TLR4 ligands which may promote cardioprotection against IRI.

Enhanced Calvarial Bone Healing in CD11c-TLR4-/- and MyD88-/- Mice

BACKGROUND

Inflammation is integral to the injury response. The inflammatory response is essential to the host defense against infection and also to tissue regeneration and repair. Toll-like receptors (TLRs) are critical activators of the innate immune response and present attractive therapeutic targets for inflammation-modulated tissue regeneration. The authors' previous study showed that depletion of TLR4 resulted in accelerated skull bone healing concurrent with increased expression of osteoclastogenic genes. As such, in the present study, the authors used various knockout mouse models for TLR4 and its associated signaling mediators as tools to further understand the role of Toll-like receptor-mediated inflammation in calvarial bone healing.

METHODS

Calvarial defects (1.8-mm diameter) were created in wild-type, TLR4 knockout (TLR4), TLR2, MyD88, TRIF, TLR4 knockout in myeloid cell (Lyz-TLR4), and TLR4 knockout in dendritic-lineage cell (CD11c-TLR4) mice. Bone healing was examined using micro-computed tomographic, histologic, and histomorphometric analyses.

RESULTS

Micro-computed tomographic and histomorphometric analyses revealed that TLR4-deficient mice (TLR4, Lyz-TLR4, and CD11c-TLR4) exhibited a faster intramembraneous healing response at postoperative day 7, whereas MyD88 and CD11c-TLR4 mice showed enhanced bone healing at day 28.

CONCLUSIONS

The authors' data suggest a detrimental role for TLR4 in CD11c cells, mediated by Myd88 signaling, during calvarial bone healing. The authors have demonstrated that Toll-like receptor signaling components affect calvarial bone healing, establishing a link between the skeletal and immune systems during craniofacial bone healing. Toll-like receptor signaling components might be used to initiate enhanced healing in bone defects to improve clinical outcomes.

An atypical role for the myeloid receptor Mincle in central nervous system injury

The C-type lectin Mincle is implicated in innate immune responses to sterile inflammation, but its contribution to associated pathologies is not well understood. Herein, we show that Mincle exacerbates neuronal loss following ischemic but not traumatic spinal cord injury. Loss of Mincle was beneficial in a model of transient middle cerebral artery occlusion but did not alter outcomes following heart or gut ischemia. High functional scores in Mincle KO animals using the focal cerebral ischemia model were accompanied by reduced lesion size, fewer infiltrating leukocytes and less neutrophil-derived cytokine production than isogenic controls. Bone marrow chimera experiments revealed that the presence of Mincle in the central nervous system, rather than recruited immune cells, was the critical regulator of a poor outcome following transient middle cerebral artery occlusion. There was no evidence for a direct role for Mincle in microglia or neural activation, but expression in a subset of macrophages resident in the perivascular niche provided new clues on Mincle's role in ischemic stroke.

Non-coding RNAs and neuroprotection after acute CNS injuries

Accumulating evidence indicates that various classes of non-coding RNAs (ncRNAs) including microRNAs (miRNAs), PIWI-interacting RNAs (piRNAs) and long non-coding RNAs (lncRNAs) play important roles in normal state as well as the diseases of the CNS. Interestingly, ncRNAs have been shown to interact with messenger RNA, DNA and proteins, and these interactions could induce epigenetic modifications and control transcription and translation, thereby adding a new layer of genomic regulation. The ncRNA expression profiles are known to be altered after acute CNS injuries including stroke, traumatic brain injury and spinal cord injury that are major contributors of morbidity and mortality worldwide. Hence, a better understanding of the functional significance of ncRNAs following CNS injuries could help in developing potential therapeutic strategies to minimize the neuronal damage in those conditions. The potential of ncRNAs in blood and CSF as biomarkers for diagnosis and/or prognosis of acute CNS injuries has also gained importance in the recent years. This review highlighted the current progress in the understanding of the role of ncRNAs in initiation and progression of secondary neuronal damage and their application as biomarkers after acute CNS injuries.

