Central nervous system diseases related to pathological microglial phagocytosis

High-fat Fructose diet induces neuroinflammation and anxiety-like behaviors by modulating liver-brain axis communication

RATIONALE

Patients with non-alcoholic fatty liver disease (NAFLD) may experience non-cognitive impairments such as anxiety and depression. However, the specific mechanism of the association between liver injury and neurological disorders is unclear.

OBJECTIVES

In this study, we aimed to explore the relationship and underlying mechanism between high-fat fructose diet (HFFD)-induced liver injury and anxiety-like behavior in mice.

METHODS

A mouse model of NAFLD was established using an HFFD, and behavioral tests were performed to detect anxiety-like behaviors in mice; moreover, we used enzyme linked immunosorbent assay (ELISA) to detect glutamate levels in treated and normal diet (ND) mice, as well as to explore inflammation levels in mice using immunofluorescence and other methods.

RESULTS

Mice in the HFFD-treated group exhibited anxiety-like behaviors, as well as elevated serum lipid and glutamate levels, increased liver injury, and hepatic tissue fat accumulation. Additionally, HFFD-fed mice exhibited elevated levels of IL-6, IL-1β, and TNF-α in the liver, hippocampus, and cortex compared with the ND counterparts; HFFD-induced astrocyte and microglial activation was detected in the cortical and hippocampal regions. However, corilagin treatment alleviated these HFFD-associated pathological changes. Corilagin did not ameliorate anxiety behaviors in mice in the absence of liver injury.

CONCLUSION

Our results indicated that the HFFD-induced NAFLD and mild hepatic fibrosis led to elevated levels of glutamate and aminotransferases, which infiltrated the brain, causing inflammation, and subsequently induced anxiety-like behaviors in mice. These pathological and behavioral manifestations were ameliorated through corilagin intervention. This study provides a possible underlying mechanism between HFFD and neurological disorders.

Loss of insulin signaling in microglia impairs cellular uptake of Aβ and neuroinflammatory response exacerbating AD-like neuropathology

Insulin receptors are present on cells throughout the body, including the brain. Dysregulation of insulin signaling in neurons and astrocytes has been implicated in altered mood, cognition, and the pathogenesis of Alzheimer's disease (AD). To define the role of insulin signaling in microglia, the primary phagocytes in the brain critical for maintenance and damage repair, we created mice with an inducible microglia-specific insulin receptor knockout (MG-IRKO). RiboTag profiling of microglial mRNAs revealed that loss of insulin signaling results in alterations of gene expression in pathways related to innate immunity and cellular metabolism. In vitro, loss of insulin signaling in microglia results in metabolic reprogramming with an increase in glycolysis and impaired uptake of Aβ. In vivo, MG-IRKO mice exhibit alterations in mood and social behavior, and when crossed with the 5xFAD mouse model of AD, the resultant mice exhibit increased levels of Aβ plaque and elevated neuroinflammation. Thus, insulin signaling in microglia plays a key role in microglial cellular metabolism and the ability of the cells to take up Aβ, such that reduced insulin signaling in microglia alters mood and social behavior and accelerates AD pathogenesis. Together, these data indicate key roles of insulin action in microglia and the potential of targeting insulin signaling in microglia in treatment of AD.

NLRX1 mediated impaired microglial phagocytosis of NETs in cerebral ischemia and reperfusion injury

