The role of microglia in ischemic preconditioning

Naoqing formula alleviates cerebral ischemia/reperfusion injury induced inflammatory injury by regulating Csf3 mediated JAK/STAT pathway and macrophage polarization

BACKGROUND

Upon cerebral ischemia/reperfusion injury (CIRI), the brain tissue experiences excessive inflammatory responses, which fuel the activation of immune cells, thereby intensifying cellular damage and inflammatory reactions. Naoqing formula (NQ), a traditional Chinese medicinal compound formulated with musk as the primary component, has been extensively utilized in China for the clinical treatment of ischaemic stroke (IS).

PURPOSE

The precise pharmacological mechanism underlying NQ's efficacy in managing IS remains elusive. In this study, we investigate the protective effect and molecular mechanism of NQ against CIRI.

METHODS

C57BL/6 mice were utilized to investigate the protective effects of NQ (130, 260 and 520mg/kg) against middle cerebral artery occlusion (MCAO) induced CIRI and the underlying mechanism. Employing molecular biology techniques, transcriptomics, proteomics, and network pharmacological analyses, the study assessed the role of NQ in the inflammatory response of neuronal cells by establishing a model for neuronal cell and microglia inflammatory injury induced by oxygen-glucose deprivation/reperfusion (OGD/R) and lipopolysaccharide (LPS) stimulation.

RESULTS

NQ demonstrated significant efficacy in mitigating neuronal damage and cerebral infarction induced by CIRI, achieved through the enhancement of cortical blood flow. Transcriptomic and network pharmacological analyses revealed that NQ mitigated the inflammatory damage caused by CIRI by modulating the Csf3-mediated JAK/STAT pathway. Proteomic analysis further corroborated this finding, indicating that NQ reduced the impact of CIRI by regulating macrophage polarization. Notably, in CIRI mice treated with NQ, there was a notable downregulation of Csf3, JAK2, STAT3, and STAT6, along with a co-localization of Csf3 and CD206. These observations suggested that NQ inhibited the activation of the JAK/STAT pathway and exerted its anti-inflammatory effects by orchestrating the transition of macrophages from the M1 phenotype to the M2 phenotype, triggered by Csf3. Consistent with the in vivo findings, NQ also inhibited the activation of the JAK/STAT pathway in neuronal cells and microglial polarization in vitro, thereby protecting against OGD/R- and LPS-induced inflammatory injury.

CONCLUSION

This study confirmed that NQ prevented CIRI induced inflammatory injury by inhibiting Csf3-mediated activation of the JAK/STAT pathway and modulating Csf3-mediated macrophage polarization. This study provided a new perspective on the use of NQ in the treatment of IS.

M2 microglia-derived small extracellular vesicles modulate NSC fate after ischemic stroke via miR-25-3p/miR-93-5p-TGFBR/PTEN/FOXO3 axis

BACKGROUND

Endogenous neurogenesis could promote stroke recovery. Furthermore, anti-inflammatory phenotypical microglia (M2-microglia) could facilitate Neural Stem Cell (NSC)-mediated neurogenesis following Ischemic Stroke (IS). Nonetheless, the mechanisms through which M2 microglia influence NSC-mediated neurogenesis post-IS remain unclear. On the other hand, M2 microglia-derived small Extracellular Vesicles (M2-sEVs) could exert phenomenal biological effects and play significant roles in cell-to-cell interactions, highlighting their potential involvement in NSC-mediated neurogenesis post-IS, forming the basis of this study.

METHODS

M2-sEVs were first isolated from IL-4-stimulated microglia. For in vivo tests, M2-sEVs were intravenously injected into mice every day for 14 days after transient Middle Cerebral Artery Occlusion (tMCAO). Following that, the infarct volume and neurological function, as well as NSC proliferation in the Subventricular Zone and dentate gyrus, migration, and differentiation in the infarct area, were examined. For in vitro tests, M2-sEVs were administered to NSC subjected to Oxygen-Glucose Deprivation (OGD) and then reoxygenation, after which NSC proliferation and differentiation were assessed. Finally, M2-sEVs were subjected to microRNA sequencing to explore the regulatory mechanisms.

RESULTS

Our findings revealed that M2-sEVs reduced the infarct volume and increased the neurological score in mice post-tMCAO. Furthermore, M2-sEV treatment promoted NSC proliferation and neuronal differentiation both in vivo and in vitro. Additionally, microRNA sequencing revealed miR-93-5p and miR-25-3p enrichment in M2-sEVs. Inhibitors of these miRNAs prevented TGFBR, PTEN, and FOXO3 downregulation in NSC, reversing M2-sEVs' beneficial effects on neurogenesis and sensorimotor recovery.

CONCLUSIONS

M2-sEVs increased NSC proliferation and neuronal differentiation, and protected against IS, at least partially, via delivering miR-25-3p and miR-93-5p to downregulate TGFBR, PTEN, and FOXO3 expression in NSC.

