Lipopolysaccharide pre-treatment induces resistance against subsequent focal cerebral ischemic damage in spontaneously hypertensive rats

Effectiveness of remote ischaemic conditioning is not affected by hyper-inflammation in a rat model of stroke

The inflammation and coagulopathy during coronavirus disease (COVID-19) impairs the efficiency of the current stroke treatments. Remote ischaemic conditioning (RIC) has shown potential in recent years to protect the brain and other organs against pathological conditions. This study aimed to evaluate the efficiency of RIC in brain infarct size using TTC staining and lung injury reduction by H&E staining during the hyper-inflammatory response in rats. The inflammation and coagulopathy were assessed by sedimentation rate, haematocrit, systemic oxidative stress and clotting time. Moreover, we observed changes in the cytokine profile. The results of the first part of the experiment showed that the inflammation and lung injury are fully developed after 24 h of intratracheal LPS administration. At this time, we induced focal brain ischaemia and examined the effect of RIC pre- and post-treatment. Our results showed that RIPre-C reduced the infarct size by about 23%, while RIPost-C by about 30%. The lung injury was also reduced following both treatments. Moreover, RIC modulated systemic inflammation. The level of chemokines CINC-1, LIX and RANTES decreased after 24 h of post-ischaemic reperfusion in treated animals compared to non-treated. The RIC-mediated decrease of inflammation was reflected in improved sedimentation rate and hematocrit, as well as reduced systemic oxidative stress. The results of this work showed neuroprotective and lung protective effects of RIC with a decrease in inflammation response. On the basis of our results, we assume that immunomodulation through the chemokines CINC-1, LIX, and RANTES play a role in RIC-mediated protection.

Role of microglia in stroke

Microglia play key roles in the post-ischemic inflammatory response and damaged tissue removal reacting rapidly to the disturbances caused by ischemia and working to restore the lost homeostasis. However, the modified environment, encompassing ionic imbalances, disruption of crucial neuron-microglia interactions, spreading depolarization, and generation of danger signals from necrotic neurons, induce morphological and phenotypic shifts in microglia. This leads them to adopt a proinflammatory profile and heighten their phagocytic activity. From day three post-ischemia, macrophages infiltrate the necrotic core while microglia amass at the periphery. Further, inflammation prompts a metabolic shift favoring glycolysis, the pentose-phosphate shunt, and lipid synthesis. These shifts, combined with phagocytic lipid intake, drive lipid droplet biogenesis, fuel anabolism, and enable microglia proliferation. Proliferating microglia release trophic factors contributing to protection and repair. However, some microglia accumulate lipids persistently and transform into dysfunctional and potentially harmful foam cells. Studies also showed microglia that either display impaired apoptotic cell clearance, or eliminate synapses, viable neurons, or endothelial cells. Yet, it will be essential to elucidate the viability of engulfed cells, the features of the local environment, the extent of tissue damage, and the temporal sequence. Ischemia provides a rich variety of region- and injury-dependent stimuli for microglia, evolving with time and generating distinct microglia phenotypes including those exhibiting proinflammatory or dysfunctional traits and others showing pro-repair features. Accurate profiling of microglia phenotypes, alongside with a more precise understanding of the associated post-ischemic tissue conditions, is a necessary step to serve as the potential foundation for focused interventions in human stroke.

Impact of inflammatory preconditioning on murine microglial proteome response induced by focal ischemic brain injury

Preconditioning with lipopolysaccharide (LPS) induces neuroprotection against subsequent cerebral ischemic injury, mainly involving innate immune pathways. Microglia are resident immune cells of the central nervous system (CNS) that respond early to danger signals through memory-like differential reprogramming. However, the cell-specific molecular mechanisms underlying preconditioning are not fully understood. To elucidate the distinct molecular mechanisms of preconditioning on microglia, we compared these cell-specific proteomic profiles in response to LPS preconditioning and without preconditioning and subsequent transient focal brain ischemia and reperfusion, - using an established mouse model of transient focal brain ischemia and reperfusion. A proteomic workflow, based on isolated microglia obtained from mouse brains by cell sorting and coupled to mass spectrometry for identification and quantification, was applied. Our data confirm that LPS preconditioning induces marked neuroprotection, as indicated by a significant reduction in brain infarct volume. The established brain cell separation method was suitable for obtaining an enriched microglial cell fraction for valid proteomic analysis. The results show a significant impact of LPS preconditioning on microglial proteome patterns by type I interferons, presumably driven by the interferon cluster regulator proteins signal transducer and activator of transcription1/2 (STAT1/2).

Severity of Peripheral Infection Differentially Affects Brain Functions in Mice via Microglia-Dependent and -Independent Mechanisms

Peripheral infection induces inflammation in peripheral tissues and the brain, impacting brain function. Glial cells are key players in this process. However, the effects of peripheral infection on glial activation and brain function remain unknown. Here, we showed that varying degrees of peripheral infection had different effects on the regulation of brain functions by microglia-dependent and -independent mechanisms. Acute mild infection (one-day LPS challenge: 1LPS) exacerbated middle cerebral artery occlusion (MCAO) injury, and severe infection (four-day LPS challenge: 4LPS) for one week suppressed it. MCAO injury was assessed by triphenyltetrazolium chloride staining. We observed early activation of microglia in the 1LPS and 4LPS groups. Depleting microglia with a colony-stimulating factor-1 receptor (CSF1R) antagonist had no effect on 1LPS-induced brain injury exacerbation but abolished 4LPS-induced protection, indicating microglial independence and dependence, respectively. Microglia-independent exacerbation caused by 1LPS involved peripheral immune cells including macrophages. RNA sequencing analysis of 4LPS-treated microglia revealed increased factors related to anti-inflammatory and neuronal tissue repair, suggesting their association with the protective effect. In conclusion, varying degrees of peripheral inflammation had contradictory effects (exacerbation vs. protection) on MCAO, which may be attributed to microglial dependence. Our findings highlight the significant impact of peripheral infection on brain function, particularly in relation to glial cells.

