Resident microglia, rather than blood-derived macrophages, contribute to the earlier and more pronounced inflammatory reaction in the immature compared with the adult hippocampus after hypoxia-ischemia

Atorvastatin Promotes Pro/anti-inflammatory Phenotypic Transformation of Microglia via Wnt/β-catenin Pathway in Hypoxic-Ischemic Neonatal Rats

Inflammatory reaction plays a key role in the pathogenesis of hypoxic-ischemic encephalopathy (HIE) in neonates. Microglia are resident innate immune cells in the central nervous system and are profoundly involved in neuroinflammation. Studies have revealed that atorvastatin exerts a neuroprotective effect by regulating neuroinflammation in adult animal models of brain stroke and traumatic brain injury, but its role regarding damage to the developing brain remains unclear. This study aimed to clarify the effect and mechanism of atorvastatin on the regulation of microglia function in neonatal hypoxic-ischemic brain damage (HIBD). The oxygen glucose deprivation (OGD) of microglia and neonatal rat HIBD model was established. Atorvastatin, recombinant sclerostin protein (SOST), and XAV939 (degradation of β-catenin) were administered to OGD microglia and HIBD rats. The pathological changes of brain tissue, cerebral infarction volume, learning and memory ability of rats, pro-inflammatory (CD16+/Iba1+) and anti-inflammatory (CD206+/Iba1+) microglia markers, inflammation-related indicators (Inos, Tnfα, Il6, Arg1, Tgfb, and Mrc1), and Wnt/β-catenin signaling molecules were examined. Atorvastatin reduced OGD-induced pro-inflammatory microglia and pro-inflammatory factors, while increasing anti-inflammatory microglia and anti-inflammatory factors. In vivo, atorvastatin attenuated hypoxia-ischemia (HI)-induced neuroinflammation and brain damage. Mechanistically, atorvastatin decreased SOST expression and activated the Wnt/β-catenin signaling pathway, and the administration of recombinant SOST protein or XAV939 inhibited Wnt/β-catenin signaling and attenuated the anti-inflammatory effect of atorvastatin. Atorvastatin promotes the pro/anti-inflammatory phenotypic transformation of microglia via the Wnt/β-catenin pathway in HI neonatal rats. Atorvastatin may be developed as a potent agent for the treatment of HIE in neonates.

Inflammatory, metabolic, and sex-dependent gene-regulatory dynamics of microglia and macrophages in neonatal hippocampus after hypoxia-ischemia

Neonatal hypoxia-ischemia (HI) is a major cause of perinatal death and long-term disabilities worldwide. Post-ischemic neuroinflammation plays a pivotal role in HI pathophysiology. In the present study, we investigated the temporal dynamics of microglia (CX3CR1GFP/+) and infiltrating macrophages (CCR2RFP/+) in the hippocampi of mice subjected to HI at postnatal day 9. Using inflammatory pathway and transcription factor (TF) analyses, we identified a distinct post-ischemic response in CCR2RFP/+ cells characterized by differential gene expression in sensome, homeostatic, matrisome, lipid metabolic, and inflammatory molecular signatures. Three days after injury, transcriptomic signatures of CX3CR1GFP/+ and CCR2RFP/+ cells isolated from hippocampi showed a partial convergence. Interestingly, microglia-specific genes in CX3CR1GFP/+ cells showed a sexual dimorphism, where expression returned to control levels in males but not in females during the experimental time frame. These results highlight the importance of further investigations on metabolic rewiring to pave the way for future interventions in asphyxiated neonates.

Long-term recruitment of peripheral immune cells to brain scars after a neonatal insult

Although brain scars in adults have been extensively studied, there is less data available regarding scar formation during the neonatal period, and the involvement of peripheral immune cells in this process remains unexplored in neonates. Using a murine model of neonatal hypoxic-ischemic encephalopathy (HIE) and confocal microscopy, we characterized the scarring process and examined the recruitment of peripheral immune cells to cortical and hippocampal scars for up to 1 year post-insult. Regional differences in scar formation were observed, including the presence of reticular fibrotic networks in the cortex and perivascular fibrosis in the hippocampus. We identified chemokines with chronically elevated levels in both regions and demonstrated, through a parabiosis-based strategy, the recruitment of lymphocytes, neutrophils, and monocyte-derived macrophages to the scars several weeks after the neonatal insult. After 1 year, however, neutrophils and lymphocytes were absent from the scars. Our data indicate that peripheral immune cells are transient components of HIE-induced brain scars, opening up new possibilities for late therapeutic interventions.

NKG2D knockdown improves hypoxic-ischemic brain damage by inhibiting neuroinflammation in neonatal mice

Hypoxic-ischemic brain damage (HIBD) is a leading cause of neonatal death and neurological dysfunction. Neuroinflammation is identified as one of the crucial pathological mechanisms after HIBD, and natural killer group 2 member D (NKG2D) is reported to be implicated in the pathogenesis of immunoinflammatory diseases. However, the role of NKG2D in neonatal HIBD is seldomly investigated. In this study, a neonatal mice model of HIBD was induced, and the role of the NKG2D in neuroinflammation and brain injury was explored by intracerebroventricular injection of lentivirus to knockdown NKG2D in neonatal mice with HIBD. The results showed that a significant increase in NKG2D protein level in the brain of neonatal mice with HIBD. The NKG2D knockdown in the brain significantly alleviated cerebral infarction, neurobehavioral deficits, and neuronal loss in neuronal HIBD. Moreover, the neuroprotective effect of NKG2D knockdown was associated with inhibition of the activation of microglia and astrocytes, expression of NKG2D ligands (NKG2DLs) and DAP10, and the nuclear translocation of NF-κB p65. Our findings reveal NKG2D knockdown may exert anti-inflammatory and neuroprotective effects in the neonatal mice with HIBD through downregulation of NKG2D/NKG2DLs/DAP10/NF-κB pathway. These results suggest that NKG2D may be a potential target for the treatment of neonatal HIBD.

Clinical Potential of Immunotherapies in Subarachnoid Hemorrhage Treatment: Mechanistic Dissection of Innate and Adaptive Immune Responses

Subarachnoid hemorrhage (SAH), classified as a medical emergency, is a devastating and severe subtype of stroke. SAH induces an immune response, which further triggers brain injury; however, the underlying mechanisms need to be further elucidated. The current research is predominantly focused on the production of specific subtypes of immune cells, especially innate immune cells, post-SAH onset. Increasing evidence suggests the critical role of immune responses in SAH pathophysiology; however, studies on the role and clinical significance of adaptive immunity post-SAH are limited. In this present study, we briefly review the mechanistic dissection of innate and adaptive immune responses post-SAH. Additionally, we summarized the experimental studies and clinical trials of immunotherapies for SAH treatment, which may form the basis for the development of improved therapeutic approaches for the clinical management of SAH in the future.

RU.521 mitigates subarachnoid hemorrhage-induced brain injury via regulating microglial polarization and neuroinflammation mediated by the cGAS/STING/NF-κB pathway

BACKGROUND

The poor prognosis of subarachnoid hemorrhage (SAH) is often attributed to neuroinflammation. The cGAS-STING axis, a cytoplasmic pathway responsible for detecting dsDNA, plays a significant role in mediating neuroinflammation in neurological diseases. However, the effects of inhibiting cGAS with the selective small molecule inhibitor RU.521 on brain injury and the underlying mechanisms after SAH are still unclear.

METHODS

The expression and microglial localization of cGAS following SAH were investigated with western blot analysis and immunofluorescent double-staining, respectively. RU.521 was administered after SAH. 2'3'-cGAMP, a second messenger converted by activated cGAS, was used to activate cGAS-STING. The assessments were carried out by adopting various techniques including neurological function scores, brain water content, blood-brain barrier permeability, western blot analysis, TUNEL staining, Nissl staining, immunofluorescence, morphological analysis, Morris water maze test, Golgi staining, CCK8, flow cytometry in the in vivo and in vitro settings.

RESULTS

Following SAH, there was an observed increase in the expression levels of cGAS in rat brain tissue, with peak levels observed at 24 h post-SAH. RU.521 resulted in a reduction of brain water content and blood-brain barrier permeability, leading to an improvement in neurological deficits after SAH. RU.521 had beneficial effects on neuronal apoptosis and microglia activation, as well as improvements in microglial morphology. Additionally, RU.521 prompted a shift in microglial phenotype from M1 to M2. We also noted a decrease in the production of pro-inflammatory cytokines TNF-α, IL-1β, and IL-6, and an increase in the level of the anti-inflammatory cytokine IL-10. Finally, RU.521 treatment was associated with improvements in cognitive function and an increase in the number of dendritic spines in the hippocampus. The therapeutic effects were mediated by the cGAS/STING/NF-κB pathway and were found to be abolished by 2'3'-cGAMP. In vitro, RU.521 significantly reduced apoptosis and neuroinflammation.

CONCLUSION

The study showed that SAH leads to neuroinflammation caused by microglial activation, which contributes to early brain injury. RU.521 improved neurological outcomes and reduced neuroinflammation by regulating microglial polarization through the cGAS/STING/NF-κB pathway in early brain injury after SAH. RU.521 may be a promising candidate for the treatment of neuroinflammatory pathology after SAH. Video Abstract.

Downregulation of TRIM27 alleviates hypoxic-ischemic encephalopathy through inhibiting inflammation and microglia cell activation by regulating STAT3/HMGB1 axis

TRIM27 expression was increased in the Parkinson's disease (PD), and knockdown of TRIM27 in PC12 cells significantly inhibited cell apoptosis, indicating that downregulation of TRIM27 exerts a neuroprotective effect. Herein, we investigated TRIM27 role in hypoxic-ischemic encephalopathy (HIE) and the underlying mechanisms. HIE models were constructed in newborn rats using hypoxic ischemic (HI) treatment and PC-12/BV2 cells with oxygen glucose deprivation (OGD), respectively. The results demonstrated that TRIM27 expression was increased in the brain tissues of HIE rats and OGD-treated PC-12/BV2 cells. Downregulation of TRIM27 reduced the brain infarct volume, inflammatory factor levels and brain injury, as well as decreased the number of M1 subtype of microglia cells while increased the number of M2 microglia cells. Moreover, deletion of TRIM27 expression inhibited the expression of p-STAT3, p-NF-κB and HMGB1 in vivo and in vitro. In addition, overexpression of HMGB1 impaired the effects of TRIM27 downregulation on improving OGD-induced cell viability, inhibiting inflammatory reactions and microglia activation. Collectively, this study revealed that TRIM27 was overexpressed in HIE, and downregulation of TRIM27 could alleviate HI-induced brain injury through repressing inflammation and microglia cell activation via the STAT3/HMGB1 axis.