Dual role of histamine on microglia-induced neurodegeneration

Several hypotheses have been raised about the dual role of histamine in neurological disorders, and evidences have shown its crucial involvement in the modulation of microglia-mediated neuroinflammation. Previously, we reported that the administration of histamine induces a deleterious effect by promoting a pro-inflammatory phenotype on microglia that in turn compromises dopaminergic neuronal survival. Contrary, under lipopolysaccharide challenge, histamine inhibits the injurious effect of microglia-mediated inflammation, protecting dopaminergic neurons, suggesting that the modulation of microglial activity is dependent on the environmental context. Thus, histamine and/or histamine receptor agonists may serve to develop new therapeutic approaches to overcome neurodegenerative disorders.

Tollip, an early regulator of the acute inflammatory response in the substantia nigra

BACKGROUND

Tollip is a ubiquitously expressed protein, originally described as a modulator of the IL-1R/TLR-NF-κB signaling pathways. Although this property has been well characterized in peripheral cells, and despite some evidence of its expression in the central nervous system, the role of Tollip in neuroinflammation remains poorly understood. The present study sought to explore the implication of Tollip in inflammation in the substantia nigra pars compacta, the structure affected in Parkinson's disease.

METHODS

We first investigated Tollip distribution in the midbrain by immunohistochemistry. Then, we addressed TLR4-mediated response by intra-nigral injections of lipopolysaccharide (LPS), a TLR4 agonist, on inflammatory markers in Tollip knockout (KO) and wild-type (WT) mice.

RESULTS

We report an unexpectedly high Tollip immunostaining in dopaminergic neurons of the mice brain. Second, intra-nigral injection of LPS led to increased susceptibility to neuroinflammation in Tollip KO compared to Tollip WT mice. This was demonstrated by a significant increase of tumor necrosis factor alpha (TNF-α), interleukin 1 beta (IL-1β), interleukin 6 (IL-6), and interferon gamma (IFN-γ) messenger RNA (mRNA) in the midbrain of Tollip KO mice upon LPS injection. Consistently, brain rAAV viral vector transduction with a nuclear factor kappa B (NF-κB)-inducible reporter gene confirmed increased NF-κB activation in Tollip KO mice. Lastly, Tollip KO mice displayed higher inducible NO synthase (iNOS) production, both at the messenger and protein level when compared to LPS-injected WT mice. Tollip deletion also aggravated LPS-induced oxidative and nitrosative damages, as indicated by an increase of 8-oxo-2'-deoxyguanosine and nitrotyrosine immunostaining, respectively.

CONCLUSIONS

Altogether, these findings highlight a critical role of Tollip in the early phase of TLR4-mediated neuroinflammation. As brain inflammation is known to contribute to Parkinson's disease, Tollip may be a potential target for neuroprotection.

Inflammation and Stroke: An Overview

The immune response to acute cerebral ischemia is a major factor in stroke pathobiology and outcome. While the immune response starts locally in occluded and hypoperfused vessels and the ischemic brain parenchyma, inflammatory mediators generated in situ propagate through the organism as a whole. This "spillover" leads to a systemic inflammatory response first, followed by immunosuppression aimed at dampening the potentially harmful proinflammatory milieu. In this overview we will outline the inflammatory cascade from its starting point in the vasculature of the ischemic brain to the systemic immune response elicited by brain ischemia. Potential immunomodulatory therapeutic approaches, including preconditioning and immune cell therapy will also be discussed.