Intermittent systemic exposure to lipopolysaccharide-induced inflammation disrupts hippocampal long-term potentiation and impairs cognition in aging male mice

Age-related cognitive decline, a common component of the brain aging process, is associated with significant impairment in daily functioning and quality of life among geriatric adults. While the complexity of mechanisms underlying cognitive aging are still being elucidated, microbial exposure and the multifactorial inflammatory cascades associated with systemic infections are emerging as potential drivers of neurological senescence. The negative cognitive and neurobiological consequences of a single pathogen-associated inflammatory experience, such as that modeled through treatment with lipopolysaccharide (LPS), are well documented. Yet, the brain aging impacts of repeated, intermittent inflammatory challenges are less well studied. To extend the emerging literature assessing the impact of infection burden on cognitive function among normally aging mice, here, we repeatedly exposed adult mice to intermittent LPS challenges during the aging period. Male 10-month-old C57BL6 mice were systemically administered escalating doses of LPS once every two weeks for 2.5 months. We evaluated cognitive consequences using the non-spatial step-through inhibitory avoidance task, and both spatial working and reference memory versions of the Morris water maze. We also probed several potential mechanisms, including cortical and hippocampal cytokine/chemokine gene expression, as well as hippocampal neuronal function via extracellular field potential recordings. Though there was limited evidence for an ongoing inflammatory state in cortex and hippocampus, we observed impaired learning and memory and a disruption of hippocampal long-term potentiation. These data suggest that a history of intermittent exposure to LPS-induced inflammation is associated with subtle but significantly impaired cognition among normally aging mice. The broader impact of these findings may have important implications for standard of care involving infections in aging individuals or populations at-risk for dementia.

Ischemic Tolerance Induced by Glial Cells

Ischemic tolerance is a phenomenon in which resistance to subsequent invasive ischemia is acquired by a preceding noninvasive ischemic application, and is observed in many organs, including the brain, the organ most vulnerable to ischemic insult. To date, much research has been conducted on cerebral ischemic tolerance as a cell-autonomous action of neurons. In this article, we review the essential roles of microglia and astrocytes in the acquisition of ischemic tolerance through neuron-non-autonomous mechanisms, where the two types of glial cells function in a concerted manner to induce ischemic tolerance.

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.

Differential Regional Vulnerability of the Brain to Mild Neuroinflammation Induced by Systemic LPS Treatment in Mice

Background

Methods

Results

Conclusion

Tissue-type plasminogen activator induces TNF-α-mediated preconditioning of the blood-brain barrier

Ischemic tolerance is a phenomenon whereby transient exposure to a non-injurious preconditioning stimulus triggers resistance to a subsequent lethal ischemic insult. Despite the fact that not only neurons but also astrocytes and endothelial cells have a unique response to preconditioning stimuli, current research has been focused mostly on the effect of preconditioning on neuronal death. Thus, it is unclear if the blood-brain barrier (BBB) can be preconditioned independently of an effect on neuronal survival. The release of tissue-type plasminogen activator (tPA) from perivascular astrocytes in response to an ischemic insult increases the permeability of the BBB. In line with these observations, treatment with recombinant tPA increases the permeability of the BBB and genetic deficiency of tPA attenuates the development of post-ischemic edema. Here we show that tPA induces ischemic tolerance in the BBB independently of an effect on neuronal survival. We found that tPA renders the BBB resistant to an ischemic injury by inducing TNF-α-mediated astrocytic activation and increasing the abundance of aquaporin-4-immunoreactive astrocytic end-feet processes in the neurovascular unit. This is a new role for tPA, that does not require plasmin generation, and with potential therapeutic implications for patients with cerebrovascular disease.

A Levee to the Flood: Pre-injury Neuroinflammation and Immune Stress Influence Traumatic Brain Injury Outcome

Increasing evidence demonstrates that aging influences the brain's response to traumatic brain injury (TBI), setting the stage for neurodegenerative pathology like Alzheimer's disease (AD). This topic is often dominated by discussions of post-injury aging and inflammation, which can diminish the consideration of those same factors before TBI. In fact, pre-TBI aging and inflammation may be just as critical in mediating outcomes. For example, elderly individuals suffer from the highest rates of TBI of all severities. Additionally, pre-injury immune challenges or stressors may alter pathology and outcome independent of age. The inflammatory response to TBI is malleable and influenced by previous, coincident, and subsequent immune insults. Therefore, pre-existing conditions that elicit or include an inflammatory response could substantially influence the brain's ability to respond to traumatic injury and ultimately affect chronic outcome. The purpose of this review is to detail how age-related cellular and molecular changes, as well as genetic risk variants for AD affect the neuroinflammatory response to TBI. First, we will review the sources and pathology of neuroinflammation following TBI. Then, we will highlight the significance of age-related, endogenous sources of inflammation, including changes in cytokine expression, reactive oxygen species processing, and mitochondrial function. Heightened focus is placed on the mitochondria as an integral link between inflammation and various genetic risk factors for AD. Together, this review will compile current clinical and experimental research to highlight how pre-existing inflammatory changes associated with infection and stress, aging, and genetic risk factors can alter response to TBI.