Glial 'omics in ischemia: Acute stroke and chronic cerebral small vessel disease

Vascular injury and pathologies underlie common diseases including ischemic stroke and cerebral small vessel disease (CSVD). Prior work has identified a key role for glial cells, including microglia, in the multifaceted and temporally evolving neuroimmune response to both stroke and CSVD. Transcriptional profiling has led to important advances including identification of distinct gene expression signatures in ischemia-exposed, flow cytometrically sorted microglia and more recently single cell RNA sequencing-identified microglial subpopulations or clusters. There is a reassuring degree of overlap in the results from these two distinct methodologies with both identifying a proliferative and a separate type I interferon responsive microglial element. Similar patterns were later seen using multimodal and spatial transcriptomal profiling in ischemia-exposed microglia and astrocytes. Methodological advances including enrichment of specific neuroanatomic/functional regions (such as the neurovascular unit) prior to single cell RNA sequencing has led to identification of novel cellular subtypes and generation of new credible hypotheses as to cellular function based on the enhanced cell sub-type specific gene expression patterns. A ribosomal tagging strategy focusing on the cellular translatome analyses carried out in the acute phases post stroke has revealed distinct inflammation-regulating roles for microglia and astrocytes in this setting. Early spatial transcriptomics experiments using cerebral ischemia models have identified regionally distinct microglial cell clusters in ischemic core versus penumbra. There is great potential for combination of these methods for multi-omics approaches to further elucidate glial responses in the context of both acute ischemic stroke and chronic CSVD.

Remote Preconditioning Provides Protection Against Retinal Cell Death From Retinal Detachment

Purpose

Remote preconditioning involves injury to a tissue that results in protection to a subsequent injury in a distal tissue. Here, we investigated the impact of remote preconditioning on retinal detachment (RD) injury, hypothesizing that a previous contralateral RD would protect the fellow retina against inflammation and cell death following its detachment.

Methods

RD was created in adult C57BL/6J mice with subretinal sodium hyaluronate injection. Preconditioning involved RD in the right eye at 1, 3, 7, or 28 days before left eye detachment, whereas the control group only received RD to the left eye. Retinas were harvested 24 hours post-left eye detachment in both groups. Cell death was assessed using Cell Death Detection ELISA and mRNA expression was evaluated via qRT-PCR.

Results

Contralateral RD promoted a transient protection against retinal cell death from 1 to 3 days and waned by 7 days compared with control RD retinas with intact fellow retinas. Contralateral RD significantly protected against post-RD cell death (P = 0.0002) and caspase 3 cleavage (P = 0.0449), compared with control RD retinas with intact fellow retinas 1-day post-RD. Detached fellow retinas from the preconditioning group expressed significantly less Tnfa (P = 0.0066), Cxcl10 (P = 0.0099), and Fas (P = 0.0223) mRNAs, compared with the detached retinas of the control group. In contrast, upregulation of type-I-IFN pathway genes, including Irf7 (P = 0.0106) and Ifit1 (P = 0.0740), following RD was higher in the preconditioning group.

Conclusions

RD in one eye produces a transient remote preconditioning effect that protects the fellow retina against retinal cell death following subsequent RD.

Systematic Review and Phenome-Wide Scans of Genetic Associations with Vascular Cognitive Impairment

Vascular cognitive impairment (VCI) is a heterogenous form of cognitive impairment that results from cerebrovascular disease. It is a result of both genetic and non-genetic factors. Although much research has been conducted on the genetic contributors to other forms of cognitive impairment (e.g. Alzheimer's disease), knowledge is lacking on the genetic factors associated with VCI. A better understanding of the genetics of VCI will be critical for prevention and treatment. To begin to fill this gap, the genetic contributors are reviewed with VCI from the literature. Phenome-wide scans of the identified genes are conducted and genetic variants identified in the review in large-scale resources displaying genetic variant-trait association information. Gene set are also carried out enrichment analysis using the genes identified from the review. Thirty one articles are identified meeting the search criteria and filters, from which 107 unique protein-coding genes are noted related to VCI. The phenome-wide scans and gene set enrichment analysis identify pathways associated with a diverse set of biological systems. This results indicate that genes with evidence of involvement in VCI are involved in a diverse set of biological functions. This information can facilitate downstream research to better dissect possible shared biological mechanisms for future therapies.

CSF1R inhibitor PLX3397 depletes microglia in Mongolian gerbil Meriones unguiculatus, but not in syrian hamster Mesocricetus auratus

Microglia are the residential immune cells in the central nervous system. Their roles as innate immune cells and regulators of synaptic remodeling are critical to the development and the maintenance of the brain. Numerous studies have depleted microglia to elucidate their involvement in healthy and pathological conditions. PLX3397, a blocker of colony stimulating factor 1 receptor (CSF1R), is widely used to deplete mouse microglia due to its non-invasiveness and convenience. Recently, other small rodents, including Syrian hamsters (Mesocricetus auratus) and Mongolian gerbils (Meriones unguiculatus), have been recognized as valuable animal models for studying brain functions and diseases. However, whether microglia depletion via PLX3397 is feasible in these species remains unclear. Here, we administered PLX3397 orally via food pellets to hamsters and gerbils. PLX3397 successfully depleted gerbil microglia but had no effect on microglial density in hamsters. Comparative analysis of the CSF1R amino acid sequence in different species hints that amino acid substitutions in the juxtamembrane domain may potentially contribute to the inefficacy of PLX3397 in hamsters.