Ischemic stroke is one of the common causes of disability and death, and subsequent pathological processes consequent to revascularization could promote secondary tissue damage leading to neuronal death, namely cerebral ischemia and reperfusion injury. Neutrophils could invade injured brain parenchyma after vascularization and exert neurotoxicity by forming neutrophil extracellular traps (NETs). However, unwanted NETs were accumulated in the infarcted core of transient middle cerebral artery occlusion (tMCAO) rats and the mechanism is unknown. Efficient microglial phagocytosis is crucial for the homeostasis of cerebral parenchyma after stroke, and dysfunction of microglial phagocytosis of NETs were observed in the infarcted core cortex at tMCAO 1 d and the accumulation of NETs persisted to 7 d, which exerting deleterious neuronal damage after stroke. However, the detailed mechanisms underlying the dysfunction of microglial phagocytosis of NETs remained unclear. Our results further demonstrated that NLRX1 was mainly enhanced in the microglial cells in the infarcted core cortex at tMCAO 1 d and promoted galectin-3 expression on the lysosomes, facilitating the lysosomal dysfunction and impaired microglial phagocytosis via mTOR/TFEB signaling. NLRX1-silencing was able to suppress the galectin-3 intensity, inhibit the phosphorylation of mTOR and facilitate the nuclear localization of TFEB, ameliorating the lysosomal dysfunction and microglial phagocytosis of NETs. Our results uncovered the regulation of NLRX1 in the dysfunctional microglial phagocytosis of NETs and provided insights into the therapeutic potential for targeting at microglial lysosomal function in cerebral ischemia and reperfusion injury.

Neuron-to-glia and glia-to-glia signaling directs critical period experience-dependent synapse pruning

Experience-dependent glial synapse pruning plays a pivotal role in sculpting brain circuit connectivity during early-life critical periods of development. Recent advances suggest a layered cascade of intercellular communication between neurons and glial phagocytes orchestrates this precise, targeted synapse elimination. We focus here on studies from the powerful Drosophila forward genetic model, with reference to complementary findings from mouse work. We present both neuron-to-glia and glia-to-glia intercellular signaling pathways directing experience-dependent glial synapse pruning. We discuss a putative hierarchy of secreted long-distance cues and cell surface short-distance cues that act to sequentially orchestrate glia activation, infiltration, target recognition, engulfment, and then phagocytosis for synapse pruning. Ligand-receptor partners mediating these stages in different contexts are discussed from recent Drosophila and mouse studies. Signaling cues include phospholipids, small neurotransmitters, insulin-like peptides, and proteins. Conserved receptors for these ligands are discussed, together with mechanisms where the receptor identity remains unknown. Potential mechanisms are proposed for the tight temporal-restriction of heightened experience-dependent glial synapse elimination during early-life critical periods, as well as potential means to re-open such plasticity at maturity.

ATP8A2 expression is reduced in the mPFC of offspring mice exposed to maternal immune activation and its upregulation ameliorates synapse-associated protein loss and behavioral abnormalities

Prenatal virus infection-induced maternal immune activation (MIA) is linked to a greater risk of neurodevelopmental disorders in offspring. Prenatal exposure to poly(I:C) in pregnant mice is a well-established approach to mimic virus infection-induced MIA, leading to neuropsychiatric disorders and aberrant brain development, especially in the medial prefrontal cortex (mPFC). ATPase phospholipid flippase 8A2 (ATP8A2) is the main phospholipid lipase, expressed in the mPFC and is crucial for maintaining cell membrane stability by flipping phosphatidylserine from the outer leaflet to the inner leaflet of the cell membrane. Atp8a2 knockout or mutation causes a series of phenotypes, including impaired neuronal cell survival, neuroinflammation, altered synaptic plasticity, and behavioral abnormalities. These findings suggest that ATP8A2 expression in the mPFC may be impaired in MIA offspring and that the decrease in ATP8A2 expression may be involved in the development of MIA-induced neuropsychiatric disorders in offspring. No reports addressing this issue have been published. Here, after confirming abnormal affective-/social-related behaviors in adulthood and reduced synapse-associated protein expression on the birth day (P0) and the fourth postnatal day (P4) in the mPFC of MIA offspring that were born to dams exposed prenatally to a single dose of poly(I:C) (10 mg/kg, i.p.), decreased ATP8A2 expression was also observed in the mPFC of MIA offspring at P0 and P4.Upregulating ATP8A2 in the mPFC restored synapse-associated protein levels, along with a partial improvement in the behavioral performance of MIA offspring. Upregulation of ATP8A2 also blocked neuronal phosphatidylserine externalization and eliminated the excitation/inhibition (E/I) imbalance in the mPFC of MIA offspring. This study revealed that the low expression of ATP8A2 following MIA exposure may play a role in mediating abnormal brain development and function in offspring. ATP8A2 potentially represents a novel molecule involved in MIA-induced neuropsychiatric disorders in offspring, and may serve as a novel therapeutic target for the intervention of psychiatric disorders.