Ischemic tolerance - blessing or curse

Application of knowledge about ischemic tolerance to clinic requires the solid understanding of mechanism of creation of this phenomenon. This review summarizes research that has been carried out in many laboratories over a long period of time, but the main focus will be on own experimental research. The main emphasis is devoted to the possibility of preparing full tolerance in the donor's body and its transfer to the patient in the form of activated blood plasma. Such plasma could be administered as soon as the patient is transported to the hospital and would take effect immediately after administration to the patient's bloodstream. One chapter is also devoted to anticonditioning, i.e. the possibility of preventing the activation of tolerance. Anticonditioning could be used to treat oncologic patients. We expect that this method could increase effectiveness of cancer treatment. Cross-tolerance with a wide range of diverse stressors gives us the courage to assume that activated plasma can significantly help with a wide range of pathological events.

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.

Toll-Like Receptor Signaling Pathways: Novel Therapeutic Targets for Cerebrovascular Disorders

Toll-like receptors (TLRs), a class of pattern recognition proteins, play an integral role in the modulation of systemic inflammatory responses. Cerebrovascular diseases (CVDs) are a group of pathological conditions that temporarily or permanently affect the brain tissue mostly via the decrease of oxygen and glucose supply. TLRs have a critical role in the activation of inflammatory cascades following hypoxic-ischemic events and subsequently contribute to neuroprotective or detrimental effects of CVD-induced neuroinflammation. The TLR signaling pathway and downstream cascades trigger immune responses via the production and release of various inflammatory mediators. The present review describes the modulatory role of the TLR signaling pathway in the inflammatory responses developed following various CVDs and discusses the potential benefits of the modulation of different TLRs in the improvement of functional outcomes after brain ischemia.

LPS Preconditioning Attenuates Apoptosis Mechanism by Inhibiting NF-κB and Caspase-3 Activity: TLR4 Pre-activation in the Signaling Pathway of LPS-Induced Neuroprotection

Neuroinflammation, an inflammatory response within the nervous system, has been shown to be implicated in the progression of various neurodegenerative diseases. Recent in vivo studies showed that lipopolysaccharide (LPS) preconditioning provides neuroprotection by activating Toll-like receptor 4 (TLR4), one of the members for pattern recognition receptor (PRR) family that play critical role in host response to tissue injury, infection, and inflammation. Pre-exposure to low dose of LPS could confer a protective state against cellular apoptosis following subsequent stimulation with LPS at higher concentration, suggesting a role for TLR4 pre-activation in the signaling pathway of LPS-induced neuroprotection. However, the precise molecular mechanism associated with this protective effect is not well understood. In this article, we provide an overall review of the current state of our knowledge about LPS preconditioning in attenuating apoptosis mechanism and conferring neuroprotection via TLR4 signaling pathway.

Serial Systemic Injections of Endotoxin (LPS) Elicit Neuroprotective Spinal Cord Microglia through IL-1-Dependent Cross Talk with Endothelial Cells

Microglia are dynamic immunosurveillance cells in the CNS. Whether microglia are protective or pathologic is context dependent; the outcome varies as a function of time relative to the stimulus, activation state of neighboring cells in the microenvironment or within progression of a particular disease. Although brain microglia can be "primed" using bacterial lipopolysaccharide (LPS)/endotoxin, it is unknown whether LPS delivered systemically can also induce neuroprotective microglia in the spinal cord. Here, we show that serial systemic injections of LPS (1 mg/kg, i.p., daily) for 4 consecutive days (LPSx4) consistently elicit a reactive spinal cord microglia response marked by dramatic morphologic changes, increased production of IL-1, and enhanced proliferation without triggering leukocyte recruitment or overt neuropathology. Following LPSx4, reactive microglia frequently contact spinal cord endothelial cells. Targeted ablation or selective expression of IL-1 and IL-1 receptor (IL-1R) in either microglia or endothelia reveal that IL-1-dependent signaling between these cells mediates microglia activation. Using a mouse model of ischemic spinal cord injury in male and female mice, we show that preoperative LPSx4 provides complete protection from ischemia-induced neuron loss and hindlimb paralysis. Neuroprotection is partly reversed by either pharmacological elimination of microglia or selective removal of IL-1R in microglia or endothelia. These data indicate that spinal cord microglia are amenable to therapeutic reprogramming via systemic manipulation and that this potential can be harnessed to protect the spinal cord from injury.SIGNIFICANCE STATEMENT Data in this report indicate that a neuroprotective spinal cord microglia response can be triggered by daily systemic injections of LPS over a period of 4 d (LPSx4). The LPSx4 regimen induces morphologic transformation and enhances proliferation of spinal cord microglia without causing neuropathology. Using advanced transgenic mouse technology, we show that IL-1-dependent microglia-endothelia cross talk is necessary for eliciting this spinal cord microglia phenotype and also for conferring optimal protection to spinal motor neurons from ischemic spinal cord injury (ISCI). Collectively, these novel data show that it is possible to consistently elicit spinal cord microglia via systemic delivery of inflammogens to achieve a therapeutically effective neuroprotective response against ISCI.

The role of Toll-like receptor signaling pathways in cerebrovascular disorders: the impact of spreading depolarization

Cerebral vascular diseases (CVDs) are a group of disorders that affect the blood supply to the brain and lead to the reduction of oxygen and glucose supply to the neurons and the supporting cells. Spreading depolarization (SD), a propagating wave of neuroglial depolarization, occurs in different CVDs. A growing amount of evidence suggests that the inflammatory responses following hypoxic-ischemic insults and after SD plays a double-edged role in brain tissue injury and clinical outcome; a beneficial effect in the acute phase and a destructive role in the late phase. Toll-like receptors (TLRs) play a crucial role in the activation of inflammatory cascades and subsequent neuroprotective or harmful effects after CVDs and SD. Here, we review current data regarding the pathophysiological role of TLR signaling pathways in different CVDs and discuss the role of SD in the potentiation of the inflammatory cascade in CVDs through the modulation of TLRs.