Microglial phagocytosis dysfunction in stroke is driven by energy depletion and induction of autophagy

Microglial phagocytosis of apoptotic debris prevents buildup damage of neighbor neurons and inflammatory responses. Whereas microglia are very competent phagocytes under physiological conditions, we report their dysfunction in mouse and preclinical monkey models of stroke (macaques and marmosets) by transient occlusion of the medial cerebral artery (tMCAo). By analyzing recently published bulk and single cell RNA sequencing databases, we show that the phagocytosis dysfunction was not explained by transcriptional changes. In contrast, we demonstrate that the impairment of both engulfment and degradation was related to energy depletion triggered by oxygen and nutrient deprivation (OND), which led to reduced process motility, lysosomal exhaustion, and the induction of a protective macroautophagy/autophagy response in microglia. Basal autophagy, in charge of removing and recycling intracellular elements, was critical to maintain microglial physiology, including survival and phagocytosis, as we determined both in vivo and in vitro using pharmacological and transgenic approaches. Notably, the autophagy inducer rapamycin partially prevented the phagocytosis impairment induced by tMCAo in vivo but not by OND in vitro, where it even had a detrimental effect on microglia, suggesting that modulating microglial autophagy to optimal levels may be a hard to achieve goal. Nonetheless, our results show that pharmacological interventions, acting directly on microglia or indirectly on the brain environment, have the potential to recover phagocytosis efficiency in the diseased brain. We propose that phagocytosis is a therapeutic target yet to be explored in stroke and other brain disorders and provide evidence that it can be modulated in vivo using rapamycin.Abbreviations: AIF1/IBA1: allograft inflammatory factor 1; AMBRA1: autophagy/beclin 1 regulator 1; ATG4B: autophagy related 4B, cysteine peptidase; ATP: adenosine triphosphate; BECN1: beclin 1, autophagy related; CASP3: caspase 3; CBF: cerebral blood flow; CCA: common carotid artery; CCR2: chemokine (C-C motif) receptor 2; CIR: cranial irradiation; Csf1r/v-fms: colony stimulating factor 1 receptor; CX3CR1: chemokine (C-X3-C motif) receptor 1; DAPI: 4',6-diamidino-2-phenylindole; DG: dentate gyrus; GO: Gene Ontology; HBSS: Hanks' balanced salt solution; HI: hypoxia-ischemia; LAMP1: lysosomal-associated membrane protein 1; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MCA: medial cerebral artery; MTOR: mechanistic target of rapamycin kinase; OND: oxygen and nutrient deprivation; Ph/A coupling: phagocytosis-apoptosis coupling; Ph capacity: phagocytic capacity; Ph index: phagocytic index; SQSTM1: sequestosome 1; RNA-Seq: RNA sequencing; TEM: transmission electron microscopy; tMCAo: transient medial cerebral artery occlusion; ULK1: unc-51 like kinase 1.

Functional reconstruction of the basal ganglia neural circuit by human striatal neurons in hypoxic-ischaemic injured brain

Perinatal hypoxic-ischaemic encephalopathy is the leading cause of neonatal death and permanent neurological deficits, while the basal ganglia is one of the major nuclei that is selectively and greatly affected in the brains of hypoxic-ischaemic encephalopathy patients, especially in severe cases. Human embryonic stem cell-derived neurons have shown great potential in different types of brain disorders in adults. However, it remains unknown whether and how grafted human embryonic stem cell-derived neurons can repair immature brains with hypoxic-ischaemic encephalopathy. Here, by administrating genetically labelled human embryonic stem cell-derived striatal neural progenitors into the ipsilateral striatum of hypoxic-ischaemic encephalopathy-injured mice, we found that the grafted cells gradually matured into GABA spiny projection neurons morphologically and electrophysiologically, and significantly rescued the area loss of hypoxic-ischaemic encephalopathy-injured brains. Intriguingly, using immunohistochemical staining combined with enhanced ascorbate peroxidase-based immunoelectron microscopy and rabies virus-mediated trans-synaptic tracing, we show that the grafts start to extend axonal projections to the endogenous target areas (globus pallidus externa, globus pallidus internus, substantia nigra), form synapses with host striatal, globus pallidus and nigra neurons, and receive extensive and stable synaptic inputs as early as 2 months post-transplantation. Importantly, we further demonstrated functional neural circuits re-established between the grafted neurons and host cortical, striatal and substantial nigra neurons at 3-6 months post-transplantation in the hypoxic-ischaemic encephalopathy-injured brain by optogenetics combined with electrophysiological recording. Finally, the transplanted striatal spiny projection neurons but not spinal GABA neurons restored the motor defects of hypoxic-ischaemic encephalopathy, which were reversed by clozapine-N-oxide-based inhibition of graft function. These findings demonstrate anatomical and functional reconstruction of the basal ganglia neural circuit including multiple loops by striatal spiny projection neurons in hypoxic-ischaemic encephalopathy-injured immature brains, which raises the possibility of such a cell replacement therapeutic strategy for hypoxic-ischaemic encephalopathy in neonates.

The SIRT1-HMGB1 axis: Therapeutic potential to ameliorate inflammatory responses and tumor occurrence

Inflammation is a common complication of many chronic diseases. It includes inflammation of the parenchyma and vascular systems. Sirtuin 1 (SIRT1) is a nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylase, which can directly participate in the suppression of inflammation. It can also regulate the activity of other proteins. Among them, high mobility group box 1 (HMGB1) signaling can be inhibited by deacetylating four lysine residues (55, 88, 90, and 177) in quiescent endothelial cells. HMGB1 is a ubiquitous nuclear protein, once translocated outside the cell, which can interact with various target cell receptors including the receptor for advanced glycation end-products (RAGE), toll-like receptor (TLR) 2, and TLR4 and stimulates the release of pro-inflammatory cyto-/chemokines. And SIRT1 has been reported to inhibit the activity of HMGB1. Both are related to the occurrence and development of inflammation and associated diseases but show an antagonistic relationship in controlling inflammation. Therefore, in this review, we introduce how this signaling axis regulates the emergence of inflammation-related responses and tumor occurrence, providing a new experimental perspective for future inflammation research. In addition, it explores diverse upstream regulators and some natural/synthetic activators of SIRT1 as a possible treatment for inflammatory responses and tumor occurrence which may encourage the development of new anti-inflammatory drugs. Meanwhile, this review also introduces the potential molecular mechanism of the SIRT1-HMGB1 pathway to improve inflammation, suggesting that SIRT1 and HMGB1 proteins may be potential targets for treating inflammation.

Hypoxia-ischemia-mediated effects on neurodevelopmentally regulated cold-shock proteins in neonatal mice under strict temperature control

BACKGROUND

Neonates have high levels of cold-shock proteins (CSPs) in the normothermic brain for a limited period following birth. Hypoxic-ischemic (HI) insults in term infants produce neonatal encephalopathy (NE), and it remains unclear whether HI-induced pathology alters baseline CSP expression in the normothermic brain.

METHODS

Here we established a version of the Rice-Vannucci model in PND 10 mice that incorporates rigorous temperature control.

RESULTS

Common carotid artery (CCA)-ligation plus 25 min hypoxia (8% O2) in pups with targeted normothermia resulted in classic histopathological changes including increased hippocampal degeneration, astrogliosis, microgliosis, white matter changes, and cell signaling perturbations. Serial assessment of cortical, thalamic, and hippocampal RNA-binding motif 3 (RBM3), cold-inducible RNA binding protein (CIRBP), and reticulon-3 (RTN3) revealed a rapid age-dependent decrease in levels in sham and injured pups. CSPs were minimally affected by HI and the age point of lowest expression (PND 18) coincided with the timing at which heat-generating mechanisms mature in mice.

CONCLUSIONS

The findings suggest the need to determine whether optimized therapeutic hypothermia (depth and duration) can prevent the age-related decline in neuroprotective CSPs like RBM3 in the brain, and improve outcomes during critical phases of secondary injury and recovery after NE.

IMPACT

The rapid decrease in endogenous neuroprotective cold-shock proteins (CSPs) in the normothermic cortex, thalamus, and hippocampus from postnatal day (PND) 11-18, coincides with the timing of thermogenesis maturation in neonatal mice. Hypoxia-ischemia (HI) has a minor impact on the normal age-dependent decline in brain CSP levels in neonates maintained normothermic post-injury. HI robustly disrupts the expected correlation in RNA-binding motif 3 (RBM3) and reticulon-3 (RTN3). The potent neuroprotectant RBM3 is not increased 1-4 days after HI in a mouse model of neonatal encephalopathy (NE) in the term newborn and in which rigorous temperature control prevents the manifestation of endogenous post-insult hypothermia.

Activation of RARα Receptor Attenuates Neuroinflammation After SAH via Promoting M1-to-M2 Phenotypic Polarization of Microglia and Regulating Mafb/Msr1/PI3K-Akt/NF-κB Pathway

Background and Purpose

Subarachnoid hemorrhage (SAH) is a life-threatening subtype of stroke with high rates of mortality. In the early stages of SAH, neuroinflammation is one of the important mechanisms leading to brain injury after SAH. In various central nervous system diseases, activation of RARα receptor has been proven to demonstrate neuroprotective effects. This study aimed to investigate the anti-inflammatory effects of RARα receptor activation after SAH.