Biased signalling is an essential feature of TLR4 in glioma cells

A distinct feature of the Toll-like receptor 4 (TLR4) is its ability to trigger both MyD88-dependent and MyD88-independent signalling, culminating in activation of pro-inflammatory NF-κB and/or the antiviral IRF3. Although TLR4 agonists (lipopolysaccharides; LPSs) derived from different bacterial species have different endotoxic activity, the impact of LPS chemotype on the downstream signalling is not fully understood. Notably, different TLR4 agonists exhibit anti-tumoural activity in animal models of glioma, but the underlying molecular mechanisms are largely unknown. Thus, we investigated the impact of LPS chemotype on the signalling events in the human glioma cell line U251. We found that LPS of Escherichia coli origin (LPS_EC) leads to NF-κB-biased downstream signalling compared to Salmonella minnesota-derived LPS (LPS_SM). Exposure of U251 cells to LPS_EC resulted in faster nuclear translocation of the NF-κB subunit p65, higher NF-κB-activity and expression of its targets genes, and higher amount of secreted IL-6 compared to LPS_SM. Using super-resolution microscopy we showed that the biased agonism of TLR4 in glioma cells is neither a result of differential regulation of receptor density nor of formation of higher order oligomers. Consistent with previous reports, LPS_EC-mediated NF-κB activation led to significantly increased U251 proliferation, whereas LPS_SM-induced IRF3 activity negatively influenced their invasiveness. Finally, treatment with methyl-β-cyclodextrin (MCD) selectively increased LPS_SM-induced nuclear translocation of p65 and NF-κB activity without affecting IRF3. Our data may explain how TLR4 agonists differently affect glioma cell proliferation and migration.

TLR3 ligand Poly IC Attenuates Reactive Astrogliosis and Improves Recovery of Rats after Focal Cerebral Ischemia

AIMS

Brain ischemia activates astrocytes in a process known as astrogliosis. Although this process has beneficial effects, excessive astrogliosis can impair neuronal recovery. Polyinosinic-polycytidylic acid (Poly IC) has shown neuroprotection against cerebral ischemia-reperfusion injury, but whether it regulates reactive astrogliosis and glial scar formation is not clear.

METHODS

We exposed cultured astrocytes to oxygen-glucose deprivation/reoxygenation (OGD/R) and used a rat middle cerebral artery occlusion (MCAO)/reperfusion model to investigate the effects of Poly IC. Astrocyte proliferation and proliferation-related molecules were evaluated by immunostaining and Western blotting. Neurological deficit scores, infarct volumes and neuroplasticity were evaluated in rats after transient MCAO.

RESULTS

In vitro, Poly IC inhibited astrocyte proliferation, upregulated Toll-like receptor 3 (TLR3) expression, upregulated interferon-β, and downregulated interleukin-6 production. These changes were blocked by a neutralizing antibody against TLR3, suggesting that Poly IC function is TLR3-dependent. Moreover, in the MCAO model, Poly IC attenuated reactive astrogliosis, reduced brain infarction volume, and improved neurological function. In addition, Poly IC prevented MCAO-induced reductions in soma size, dendrite length, and number of dendritic bifurcations in cortical neurons of the infarct penumbra.

CONCLUSIONS

By ameliorating astrogliosis-related damage, Poly IC is a potential therapeutic agent for attenuating neuronal damage and promoting recovery after brain ischemia.

Chemical Conditioning as an Approach to Ischemic Stroke Tolerance: Mitochondria as the Target

It is well established that the brain can be prepared to resist or tolerate ischemic stroke injury, and mitochondrion is a major target for this tolerance. The preparation of ischemic stroke tolerance can be achieved by three major approaches: ischemic conditioning, hypoxic conditioning and chemical conditioning. In each conditioning approach, there are often two strategies that can be used to achieve the conditioning effects, namely preconditioning (Pre-C) and postconditioning (Post-C). In this review, we focus on chemical conditioning of mitochondrial proteins as targets for neuroprotection against ischemic stroke injury. Mitochondrial targets covered include complexes I, II, IV, the ATP-sensitive potassium channel (mitoKATP), adenine dinucleotide translocase (ANT) and the mitochondrial permeability transition pore (mPTP). While numerous mitochondrial proteins have not been evaluated in the context of chemical conditioning and ischemic stroke tolerance, the paradigms and approaches reviewed in this article should provide general guidelines on testing those mitochondrial components that have not been investigated. A deep understanding of mitochondria as the target of chemical conditioning for ischemic stroke tolerance should provide valuable insights into strategies for fighting ischemic stroke, a leading cause of death in the world.