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.

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.

The Janus Face of VEGF in Stroke

The family of vascular endothelial growth factors (VEGFs) are known for their regulation of vascularization. In the brain, VEGFs are important regulators of angiogenesis, neuroprotection and neurogenesis. Dysregulation of VEGFs is involved in a large number of neurodegenerative diseases and acute neurological insults, including stroke. Stroke is the main cause of acquired disabilities, and normally results from an occlusion of a cerebral artery or a hemorrhage, both leading to focal ischemia. Neurons in the ischemic core rapidly undergo necrosis. Cells in the penumbra are exposed to ischemia, but may be rescued if adequate perfusion is restored in time. The neuroprotective and angiogenic effects of VEGFs would theoretically make VEGFs ideal candidates for drug therapy in stroke. However, contradictory to what one might expect, endogenously upregulated levels of VEGF as well as the administration of exogenous VEGF is detrimental in acute stroke. This is probably due to VEGF-mediated blood–brain-barrier breakdown and vascular leakage, leading to edema and increased intracranial pressure as well as neuroinflammation. The key to understanding this Janus face of VEGF function in stroke may lie in the timing; the harmful effect of VEGFs on vessel integrity is transient, as both VEGF preconditioning and increased VEGF after the acute phase has a neuroprotective effect. The present review discusses the multifaceted action of VEGFs in stroke prevention and therapy.

New roles of reactive astrocytes in the brain; an organizer of cerebral ischemia

The brain consists of neurons and much higher number of glial cells. They communicate each other, by which they control brain functions. The brain is highly vulnerable to several insults such as ischemia, but has a self-protective and self-repairing mechanisms against these. Ischemic tolerance or preconditioning is an endogenous neuroprotective phenomenon, where a mild non-lethal ischemic episode can induce resistance to a subsequent severe ischemic injury in the brain. Because of its neuroprotective effects against cerebral ischemia or stroke, ischemic tolerance has been widely studied. However, almost all studies have been performed from the viewpoint of neurons. Glial cells are structurally in close association with synapses. Recent studies have uncovered the active roles of astrocytes in modulating synaptic connectivity, such as synapse formation, elimination and maturation, during development or pathology. However, glia-mediated ischemic tolerance and/or neuronal repairing have received only limited attention. We and others have demonstrated that glial cells, especially astrocytes, play a pivotal role in regulation of induction of ischemic tolerance as well as repairing/remodeling of neuronal networks by phagocytosis. Here, we review our current understanding of (1) glial-mediated ischemic tolerance and (2) glia-mediated repairing/remodeling of the penumbra neuronal networks, and highlight their mechanisms as well as their potential benefits, problems, and therapeutic application.

Volumetric response of the adult brain to seizures depends on the developmental stage when systemic inflammation was induced

Inflammation has detrimental influences on the developing brain including triggering the epileptogenesis. On the other hand, seizure episodes may induce inflammatory processes and further increase of brain excitability. The present study focuses on the problem whether transitory systemic inflammation during developmental period may have critical importance to functional and/or structural features of the adult brain. An inflammatory status was induced with lipopolysaccharide (LPS) in 6- or 30-day-old rats. Two-month-old rats which experienced the inflammation and untreated controls received injections of pilocarpine, and the intensity of their seizure behavior was rated during a 6-hour period. Three days thereafter, the animals were perfused; their brains were postfixed and subjected to magnetic resonance imaging (MRI) scans. Then, volumes of the brain and of its main regions were assessed. LPS injections alone performed at different developmental stages led to different changes in the volume of adult brain and also to different susceptibility to seizures induced in adulthood. Moreover, the LPS pretreatments modified different volumetric responses of the brain and of its regions to seizures. The responses showed strong inverse correlations with the intensity of seizures but exclusively in rats treated with LPS on postnatal day 30. It could be concluded that generalized inflammation elicited at developmental stages may have strong age-dependent effects on the adult brain regarding not only its susceptibility to action of a seizuregenic agent but also its volumetric reactivity to seizures.

Astrocytes and ischemic tolerance

A mild non-lethal ischemic episode can induce resistance to a subsequent severe ischemic injury in the brain. This phenomenon is termed ischemic tolerance or ischemic preconditioning, and is an endogenous mechanism that can provide robust neuroprotection. Because of its neuroprotective effects against cerebral ischemia or stroke, ischemic tolerance has been widely studied. However, almost all studies have been performed from the viewpoint of neurons. Accumulating evidence suggests that glial cells have various roles in regulation of brain function, including modulation of synaptic transmission, neuronal excitation, and neuronal structure. In addition, astrocytes are closely related to homeostasis, stability of brain function, and protection of neurons. However, glial cells have received only limited attention with regard to ischemic tolerance. Cross-ischemic preconditioning is a phenomenon whereby non-ischemic preconditioning such as mechanical, thermal, and chemical treatment can induce ischemic tolerance. Of these, chemical treatments that affect the immune system can strongly induce ischemic tolerance, suggesting that glial cells may have important roles in this process. Indeed, we and others have demonstrated that glial cells, especially astrocytes, play a pivotal role in the induction of ischemic tolerance. This glial-mediated ischemic tolerance provides a robust and long-lasting neuroprotection against ischemic injury. In this review, we discuss the mechanisms underlying glial-mediated ischemic tolerance, as well as its potential benefits, problems, and therapeutic application.