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.

Role of microglia after subarachnoid hemorrhage

Subarachnoid hemorrhage is a devastating sequela of aneurysm rupture. Because it disproportionately affects younger patients, the population impact of hemorrhagic stroke from subarachnoid hemorrhage is substantial. Secondary brain injury is a significant contributor to morbidity after subarachnoid hemorrhage. Initial hemorrhage causes intracranial pressure elevations, disrupted cerebral perfusion pressure, global ischemia, and systemic dysfunction. These initial events are followed by two characterized timespans of secondary brain injury: the early brain injury period and the delayed cerebral ischemia period. The identification of varying microglial phenotypes across phases of secondary brain injury paired with the functions of microglia during each phase provides a basis for microglia serving a critical role in both promoting and attenuating subarachnoid hemorrhage-induced morbidity. The duality of microglial effects on outcomes following SAH is highlighted by the pleiotropic features of these cells. Here, we provide an overview of the key role of microglia in subarachnoid hemorrhage-induced secondary brain injury as both cytotoxic and restorative effectors. We first describe the ontogeny of microglial populations that respond to subarachnoid hemorrhage. We then correlate the phenotypic development of secondary brain injury after subarachnoid hemorrhage to microglial functions, synthesizing experimental data in this area.

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).

Role of myeloid cells in ischemic retinopathies: recent advances and unanswered questions

Myeloid cells including microglia and macrophages play crucial roles in retinal homeostasis by clearing cellular debris and regulating inflammation. These cells are activated in several blinding ischemic retinal diseases including diabetic retinopathy, where they may exert both beneficial and detrimental effects on neurovascular function and angiogenesis. Myeloid cells impact the progression of retinal pathologies and recent studies suggest that targeting myeloid cells is a promising therapeutic strategy to mitigate diabetic retinopathy and other ischemic retinal diseases. This review summarizes the recent advances in our understanding of the role of microglia and macrophages in retinal diseases and focuses on the effects of myeloid cells on neurovascular injury and angiogenesis in ischemic retinopathies. We highlight gaps in knowledge and advocate for a more detailed understanding of the role of myeloid cells in retinal ischemic injury to fully unlock the potential of targeting myeloid cells as a therapeutic strategy for retinal ischemia.

Microglia dynamic response and phenotype heterogeneity in neural regeneration following hypoxic-ischemic brain injury

Hypoxic-ischemic brain injury poses a significant threat to the neural niche within the central nervous system. In response to this pathological process, microglia, as innate immune cells in the central nervous system, undergo rapid morphological, molecular and functional changes. Here, we comprehensively review these dynamic changes in microglial response to hypoxic-ischemic brain injury under pathological conditions, including stroke, chronic intermittent hypoxia and neonatal hypoxic-ischemic brain injury. We focus on the regulation of signaling pathways under hypoxic-ischemic brain injury and further describe the process of microenvironment remodeling and neural tissue regeneration mediated by microglia after hypoxic-ischemic injury.

Protective effects of limb remote ischemic per-conditioning on the heart injury induced by renal ischemic-reperfusion through the interaction of the apelin with the RAS/iNOS pathway

Introduction

Remote ischemic conditioning upregulates endogenous protective pathways in response to ischemia-reperfusion injury. This study tested the hypothesis that limb remote ischemic per- conditioning (RIPerC) exerts cardioprotective effects via the renin-angiotensin system (RAS)/inducible nitric oxide synthase (iNOS)/apelin pathway.

Methods

Renal ischemia-reperfusion injury (I/R) was induced by bilateral occlusion of the renal pedicles for 60 minutes, followed by 24 hours of reperfusion; sham-operated rats served as controls. RIPerC was induced by four cycles (5 minutes) of limb ischemia-reperfusion along with bilateral renal ischemia. The functional disturbance was evaluated by renal (BUN and creatinine) and cardiac (troponin I and lactate dehydrogenase) injury biomarkers.