Neuroinflammation-targeted magnetic resonance imaging nanoprobes for the early diagnosis of Alzheimer's disease

Alzheimer's disease (AD) that is an important contributor to dementia, is a chronic and irreversible neurodegenerative disease, with high rates of disability and mortality. Recently, more and more therapeutic methods have been developed to delay the progression of AD, but it remains a great challenge to achieve the early diagnosis of AD. In this work, we developed a magnetic resonance imaging (MRI) nanoprobe (NP@angiopep-2/CD137) based on angiopep-2 peptide and CD137 antibody with a NaGdF4 nanoparticle as the core and realized neuroinflammation-targeted imaging on APP/PS1 model mice using a clinical 7.0 T MRI scanner. CD137 expression was upregulated in neuroglial cells and cerebral vascular endothelial cells in inflammatory state. In the APP/PS1 mouse model, after administration, the nanoprobe-enhanced images showed specific dot-like signals in the susceptibility-weighted imaging (SWI) sequence. In summary, we designed and synthesized NP@angiopep-2/CD137 nanoprobes using the activation-dependent expression of CD137, which were applied to the pathological assessment of AD based on the hypothesis of AD neuroinflammation, and provided a reliable idea for the early molecular imaging diagnosis of AD.

Role of PI3Kγ in the polarization, migration, and phagocytosis of microglia

Phosphoinositide 3-kinase γ (PI3Kγ) is a signaling protein that is constitutively expressed in immune competent cells and plays a crucial role in cell proliferation, apoptosis, migration, deformation, and immunology. Several studies have shown that high expression of PI3Kγ can inhibit the occurrence of inflammation in microglia while also regulating the polarization of microglia to inhibit inflammation and enhance microglial migration and phagocytosis. It is well known that the regulation of microglial polarization, migration, and phagocytosis is key to the treatment of most neurodegenerative diseases. Therefore, in this article, we review the important regulatory role of PI3Kγ in microglia to provide a basis for the treatment of neurodegenerative diseases.

μMap-Interface: Temporal Photoproximity Labeling Identifies F11R as a Functional Member of the Transient Phagocytic Surfaceome

Phagocytosis is usually carried out by professional phagocytic cells in the context of pathogen response or wound healing. The transient surface proteins that regulate phagocytosis pose a challenging proteomics target; knowledge thereof could lead to new therapeutic insights. Herein, we describe a novel photocatalytic proximity labeling method: "μMap-Interface", allowing for spatiotemporal mapping of phagocytosis. Utilizing photocatalyst-conjugated IGG-opsonized beads and initiating phagocytosis in a synchronized manner, we capture phagocytic interactome "snapshots" at the interface of the phagocyte and its target. This allows profiling of the dynamic surface proteome of human macrophages during the engulfment process. We reveal previously known phagocytic mediators as well as potential novel interactors and validate their presence with super-resolution microscopy. This includes F11R, an important cancer target yet to be investigated in the context of phagocytosis. Further, we demonstrate that knocking down F11R leads to an increased degree of phagocytosis; this insight could contribute to explaining its oncogenic activity. Lastly, we show capture of orthogonal phagocytic surfaceomes across different cells, using a neutrophil-like model. We believe this method will enable new insights into phagocytic processes in a variety of contexts.