Methods

Internal carotid artery puncture method used to established SAH model in Sprague-Dawley rats. The RARα specific agonist Am80 was injected intraperitoneally 1 hour after SAH. AGN196996 (specific RARα inhibitor), Msr1 siRNA and LY294002 (PI3K-Akt inhibitor) were administered via the lateral ventricle before SAH. Evaluation SAH grade, neurological function score, blood-brain barrier permeability. BV2 cells and SH-SY5Y cells were co-cultured and stimulated by oxyhemoglobin to establish an in vitro model of SAH. RT-PCR, Western blotting, and immunofluorescence staining were used to investigate pathway-related proteins, microglia activation and inflammatory response. Results: The expression of RARα, Mafb, and Msr1 increased in rat brain tissue after SAH. Activation of the RARα receptor with Am80 improved neurological deficits and attenuated brain edema, blood brain barrier permeability. Am80 increased the expression of Mafb and Msr1, and reduced neuroinflammation by enhancing the phosphorylation of Akt and by inhibiting the phosphorylation of NF-κB. AGN196996, Msr1 siRNA, and LY294002 reversed the therapeutic effects of Am80 by reducing the expression of Msr1 and the phosphorylation of Akt. In vitro model of SAH, Am80 promoted M1-to-M2 phenotypic polarization in microglia and suppressed the nuclear transcription of NF-κB.

Conclusion

Activation of the RARα receptor attenuated neuroinflammation by promoting M1-to-M2 phenotypic polarization in microglia and regulating the Mafb/Msr1/PI3K-Akt/NF-κB pathway. RARα might serve as a potential target for SAH therapy.

The natural (poly)phenols as modulators of microglia polarization via TLR4/NF-κB pathway exert anti-inflammatory activity in ischemic stroke

Increasing evidences suggest that inflammation plays a key role in the pathogenesis of stroke, a devastating disease second only to cardiac ischemia as a cause of death worldwide. Microglia are the first non-neuronal cells on the scene during the innate immune response to acute ischemic stroke. Microglia respond to acute brain injury by activating and developing classic M1-like (pro-inflammatory) or alternative M2-like (anti-inflammatory) phenotypes. M1 microglia produce pro-inflammatory cytokines to exacerbate neural death, astrocyte apoptosis, and blood brain barrier (BBB) disruption, while M2 microglia play the opposite role. NF-κB, a central regulator of the inflammatory response, was responsible for microglia M1 and M2 polarization. NF-κB p65 and p50 form a heterodimer to initiate a pro-inflammatory cytokine response, which enhances M1 activation and impair M2 response of microglia. TLR4, expressed on the surface of microglia, plays an important role in activating NF-κB, ultimately causing the M1 response of microglia. Therefore, modulation of microglial phenotypes via TLR4/NF-κB signaling pathway may be a promising therapeutic approach for ischemic stroke. Dietary (poly)phenols are present in various foods, which have shown promising protective effects on ischemic stroke. In vivo studies strongly suggest that many (poly)phenols have a pronounced impact on ischemic stroke, as demonstrated by lower neuroinflammation. Thus, this review focuses on the anti-inflammatory properties of dietary (poly)phenols and discusses their effects on the polarization of microglia through modulating TLR4/NF-κB signaling pathway in the ischemic stroke.

Monocytes promote acute neuroinflammation and become pathological microglia in neonatal hypoxic-ischemic brain injury

Rationale: Monocytes belong to the mononuclear phagocyte system and are immune responders to tissue injury and infection. There were also reports of monocytes transforming to microglia-like cells. Here we explore the roles of monocytes in microglia ontogeny and the pathogenesis of neonatal cerebral hypoxic-ischemic (HI) brain injury in mice. Methods: We used three genetic methods to track the development of monocytes, including CX3CR1GFP/+; CCR2RFP/+ reporter mice, adoptive transfer of GFP+ monocytes, and fate-mapping with CCR2-CreER mice, in neonatal mouse brains with or without lipopolysaccharide (LPS, 0.3 mg/kg)-sensitized Vannucci HI. We also used genetic (CCR2RFP/ RFP, CCR2 knockout) and pharmacological methods (RS102895, a CCR2 antagonist) to test the roles of monocytic influx in LPS/HI brain injury. Results: CCR2+ monocytes entered the late-embryonic brains via choroid plexus, but rapidly became CX3CR1+ amoeboid microglial cells (AMCs). The influx of CCR2+ monocytes declined after birth, but recurred after HI or LPS-sensitized HI (LPS/HI) brain injury, particularly in the hippocampus. The CCR2-CreER-based fate-mapping showed that CCR2+ monocytes became CD68+ TNFα+ macrophages within 4 d after LPS/HI, and maintained as TNFα+ MHCII+ macrophages or persisted as Tmem119+ Sall1+ P2RY12+ ramified microglia for at least five months after injury. Genetic deletion of the chemokine receptor CCR2 markedly diminished monocytic influx, the expression of pro- and anti-inflammatory cytokines, and brain damage. Post-LPS/HI application of RS102895 also reduced inflammatory responses and brain damage, leading to better cognitive functions. Conclusion: These results suggest that monocytes promote acute inflammatory responses and may become pathological microglia long after the neonatal LPS/HI insult. Further, blocking the influx of monocytes may be a potential therapy for neonatal brain injury.

Peripheral immune cells and perinatal brain injury: a double-edged sword?

Perinatal brain injury is the leading cause of neurological mortality and morbidity in childhood ranging from motor and cognitive impairment to behavioural and neuropsychiatric disorders. Various noxious stimuli, including perinatal inflammation, chronic and acute hypoxia, hyperoxia, stress and drug exposure contribute to the pathogenesis. Among a variety of pathological phenomena, the unique developing immune system plays an important role in the understanding of mechanisms of injury to the immature brain. Neuroinflammation following a perinatal insult largely contributes to evolution of damage to resident brain cells, but may also be beneficial for repair activities. The present review will focus on the role of peripheral immune cells and discuss processes involved in neuroinflammation under two frequent perinatal conditions, systemic infection/inflammation associated with encephalopathy of prematurity (EoP) and hypoxia/ischaemia in the context of neonatal encephalopathy (NE) and stroke at term. Different immune cell subsets in perinatal brain injury including their infiltration routes will be reviewed and critical aspects such as sex differences and maturational stage will be discussed. Interactions with existing regenerative therapies such as stem cells and also potentials to develop novel immunomodulatory targets are considered. IMPACT: Comprehensive summary of current knowledge on the role of different immune cell subsets in perinatal brain injury including discussion of critical aspects to be considered for development of immunomodulatory therapies.

Ambient Temperature Is Correlated With the Severity of Neonatal Hypoxic-Ischemic Brain Injury via Microglial Accumulation in Mice

BACKGROUND

The pathophysiology of neonatal hypoxic-ischemic encephalopathy (HIE) has been studied in several rodent models to develop novel treatments. Although it is well known that high ambient temperature results in severe HIE, the effect of subtle changes in ambient temperature during a hypoxic-ischemic (HI) insult has not been studied. Therefore, in order to clarify the difference of pathophysiological change among the HIE models due to the influence of small changes in chamber temperature, three-step gradual change of 0.5°C each were prepared in ambient temperature during hypoxic exposure.

METHODS

Blood flow in the left common carotid artery (CCA) of neonatal mice was interrupted using bipolar electronic forceps under general and local anesthesia. The mice were subsequently subjected to 10% hypoxic exposure for 50 min at 36.0, 36.5, or 37.0°C. A control group was also included in the study. The size of the striatum and hippocampus and the volume reduction rate of the hemisphere in the section containing them on the ischemic side were evaluated using microtubule associated protein 2 (MAP2) immunostaining. The accumulation of Iba1-positive cells was investigated to assess inflammation. Additionally, rotarod and open-field tests were performed 2 weeks after HI insult to assess its effect on physiological conditions.

RESULTS

MAP2 staining revealed that the higher the temperature during hypoxia, the more severe the volume reduction rate in the hemisphere, striatum, and hippocampus. The number of Iba1-positive cells in the ipsilateral lesion gradually increased with increasing temperature, and there was a significant difference in motor function in the 36.5 and 37.0°C groups compared with the sham group. In the open-field tests, there was a significant decrease in performance in the 37.0°C groups compared with the 36.0°C and sham groups.

CONCLUSIONS

Even a small gradual change of 0.5°C produced a significant difference in pathological and behavioral changes and contributed to the accumulation of Iba1-positive cells. The arrangement of ambient temperature is useful for creating a rodent model with the appropriate severity of the targeted neuropsychological symptoms to establish a novel therapy for HIE.

Hydrogen Attenuated Inflammation Response and Oxidative in Hypoxic Ischemic Encephalopathy via Nrf2 Mediated the Inhibition of NLRP3 and NF-κB

Hypoxia and ischemia cause neonatal encephalopathy and brain injury and can further result in cerebral palsy, cognitive impairment, growth restriction, and epilepsy. Induction of neuroprotection is a crucial therapeutic strategy for the treatment of perinatal hypoxic-ischaemic encephalopathy (HIE). Hydrogen has neuroprotective effects against brain-related diseases. Inflammation and oxidative stress are the two main pathophysiological mechanisms in neonatal hypoxic-ischaemic injury. Nuclear factor erythroid 2-related factor 2 (Nrf2) is an endogenous redox-sensitive transcription factor that participates in the antioxidant defence system through its effects on inflammation and oxidative stress. Herein, the research focuses on the mechanisms by which Nrf2 participates in the protection of hydrogen against HIE. The model of HIE was established by ligation of the right carotid artery and hypoxia in wild-type (WT) and Nrf2-/- mice. First, Nrf2 pathway activity was detected after hypoxia-ischaemia (HI) followed or not by hydrogen treatment. Brain injury, apoptosis, the inflammatory response, oxidative stress injury, and learning and memory function were assayed. We found that HI induced Nrf2 expression and signalling activation. Hydrogen alleviated the infarction volume, brain water content, neurological scores, apoptosis and long-term learning and memory functions after HI in WT mice but not in Nrf2-/- mice. Moreover, the oxidative products reactive oxygen species (ROS) and malondialdehyde (MDA) and the cytokines tumor necrosis factor-alpha (TNF-α), interleukin 6 (IL-6) and High mobility group box 1 (HMGB1) were reduced and the antioxidant enzymes Superoxide dismutase (SOD), glutathione peroxidase (GPx), catalase (CAT) were upregulated by hydrogen treatment after HI in WT mice, but not in Nrf2-/- mice. In addition, the absence of Nrf2 abolished the suppressive effect of hydrogen on the expression of Nacht, Lrr, and Pyd domains-containing protein 3 (NLRP3) pathway members and p65 NF-κB after HI. Taken together, our findings showed that hydrogen alleviated cellular injury and apoptosis, neurobehavioural deficits, the inflammatory response and oxidative stress via the Nrf2-mediated NLRP3 and NF-κB pathways.