Method parameters' impact on mortality and variability in mouse stroke experiments: a meta-analysis

Although hundreds of promising substances have been tested in clinical trials, thrombolysis currently remains the only specific pharmacological treatment for ischemic stroke. Poor quality, e.g. low statistical power, in the preclinical studies has been suggested to play an important role in these failures. Therefore, it would be attractive to use animal models optimized to minimize unnecessary mortality and outcome variability, or at least to be able to power studies more exactly by predicting variability and mortality given a certain experimental setup. The possible combinations of methodological parameters are innumerous, and an experimental comparison of them all is therefore not feasible. As an alternative approach, we extracted data from 334 experimental mouse stroke articles and, using a hypothesis-driven meta-analysis, investigated the method parameters’ impact on infarct size variability and mortality. The use of Swiss and C57BL6 mice as well as permanent occlusion of the middle cerebral artery rendered the lowest variability of the infarct size while the emboli methods increased variability. The use of Swiss mice increased mortality. Our study offers guidance for researchers striving to optimize mouse stroke models.

LPS-Primed Release of HMGB-1 from Cortical Astrocytes is Modulated Through PI3K/AKT Pathway

Studies have shown that LPS-preconditioned tolerant state could protect against brain injury to subsequent challenges. We hypothesized astrocytes were directly involved in the readjustment to confer neuroprotective effects with LPS pretreatment. High-mobility group box 1(HMGB-1) from LPS-preconditioned astrocytes, presumably serving as a positive regulator, might contribute to the favorable preconditioned effects. Furthermore, a potential cellular pathway (PI3K/AKT pathway), has been proposed for the active regulation of LPS-primed reactive astrocytes to secrete HMGB-1. In the present study, we used a low concentration of LPS to directly prime the astrocytes in vitro, and the subsequent astrocytic reactions, including cytokine secretion, the expression of transcription factors, and the release of HMGB-1 were examined after the blockade of the PI3K pathway. The data showed that LPS preconditioning could reduce some capacity of astrocytes to subsequent challenge in vitro. PI3K/AKT pathway was partially involved in the modulation of the release HMGB-1 from reactive astrocytes. These findings offer direct evidence supporting the flexible roles of astrocytes in mediating LPS-primed neuroprotection, and highlight additional targets for future attempts to modify the protective effects of astrocytes through LPS preconditioning.

Simvastatin Reduces Lipopolysaccharides-Accelerated Cerebral Ischemic Injury via Inhibition of Nuclear Factor-kappa B Activity

Preceding infection or inflammation such as bacterial meningitis has been associated with poor outcomes after stroke. Previously, we reported that intracorpus callosum microinjection of lipopolysaccharides (LPS) strongly accelerated the ischemia/reperfusion-evoked brain tissue damage via recruiting inflammatory cells into the ischemic lesion. Simvastatin, 3-hydroxy-3-methylgultaryl (HMG)-CoA reductase inhibitor, has been shown to reduce inflammatory responses in vascular diseases. Thus, we investigated whether simvastatin could reduce the LPS-accelerated ischemic injury. Simvastatin (20 mg/kg) was orally administered to rats prior to cerebral ischemic insults (4 times at 72, 48, 25, and 1-h pre-ischemia). LPS was microinjected into rat corpus callosum 1 day before the ischemic injury. Treatment of simvastatin reduced the LPS-accelerated infarct size by 73%, and decreased the ischemia/reperfusion-induced expressions of pro-inflammatory mediators such as iNOS, COX-2 and IL-1β in LPS-injected rat brains. However, simvastatin did not reduce the infiltration of microglial/macrophageal cells into the LPS-pretreated brain lesion. In vitro migration assay also showed that simvastatin did not inhibit the monocyte chemoattractant protein-1-evoked migration of microglial/macrophageal cells. Instead, simvastatin inhibited the nuclear translocation of NF-κB, a key signaling event in expressions of various proinflammatory mediators, by decreasing the degradation of IκB. The present results indicate that simvastatin may be beneficial particularly to the accelerated cerebral ischemic injury under inflammatory or infectious conditions.