Microglial Melatonin Receptor 1 Degrades Pathological Alpha-Synuclein Through Activating LC3-Associated Phagocytosis In Vitro

AIMS

Parkinson's disease (PD) is characterized by the formation of Lewy bodies (LBs), primarily constituted of α-synuclein (α-Syn). Microglial cells exhibit specific reactivity toward misfolded proteins such as α-Syn. However, the exact clearance mechanism and related molecular targets remain elusive.

METHODS

BV2 cells, primary microglia from wild-type and MT1 knockout mice, and primary cortical neurons were utilized as experimental models. The study investigated relevant mechanisms by modulating microglial MT1 expression through small RNA interference (RNAi) and lentiviral overexpression techniques. Furthermore, pathological aggregation of α-Syn was induced using pre-formed fibrils (PFF) α-Syn. Co-immunoprecipitation, immunofluorescence, Western blot (WB), and quantitative real-time PCR were used to elucidate the mechanisms of molecular regulation.

RESULTS

In this study, we elucidated the regulatory role of the melatonin receptor 1 (MT1) in the microglial phagocytic process. Following MT1 knockout, the ability of microglial cells to engulf latex beads and zymosan particles decreased, subsequently affecting the phagocytic degradation of fibrillar α-Syn by microglial cells. Furthermore, the loss of MT1 receptors in microglial cells exacerbates the aggregation of α-Syn in neurons induced by pre-formed fibrils (PFF) α-Syn. Mechanistically, MT1 influences the phagocytic function of microglial cells by regulating the Rubicon-dependent LC3-associated phagocytosis (LAP) pathway.

CONCLUSION

Taken together, the results suggest the neuroprotective function of microglial cells in clearing α-Syn through MT1-mediated LAP, highlighting the potential key role of MT1 in pathogenic mechanisms associated with α-Syn.

Prebiotic diet normalizes aberrant immune and behavioral phenotypes in a mouse model of autism spectrum disorder

Autism spectrum disorder (ASD) is a cluster of neurodevelopmental disorders characterized by deficits in communication and behavior. Increasing evidence suggests that the microbiota-gut-brain axis and the likely related immune imbalance may play a role in the development of this disorder. Gastrointestinal deficits and gut microbiota dysfunction have been linked to the development or severity of autistic behavior. Therefore, treatments that focus on specific diets may improve gastrointestinal function and aberrant behavior in individuals with ASD. In this study, we investigated whether a diet containing specific prebiotic fibers, namely, 3% galacto-oligosaccharide/fructo-oligosaccharide (GOS/FOS; 9:1), can mitigate the adverse effects of in utero exposure to valproic acid (VPA) in mice. Pregnant BALB/cByJ dams were injected with VPA (600 mg/kg, sc.) or phosphate-buffered saline (PBS) on gestational day 11 (G11). Male offspring were divided into four groups: (1) in utero PBS-exposed with a control diet, (2) in utero PBS-exposed with GOS/FOS diet, (3) in utero VPA-exposed with a control diet, and (4) in utero VPA-exposed with GOS/FOS diet. Dietary intervention started from birth and continued throughout the duration of the experiment. We showed that the prebiotic diet normalized VPA-induced alterations in male offspring, including restoration of key microbial taxa, intestinal permeability, peripheral immune homeostasis, reduction of neuroinflammation in the cerebellum, and impairments in social behavior and cognition in mice. Overall, our research provides valuable insights into the gut-brain axis involvement in ASD development. In addition, dietary interventions might correct the disbalance in gut microbiota and immune responses and, ultimately, might improve detrimental behavioral outcomes in ASD.

KLOTHO KL-VS heterozygosity is associated with diminished age-related neuroinflammation, neurodegeneration, and synaptic dysfunction in older cognitively unimpaired adults

INTRODUCTION

We examined whether the aging suppressor KLOTHO gene's functionally advantageous KL-VS variant (KL-VS heterozygosity [KL-VSHET]) confers resilience against deleterious effects of aging indexed by cerebrospinal fluid (CSF) biomarkers of neuroinflammation (interleukin-6 [IL-6], S100 calcium-binding protein B [S100B], triggering receptor expressed on myeloid cells [sTREM2], chitinase-3-like protein 1 [YKL-40], glial fibrillary acidic protein [GFAP]), neurodegeneration (total α-synuclein [α-Syn], neurofilament light chain protein), and synaptic dysfunction (neurogranin [Ng]).