Biomaterial and Therapeutic Approaches for the Manipulation of Macrophage Phenotype in Peripheral and Central Nerve Repair

Injury to the peripheral or central nervous systems often results in extensive loss of motor and sensory function that can greatly diminish quality of life. In both cases, macrophage infiltration into the injury site plays an integral role in the host tissue inflammatory response. In particular, the temporally related transition of macrophage phenotype between the M1/M2 inflammatory/repair states is critical for successful tissue repair. In recent years, biomaterial implants have emerged as a novel approach to bridge lesion sites and provide a growth-inductive environment for regenerating axons. This has more recently seen these two areas of research increasingly intersecting in the creation of 'immune-modulatory' biomaterials. These synthetic or naturally derived materials are fabricated to drive macrophages towards a pro-repair phenotype. This review considers the macrophage-mediated inflammatory events that occur following nervous tissue injury and outlines the latest developments in biomaterial-based strategies to influence macrophage phenotype and enhance repair.

Differential Expression and Distinct Roles of Proteinase-Activated Receptor 2 in Microglia and Neurons in Neonatal Mouse Brain After Hypoxia-Ischemic Injury

Regulation of microglial activation and neuroinflammation are critical factors in the pathogenesis of ischemic brain injury. Interest in protease-activated receptor 2 (PAR2) as a pharmaceutical target for various diseases is creasing. However, it is unclear the expression and functions of PAR2 in hypoxia-ischemic (HI) brain injury. Mice with HI and cells with oxygen-glucose deprivation and reoxygenation (OGD/R) were studied. Immunoblot and qRT-PCR were used to study the differential gene expression in cultured microglia and neurons. Immunofluorescent staining was used to study the expression pattern of PAR2 in the HI brain and phagocytotic activity of microglia after OGD/R. In neonatal mice brain after HI, we found PAR2 expression was abundant in neurons, but barely in microglia from the contralateral side of cortex and hippocampus. Conversely, PAR2 expression was barely in neurons while significantly increased in activated microglia from the ipsilateral side of cortex and hippocampus. The activations of PAR2 were increased in both microglia and neuron in a cell model of OGD/R. PAR2 activation mediated the cross-talk between microglia and neurons including the following: microglial PAR2 mediated inflammatory responses that induced neuronal damage; neuronal PAR2 regulated chemokines that recruited activated microglia to damage area; microglia PAR2 controlled the phagocytosis of degenerating neurons. These data suggested differential expression and distinct roles of PAR2 in microglia and neurons after HI injury; thereby, interventions targeting PAR2 may provide insights into the inflammatory-related diseases.

Microglia and Stem-Cell Mediated Neuroprotection after Neonatal Hypoxia-Ischemia

Neonatal hypoxia-ischemia encephalopathy (HIE) refers to a brain injury in term infants that can lead to death or lifelong neurological deficits such as cerebral palsy (CP). The pathogenesis of this disease involves multiple cellular and molecular events, notably a neuroinflammatory response driven partly by microglia, the brain resident macrophages. Treatment options are currently very limited, but stem cell (SC) therapy holds promise, as beneficial outcomes are reported in animal studies and to a lesser degree in human trials. Among putative mechanisms of action, immunomodulation is considered a major contributor to SC associated benefits. The goal of this review is to examine whether microglia is a cellular target of SC-mediated immunomodulation and whether the recruitment of microglia is linked to brain repair. We will first provide an overview on microglial activation in the rodent model of neonatal HI, and highlight its sensitivity to developmental age. Two complementary questions are then addressed: (i) do immune-related treatments impact microglia and provide neuroprotection, (ii) does stem cell treatment modulates microglia? Finally, the immune-related findings in patients enrolled in SC based clinical trials are discussed. Our review points to an impact of SCs on the microglial phenotype, but heterogeneity in experimental designs and methodological limitations hamper our understanding of a potential contribution of microglia to SC associated benefits. Thorough analyses of the microglial phenotype are warranted to better address the relevance of the neuroimmune crosstalk in brain repair and improve or advance the development of SC protocols in humans.

Cathepsin H deficiency decreases hypoxia-ischemia-induced hippocampal atrophy in neonatal mice through attenuated TLR3/IFN-β signaling

Background

Cathepsin H (CatH) is a lysosomal cysteine protease with a unique aminopeptidase activity. Its expression level is increased in activated immune cells including dendritic cells, macrophages, and microglia. We have previously reported that CatH deficiency impairs toll-like receptor 3 (TLR3)-mediated activation of interferon regulatory factor 3 (IRF3), and the subsequent secretion of interferon (IFN)-β from dendritic cells. Furthermore, there is increasing evidence that IFN-β secreted from microglia/macrophages has neuroprotective effects. These observations prompted further investigation into the effects of CatH deficiency on neuropathological changes.

Methods

In this study, neuropathological changes were examined using histochemical staining (both hematoxylin-eosin (H&E) and Nissl) of the hippocampus of wild-type (WT) and CatH-deficient (CatH^−/−) mice after hypoxia-ischemia (HI). The density and the localization of CatH and TLR3 were examined by immunofluorescent staining. CatH processing in microglia was assayed by pulse-chase experiments, while immunoblotting was used to examine TLR3 expression and IRF3 activation in microglia/macrophages in the presence of poly(I:C). Microglial cell death was examined by fluorescence-activated cell sorting (FACS), and primary astrocyte proliferation in the presence of IFN-β was examined using scratch wound assay.

Results

WT mice displayed severe atrophy in association with neuronal death and moderate astrogliosis in the hippocampus following neonatal HI. Somewhat surprisingly, CatH^−/− mice showed marked neuronal death without severe atrophy in the hippocampus following HI. Furthermore, there was notable microglia/macrophages cell death and strong astrogliosis in the hippocampus. The TLR3 and phosphorylated IRF3 expression level in the hippocampus or splenocytes (mainly splenic macrophages); from CatH^−/− mice was lower than in WT mice. In vitro experiments demonstrated that recombinant IFN-β suppressed HI-induced microglial cell death and astrocyte proliferation.

Conclusion

These observations suggest that CatH plays a critical role in the proteolytic maturation and stabilization of TLR3, which is necessary for IFN-β production. Therefore, impaired TLR3/IFN-β signaling resulting from CatH deficiency may induce microglial cell death after activation and astrogliosis/glial scar formation in the hippocampus following HI injury, leading to suppression of hippocampal atrophy.

Tracing bone marrow-derived microglia in brain metastatic tumors

Microglia are the resident macrophages in the central nervous system (CNS), and they constitute 15-20% of the total glial populations. They have wide developmental and protective functions during brain injury, infection and tumorigenesis. Originally thought to derive from postnatal hematopoietic progenitors, it has recently been demonstrated that microglia originate from primitive myeloid progenitor cells that arise during early development from the embryonic yolk sac. Circulating monocytes infiltrate the CNS upon inflammatory conditions, such as cancer, primarily differentiating into macrophages and dendritic cells. Both resident and recruited microglia respond to environmental cues and actively participate in pathogenic processes, albeit their transcriptomic profiles contain significant differences suggesting distinctive roles. Metastatic brain tumors are the most common intracranial neoplasm in adults, with an estimate incidence 10 times higher than all primary brain neoplasms combined, and with dismal prognosis. Microglia is a major immune population associated with brain metastatic tumors in patients. They are proposed to play multiple, and sometimes opposing roles, in tumor progression. However, our ability to evaluate individual contribution of resident and recruited populations is hindered by the fact that they express overlapping sets of surface markers. Tracking and interrogating tissue-resident vs recruited microglia in the brain tumor microenvironment becomes critical to dissect their respective roles and gain a better understanding of the mechanism governing their interaction. In this chapter, we describe the utilization of genetic reporter mice to identify recruited brain microglia, offer a comparison between the genetic method and the most widely used flow cytometric approach, and discuss potential downstream applications to interrogate BMDM function in brain metastatic disease.

Galectin-3 modulates microglia inflammation in vitro but not neonatal brain injury in vivo under inflammatory conditions

Microglia may contribute to injury but may also have neuroprotective properties. Galectin-3 has immunomodulatory properties that may affect the microglia phenotype and subsequent development of injury. Galectin-3 contributes to experimental hypoxic-ischemic (HI) injury in the neonatal brain, but it is unclear if galectin-3 has similar effects on infectious and sterile inflammation. Thus, we investigated the effect of galectin-3 on microglia in vitro under normal as well as infectious and sterile inflammatory conditions, and the effect of galectin-3 on neonatal brain injury following an infectious challenge in vivo. Conditions mimicking infectious or sterile inflammation were evaluated in primary microglia cell cultures from newborn mice, using LPS (10 ng/mL) and TNF-α (100 ng/mL). The response to galectin-3 was tested alone or together with LPS or TNF-α. Supernatants were collected 24 h after treatment and analyzed for 23 inflammatory mediators including pro- and anti-inflammatory cytokines and chemokines using multiplex protein analysis, as well as ELISA for MCP-1 and insulin-like growth factor (IGF)-1. Phosphorylation of proteins (AKT, ERK1/2, IκB-α, JNK, and p38) was determined in microglia cells. Neonatal brain injury was induced by a combination of LPS and HI (LPS + HI) in postnatal day 9 transgenic mice lacking functional galectin-3 and wild-type controls. LPS and TNF-α induced pro-inflammatory (9/11 vs. 9/10) and anti-inflammatory (6/6 vs. 2/6) cytokines, as well as chemokines (6/6 vs. 4/6) in a similar manner, except generally lower amplitude of the TNF-α-induced response. Galectin-3 alone had no effect on any of the proteins analyzed. Galectin-3 reduced the LPS- and TNF-α-induced microglia response for cytokines, chemokines, and phosphorylation of IκB-α. LPS decreased baseline IGF-1 levels, and the levels were restored by galectin-3. Brain injury or microglia response after LPS + HI was not affected by galectin-3 deficiency. Galectin-3 has no independent effect on microglia but modulates inflammatory activation in vitro. The effect was similar under infectious and sterile inflammatory conditions, suggesting that galectin-3 regulates inflammation not just by binding to LPS or toll-like receptor-4. Galectin-3 restores IGF-1 levels reduced by LPS-induced inflammation, suggesting a potential protective effect on infectious injury. However, galectin-3 deficiency did not affect microglia activation and was not beneficial in an injury model encompassing an infectious challenge.