LPS Pretreatment Provides Neuroprotective Roles in Rats with Subarachnoid Hemorrhage by Downregulating MMP9 and Caspase3 Associated with TLR4 Signaling Activation

Subarachnoid hemorrhage (SAH), as a severe brain disease, has high morbidity and mortality. SAH usually induced neurological dysfunction or death and the treatment is far from satisfaction. Here, we investigated the effect of low dose of LPS pretreatment and underlying molecular mechanism in rat SAH model. Firstly, SAH model was induced by prechiasmal cistern injection method (SAH1) and common carotid artery-prechiasmal cistern shunt method (SAH2), respectively, to select the more suitable SAH model. At 6, 12, 24, 48, and 72 h after SAH, brain injury including neurological dysfunction, blood-brain barrier disruption, brain edema, and cell apoptosis were detected. And the expression of MMP9, HMGB1/TLR4, and caspase3 in cortex were also explored. Then, SB-3CT, an inhibitor of MMP9, was administrated to investigate the exact function of MMP9 in the brain injury at 24 h after SAH. Moreover, low dose of LPS was used to verify whether it had nerve protection after SAH and the mechanism involving in MMP9 and caspase 3 was investigated. Our results showed SAH1 seems to be the most suitable SAH model. In addition, MMP9 activated by HMGB1/TLR4 may promote or aggravate brain injury, while inhibiting MMP9 via SB-3CT exerted a neuroprotective effect. Moreover, LPS improved the neurological dysfunction, reduced Evans blue extravasation and brain edema, and inhibited cell apoptosis of cortex in rats with brain injury induced by SAH. Importantly, LPS pretreatment increased the expression level of TLR4, and decreased the level of MMP9 and caspase3. Therefore, the present study revealed that low dose of LPS pretreatment could provide neuroprotective effects on brain injury caused by SAH via downregulating MMP9 and caspase3 and activating TLR4 signal pathway.

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.

Crosstalk between TLR2 and Sphk1 in microglia in the cerebral ischemia/reperfusion-induced inflammatory response

Stroke is associated with high morbidity and mortality, and much remains unknown about the injury-related mechanisms that occur following reperfusion. This study aimed to explore the roles of Toll-like receptor 2 (TLR2) and sphingosine kinase 1 (Sphk1) in microglial cells in inflammatory responses induced by cerebral ischemia/reperfusion (I/R). For this purpose, C57BL/6 mice were randomly divided into 4 groups as follows: the sham-operated group, the I/R group, the I/R group treated with TLR2 antibody, and the I/R group treated with N,N-dimethylsphingosine. Focal cerebral I/R was induced by middle cerebral artery occlusion. Double-labeling immunofluorescence was used to observe the protein expression of TLR2 and Sphk1 in the ischemic brain tissue. Quantitative polymerase chain reaction was performed to determine the mRNA levels of TLR2 and Sphkl in ischemic brain tissue. Enzyme-linked immunosorbent assay was carried out to detect the protein contents of interleukin (IL)-1β, tumor necrosis factor-α (TNF-α), IL-17 and IL-23 in ischemic brain tissue. The results revealed that I/R upregulated TLR2 and Sphk1 expression in microglial cells, and the inhibition of either TLR2 or Sphk1 inhibited the expression of the pro-inflammatory cytokines, IL-1β, TNF-α, IL-17 and IL-23. Notably, the inhibition of TLR2 activity also decreased Sphk1 expression. These results thus indicate that the activation of microglial cells, via a TLR2→Sphk1→pro-inflammatory cytokine (IL-1β, TNF-α, IL-17 and IL-23) pathway, may participate in I/R injury.

Ischemic Tolerance of the Brain and Spinal Cord: A Review

Ischemic tolerance is an endogenous neuroprotective phenomenon induced by sublethal ischemia. Ischemic preconditioning (IPC), the first discovered form of ischemic tolerance, is widely seen in many species and in various organs including the brain and the spinal cord. Ischemic tolerance of the spinal cord is less familiar among neurosurgeons, although it has been reported from the viewpoint of preventing ischemic spinal cord injury during aortic surgery. It is important for neurosurgeons to have opportunities to see patients with spinal cord ischemia, and to understand ischemic tolerance of the spinal cord as well as the brain. IPC has a strong neuroprotective effect in animal models of ischemia; however, clinical application of IPC for ischemic brain and spinal diseases is difficult because they cannot be predicted. In addition, one drawback of preconditioning stimuli is that they are also capable of producing injury with only minor changes to their intensity or duration. Numerous methods to induce ischemic tolerance have been discovered that vary in their timing and the site at which short-term ischemia occurs. These methods include ischemic postconditioning (IPoC), remote ischemic preconditioning (RIPC), remote ischemic perconditioning (RIPerC) and remote ischemic postconditioning (RIPoC), which has had a great impact on clinical approaches to treatment of ischemic brain and spinal cord injury. Especially RIPerC and RIPoC to induce spinal cord tolerance are considered clinically useful, however the evidence supporting these methods is currently insufficient; further experimental or clinical research in this area is thus necessary.

Hyperbaric oxygen preconditioning ameliorates blood-brain barrier damage induced by hypoxia through modulation of tight junction proteins in an in vitro model

Aim

To explore the effects of hyperbaric oxygen preconditioning (HBOP) on the permeability of blood-brain barrier (BBB) and expression of tight junction proteins under hypoxic conditions in vitro.