Results

Renal I/R injury increased renal and cardiac injury biomarkers that were reduced in the RIPerC group. Histopathological findings of the kidney and heart were also suggestive of amelioration injury-induced changes in the RIPerC group. Assessment of cardiac electrophysiology revealed that RIPerC ameliorated the decline in P wave duration without significantly affecting other cardiac electrophysiological changes. Further, renal I/R injury increased the plasma (322.40±34.01 IU/L), renal (8.27±1.10 mIU/mg of Protein), and cardiac (68.28±10.28 mIU/mg of protein) angiotensin-converting enzyme (ACE) activities in association with elevations in the plasma and urine nitrite (25.47±2.01 & 16.62±3.05 μmol/L) and nitrate (15.47±1.33 & 5.01±0.96 μmol/L) levels; these changes were reversed by RIPerC. Further, renal ischemia-reperfusion injury significantly (P=0.047) decreased the renal (but not cardiac) apelin mRNA expression, while renal and cardiac ACE2 (P<0.05) and iNOS (P=0.043) mRNA expressions were significantly increased compared to the sham group; these effects were largely reversed by RIPerC.

Conclusion

Our results indicated that RIPerC protects the heart against renal ischemia- reperfusion injury, likely via interaction of the apelin with the RAS/iNOS pathway.

cPKCγ-Modulated Autophagy Contributes to Ischemic Preconditioning-Induced Neuroprotection in Mice with Ischemic Stroke via mTOR-ULK1 Pathway

Neuron-specific conventional protein kinase C (cPKC)γ mediates cerebral hypoxic preconditioning (HPC). In parallel, autophagy plays a prosurvival role in ischemic preconditioning (IPC) against ischemic stroke. However, the effect of cPKCγ on autophagy in IPC still remains to be addressed. In this study, adult and postnatal 1-day-old C57BL/6 J wild-type (cPKCγ+/+) and knockout (cPKCγ-/-) mice were used to establish in vivo and in vitro IPC models. The results showed that IPC pretreatment alleviated neuronal damage caused by lethal ischemia, which could be suppressed by autophagy inhibitor 3-MA or bafilomycin A1. Meanwhile, cPKCγ knockout blocked IPC-induced neuroprotection, accompanied by significant increase of LC3-I to LC3-II conversion and Beclin 1 protein level, and a significant decrease in p62 protein level. Immunofluorescent staining results showed a decrease of LC3 puncta numbers in IPC-treated cPKCγ+/+ neurons with fatal ischemia, which was reversed in cPKCγ-/- neurons. In addition, cPKCγ-modulated phosphorylation of mTOR at Ser 2448 and ULK1 at Ser 555, rather than p-Thr-172 AMPK, was detected in IPC-pretreated neurons upon lethal ischemic exposure. The present data demonstrated that cPKCγ-modulated autophagy via the mTOR-ULK1 pathway likely modulated IPC-induced neuroprotection.

Recurrent Transient Ischemic Attack Induces Neural Cytoskeleton Modification and Gliosis in an Experimental Model

Transient ischemic attack (TIA) presents a high risk for subsequent stroke, Alzheimer's disease (AD), and related dementia (ADRD). However, the neuropathophysiology of TIA has been rarely studied. By evaluating recurrent TIA-induced neuropathological changes, our study aimed to explore the potential mechanisms underlying the contribution of TIA to ADRD. In the current study, we established a recurrent TIA model by three times 10-min middle cerebral artery occlusion within a week in rat. Neither permanent neurological deficit nor apoptosis was observed following recurrent TIA. No increase of AD-related biomarkers was indicated after TIA, including increase of tau hyperphosphorylation and β-site APP cleaving enzyme 1 (BACE1). Neuronal cytoskeleton modification and neuroinflammation was found at 1, 3, and 7 days after recurrent TIA, evidenced by the reduction of microtubule-associated protein 2 (MAP2), elevation of neurofilament-light chain (NFL), and increase of glial fibrillary acidic protein (GFAP)-positive astrocytes and ionized calcium binding adaptor molecule 1 (Iba1)-positive microglia at the TIA-affected cerebral cortex and basal ganglion. Similar NFL, GFAP and Iba1 alteration was found in the white matter of corpus callosum. In summary, the current study demonstrated that recurrent TIA may trigger neuronal cytoskeleton change, astrogliosis, and microgliosis without induction of cell death at the acute and subacute stage. Our study indicates that TIA-induced neuronal cytoskeleton modification and neuroinflammation may be involved in the vascular contribution to cognitive impairment and dementia.

Microglia-mediated neuroinflammation and neuroplasticity after stroke

Stroke remains a major cause of long-term disability and mortality worldwide. The immune system plays an important role in determining the condition of the brain following stroke. As the resident innate immune cells of the central nervous system, microglia are the primary responders in a defense network covering the entire brain parenchyma, and exert various functions depending on dynamic communications with neurons, astrocytes, and other neighboring cells under both physiological or pathological conditions. Microglia activation and polarization is crucial for brain damage and repair following ischemic stroke, and is considered a double-edged sword for neurological recovery. Microglia can exist in pro-inflammatory states and promote secondary brain damage, but they can also secrete anti-inflammatory cytokines and neurotrophic factors and facilitate recovery following stroke. In this review, we focus on the role and mechanisms of microglia-mediated neuroinflammation and neuroplasticity after ischemia and relevant potential microglia-based interventions for stroke therapy.