Radiation Triggers a Dynamic Sequence of Transient Microglial Alterations in Juvenile Brain

Cranial irradiation (IR), an effective tool to treat malignant brain tumors, triggers a chronic pro-inflammatory microglial response, at least in the adult brain. Using single-cell and bulk RNA sequencing, combined with histology, we show that the microglial response in the juvenile mouse hippocampus is rapid but returns toward normal within 1 week. The response is characterized by a series of temporally distinct homeostasis-, sensome-, and inflammation-related molecular signatures. We find that a single microglial cell simultaneously upregulates transcripts associated with pro- and anti-inflammatory microglial phenotypes. Finally, we show that juvenile and adult irradiated microglia are already transcriptionally distinct in the early phase after IR. Our results indicate that microglia are involved in the initial stages but may not be responsible for driving long-term inflammation in the juvenile brain.

Oxiracetam Mediates Neuroprotection Through the Regulation of Microglia Under Hypoxia-Ischemia Neonatal Brain Injury in Mice

In neonatal hypoxic-ischemic brain damage (HIBD), in addition to damage caused by hypoxia and ischemia, over-activation of inflammation leads to further deterioration of the condition, thus greatly shortening the optimal treatment time window. Ischemic penumbra, the edematous area encompassing the infarct core, is characterized by typical activation of microglia and overt inflammation, and prone to incorporate into the infarct core gradually after ischemia onset. If treated in time, the cells located in the penumbra can survive, thereby impeding the expansion of the infarction. We demonstrated for the first time that in the acute phase of HIBD in neonatal mice, treatment of Oxiracetam (ORC) significantly curtailed the size of ischemic penumbra together with drastic reduction of infarction. By staining various cellular markers, we found that the penumbra was defined and concentrated with activated microglia. We also analyzed transmission electron microscopy and Luminex assay results to elucidate the mechanisms involved. We further confirmed that ORC switched polarization of microglia from the inflammatory towards the alternatively activated phenotype, thus promoting microglia from being neurotoxic into neuroprotective. Meanwhile, ORC decreased proliferation of microglia; however, their functions of phagocytosis and autophagy were otherwise enhanced. Last, we clarified that ORC promoted autophagy through the AMPK/mTOR pathway, which further induced the transition of the inflammatory to the alternatively activated phenotype in microglia. The pro-inflammatory factors secretion was inhibited as well, thereby reducing the progression of the infarction. Taken together, it is concluded that Oxiracetam reduced the expansion of ischemic infarction in part via regulating the interplay between microglia activation and autophagy, which would delay the progression of HIBD and effectively prolong the time window for the clinical treatment of HIBD.

Inhibition of Colony Stimulating Factor 1 Receptor Suppresses Neuroinflammation and Neonatal Hypoxic-Ischemic Brain Injury

Hypoxic-ischemic (HI) brain injury is a major cause of neonatal death or lifetime disability without widely accepted effective pharmacological treatments. It has been shown that the survival of microglia requires colony-stimulating factor 1 receptor (CSF1R) signaling and microglia participate in neonatal HI brain injury. We therefore hypothesize that microglia depletion during a HI insult period could reduce immature brain injury. In this study, CD1 mouse pups were treated with a CSF1R inhibitor (PLX3397, 25 mg/kg/daily) or a vehicle from postnatal day 4 to day 11 (P4-11), and over 90% of total brain microglia were deleted at P9. Unilateral hemisphere HI injury was induced at P9 by permanently ligating the left common carotid arteries and exposing the pups to 10% oxygen for 30 min to produce moderate left hemisphere injury. We found that the PLX3397 treatment reduced HI brain injury by 46.4%, as evaluated by the percentage of brain infarction at 48 h after HI. Furthermore, CSF1R inhibition suppressed the infiltration of neutrophils (69.7% reduction, p = 0.038), macrophages (77.4% reduction, p = 0.009), and T cells (72.9% reduction, p = 0.008) to the brain, the production of cytokines and chemokines (such as CCL12, CCL6, CCL21, CCL22, CCL19, IL7, CD14, and WISP-1), and reduced neuronal apoptosis as indicated by active caspase-3 labeled cells at 48 h after HI (615.20 ± 156.84/mm² vs. 1,205.00 ± 99.15/mm², p = 0.013). Our results suggest that CSF1R inhibition suppresses neuroinflammation and neonatal brain injury after acute cerebral hypoxia-ischemia in neonatal mice.

Activation of MC1R with BMS-470539 attenuates neuroinflammation via cAMP/PKA/Nurr1 pathway after neonatal hypoxic-ischemic brain injury in rats

Background

Microglia-mediated neuroinflammation plays a crucial role in the pathogenesis of hypoxic-ischemic (HI)-induced brain injury. Activation of melanocortin-1 receptor (MC1R) has been shown to exert anti-inflammatory and neuroprotective effects in several neurological diseases. In the present study, we have explored the role of MC1R activation on neuroinflammation and the potential underlying mechanisms after neonatal hypoxic-ischemic brain injury in rats.

Methods

A total of 169 post-natal day 10 unsexed rat pups were used. HI was induced by right common carotid artery ligation followed by 2.5 h of hypoxia. BMS-470539, a specific selective MC1R agonist, was administered intranasally at 1 h after HI induction. To elucidate the potential underlying mechanism, MC1R CRISPR KO plasmid or Nurr1 CRISPR KO plasmid was administered via intracerebroventricular injection at 48 h before HI induction. Percent brain infarct area, short- and long-term neurobehavioral tests, Nissl staining, immunofluorescence staining, and Western blot were conducted.

Results

The expression levels of MC1R and Nurr1 increased over time post-HI. MC1R and Nurr1 were expressed on microglia at 48 h post-HI. Activation of MC1R with BMS-470539 significantly reduced the percent infarct area, brain atrophy, and inflammation, and improved short- and long-term neurological deficits at 48 h and 28 days post-HI. MC1R activation increased the expression of CD206 (a microglial M2 marker) and reduced the expression of MPO. Moreover, activation of MC1R with BMS-470539 significantly increased the expression levels of MC1R, cAMP, p-PKA, and Nurr1, while downregulating the expression of pro-inflammatory cytokines (TNFα, IL-6, and IL-1β) at 48 h post-HI. However, knockout of MC1R or Nurr1 by specific CRISPR reversed the neuroprotective effects of MC1R activation post-HI.

Conclusions

Our study demonstrated that activation of MC1R with BMS-470539 attenuated neuroinflammation, and improved neurological deficits after neonatal hypoxic-ischemic brain injury in rats. Such anti-inflammatory and neuroprotective effects were mediated, at least in part, via the cAMP/PKA/Nurr1 signaling pathway. Therefore, MC1R activation might be a promising therapeutic target for infants with hypoxic-ischemic encephalopathy (HIE).

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02078-2.

Intranasal mesenchymal stem cell therapy to boost myelination after encephalopathy of prematurity

Encephalopathy of prematurity (EoP) is a common cause of long-term neurodevelopmental morbidity in extreme preterm infants. Diffuse white matter injury (dWMI) is currently the most commonly observed form of EoP. Impaired maturation of oligodendrocytes (OLs) is the main underlying pathophysiological mechanism. No therapies are currently available to combat dWMI. Intranasal application of mesenchymal stem cells (MSCs) is a promising therapeutic option to boost neuroregeneration after injury. Here, we developed a double-hit dWMI mouse model and investigated the therapeutic potential of intranasal MSC therapy. Postnatal systemic inflammation and hypoxia-ischemia led to transient deficits in cortical myelination and OL maturation, functional deficits and neuroinflammation. Intranasal MSCs migrated dispersedly into the injured brain and potently improved myelination and functional outcome, dampened cerebral inflammationand rescued OL maturation after dWMI. Cocultures of MSCs with primary microglia or OLs show that MSCs secrete factors that directly promote OL maturation and dampen neuroinflammation. We show that MSCs adapt their secretome after ex vivo exposure to dWMI milieu and identified several factors including IGF1, EGF, LIF, and IL11 that potently boost OL maturation. Additionally, we showed that MSC-treated dWMI brains express different levels of these beneficial secreted factors. In conclusion, the combination of postnatal systemic inflammation and hypoxia-ischemia leads to a pattern of developmental brain abnormalities that mimics the clinical situation. Intranasal delivery of MSCs, that secrete several beneficial factors in situ, is a promising strategy to restore myelination after dWMI and subsequently improve the neurodevelopmental outcome of extreme preterm infants in the future.

Quercetin alleviates neonatal hypoxic-ischemic brain injury by inhibiting microglia-derived oxidative stress and TLR4-mediated inflammation

OBJECTIVE AND DESIGN

Microglia stimulated by oxygen glucose deprivation (OGD) were treated with quercetin to investigate the effect on oxidative stress and the inflammatory response and to explore whether toll-like receptor 4 (TLR4) signaling was involved. In addition, the effect of quercetin on the neurological functions of neonatal mice with hypoxic-ischemic brain injury (HIBI) was examined.

MATERIALS AND SUBJECTS

Mouse BV2 microglial cells and postnatal day 7 neonatal mice were used.

TREATMENT

A predetermined concentration of quercetin was used in cell experiments. Quercetin was injected i.p. (50 mg/kg) at three time points after HI insult: 0, 24, and 48 h.

METHODS

Cell viability assay, Western blotting, qRT-RCR, ELISA, HIBI model construction and behavioral tests.

RESULTS

This study first showed that quercetin protected BV2 cells from OGD-induced damage and reversed the changes in microglial oxidative stress-related molecules. Second, quercetin inhibited OGD-induced expression of inflammatory factors in BV2 cells and suppressed TLR4/MyD88/NF-κB signaling. Finally, quercetin was disclosed to be effective in mitigating cerebral infarct volume and cognitive and motor function deficits in HIBI mice.

CONCLUSION

These results suggest that the neuroprotective effect of quercetin in HIBI mice is partially due to the inhibition of oxidative stress and TLR4-mediated inflammatory responses in activated microglia.