METHODS

This Alzheimer disease risk-enriched cohort consisted of 454 cognitively unimpaired adults (Mage = 61.5 ± 7.75). Covariate-adjusted multivariate regression examined relationships between age (mean-split[age ≥ 62]) and CSF biomarkers (Roche/NeuroToolKit), and whether they differed between KL-VSHET (N = 122) and non-carriers (KL-VSNC; N = 332).

RESULTS

Older age was associated with a poorer biomarker profile across all analytes (Ps ≤ 0.03). In age-stratified analyses, KL-VSNC exhibited this same pattern (Ps ≤ 0.05) which was not significant for IL-6, S100B, Ng, and α-Syn (Ps ≥ 0.13) in KL-VSHET. Although age-related differences in GFAP, sTREM2, and YKL-40 were evident for both groups (Ps ≤ 0.01), the effect magnitude was markedly stronger for KL-VSNC.

DISCUSSION

Higher levels of neuroinflammation, neurodegeneration, and synaptic dysfunction in older adults were attenuated in KL-VSHET.

HIGHLIGHTS

Older age was associated with poorer profiles across all cerebrospinal fluid biomarkers of neuroinflammation, neurodegeneration, and synaptic dysfunction. KLOTHO KL-VS non-carriers exhibit this same pattern, which is does not significantly differ between younger and older KL-VS heterozygotes for interleukin-6, S100 calcium-binding protein B, neurogranin, and total α-synuclein. Although age-related differences in glial fibrillary acidic protein, triggering receptor expressed on myeloid cells, and chitinase-3-like protein 1 are evident for both KL-VS groups, the magnitude of the effect is markedly stronger for KL-VS non-carriers. Higher levels of neuroinflammation, neurodegeneration, and synaptic dysfunction in older adults are attenuated in KL-VS heterozygotes.

Role of MicroRNA-21 in Regulating Intracellular Pathways Associated With Phagocytosis in Human Macrophages: An In Vitro Study

Introduction The efficient clearance of bacteria by macrophages is crucial for the timely resolution of inflammation. In this study, we investigated the role of microRNA-21 (miR-21)-induced phagocytosis and its intracellular signaling pathways in human macrophages in vitro. Methods Human peripheral blood mononuclear cells (PBMCs) were isolated from whole blood collected from 15 healthy volunteers. Differentiated human macrophages were incubated with lipopolysaccharide (LPS) to determine the time course of changes in phagocytic activity and miR-21 expression. The expression of candidate genes targeted by miR-21 and its downstream effectors was quantitatively assessed. The effects of miR-21 modulation were also examined via transfection with miR-21 mimics and inhibitors. Results Incubation of human macrophages with LPS upregulated both phagocytosis and miR-21 expression. Notably, changing miR-21 expression levels using miR-21 mimics or inhibitors led to significant and opposite changes in the expression of its downstream effectors. miR-21 induction in macrophages downregulated PDCD4 and PTEN, promoted the phosphorylation of Akt and the production of the anti-inflammatory cytokine IL-10, and facilitated phagocytosis. Conclusion This study directly confirms that LPS upregulates macrophage phagocytosis and miR-21 expression. Elevated miR-21 levels in macrophages enhanced phagocytosis, contributing to an anti-inflammatory phenotype. These findings underscore the importance of miR-21 in resolving inflammation.