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.

Granulocyte-macrophage colony-stimulating factor-triggered innate immune tolerance against chronic stress-induced behavioral abnormalities in mice

Pre-stimulation of the innate immune is considered a potential strategy to prevent chronic stress-induced behavioral abnormalities in animals. In this study, we investigated whether granulocyte-macrophage colony-stimulating factor (GM-CSF), an immunostimulant used in the clinic to treat diseases of the hematopoietic system, can prevent chronic stress-induced behavioral abnormalities in mice. Our results showed that a single intraperitoneal injection of GM-CSF (100 μg/kg) one day before stress exposure prevented the depression- and anxiety-like behaviors induced by chronic social defeat stress (CSDS) in mice, including preventing the CSDS-induced increase in the immobility time in the tail suspension test and forced swimming test and decrease in the time spent in the interaction zone in the social interaction test, as well as preventing the CSDS-induced decrease in the time spent (i) in open arms in the elevated plus maze test, (ii) on the illuminated side in the light-dark test, and (iii) in the central region of the open field test. The single GM-CSF preinjection (100 μg/kg) also prevented the CSDS-induced increase in the expression levels of interleukin-6 (IL-6), IL-1β, and tumor necrosis factor-α (TNF-α) mRNA in the hippocampus and medial prefrontal cortex of the mice. Further analysis showed that the preventive effect of GM-CSF on CSDS-induced depression- and anxiety-like behaviors and neuroinflammatory responses was abolished by pretreatment with minocycline (an innate immune inhibitor). These results indicate that a single low dose of GM-CSF before injection could be a potential way to prevent behavioral abnormalities induced by chronic stress in mice.

P2X7 Receptors in Astrocytes: A Switch for Ischemic Tolerance

A sub-lethal ischemic episode (preconditioning [PC]) protects neurons against a subsequent lethal ischemic injury. This phenomenon is known as ischemic tolerance. PC itself does not cause brain damage, but affects glial responses, especially astrocytes, and transforms them into an ischemia-resistant phenotype. P2X7 receptors (P2X7Rs) in astrocytes play essential roles in PC. Although P2X7Rs trigger inflammatory and toxic responses, PC-induced P2X7Rs in astrocytes function as a switch to protect the brain against ischemia. In this review, we focus on P2X7Rs and summarize recent developments on how astrocytes control P2X7Rs and what molecular mechanisms they use to induce ischemic tolerance.

Ischemic Preconditioning Modulates the Peripheral Innate Immune System to Promote Anti-Inflammatory and Protective Responses in Mice Subjected to Focal Cerebral Ischemia

The development of tolerance triggered by a sublethal ischemic episode (preconditioning, PC) involves a complex crosstalk between neurons, astrocytes and microglia, although the role of the peripheral immune system in this context is largely unexplored. Here, we report that severe cerebral ischemia caused by transient middle cerebral artery occlusion (MCAo) in adult male mice elevates blood counts of inflammatory neutrophils and monocytes, and plasma levels of miRNA-329-5p. These inflammatory responses are prevented by ischemic PC induced by 15 min MCAo, 72h before the severe insult (1h MCAo). As compared with sham-operated animals, mice subjected to either ischemic PC, MCAo or a combination of both (PC+MCAo) display spleen contraction. However, protein levels of Ym1 (a marker of polarization of myeloid cells towards M2/N2 protective phenotypes) are elevated only in spleen from the experimental groups PC and PC+MCAo, but not MCAo. Conversely, Ym1 protein levels only increase in circulating leukocytes from mice subjected to 1h MCAo, but not in preconditioned animals, which is coincident with a dramatic elevation of Ym1 expression in the ipsilateral cortex. By immunofluorescence analysis, we observe that expression of Ym1 occurs in amoeboid-shaped myeloid cells, mainly representing inflammatory monocytes/macrophages and neutrophils. As a result of its immune-regulatory functions, ischemic PC prevents elevation of mRNA levels of the pro-inflammatory cytokine interleukin (IL)-1β in the ipsilateral cortex, while not affecting IL-10 mRNA increase induced by MCAo. Overall, the elevated anti-inflammatory/pro-inflammatory ratio observed in the brain of mice pre-exposed to PC is associated with reduced brain infarct volume and ischemic edema, and with amelioration of functional outcome. These findings reaffirm the crucial and dualistic role of the innate immune system in ischemic stroke pathobiology, extending these concepts to the context of ischemic tolerance and underscoring their relevance for the identification of novel therapeutic targets for effective stroke treatment.