Galectin-3: Roles in Neurodevelopment, Neuroinflammation, and Behavior

There is a plethora of evidence to suggest that Galectin-3 plays an important role in normal functions of mammalian cells, as well as in different pathogenic conditions. This review highlights recent data published by researchers, including our own team, on roles of Galectin-3 in the nervous system. Here, we discuss the roles of Galectin-3 in brain development, its roles in glial cells, as well as the interactions of glial cells with other neural and invading cells in pathological conditions. Galectin-3 plays an important role in the pathogenesis of neuroinflammatory and neurodegenerative disorders, such as multiple sclerosis, Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease. On the other hand, there is also evidence of the protective role of Galectin-3 due to its anti-apoptotic effect in target cells. Interestingly, genetic deletion of Galectin-3 affects behavioral patterns in maturing and adult mice. The results reviewed in this paper and recent development of highly specific inhibitors suggests that Galectin-3 may be an important therapeutic target in pathological conditions including the disorders of the central nervous system.

Aggravated brain injury after neonatal hypoxic ischemia in microglia-depleted mice

Background

Neuroinflammation plays an important role in neonatal hypoxic-ischemic encephalopathy (HIE). Although microglia are largely responsible for injury-induced inflammatory response, they play beneficial roles in both normal and disease states. However, the effects of microglial depletion on neonatal HIE remain unclear.

Methods

Tamoxifen was administered to Cx3cr1^CreER/+Rosa26^DTA/+ (microglia-depleted model) and Cx3cr1^CreER/+Rosa26^DTA/− (control) mice at P8 and P9 to assess the effect of microglial depletion. The density of microglia was quantified using Iba-1 staining. Moreover, the proportion of resident microglia after the HI insult was analyzed using flow cytometric analysis. At P10, the HI insult was conducted using the Rice-Vannucci procedure at P10. The infarct size and apoptotic cells were analyzed at P13. Cytokine analyses were performed using quantitative polymerase chain reaction and enzyme-linked immunosorbent assay (ELISA) at P13.

Results

At P10, tamoxifen administration induced > 99% microglial depletion in DTA⁺ mice. Following HI insult, there was persisted microglial depletion over 97% at P13. Compared to male DTA⁻ mice, male DTA⁺ mice exhibited significantly larger infarct volumes; however, there were no significant differences among females. Moreover, compared to male DTA⁻ mice, male DTA⁺ mice had a significantly higher density of TUNEL⁺ cells in the caudoputamen, cerebral cortex, and thalamus. Moreover, compared to female DTA⁻ mice, female DTA⁺ mice showed a significantly greater number of TUNEL⁺ cells in the hippocampus and thalamus. Compared to DTA⁻ mice, ELISA revealed significantly lower IL-10 and TGF-β levels in both male and female DTA⁺ mice under both normal conditions and after HI (more pronounced).

Conclusion

We established a microglial depletion model that aggravated neuronal damage and apoptosis after the HI insult, which was predominantly observed in males.

Alarmin HMGB1 Plays a Detrimental Role in Hippocampal Dysfunction Caused by Hypoxia-Ischemia Insult in Neonatal Mice: Evidence from the Application of the HMGB1 Inhibitor Glycyrrhizin

Hippocampal dysfunction related to cognitive impairment and emotional disorders in young children and adolescents caused by neonatal hypoxic-ischemic brain injury (HIBI) has attracted increasing attention in recent years. Crosstalk between the nervous and immune systems organized by hypoxia-ischemia (HI) insult may contribute to hippocampal dysfunction after HIBI. Extracellular HMGB1 functions as a damage-associated molecular pattern to instigate and amplify inflammatory responses, but whether this molecule is correlated with hippocampal dysfunction after HIBI is largely unknown. Therefore, this study examined hippocampal function after HMGB1 inhibition in an experimental HIBI model to verify the hypothesis that HMGB1 is a key mediator of hippocampal neuropathology in neonatal HIBI. By administering different doses of the HMGB1-specific inhibitor glycyrrhizin (GLY), we first found that GLY reversed the HI insult-induced loss of neurons and myelin in the hippocampal region and neurobehavioral impairments, partially in a dose-dependent manner, and based on this, we determined the optimal drug concentration to be 50 mg/kg. Subsequent analysis found that this neuroprotective effect was achieved through the inhibition of HMGB1 expression and nucleocytoplasmic translocation, a reduction in the abnormal expression of proteins associated with the downstream signaling pathway of HMGB1, a decrease in the inflammatory response, the suppression of increases in microglia/astrocytes, and the inhibition of hippocampal cell apoptosis. Collectively, our discoveries contribute to the rising appreciation of the role of HMGB1 in the neuropathology of hippocampal dysfunction and related behavioral outcomes following HIBI.

TSPO Modulates IL-4-Induced Microglia/Macrophage M2 Polarization via PPAR-γ Pathway

Microglia activation has been reported to be associated with pathogenesis of neuroinflammation, central nervous system damage, and degeneration diseases. With various damage-associated molecules released, M1 polarization of microglia emerges early after injury and followed by M2 polarization. In this study, we demonstrate using a primary microglia polarization model that, during the M2 polarization of microglia, the protein expression of translocator protein (TSPO) was decreased and peroxisome proliferator-activated receptor (PPAR-γ) activation was observed. In addition, we found TSPO antagonist PK11195 treatment enhanced PPAR-γ expression in M2-polarized microglia, while TSPO agonist FGIN-1-27 and TSPO overexpression in microglia significantly suppressed PPAR-γ expression in both the cytoplasm and nucleus. Then, real-time quantitative PCR was used to detect the expression of M2 polarization markers in microglia after TSPO ligand treatment, the data showed that PK11195 promoted the expression of CD206, Arg-1, YM-1, and FIZZ-1 induced by interleukin-4 (IL-4), and FGIN-1-27 and TSPO overexpression inhibited the expression of these molecules. Furthermore, the release of BDNF, CNTF-1, IGF-1, and NGF-1 from microglia was determined by enzyme-linked immunosorbent assay; these trophic factors showed similar trends with expression of M2 polarization markers. Levels of BDNF, CNTF-1, IGF-1, and NGF-1 were obviously upregulated by PK11195 and downregulated by FGIN-1-27 and TSPO overexpression. We propose that IL-4 in the hypoxic ischemia brain site induces the M2 polarization of microglia, and TSPO inhibits the M2 polarization and trophic factor release through PPAR-γ pathway.

Reducing Th2 inflammation through neutralizing IL-4 antibody rescues myelination in IUGR rat brain

BACKGROUND

Intrauterine growth restriction (IUGR) is a common complication of pregnancy and is associated with significant neurological deficits in infants, including white matter damage. Previous work using an animal model of IUGR has demonstrated that IUGR rats exhibit neurobehavioral deficits and developmental delays in oligodendrocyte maturation and myelination, but the mechanisms which cause this delay are unknown. Inflammation may be an important etiological factor in IUGR and has been recognized as playing a fundamental role in the pathogenesis of myelin disorders, including cerebral palsy.

METHODS

To create the model, the uterine arteries of pregnant rats were ligated at embryonic day 15. Rats delivered spontaneously. Cytokine and chemokine expression was evaluated at one prenatal and three postnatal time points, and myelin protein expression and oligodendrocyte cell numbers were evaluated by several methods at postnatal day 14. IL-4 was identified as a potential inhibitor of myelination, and rat pups were injected with IL-4 function blocking antibody from postnatal days 1-5 and myelination was assessed.

RESULTS

Here, we show a novel mechanism of white matter injury. IUGR induces an exaggerated Th2 response in the developing rat brain, including upregulation of several Th2 cytokines. Of these, IL-4 is significantly increased during the period corresponding to robust developmental myelination. We show that neutralizing IL-4 antibody therapy given in the newborn period ameliorates inflammation and restores myelin protein expression and oligodendrocyte cell number in the IUGR brain to control levels, demonstrating a novel role for Th2 responses and IL-4 in IUGR and white matter injury. In addition, IL-4 directly affects oligodendrocytes in vitro decreasing differentiation.

CONCLUSIONS

In this study, we have identified inflammation as a factor in the decrease in myelin seen in an animal model of IUGR. IL-4, an inflammatory protein often thought to be protective in the adult, is specifically increased, and treatment of these animals to prevent this increase ameliorates white matter damage. Our results suggest that the immune system plays a role in IUGR that is different in the perinatal period than in the adult and preventing this exaggerated Th2 response may be a potential therapeutic target.

High-Mobility Group Box 1 Contributes to Cerebral Cortex Injury in a Neonatal Hypoxic-Ischemic Rat Model by Regulating the Phenotypic Polarization of Microglia

Neonatal hypoxic-ischemic (HI) encephalopathy is a severe disease for which there is currently no curative treatment. Recent evidence suggests that high-mobility group box 1 (HMGB1) protein can promote neuroinflammation after stroke in adult rodents, but its role in perinatal hypoxic-ischemic brain damage (HIBD) remains largely uninvestigated. In the present work, the potential role of HMGB1 in the pathogenesis of HIBD was explored. A HIBD model was established in postpartum day 7 rat pups. HMGB1 expression, the cellular distribution of HMGB1, and microglial activation were all evaluated. Glycyrrhizin (GL), an inhibitor of HMGB1, was used to investigate whether the inhibition of HMGB1 modulated microglial M1/M2 polarization or attenuated brain damage after HI. HAPI microglial cells and primary neurons were cultured in vitro and an oxygen-glucose deprivation model was established to evaluate the effects of different microglial-conditioned media on neurons using GL and recombinant HMGB1. Results showed that the expression of HMGB1 was increased in both the ipsilateral cortex and peripheral blood 72 h after HI. Immunofluorescence analyses showed that HMGB1 in the cortex was primarily expressed in neurons. This increase in cortical HMGB1 expression 72 h after HI was characterized by increased co-expression with microglia, rather than neurons or astrocytes. The expression of both M1 and M2 microglia was upregulated 72 h after HI. The administration of GL significantly suppressed M1 microglial polarization and promoted M2 microglial polarization. Meanwhile, GL pretreatment significantly alleviated brain edema and cerebral infarction. In vitro experimentation showed that HMGB1-induced M1-conditioned media aggravated neuronal damage, but this effect was neutralized by GL. These findings suggest that HMGB1 may result in an imbalance of M1/M2 microglial polarization in the cortex and thus cause neuronal injury. Pharmacological blockade of HMGB1 signaling may attenuate this imbalanced polarization of microglia and thus could be used as a therapeutic strategy against brain injury in HIBD.