Methods

A BBB in vitro model was constructed using the hCMEC/D3 cell line and used when its trans-endothelial electrical resistance (TEER) reached 80-120 Ω · cm² (tested by Millicell-Electrical Resistance System). The cells were randomly divided into the control group cultured under normal conditions, the group cultured under hypoxic conditions (2%O₂) for 24 h (hypoxia group), and the group first subjected to HBOP for 2 h and then to hypoxia (HBOP group). Occludin and ZO-1 expression were analyzed by immunofluorescence assay.

Results

Normal hCMEC/D3 was spindle-shaped and tightly integrated. TEER was significantly reduced in the hypoxia (P = 0.001) and HBOP group (P = 0.014) compared to control group, with a greater decrease in the hypoxia group. Occludin membranous expression was significantly decreased in the hypoxia group (P = 0.001) compared to the control group, but there was no change in the HBOP group. ZO-1 membranous expression was significantly decreased (P = 0.002) and cytoplasmic expression was significantly increased (P = 0.001) in the hypoxia group compared to the control group, although overall expression levels did not change. In the HBOP group, there was no significant change in ZO-1 expression compared to the control group.

Conclusion

Hyperbaric oxygen preconditioning protected the integrity of BBB in an in vitro model through modulation of occludin and ZO-1 expression under hypoxic conditions.

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.

Ethanol-Induced Neurodegeneration and Glial Activation in the Developing Brain

Ethanol induces neurodegeneration in the developing brain, which may partially explain the long-lasting adverse effects of prenatal ethanol exposure in fetal alcohol spectrum disorders (FASD). While animal models of FASD show that ethanol-induced neurodegeneration is associated with glial activation, the relationship between glial activation and neurodegeneration has not been clarified. This review focuses on the roles of activated microglia and astrocytes in neurodegeneration triggered by ethanol in rodents during the early postnatal period (equivalent to the third trimester of human pregnancy). Previous literature indicates that acute binge-like ethanol exposure in postnatal day 7 (P7) mice induces apoptotic neurodegeneration, transient activation of microglia resulting in phagocytosis of degenerating neurons, and a prolonged increase in glial fibrillary acidic protein-positive astrocytes. In our present study, systemic administration of a moderate dose of lipopolysaccharides, which causes glial activation, attenuates ethanol-induced neurodegeneration. These studies suggest that activation of microglia and astrocytes by acute ethanol in the neonatal brain may provide neuroprotection. However, repeated or chronic ethanol can induce significant proinflammatory glial reaction and neurotoxicity. Further studies are necessary to elucidate whether acute or sustained glial activation caused by ethanol exposure in the developing brain can affect long-lasting cellular and behavioral abnormalities observed in the adult brain.

Neuroinflammation: the devil is in the details

There is significant interest in understanding inflammatory responses within the brain and spinal cord. Inflammatory responses that are centralized within the brain and spinal cord are generally referred to as 'neuroinflammatory'. Aspects of neuroinflammation vary within the context of disease, injury, infection, or stress. The context, course, and duration of these inflammatory responses are all critical aspects in the understanding of these processes and their corresponding physiological, biochemical, and behavioral consequences. Microglia, innate immune cells of the CNS, play key roles in mediating these neuroinflammatory responses. Because the connotation of neuroinflammation is inherently negative and maladaptive, the majority of research focus is on the pathological aspects of neuroinflammation. There are, however, several degrees of neuroinflammatory responses, some of which are positive. In many circumstances including CNS injury, there is a balance of inflammatory and intrinsic repair processes that influences functional recovery. In addition, there are several other examples where communication between the brain and immune system involves neuroinflammatory processes that are beneficial and adaptive. The purpose of this review is to distinguish different variations of neuroinflammation in a context-specific manner and detail both positive and negative aspects of neuroinflammatory processes. In this review, we will use brain and spinal cord injury, stress, aging, and other inflammatory events to illustrate the potential harm and benefits inherent to neuroinflammation. Context, course, and duration of the inflammation are highly important to the interpretation of these events, and we aim to provide insight into this by detailing several commonly studied insults. This article is part of the 60th anniversary supplemental issue.

Help-me signaling: Non-cell autonomous mechanisms of neuroprotection and neurorecovery

Self-preservation is required for life. At the cellular level, this fundamental principle is expressed in the form of molecular mechanisms for preconditioning and tolerance. When the cell is threatened, internal cascades of survival signaling become triggered to protect against cell death and defend against future insults. Recently, however, emerging findings suggest that this principle of self-preservation may involve not only intracellular signals; the release of extracellular signals may provide a way to recruit adjacent cells into an amplified protective program. In the central nervous system where multiple cell types co-exist, this mechanism would allow threatened neurons to "ask for help" from glial and vascular compartments. In this review, we describe this new concept of help-me signaling, wherein damaged or diseased neurons release signals that may shift glial and vascular cells into potentially beneficial phenotypes, and help remodel the neurovascular unit. Understanding and dissecting these non-cell autonomous mechanisms of self-preservation in the CNS may lead to novel opportunities for neuroprotection and neurorecovery.

Epigenetic modulation of gene expression governs the brain's response to injury

Mild stress from ischemia, seizure, hypothermia, or infection can produce a transient neuroprotected state in the brain. In the neuroprotected state, the brain responds differently to a severe stress and sustains less injury. At the genomic level, the response of the neuroprotected brain to a severe stress is characterized by widespread differential regulation of genes with diverse functions. This reprogramming of gene expression observed in the neuroprotected brain in response to a stress is consistent with an epigenetic model of regulation mediated by changes in DNA methylation and histone modification. Here, we summarize our evolving understanding of the molecular basis for endogenous neuroprotection and review recent findings that implicate DNA methylation and protein mediators of histone modification as epigenetic regulators of the brain's response to injury.