Neuron–Microglia Contacts Govern the PGE2 Tolerance through TLR4-Mediated de Novo Protein Synthesis

Cellular and molecular mechanisms of the peripheral immune system (e.g., macrophage and monocyte) in programming endotoxin tolerance (ET) have been well studied. However, regulatory mechanism in development of brain immune tolerance remains unclear. The inducible COX-2/PGE₂ axis in microglia, the primary innate immune cells of the brain, is a pivotal feature in causing inflammation and neuronal injury, both in acute excitotoxic insults and chronic neurodegenerative diseases. This present study investigated the regulatory mechanism of PGE₂ tolerance in microglia. Multiple reconstituted primary brain cells cultures, including neuron–glial (NG), mixed glial (MG), neuron-enriched, and microglia-enriched cultures, were performed and consequently applied to a treatment regimen for ET induction. Our results revealed that the levels of COX-2 mRNA and supernatant PGE₂ in NG cultures, but not in microglia-enriched and MG cultures, were drastically reduced in response to the ET challenge, suggesting that the presence of neurons, rather than astroglia, is required for PGE₂ tolerance in microglia. Furthermore, our data showed that neural contact, instead of its soluble factors, is sufficient for developing microglial PGE₂ tolerance. Simultaneously, this finding determined how neurons regulated microglial PGE₂ tolerance. Moreover, by inhibiting TLR4 activation and de novo protein synthesis by LPS-binding protein (LBP) manipulation and cycloheximide, our data showed that the TLR4 signal and de novo protein synthesis are necessary for microglia to develop PGE₂ tolerance in NG cells under the ET challenge. Altogether, our findings demonstrated that neuron–microglia contacts are indispensable in emerging PGE₂ tolerance through the regulation of TLR4-mediated de novo protein synthesis.

Monophosphoryl Lipid A Tolerance Against Chronic Stress-Induced Depression-Like Behaviors in Mice

BACKGROUNDS

Our recent studies reported that a single injection with lipopolysaccharide (LPS) before stress exposure prevents depression-like behaviors in stressed mice. Monophosphoryl lipid A (MPL) is a derivative of LPS that lacks the undesirable properties of LPS. We hypothesize that MPL can exert a prophylactic effect on depression.

METHODS

The experimental mice were pre-injected with MPL before stress exposure. Depression in mice was induced through chronic social defeat stress (CSDS). Behavioral tests were conducted to identify depression-like behaviors. Real-time polymerase chain reaction and biochemical assays were performed to examine the gene and protein expression levels of pro-inflammatory cytokines.

RESULTS

A single MPL injection 1 day before stress exposure at the dosages of 400, 800, and 1600 μg/kg but not 200 μg/kg prevented CSDS-induced depression-like behaviors in mice. This effect of MPL, however, vanished with the extension of the interval time between drug injection and stress exposure from 1 day or 5 days to 10 days, which was rescued by a second MPL injection 10 days after the first MPL injection or by a 4× MPL injection 10 days before stress exposure. A single MPL injection (800 μg/kg) before stress exposure prevented CSDS-induced increases in the gene expression levels of pro-inflammatory cytokines in the hippocampus and prefrontal cortex. Pre-inhibiting the innate immune stimulation by minocycline pretreatment (40 mg/kg) abrogated the preventive effect of MPL on CSDS-induced depression-like behaviors and neuroinflammatory responses in animal brains.

CONCLUSIONS

MPL, through innate immune stimulation, prevents stress-induced depression-like behaviors in mice by preventing neuroinflammatory responses.

Regulatory T cells in ischemic stroke

Recent evidence shows that when ischemic stroke (IS) occurs, the BBB would be destructed, thereby promoting the immune cells to migrate into the brain, suggesting that the immune responses can play a vital role in the pathology of IS. As an essential subpopulation of immunosuppressive T cells, regulatory T (Treg) cells are involved in maintaining immune homeostasis and suppressing immune responses in the pathophysiological conditions of IS. During the past decades, the regulatory role of Treg cells has attracted the interest of numerous researchers. However, whether they are beneficial or detrimental to the outcomes of IS remains controversial. Moreover, Treg cells exert distinctive effects in the different stages of IS. Therefore, it is urgent to elucidate how Treg cells modulate the immune responses induced by IS. In this review, we describe how Treg cells fluctuate and play a role in the regulation of immune responses after IS in both experimental animals and humans, and summarize their biological functions and mechanisms in both CNS and periphery. We also discuss how Treg cells participate in poststroke inflammation and immunodepression and the potential of Treg cells as a novel therapeutic approach.