The Dynamics of Microglial Polarization Reveal the Resident Neuroinflammatory Responses After Subarachnoid Hemorrhage

Neuroinflammation plays a critical role in the pathogenesis of subarachnoid hemorrhage (SAH). Microglia, as the resident immune cells, orchestrate neuroinflammation distinctly in neurological diseases with different polarization statuses. However, microglial polarizations in the neuroinflammatory responses after SAH are not fully understood. In this study, we investigated the dynamics of microglial reaction in an endovascular perforated SAH model. By using the Cx3cr1^GFP/GFP Ccr2^RFP/RFP transgenic mice, we found that the reactive immune cells were largely from resident microglia pool rather than infiltrating macrophages. Immunostaining and real-time PCR were employed to analyze the temporal microglial polarization and the resulting inflammatory responses. Our results showed that microglia accumulated immediately after SAH with a centrifugal spreading through the Cortex Adjacent to the Perforated Site (CAPS) to the remote motor cortex. Microglia polarized dynamically from M1 to M2 phenotype along with the morphological transformation from ramified to amoeboid shapes. The ramified microglia demonstrated the M1 property, which suggested the function-related microglial polarization occurred prior to morphological transformation after SAH. Bipolar-shaped microglia appeared as the intermediate and transitional status with the capacity of bidirectional polarization. The microglial polarization status is distinct in molecular inflammatory responses; M1-related pro-inflammation was predominant in the early phase and subsequently transited to the M2-related anti-inflammation. The systematic characterization of the dynamics of microglial polarization in this study contributes to the understanding of the origin of neuroinflammatory responses after SAH and provides key foundation for further investigations to develop target treatment.

Transcriptome profiling of brain myeloid cells revealed activation of Itgal, Trem1, and Spp1 in western diet-induced obesity

Background

Environmental factors are critical in the development of age-related cognitive decline and dementia. A western diet (WD) can cause nutrient deficiency and inflammation that could impact cognition directly. It is increasingly recognized that innate immune responses by brain myeloid cells, such as resident microglia, and infiltrating peripheral monocytes/macrophages may represent an essential link between a WD, cognitive decline, and dementia. Our previous data demonstrated that chronic consumption of a WD induced inflammation through brain myeloid cells in aging mice and a mouse model of Alzheimer’s disease (AD). However, the subtypes of myeloid cells that contribute to the WD-induced inflammation remain unclear.

Methods

C57BL/6J (B6), myeloid cell reporter mice (B6.Ccr2^RFP/+Cx3cr1^GFP/+), and Ccr2-deficient mice (B6.Ccr2^RFP/RFP) were fed a WD or a control chow diet (CD) from 2 to 6 or 12 months of age. CD11b+CD45^lo and CD11b+CD45^hi cells from WD- and CD-fed B6 or Ccr2-deficient mice were characterized using flow cytometry, RNA-sequencing, and immunofluorescence.

Results

Ccr2::RFP expressing myeloid cells were significantly increased in brains of WD- compared to CD-fed mice, but were not elevated in Ccr2-deficient WD-fed mice. The percent of CD11b+CD45^hi cells was significantly increased in WD- compared to CD-fed mice. Comparison of RNA-sequencing data with immune cell data in ImmGen supports that CD11b+CD45^hi cells from WD-fed mice are enriched for peripheral monocytes and neutrophils. Ingenuity pathway analysis predicted these cells elicit proinflammatory responses that may be damaging to the brain. Using stringent criteria for gene expression levels between CD11b+CD45^hi and CD11b+CD45^lo cells, we identified approximately 70 genes that we predict are uniquely expressed in infiltrating cells, including Itgal, Trem1, and Spp1 (osteopontin, OPN). Finally, we show a significantly greater number of OPN+IBA1– cells in WD- compared to CD-fed mice that we propose are activated neutrophils based on ImmGen data. OPN+IBA1– cells are not significantly increased in Ccr2-deficient WD-fed mice.

Conclusions

These data further support the model that peripheral myeloid cells enter the brain in response to diet-induced obesity. Elucidating their contribution to age-related cognitive decline and age-related neurodegenerative diseases should offer new avenues for therapeutic intervention in Alzheimer’s disease and related dementias, where diet/obesity are major risk factors.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1527-z) contains supplementary material, which is available to authorized users.

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.

SIRT1-regulated HMGB1 release is partially involved in TLR4 signal transduction: A possible anti-neuroinflammatory mechanism of resveratrol in neonatal hypoxic-ischemic brain injury

Neonatal hypoxic-ischemic brain injury (HIBI) is a knotty disease that lacks appropriate treatment. Inflammation is an important contributor to brain damage, and microglia are responsible for eliciting early and pronounced inflammatory reactions in the immature brain after hypoxic-ischemic (HI) insult. Acetylated HMGB1 can be released from immune cells into the extracellular space, where it acts as a danger-associated molecular pattern molecule to activate TLR4 signalling-mediated inflammatory responses. Resveratrol has neuroprotective and anti-inflammatory effects against HIBI, but whether these effects involve the regulation of the TLR4 signalling pathway and whether HMGB1 participates in this process is still unclear. We investigated the anti-inflammatory effects of resveratrol in HIBI and the molecular mechanisms potentially involved in the effect. The in vivo and in vitro results indicated that the level of cytoplasmic HMGB1 in microglia increased after insult and that treating experimental animals or mouse BV2 microglial cells with resveratrol attenuated HI insult-induced neuroinflammation, which was characterized by improved behavioural defects, reduced microglial activation and TLR4/MyD88/NF-κB signalling, and attenuated primary neuronal damage; this was accompanied by the inhibition of HMGB1 nucleoplasmic transfer and extracellular release. EX527 pretreatment reversed these effects. In addition, co-immunoprecipitation confirmed that SIRT1 was directly involved in the HMGB1 acetylation process in BV2 cells after oxygen glucose deprivation. These data demonstrate that resveratrol plays a neuroprotective role in neonatal HIBI by activating SIRT1 to inhibit HMGB1/TLR4/MyD88/NF-κB signalling and subsequent neuroinflammatory responses.

Phagocytosis in the Brain: Homeostasis and Disease

Microglia are resident macrophages of the central nervous system and significantly contribute to overall brain function by participating in phagocytosis during development, homeostasis, and diseased states. Phagocytosis is a highly complex process that is specialized for the uptake and removal of opsonized and non-opsonized targets, such as pathogens, apoptotic cells, and cellular debris. While the role of phagocytosis in mediating classical innate and adaptive immune responses has been known for decades, it is now appreciated that phagocytosis is also critical throughout early neural development, homeostasis, and initiating repair mechanisms. As such, modulating phagocytic processes has provided unexplored avenues with the intent of developing novel therapeutics that promote repair and regeneration in the CNS. Here, we review the functional consequences that phagocytosis plays in both the healthy and diseased CNS, and summarize how phagocytosis contributes to overall pathophysiological mechanisms involved in brain injury and repair.

Tadalafil treatment in mice for preeclampsia with fetal growth restriction has neuro-benefic effects in offspring through modulating prenatal hypoxic conditions

We have demonstrated that tadalafil facilitates fetal growth in mice with L-NG-nitroarginine methyl ester (L-NAME)-induced preeclampsia (PE) with fetal growth restriction (FGR). Tadalafil is a selective phosphodiesterase 5 inhibitor that dilates the maternal blood sinuses in the placenta, thereby facilitating the growth of the fetus. The purpose of this study was to investigate the effects of tadalafil treatment for PE and FGR on the developing brain in FGR offspring using an L-NAME-induced mouse model of PE with FGR. A control group of dams received carboxymethylcellulose (CMC). L-NAME-treated groups received L-NAME dissolved in CMC from 11 days post coitum (d.p.c.). The L-NAME-treated dams were divided into two subgroups 14 d.p.c. One subgroup continued to receive L-NAME. The other subgroup received L-NAME with tadalafil suspended in CMC. Tadalafil treatment for PE with FGR reduced the expression of hypoxia-inducible factor-2α in the placenta and in the brain of the FGR fetus. Moreover, tadalafil treatment in utero shows improved synaptogenesis and myelination in FGR offspring on postnatal day 15 (P15) and P30. These results suggest that tadalafil treatment for PE with FGR not only facilitates fetal growth, but also has neuroprotective effects on the developing brain of FGR offspring through modulating prenatal hypoxic conditions.

Resident Microglia Activate before Peripheral Monocyte Infiltration and p75NTR Blockade Reduces Microglial Activation and Early Brain Injury after Subarachnoid Hemorrhage

Early brain injury (EBI) after aneurysmal subarachnoid hemorrhage (SAH) contributes to high morbidity and mortality. Although it is well recognized that acute neuroinflammation reaction is one of the most important triggers of EBI, pharmacotherapy proved to be clinically effective against the initiating of neuroinflammation after SAH is lacking. The resident microglia and infiltrated peripheral monocyte are two main types of immune cells in central nervous system (CNS) and control the inflammation process in brain after SAH. But the time course and relative contributions of these two immune cell activations after SAH are unknown. The p75 neurotrophin receptor (p75NTR), member of TNF receptor superfamily, expresses on infiltrated peripheral monocytes and suppresses their proinflammatory action after brain insults. But the p75NTR expression on resident microglia in vivo is rarely explored and their function keeps elusive. Therefore, we designed this study to investigate the time course of resident microglia activation and peripheral monocyte infiltration, as well as the microglial expression of p75NTR by using CX3C-chemokine receptor 1 (Cx3cr1) and chemokine receptor 2 (Ccr2) double transgenic mice (Cx3cr1GFP/+Ccr2RFP/+) after SAH. The results showed activated microglia was observed in cortex as early as 24 h and further increased at 48 and 72 h post SAH, while the infiltrated monocyte was not found until 72h. In addition, activated microglia expressed p75NTR acutely and p75NTR specific antagonist TAT-Pep5 significantly reduced microglia activation, neuroinflammation and EBI from 24 to 72 h. Together, these data suggest that the early neuroinflammation reaction might be initiated and intensified mainly by resident microglia rather than infiltrated monocyte at least in the first 48 h after SAH and p75NTR blockading by TAT-Pep5P might alleviate EBI through mediating microglial activation.