Prothymosin-alpha preconditioning activates TLR4-TRIF signaling to induce protection of ischemic retina

Prothymosin-alpha protects the brain and retina from ischemic damage. Although prothymosin-alpha contributes to toll-like receptor (TLR4)-mediated immnunopotentiation against viral infection, the beneficial effects of prothymosin-alpha-TLR4 signaling in protecting against ischemia remain to be elucidated. In this study, intravitreal administration of prothymosin-alpha 48 h before induction of retinal ischemia prevented retinal cellular damage as evaluated by histology, and retinal functional deficits as evaluated by electroretinography. Prothymosin-alpha preconditioning completely prevented the ischemia-induced loss of ganglion cells with partial survival of bipolar and photoreceptor cells, but not amacrine cells, in immunohistochemistry experiments. Prothymosin-alpha treatment in the absence of ischemia caused mild activation, proliferation, and migration of retinal microglia, whereas the ischemia-induced microglial activation was inhibited by prothymosin-alpha preconditioning. All these preventive effects of prothymosin-alpha preconditioning were abolished in TLR4 knock-out mice and by pre-treatments with anti-TLR4 antibodies or minocycline, a microglial inhibitor. Prothymosin-alpha preconditioning inhibited the retinal ischemia-induced up-regulation of TLR4-related injury genes, and increased expression of TLR4-related protective genes. Furthermore, the prothymosin-alpha preconditioning-induced prevention of retinal ischemic damage was abolished in TIR-domain-containing adapter-inducing interferon-β knock-out mice, but not in myeloid differentiation primary response gene 88 knock-out mice. Taken together, the results of this study suggest that prothymosin-alpha preconditioning selectively drives TLR4-TIR-domain-containing adapter-inducing interferon-β signaling and microglia in the prevention of retinal ischemic damage. We propose the following mechanism for prothymosin-alpha (ProTα) preconditioning-induced retinal prevention against ischemia: ProTα preconditioning-induced prevention of retinal ischemic damage is mediated by selective activation of the TIR-domain-containing adapter-inducing interferon-β (TRIF)- interferon regulatory factor 3 (IRF3) pathway downstream of toll-like receptor 4 (TLR4) in microglia, resulting in up-regulation of TRIF-IRF3-dependent protective genes and down-regulation of myeloid differentiation primary response gene 88 (MyD88)-Nuclear factor (NF)κB-dependent injury genes. Detailed investigations would be helpful to test the efficacy of ProTα as a therapeutic agent for the prevention of ischemic disorders.

Endotoxin Tolerance Inhibits Lyn and c-Src Phosphorylation and Association with Toll-Like Receptor 4 but Increases Expression and Activity of Protein Phosphatases

Endotoxin tolerance protects the host by limiting excessive 'cytokine storm' during sepsis, but compromises the ability to counteract infections in septic shock survivors. It reprograms Toll-like receptor (TLR) 4 responses by attenuating the expression of proinflammatory cytokines without suppressing anti-inflammatory and antimicrobial mediators, but the mechanisms of reprogramming remain unclear. In this study, we demonstrate that the induction of endotoxin tolerance in human monocytes, THP-1 and MonoMac-6 cells inhibited lipopolysaccharide (LPS)-mediated phosphorylation of Lyn, c-Src and their recruitment to TLR4, but increased total protein phosphatase (PP) activity and the expression of protein tyrosine phosphatase (PTP) 1B, PP2A, PTP nonreceptor type (PTPN) 22 and mitogen-activated protein kinase phosphatase (MKP)-1. Chemical PP inhibitors, okadaic acid, dephostatin and cantharidic acid markedly decreased or completely abolished LPS tolerance, indicating the importance of phosphatases in endotoxin tolerization. Overexpression of PTPN22 decreased LPS-mediated nuclear factor (NF)-x03BA;B activation, p38 phosphorylation and CXCL8 gene expression, while PTPN22 ablation upregulated LPS-induced p65 NF-x03BA;B and p38 phosphorylation and the expression of TNF-α and pro-IL-1β mRNA, indicating PTPN22 as an inhibitor of TLR4 signaling. Thus, LPS tolerance interferes with TLR4 signaling by inhibiting Lyn and c-Src phosphorylation and their recruitment to TLR4, while increasing the phosphatase activity and expression of PP2A, PTPN22, PTP1B and MKP1.