Microglial depletion abolishes ischemic preconditioning in white matter

Ischemic preconditioning (IPC) is a phenomenon whereby a brief, non-injurious ischemic exposure enhances tolerance to a subsequent ischemic challenge. The mechanism of IPC has mainly been studied in rodent stroke models where gray matter (GM) constitutes about 85% of the cerebrum. In humans, white matter (WM) is 50% of cerebral volume and is a critical component of stroke damage. We developed a novel CNS WM IPC model using the mouse optic nerve (MON) and identified the involved immune signaling pathways. Here we tested the hypothesis that microglia are necessary for WM IPC. Microglia were depleted by treatment with the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622. MONs were exposed to transient ischemia in vivo, acutely isolated 72 h later, and subjected to oxygen-glucose deprivation (OGD) to simulate a severe ischemic injury (i.e., stroke). Functional and structural axonal recovery was assessed by recording compound action potentials (CAPs) and by microscopy using quantitative stereology. Microglia depletion eliminated IPC-mediated protection. In control mice, CAP recovery was improved in preconditioned MONs compared with non-preconditioned MONs, however, in PLX5622-treated mice, we observed no difference in CAP recovery between preconditioned and non-preconditioned MONs. Microgliadepletion also abolished IPC protective effects on axonal integrity and survival of mature (APC⁺ ) oligodendrocytes after OGD. IPC-mediated protection was independent of retinal injury suggesting it results from mechanistic processes intrinsic to ischemia-exposed WM. We conclude that preconditioned microglia are critical for IPC in WM. The "preconditioned microglia" phenotype might protect against other CNS pathologies and is a neurotherapeutic horizon worth exploring.

The Influence of Mitochondrial-DNA-Driven Inflammation Pathways on Macrophage Polarization: A New Perspective for Targeted Immunometabolic Therapy in Cerebral Ischemia-Reperfusion Injury

Cerebral ischemia-reperfusion injury is related to inflammation driven by free mitochondrial DNA. At the same time, the pro-inflammatory activation of macrophages, that is, polarization in the M1 direction, aggravates the cycle of inflammatory damage. They promote each other and eventually transform macrophages/microglia into neurotoxic macrophages by improving macrophage glycolysis, transforming arginine metabolism, and controlling fatty acid synthesis. Therefore, we propose targeting the mtDNA-driven inflammatory response while controlling the metabolic state of macrophages in brain tissue to reduce the possibility of cerebral ischemia-reperfusion injury.

Non-Invasive Remote Ischemic Preconditioning May Protect the Gastric Mucosa Against Ischemia-Reperfusion-Induced Injury Through Involvement of Glucocorticoids

Remote ischemic preconditioning (RIPC) is one of the most effective approaches to attenuate tissue injury caused by severe ischemia-reperfusion (I/R). Experimental studies have demonstrated that RIPC is capable of producing a protective effect not only on heart, but also on brain, lungs, kidneys, liver, intestine, and stomach. We previously demonstrated that glucocorticoids participate in protective effect of local gastric ischemic preconditioning against I/R-induced gastric injury. In the present study we investigated whether RIPC may protect the gastric mucosa against I/R-induced injury through involvement of glucocorticoids. Anesthetized fasted Sprague Dawley male rats were exposed to prolonged gastric I/R (30 min occlusion of celiac artery followed by 3 h of reperfusion) alone or with preliminary brief RIPC (10 min non-invasive occlusion of right hind limb blood flow followed by reperfusion for 30 min). First, we investigated the effect of RIPC on I/R-induced injury by itself. Then to study the role of glucocorticoids similar experiments were carried out: 1) in rats pretreated with the inhibitor of glucocorticoid synthesis, metyrapone (30 mg/kg, i.p), and in control animals; 2) in adrenalectomized rats without or with corticosterone replacement (4 mg/kg, s.c.) and in sham-operated animals; 3) in rats pretreated with glucocorticoid receptor antagonist RU-38486 (20 mg/kg, s.c.) and in control animals. I/R induced corticosterone rise and resulted in the gastric erosion formation. RIPC significantly reduced the erosion area in control animals. Metyrapone injected shortly before RIPC caused a decrease in plasma corticosterone levels and prevented the gastroprotective effect of RIPC and, moreover, further aggravated the deleterious effect of I/R. Adrenalectomy performed 1 week before experiment created long-lasting corticosterone deficiency and had no effect on the gastroprotective effect of RIPC. Nevertheless, corticosterone replacement which mimics the corticosterone rise, similar to RIPS, significantly reduced erosion areas of gastric mucosa in adrenalectomized rats supporting the role of glucocorticoids in gastroprotection. RU-38486, which occupied glucocorticoid receptors, similar to metyrapone prevented the gastroprotective effect of RIPC and, moreover, further aggravated the deleterious effect of I/R. The results of the present study demonstrate for the first time that RIPC may protect the gastric mucosa against I/R-induced injury through involvement of glucocorticoids.

Statin therapy associated with decreased neuronal injury measured by serum S100β levels in patients with acute ischemic stroke

Background

Acute ischemic strokes (AIS) are a common cause of morbidity, mortality, and disability. The serum biomarker S100β correlates with poor neurological outcomes in the setting of AIS. This study describes the impact of statin treatment on S100β levels following AIS.

Methods

This was a prospective case-control study of AIS patients compared to healthy controls. Patients were stratified into three groups: (1) AIS patients on statin therapy, (2) AIS patients not on statin therapy, and (3) healthy controls. Demographics, clinical parameters, stroke risk scores (SRS), and S100β levels were recorded for all patients.