Lipopolysaccharide preconditioning prevents acceleration of kindling epileptogenesis induced by traumatic brain injury

10-20% of symptomatic epilepsies are post-traumatic. We examined effect of LPS preconditioning on epileptogenesis after controlled cortical impact (CCI). LPS (0.01, 0.1 and 0.5 mg/kg) was injected i.p. to rats 5 days before induction of CCI to parieto-temporal cortex. Kindling started 24h after CCI by i.p. injection of 30 mg/kg of pentylenetetrazole every other day until manifestation of 3 consecutive generalized seizures. CCI injury accelerated the rate of kindled seizures acquisition. LPS (0.1 and 0.5 mg/kg) prevented the acceleration of kindling. LPS preconditioning significantly decreased IL-1β and TNF-α over-expression and the number of damaged neurons in the hippocampus of traumatic rats.

Hypophosphorylation of ribosomal protein S6 is a molecular mechanism underlying ischemic tolerance induced by either hibernation or preconditioning

Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) have an extraordinary capacity to withstand prolonged and profound reductions in blood flow and oxygen delivery to the brain without incurring any cellular damage. As such, the hibernation torpor of I. tridecemlineatus provides a valuable model of tolerance to ischemic stress. Herein, we report that during hibernation torpor, a marked reduction in the phosphorylation of the ribosomal protein S6 (rpS6) occurs within the brains of I. tridecemlineatus. Of note, rpS6 phosphorylation was shown to increase in the brains of rats that underwent an occlusion of the middle cerebral artery. However, such an increase was attenuated after the implementation of an ischemic preconditioning paradigm. In addition, cultured cortical neurons treated with the rpS6 kinase (S6K) inhibitors, D-glucosamine or PF4708671, displayed a decrease in rpS6 phosphorylation and a subsequent increase in tolerance to oxygen/glucose deprivation, an in vitro model of ischemic stroke. Collectively, such evidence suggests that the down-regulation of rpS6 signal transduction may account for a substantial part of the observed increase in cellular tolerance to brain ischemia that occurs during hibernation torpor and after ischemic preconditioning. Further identification and characterization of the mechanisms used by hibernating species to increase ischemic tolerance may eventually clarify how the loss of homeostatic control that occurs during and after cerebral ischemia in the clinic can ultimately be minimized and/or prevented. Mammalian hibernation provides a valuable model of tolerance to ischemic stress. Herein, we demonstrate that marked reductions in the phosphorylation of ribosomal protein S6 (rpS6), extracellular signal-regulated kinase family of mitogen-activated protein (MAP) kinase p44/42 (p44/42MAPK) and ribosomal protein S6 kinase (S6K) occur within the brains of both hibernating squirrels and rats, which have undergone an ischemic preconditioning paradigm. We therefore propose that the down-regulation of rpS6 signal transduction may account for a substantial part of the observed increase in cellular tolerance to brain ischemia that occurs during hibernation torpor and after ischemic preconditioning, via a suppression of protein synthesis and/or energy consumption.

Inflammation induced at different developmental stages affects differently the range of microglial reactivity and the course of seizures evoked in the adult rat

BACKGROUND

In the brain, inflammation occurs following a variety of types of brain damage, including epileptic seizures. Proinflammatory cytokines, like IL-1β or TNFα, can increase neuronal excitability and initiate spontaneous seizures or epileptogenesis. Recent studies indicate that the effects can be attenuated or even abolished in animals subjected to inflammation-inducing treatments at earlier developmental stages, termed "preconditioning". Immunocompetent microglial cells display particular sensitivity to subtle brain pathologies showing a morphological continuum from resting to reactive forms. Following inflammation, multiple ramified processes of resting microglia become gradually shorter, and the cells transform into macrophages. Parameters of the morphological variations were used here as indicators of the nervous tissue reactivity to seizures in adult rats experiencing inflammation at earlier stages of postnatal development.

METHODS

Systemic inflammation was induced with lipopolysaccharide (LPS) in 6-day-old or 30-day-old rats. In two-month-old survivors of the inflammatory status, seizures were evoked with pilocarpine injection. The seizure intensity was scored during a six-hour continuous observation period following the injection. Brain sections were immunostained for Iba1 to visualize microglia. Thereafter, morphology of microglial cells located in the hippocampal formation was analyzed using parameters such as solidity, circularity, ramification index, and area.

RESULTS

In naïve rats, seizure-induced transformations of microglial cells were reflected by strong changes in the parameters of their morphology. However, in the adult rats pretreated with LPS on their 6th or 30th postnatal days, the seizure-induced changes were significantly reduced, and microglial morphology remained significantly closer to normal. Significant amelioration of the acute phase of seizures was observed only when inflammation was induced in 30-day-old, but not in 6-day-old, rats.

CONCLUSIONS

The results confirm previous reports that moderate inflammation protects the nervous tissue from subsequent damage by reducing influences of proinflammatory factors on reactive glial cells. The young-age inflammation may have age-dependent effects on susceptibility to seizures induced in adulthood. This article is part of a Special Issue entitled "Status Epilepticus".