Galectin-3-Mediated Glial Crosstalk Drives Oligodendrocyte Differentiation and (Re)myelination

Galectin-3 (Gal-3) is the only chimeric protein in the galectin family. Gal-3 structure comprises unusual tandem repeats of proline and glycine-rich short stretches bound to a carbohydrate-recognition domain (CRD). The present review summarizes Gal-3 functions in the extracellular and intracellular space, its regulation and its internalization and secretion, with a focus on the current knowledge of Gal-3 role in central nervous system (CNS) health and disease, particularly oligodendrocyte (OLG) differentiation, myelination and remyelination in experimental models of multiple sclerosis (MS). During myelination, microglia-expressed Gal-3 promotes OLG differentiation by binding glycoconjugates present only on the cell surface of OLG precursor cells (OPC). During remyelination, microglia-expressed Gal-3 favors an M2 microglial phenotype, hence fostering myelin debris phagocytosis through TREM-2b phagocytic receptor and OLG differentiation. Gal-3 is necessary for myelin integrity and function, as evidenced by myelin ultrastructural and behavioral studies from LGALS3^-/- mice. Mechanistically, Gal-3 enhances actin assembly and reduces Erk 1/2 activation, leading to early OLG branching. Gal-3 later induces Akt activation and increases MBP expression, promoting gelsolin release and actin disassembly and thus regulating OLG final differentiation. Altogether, findings indicate that Gal-3 mediates the glial crosstalk driving OLG differentiation and (re)myelination and may be regarded as a target in the design of future therapies for a variety of demyelinating diseases.

CCR2-dependent monocytes/macrophages exacerbate acute brain injury but promote functional recovery after ischemic stroke in mice

Rationale: Peripheral blood monocytes are recruited into the ischemic brain and transform into macrophages after stroke. Nevertheless, the exact role of CCR2-dependent monocytes/macrophages in brain injury after stroke remains elusive.

Methods: We used CCR2 knockout (KO) mice and the CCR2 pharmacological inhibitor, propagermanium (PG), to address the role of CCR2-dependent monocytes/macrophages in the acute stage and neurological functional recovery after middle cerebral artery (MCA) occlusion and reperfusion.

Results: CCR2 KO resulted in smaller infarct size and lower mortality than in wild type (WT) mice, when measured 3 days after stroke. However, from 5 to 28 days after stroke, the KO mice had higher mortality and showed no obvious neurological functional recovery. In addition, WT mice treated with PG had similar stroke outcomes compared with CCR2 KO, as measured by T2 weighted MRI. Flow cytometry and real-time PCR analyses suggest that monocyte-derived macrophages (MoDMs) in the stroke brains mainly polarized to pro-inflammatory macrophages at the early stage, but gradually switched to anti-inflammatory macrophages at 7 days after stroke. In addition, adoptive transfer of anti-inflammatory macrophages into CCR2 KO mice at 4 and 6 days after stroke alleviated mortality and promoted neurological recovery.

Conclusion: CCR2-dependent monocytes/macrophages are a double-edged sword; they worsen acute brain injury, but are essential for neurological recovery by promoting anti-inflammatory macrophage polarization.

Correction to: Neuroimmune Response in Ischemic Preconditioning

Ischemic preconditioning (IPC) is a robust neuroprotective phenomenon in which a brief period of cerebral ischemia confers transient tolerance to subsequent ischemic challenge. Research on IPC has implicated cellular, molecular, and systemic elements of the immune response in this phenomenon. Potent molecular mediators of IPC include innate immune signaling pathways such as Toll-like receptors and type 1 interferons. Brain ischemia results in release of pro- and anti-inflammatory cytokines and chemokines that orchestrate the neuroinflammatory response, resolution of inflammation, and transition to neurological recovery and regeneration. Cellular mediators of IPC include microglia, the resident central nervous system immune cells, astrocytes, and neurons. All of these cell types engage in cross-talk with each other using a multitude of signaling pathways that modulate activation/suppression of each of the other cell types in response to ischemia. As the postischemic neuroimmune response evolves over time there is a shift in function toward provision of trophic support and neuroprotection. Peripheral immune cells infiltrate the central nervous system en masse after stroke and are largely detrimental, with a few subtypes having beneficial, protective effects, though the role of these immune cells in IPC is largely unknown. The role of neural progenitor cells in IPC-mediated neuroprotection is another active area of investigation as is the role of microglial proliferation in this setting. A mechanistic understanding of these molecular and cellular mediators of IPC may not only facilitate more effective direct application of IPC to specific clinical scenarios, but also, more broadly, reveal novel targets for therapeutic intervention in stroke.

Glial alterations in human prion diseases: A correlative study of astroglia, reactive microglia, protein deposition, and neuropathological lesions

BACKGROUND

Neuroinflammation has recently been proposed to be a major component of neurodegenerative diseases. The aim of this study was to determine how the interaction between microglia and astroglia, which are the primary immune cell populations in the brain, and pathological prion protein (PrPsc) could influence the development and propagation of this neurodegenerative disease. Because a relevant role for glial response in prion disease has been clearly demonstrated in our previous studies using the natural animal model, a similar approach has been taken here using the natural human model.

METHODS

A morphological approach has been developed to analyze cerebellar samples from patients with Creutzfeldt-Jakob disease (CJD) in comparison with healthy control cases. Histopathological lesions were assessed, and PrPsc, glial fibrillary acidic protein (GFAP) and reactive microglia were immunolabelled by specific antibodies. Furthermore, co-location studies using confocal microscopy were performed to determine the possible relationships between both types of glial cells in all samples.

RESULTS

The results presented in this study support the involvement of both types of glial cells in CJD. Evidence of increased astrocyte and microglia reactivity can be observed in all CJD cases, and a close relationship between the types of glia is demonstrated by co-location studies.

CONCLUSION

Proteinopathies such as Alzheimer, Parkinson, and Huntington diseases, where aberrant proteins spread throughout the brain during disease progression, may share a molecular basis and mechanisms of propagation. Therefore, studies elucidating the interaction between gliosis and prion propagation may be relevant to these other neurodegenerative diseases and may provide new targets for therapeutic intervention.

Peripheral immune cells infiltrate into sites of secondary neurodegeneration after ischemic stroke

Experimental stroke leads to microglia activation and progressive neuronal loss at sites of secondary neurodegeneration (SND). These lesions are remote from, but synaptically connected to, primary infarction sites. Previous studies have demonstrated that immune cells are present in sites of infarction in the first hours and days after stroke, and are associated with increased neurodegeneration in peri-infarct regions. However, it is not known whether immune cells are also present in more distal sites where SND occurs. Our study aimed to investigate whether immune cells are present in sites of SND and, if so, how these cell populations compare to those in the peri-infarct zone. Cells were isolated from the thalamus, the main site of SND, and remaining brain tissue 14days post-stroke. Analysis was performed using flow cytometry to quantify microglia, myeloid cell and lymphocyte numbers. We identified a substantial infiltration of immune cells in the ipsilateral (stroked) compared to the contralateral (control) thalamus, with a significant increase in the percentage of CD4⁺ and CD8⁺ T cells. This result was further quantified using immunofluorescent labelling of fixed tissue. In the remaining ipsilateral hemisphere tissue, there were significant increases in the frequency of CD4⁺ and CD8⁺ T lymphocytes, B lymphocytes, Ly6G⁺ neutrophils and both Ly6G^-Ly6C^LO and Ly6G^-Ly6C^HI monocytes. Our results indicate that infiltrating immune cells persist in ischemic tissue after the acute ischemic phase, and are increased in sites of SND. Importantly, immune cells have been shown to play pivotal roles in both damage and repair processes after stroke. Our findings indicate that immune cells may also be involved in the pathogenesis of SND and further clinical studies are warranted to characterise the nature of inflammatory cell infiltrates in human disease.

The choroid plexus is a key cerebral invasion route for T cells after stroke

Neuroinflammation contributes substantially to stroke pathophysiology. Cerebral invasion of peripheral leukocytes-particularly T cells-has been shown to be a key event promoting inflammatory tissue damage after stroke. While previous research has focused on the vascular invasion of T cells into the ischemic brain, the choroid plexus (ChP) as an alternative cerebral T-cell invasion route after stroke has not been investigated. We here report specific accumulation of T cells in the peri-infarct cortex and detection of T cells as the predominant population in the ipsilateral ChP in mice as well as in human post-stroke autopsy samples. T-cell migration from the ChP to the peri-infarct cortex was confirmed by in vivo cell tracking of photoactivated T cells. In turn, significantly less T cells invaded the ischemic brain after photothrombotic lesion of the ipsilateral ChP and in a stroke model encompassing ChP ischemia. We detected a gradient of CCR2 ligands as the potential driving force and characterized the neuroanatomical pathway for the intracerebral migration. In summary, our study demonstrates that the ChP is a key invasion route for post-stroke cerebral T-cell invasion and describes a CCR2-ligand gradient between cortex and ChP as the potential driving mechanism for this invasion route.

Combined fetal inflammation and postnatal hypoxia causes myelin deficits and autism-like behavior in a rat model of diffuse white matter injury

Diffuse white matter injury (WMI) is a serious problem in extremely preterm infants, and is associated with adverse neurodevelopmental outcome, including cognitive impairments and an increased risk of autism-spectrum disorders. Important risk factors include fetal or perinatal inflammatory insults and fluctuating cerebral oxygenation. However, the exact mechanisms underlying diffuse WMI are not fully understood and no treatment options are currently available. The use of clinically relevant animal models is crucial to advance knowledge on the pathophysiology of diffuse WMI, allowing the definition of novel therapeutic targets. In the present study, we developed a multiple-hit animal model of diffuse WMI by combining fetal inflammation and postnatal hypoxia in rats. We characterized the effects on white matter development and functional outcome by immunohistochemistry, MRI and behavioral paradigms. Combined fetal inflammation and postnatal hypoxia resulted in delayed cortical myelination, microglia activation and astrogliosis at P18, together with long-term changes in oligodendrocyte maturation as observed in 10 week old animals. Furthermore, rats with WMI showed impaired motor performance, increased anxiety and signs of autism-like behavior, i.e. reduced social play behavior and increased repetitive grooming. In conclusion, the combination of fetal inflammation and postnatal hypoxia in rats induces a pattern of brain injury and functional impairments that closely resembles the clinical situation of diffuse WMI. This animal model provides the opportunity to elucidate pathophysiological mechanisms underlying WMI, and can be used to develop novel treatment options for diffuse WMI in preterm infants.