Prenatal Immune Challenge Differentiates the Effect of Aripiprazole and Risperidone on CD200-CD200R and CX3CL1-CX3CR1 Dyads and Microglial Polarization: A Study in Organotypic Cortical Cultures

Microglia are the primary innate immune cells of the central nervous system and extensively contribute to brain homeostasis. Dysfunctional or excessive activity of microglia may be associated with several neuropsychiatric disorders, including schizophrenia. Therefore, we examined whether aripiprazole and risperidone could influence the expression of the Cd200-Cd200r and Cx3cl1-Cx3cr1 axes, which are crucial for the regulation of microglial activity and interactions of these cells with neurons. Additionally, we evaluated the impact of these drugs on microglial pro- and anti-inflammatory markers (Cd40, Il-1β, Il-6, Cebpb, Cd206, Arg1, Il-10 and Tgf-β) and cytokine release (IL-6, IL-10). The research was executed in organotypic cortical cultures (OCCs) prepared from the offspring of control rats (control OCCs) or those exposed to maternal immune activation (MIA OCCs), which allows for the exploration of schizophrenia-like disturbances in animals. All experiments were performed under basal conditions and after additional stimulation with lipopolysaccharide (LPS), following the "two-hit" hypothesis of schizophrenia. We found that MIA diminished the mRNA level of Cd200r and affected the OCCs' response to additional LPS exposure in terms of this parameter. LPS downregulated the Cx3cr1 expression and profoundly changed the mRNA levels of pro- and anti-inflammatory microglial markers in both types of OCCs. Risperidone increased Cd200 expression in MIA OCCs, while aripiprazole treatment elevated the gene levels of the Cx3cl1-Cx3cr1 dyad in control OCCs. The antipsychotics limited the LPS-generated increase in the expression of proinflammatory factors (Il-1β and Il-6) and enhanced the mRNA levels of anti-inflammatory components (Cd206 and Tgf-β) of microglial polarization, mostly in the absence of the MIA procedure. Finally, we observed a more pronounced modulating impact of aripiprazole on the expression of pro- and anti-inflammatory cytokines when compared to risperidone in MIA OCCs. In conclusion, our data suggest that MIA might influence microglial activation and crosstalk of microglial cells with neurons, whereas aripiprazole and risperidone could beneficially affect these changes in OCCs.

Ferroptosis mediated by TNFSF9 interferes in acute ischaemic stroke reperfusion injury with the progression of acute ischaemic stroke

In this study, we investigated the potential involvement of TNFSF9 in reperfusion injury associated with ferroptosis in acute ischaemic stroke patients, mouse models and BV2 microglia. We first examined TNFSF9 changes in peripheral blood from stroke patients with successful reperfusion, and constructed oxygen-glucose deprivation-reperfusion (OGD-R) on BV2 microglia, oxygen-glucose deprivation for 6 h followed by reoxygenation and re-glucose for 24 h, and appropriate over-expression or knockdown of TNFSF9 manipulation on BV2 cells and found that in the case of BV2 cells encountering OGD-R over-expression of TNFSF9 resulted in increased BV2 apoptosis. Still, the knockdown of TNFSF9 ameliorated apoptosis and ferroptosis. In an in vivo experiment, we constructed TNFSF9 over-expression or knockout mice by intracerebral injection of TNFSF9-OE or sh-TNFSF9 adenovirus. We performed the middle cerebral artery occlusion (MCAO) model on day four, 24 h after ligation of the proximal artery, for half an hour to recanalize. As luck would have it, over-expression of TNFSF9 resulted in increased brain infarct volumes, neurological function scores and abnormalities in TNFSF9-related TRAF1 and ferroptosis-related pathways, but knockdown of TNFSF9 improved brain infarcts in mice as well as reversing TNFSF9-related signalling pathways. In conclusion, our data provide the first evidence that TNFSF9 triggers microglia activation by activating the ferroptosis signalling pathway following ischaemic stroke, leading to brain injury and neurological deficits.