Role of adaptor protein MyD88 in TLR-mediated preconditioning and neuroprotection after acute excitotoxicity

Excitotoxic cell death is a crucial mechanism through which neurodegeneration occurs in numerous pathologies of the central nervous system (CNS), such as Alzheimer's disease, stroke and spinal cord injury. Toll-like receptors (TLRs) are strongly expressed on microglial cells and are key regulators of the innate immune response to neuronal damage. However, it is still unclear whether their stimulation is protective or harmful in excitotoxic contexts. In this study, we demonstrate that systemic administration of lipopolysaccharide (LPS) or Pam3CSK4 24h prior to an intrastriatal injection of kainic acid (KA) significantly protected cortical neurons in the acute phase of injury. Protection could not be detected with the TLR3 ligand poly-IC. Histological analyses revealed that microglia of LPS and Pam3CSK4 pre-conditioned group were primed to react to injury and exhibited a stronger expression of Tnf and Tlr2 mRNA. We also found that mice deficient for MyD88, a critical adaptor protein for most TLR, were more vulnerable than WT mice to KA-induced excitotoxicity at early (12h and 24h) and late (10days) time points. Finally, bone-marrow chimeric mice revealed that MyD88 signaling in CNS resident cells, but not in cells of hematopoietic origin, mediates the protective effect. This study unravels the potential of TLR2 and TLR4 agonists to induce a protective state of preconditioning against KA-mediated excitotoxicity and further highlights the beneficial role of cerebral MyD88 signaling in this context.

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.

Protection of Tong-Sai-Mai Decoction against Apoptosis Induced by H2O2 in PC12 Cells: Mechanisms via Bcl-2-Mitochondria-ROS-INOS Pathway

Tong-Sai-Mai decoction (TSM) is a Chinese materia medica polyherbal formulation that has been applied in treating brain ischemia for hundreds of years. Because it could repress the oxidative stress in in vivo studies, now we focus on the in vitro studies to investigate the mechanism by targeting the oxidative stress dependent signaling. The relation between the neurogenesis and the reactive oxygen species (ROS) production remains largely unexamined. PC12 cells are excitable cell types widely used as in vitro model for neuronal cells. Most marker genes that are related to neurotoxicity, apoptosis, and cell cycles are expressed at high levels in these cells. The aim of the present study is to explore the cytoprotection of TSM against hydrogen peroxide- (H₂O₂-) induced apoptosis and the molecular mechanisms underlying PC12 cells. Our findings revealed that TSM cotreatment with H₂O₂ restores the expression of bcl-2, inducible nitric oxide synthase (INOS), and mitochondria membrane potential. Meanwhile, it reduces intracellular [Ca²⁺] concentration, lactate dehydrogenase (LDH) release, and the expression of caspase-3 and bax. The results of the present study suggested that the cytoprotective effects of the TSM might be mediated, at least in part, by the bcl-2-mitochondria-ROS-INOS pathway. Due to its nontoxic characteristics, TSM could be further developed to treat the neurodegenerative diseases which are closely associated with the oxidative stress.

A novel protective role for the innate immunity Toll-Like Receptor 3 (TLR3) in the retina via Stat3

The innate immune system and inflammatory pathways play key roles in numerous diseases of the central nervous system (CNS). Recent evidence indicates that innate immunity induces both pathogenesis and protection during neuronal injury. To test the possibility that the conflicting roles of innate immunity in the CNS depends on the cellular environment in which innate immunity is stimulated, we analyzed the effect of Toll-Like Receptor 3 (TLR3) activation on neuronal survival in the presence and absence of oxidative injury in a mouse model system. We demonstrated that activation of TLR3 by the double stranded RNA activator, Poly (I:C), during paraquat induced oxidative stress, significantly protected mouse photoreceptors, as measured by increased retinal structure, function, and improved visual acuity. In contrast, TLR3 activation without concurrent oxidative injury was neurotoxic. The neurotoxic and protective effects of Poly (I:C) stimulation were absent in TLR3 knockout animals, which indicates that protection by Poly (I:C) is dependent on the TLR3 signaling pathway. Furthermore, we identified the pro-survival transcription factor Stat3 as a necessary mechanism for protection. Knockdown of Stat3 using lentivirally delivered shRNA abolished the protective effects of TLR3 signaling in the retina during oxidative stress. Therefore, TLR3 activation in the context of oxidative stress triggers protective instead of pathogenic signaling, suggesting that TLR3 is a potential therapeutic target for neurodegeneration where oxidative stress is a significant contributor.

Systemic injection of low-dose lipopolysaccharide fails to break down the blood-brain barrier or activate the TLR4-MyD88 pathway in neonatal rat brain

We aimed to investigate whether peripheral low-dose lipopolysaccharide (LPS) induces the breakdown of the blood–brain barrier (BBB) and/or the activation of toll-like receptor 4 (TLR4) in the neonatal rat brain. Neonatal rats received intraperitoneal injections of low-dose LPS (0.3 mg/kg∙bw), and the BBB compromise was detected by Evans Blue extravasation and electron microscopy. Meanwhile, TLR4, adaptin myeloid differentiation factor 88 (MyD88), nuclear transcription factor kappa-B (NF-κB) p50 and tumor necrosis factor alpha (TNFα) in the neonatal rat brain were determined by quantitative real-time polymerase chain reaction (PCR) and Western Blot. Immunohistochemistry was used to determine the distribution and activation of microglia in the brain after LPS administration. It was demonstrated that Evans Blue extravasation was not observed in the brain parenchyma, and that tight junctions of cerebral endothelial cells remained intact after systemic injections of LPS in neonatal rats. Although intracerebroventricular injections of LPS activated microglia and up-regulated the expression of TLR4, MyD88, NF-κB p50 and TNFα in the neonatal rat brain, systemic LPS did not induce these responses. These findings indicate that while the neonatal rat brain responds to the direct intra-cerebral administration of LPS through robust TLR4 activation, systemic low-dose LPS does not induce the innate immune reaction or compromise the BBB in neonatal rats.