LPS-preconditioned mesenchymal stromal cells modify macrophage polarization for resolution of chronic inflammation via exosome-shuttled let-7b

BACKGROUND

Within the last few years, it has become evident that LPS-preconditioned mesenchymal stromal cells (LPS pre-MSCs) show enhanced paracrine effects, including increased trophic support and improved regenerative and repair properties. MSCs may release large amounts of exosomes for cell-to-cell communication and maintain a dynamic and homeostatic microenvironment for tissue repair. The present study assesses the therapeutic efficacy and mechanisms of LPS-preconditioned MSC-derived exosomes (LPS pre-Exo) for chronic inflammation and wound healing.

METHODS

We extracted exosomes from the supernatant of LPS pre-MSCs using a gradient centrifugation method. In vitro, THP-1 cells were cultured with high glucose (HG, 30 mM) as an inflammatory model and treated with LPS pre-Exo for 48 h. The expression of inflammation-related cytokines was detected by real-time RT-PCR, and the distribution of macrophage subtype was measured by immunofluorescence. Next, the miRNA expression profiles of LPS pre-Exo were evaluated using miRNA microarray analysis. The molecular signaling pathway responsible for the regenerative potential was identified by western blotting. In vivo, we established a cutaneous wound model in streptozotocin-induced diabetic rats, and LPS pre-Exo were injected dispersively into the wound edge. The curative effects of LPS pre-Exo on inflammation and wound healing were observed and evaluated.

RESULTS

LPS pre-Exo have a better ability than untreated MSC-derived exosomes (un-Exo) to modulate the balance of macrophages due to their upregulation of the expression of anti-inflammatory cytokines and promotion of M2 macrophage activation. Microarray analysis of LPS pre-Exo identified the unique expression of let-7b compared with un-Exo, and the let-7b/TLR4 pathway served as potential contributor to macrophage polarization and inflammatory ablation. Further investigation of the mechanisms that control let-7b expression demonstrated that a TLR4/NF-κB/STAT3/AKT regulatory signaling pathway plays a critical role in the regulation of macrophage plasticity. Knockdown of AKT in THP-1 cells similarly abolished the immunomodulatory effect of LPS pre-Exo. In vivo, LPS pre-Exo greatly alleviated inflammation and enhanced diabetic cutaneous wound healing.

CONCLUSION

LPS pre-Exo may have improved regulatory abilities for macrophage polarization and resolution of chronic inflammation by shuttling let-7b, and these exosomes carry much immunotherapeutic potential for wound healing.

Identification of autophagy signaling network that contributes to stroke in the ischemic rodent brain via gene expression

Autophagy plays a vital role in cerebral ischemia and may be a potential target for developing novel therapy for stroke. In this study, we constructed an autophagy-related pathway network by analyzing the genes related to autophagy and ischemic stroke, and the risk genes were screened. Two autophagy-related modules were significantly up-regulated and clustered to influence cerebral ischemia. Besides, three key modular genes (NFKB1, RELA, and STAT3) were revealed. With 5-fold cross validation, the ROC curves of NFKB1, RELA, and STAT3 were 0.8256, 0.8462, and 0.8923. They formed a complex module and competitively mediated the activation of autophagy in cerebral ischemia. In conclusion, a module containing NFKB1, RELA, and STAT3 mediates autophagy, serving as a potential biomarker for the diagnosis and therapy of ischemic stroke.