Microglia in Ischemic Stroke: Pathogenesis Insights and Therapeutic Challenges

Ischemic stroke is the most common type of stroke, which is the main cause of death and disability on a global scale. As the primary immune cells in the brain that are crucial for preserving homeostasis of the central nervous system microenvironment, microglia have been found to exhibit dual or even multiple effects at different stages of ischemic stroke. The anti-inflammatory polarization of microglia and release of neurotrophic factors may provide benefits by promoting neurological recovery at the lesion in the early phase after ischemic stroke. However, the pro-inflammatory polarization of microglia and secretion of inflammatory factors in the later phase of injury may exacerbate the ischemic lesion, suggesting the therapeutic potential of modulating the balance of microglial polarization to predispose them to anti-inflammatory transformation in ischemic stroke. Microglia-mediated signaling crosstalk with other cells may also be key to improving functional outcomes following ischemic stroke. Thus, this review provides an overview of microglial functions and responses under physiological and ischemic stroke conditions, including microglial activation, polarization, and interactions with other cells. We focus on approaches that promote anti-inflammatory polarization of microglia, inhibit microglial activation, and enhance beneficial cell-to-cell interactions. These targets may hold promise for the creation of innovative therapeutic strategies.

The role of glial cells in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) has traditionally been considered a neuron-centric disease. This view is now outdated, with increasing recognition of cell autonomous and non-cell autonomous contributions of central and peripheral nervous system glia to ALS pathomechanisms. With glial research rapidly accelerating, we comprehensively interrogate the roles of astrocytes, microglia, oligodendrocytes, ependymal cells, Schwann cells and satellite glia in nervous system physiology and ALS-associated pathology. Moreover, we highlight the inter-glial, glial-neuronal and inter-system polylogue which constitutes the healthy nervous system and destabilises in disease. We also propose classification based on function for complex glial reactive phenotypes and discuss the pre-requisite for integrative modelling to advance translation. Given the paucity of life-enhancing therapies currently available for ALS patients, we discuss the promising potential of harnessing glia in driving ALS therapeutic discovery.

P2X7 receptor: A receptor closely linked with sepsis-associated encephalopathy

Sepsis is defined as a dysregulated host response to infection resulting in life-threatening organ dysfunction. Sepsis-associated encephalopathy (SAE) is the main manifestation of sepsis. Inflammation, peroxidation stress injury, and apoptosis are the main factors involved in the pathogenesis of SAE. A growing body of evidence has proved that P2X7 receptor (P2X7R), a cationic channel receptor that is widely distributed in the body, plays a major role in the occurrence and development of inflammatory injury. Therefore, this review mainly describes the activation of P2X7R in sepsis, which leads to the recruitment of inflammatory cells to the cerebral vasculature, the destruction of the blood-brain barrier, the activation of microglial cells in the brain, the apoptosis of brain cells, and other damage processes. This review also illustrates the potential therapeutic value of P2X7R inhibition in SAE.

Oligomer-Aβ42 suppress glioma progression via potentiating phagocytosis of microglia

AIMS

Glioma is characterized by an immunosuppressed environment and a poor prognosis. The accumulation of Amyloid β (Aβ) leads to an active environment during the early stages of Alzheimer's disease (AD). Aβ is also present in glioma tissues; however, the biological and translational implications of Aβ in glioma are elusive.

METHODS

Immunohistochemical (IHC) staining, Kaplan-Meier (KM) survival analysis and Cox regression analysis on a cohort of 79 patients from our institution were performed to investigate the association between Aβ and the malignancy of glioma. Subsequently, the potential of oligomer-Aβ42 (OAβ42) to inhibit glioma growth was investigated in vivo and in vitro. Immunofluorescence staining and phagocytosis assays were performed to evaluate the activation of microglia. Finally, RNA-seq was utilized to identify the predominant signaling involved in this process and in vitro studies were performed to validate them.

RESULTS

A positive correlation between Aβ and a favorable prognosis was observed in glioma. Furthermore, OAβ42 suppressed glioma growth by enhancing the phagocytic activity of microglia. Insulin-like growth factor 1 (IGF-1) secreted by OAβ42-activated microglia was essential in the engulfment process.