Results

Blood pressure, lipids, and SRS scores were higher in stroke versus control patients (all P < 0.05), and lower in Group I versus II (all P < 0.05). S100β levels were higher in stroke versus nonstroke patients (P = 0.001), and lower in Group I versus II (P = 0.001). Furthermore, patients on atorvastatin showed greater S100β reductions than those on rosuvastatin therapy (P = 0.01).

Conclusion

In acute stroke patients, statins therapy correlated with reductions in the neuronal injury biomarker S100β, with greater reductions observed for atorvastatin than rosuvastatin therapy.

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.

A prophylactic effect of macrophage-colony stimulating factor on chronic stress-induced depression-like behaviors in mice

Innate immune activation has been shown to reduce the severity of nervous system disorders such as brain ischemia and traumatic brain damage. Macrophage-colony stimulating factor (M-CSF), a drug that is used to treat hematological system disease, is an enhancer of the innate immune response. In the present study, we evaluated the effect of M-CSF preconditioning on chronic social defeat stress (CSDS)-induced depression-like behaviors in mice. Results showed that a single M-CSF injection 1 day before stress exposure at the dose of 100 and 500 μg/kg, or a single M-CSF injection (100 μg/kg) 1 or 5 days but not 10 days before stress exposure prevented CSDS-induced depression-like behaviors in mice. Further analysis showed that a second M-CSF injection 10 days after the first M-CSF injection and a 2 × or 4 × M-CSF injections 10 days before stress exposure also prevented CSDS-induced depression-like behaviors. Molecular studies revealed that a single M-CSF injection prior to stress exposure skewed the neuroinflammatory responses in the brain in CSDS-exposed mice towards an anti-inflammatory phenotype. These behavioral and molecular actions of M-CSF were correlated with innate immune stimulation, as pre-inhibiting the innate immune activation by minocycline pretreatment (40 mg/kg) abrogated the preventive effect of M-CSF on CSDS-induced depression-like behaviors and neuroinflammatory responses. These results provide evidence to show that innate immune activation by M-CSF pretreatment may prevent chronic stress-induced depression-like behaviors via preventing the development of neuroinflammatory response in the brain, which may help to develop novel strategies for the prevention of depression.

Receptors, Channel Proteins, and Enzymes Involved in Microglia-mediated Neuroinflammation and Treatments by Targeting Microglia in Ischemic Stroke

Stroke is the largest contributor to global neurological disability-adjusted life-years, posing a huge economic and social burden to the world. Though pharmacological recanalization with recombinant tissue plasminogen activator and mechanical thrombectomy have greatly improved the prognosis of patients with ischemic stroke, clinically, there is still no effective treatment for the secondary injury caused by cerebral ischemia. In recent years, more and more evidences show that neuroinflammation plays a pivotal role in the pathogenesis and progression of ischemic cerebral injury. Microglia are brain resident innate immune cells and act the role peripheral macrophages. They play critical roles in mediating neuroinflammation after ischemic stroke. Microglia-mediated neuroinflammation is not an isolated process and has complex relationships with other pathophysiological processes as oxidative/nitrative stress, excitotoxicity, necrosis, apoptosis, pyroptosis, autophagy, and adaptive immune response. Upon activation, microglia differentially express various receptors, channel proteins, and enzymes involved in promoting or inhibiting the inflammatory processes, making them the targets of intervention for ischemic stroke. To inhibit microglia-related neuroinflammation and promote neurological recovery after ischemic stroke, numerous biochemical agents, cellular therapies, and physical methods have been demonstrated to have therapeutic potentials. Though accumulating experimental evidences have demonstrated that targeting microglia is a promising approach in the treatment of ischemic stroke, the clinical progress is slow. Till now, no clinical study could provide convincing evidence that any biochemical or physical therapies could exert neuroprotective effect by specifically targeting microglia following ischemic stroke.

Sexually dimorphic microglia and ischemic stroke

Ischemic stroke kills more women compared with men thus emphasizing a significant sexual dimorphism in ischemic pathophysiological outcomes. However, the mechanisms behind this sexual dimorphism are yet to be fully understood. It is well established that cerebral ischemia activates a variety of inflammatory cascades and that microglia are the primary immune cells of the brain. After ischemic injury, microglia are activated and play a crucial role in progression and resolution of the neuroinflammatory response. In recent years, research has focused on the role that microglia play in this sexual dimorphism that exists in the response to central nervous system (CNS) injury. Evidence suggests that the molecular mechanisms leading to microglial activation and polarization of phenotypes may be influenced by sex, therefore causing a difference in the pro/anti‐inflammatory responses after CNS injury. Here, we review advances highlighting that sex differences in microglia are an important factor in the inflammatory responses that are seen after ischemic injury. We discuss the main differences between microglia in the healthy and diseased developing, adult, and aging brain. We also focus on the dimorphism that exists between males and females in microglial‐induced inflammation and energy metabolism after CNS injury. Finally, we describe how all of the current research and literature regarding sex differences in microglia contribute to the differences in poststroke responses between males and females.