CONCLUSION

Our study proved an anti-glioma effect of Aβ, and microglia could serve as a cellular target for treating glioma with OAβ42.

Microglia and Systemic Immunity

Microglia are specialized immune cells that reside in the central nervous system (CNS) and play a crucial role in maintaining the homeostasis of the brain microenvironment. While traditionally regarded as a part of the innate immune system, recent research has highlighted their role in adaptive immunity. The CNS is no longer considered an immune-privileged organ, and increasing evidence suggests bidirectional communication between the immune system and the CNS. Microglia are sensitive to systemic immune signals and can respond to systemic inflammation by producing various inflammatory cytokines and chemokines. This response is mediated by activating pattern recognition receptors (PRRs), which recognize pathogen- and danger-associated molecular patterns in the systemic circulation. The microglial response to systemic inflammation has been implicated in several neurological conditions, including depression, anxiety, and cognitive impairment. Understanding the complex interplay between microglia and systemic immunity is crucial for developing therapeutic interventions to modulate immune responses in the CNS.

Hemicyanine-based sensor for mitochondrial viscosity imaging in BV2 cells

Mitochondrial viscosity is a critical factor affecting numerous physiological processes, including phagocytosis. Abnormal viscosity in mitochondria is related to some pathological activities and diseases. Evaluating and detecting the changes in mitochondrial viscosity in vivo is crucial. Thus, a mitochondria-targeted red-emitting fluorescent probe (VP) was prepared, and can be used to detect viscosity with high selectivity and sensitivity. The synthesis of probe VP was as simple as two steps and the cost was low. In addition, the fluorescence intensity (log I615) exhibited an excellent relationship with viscosity (log η) in the range of 0.5 - 2.5 (R2 = 0.9985) in water/glycerol mixture. It is noteworthy that the probe VP displayed the highest signal-to-noise ratio (about 50-fold) for viscosity in water and glycerol system. The probe VP can visualize the mitochondrial viscosity change in living cells. More importantly, phagocytic test for BV2 cells further demonstrated that phagocytosis decreased with increased viscosity. Furthermore, VP was successfully used for monitoring the mitophagy process induced by starvation, and mitochondrial viscosity exhibited enhancement during mitophagy. The probe was a potential tool for studying viscosity and phagocytosis.

Electroacupuncture Alleviates Neuroinflammation by Regulating Microglia Polarization via STAT6/PPARγ in Ischemic Stroke Rats

Previous study showed that electroacupuncture (EA) produced a protective effect on cerebral ischemia-reperfusion injury (CIRI) in rats and may correlate with the anti-inflammatory effects of microglia. This study aimed to investigate further whether EA could modulate neuroinflammation by targeting the Signal Transducer and Activator of Transcription 6 (STAT6) and Peroxisome Proliferator-Activated Receptor γ (PPARγ) pathway, the key regulator of microglia. Middle cerebral artery occlusion (MCAO) rats were used, and 6 h after reperfusion, EA interventions were performed in Chize (LU 5), Hegu (LI 4), Sanyinjiao (SP 6), and Zusanli (ST 36) on the affected side of the rats, the group that received EA + STAT6 phosphorylation inhibitor AS1517499 was used as a parallel control. The degree of neurological impairment, infarct volume, microglia polarization, inflammation levels and activity of STAT6/PPARγ pathway were then assessed by neurological deficit score, triphenyl tetrazolium chloride (TTC) staining, immunofluorescence, western blotting (WB), quantitative real-time PCR (qPCR) and Enzyme linked immunosorbent assay (ELISA). The data showed that EA significantly alleviated nerve injury, reduced infarct volume, enhanced the expression and activity of STAT6/PPARγ pathway, inhibited NF-κB activity, increased M2 microglia numbers and anti-inflammatory factor release, and inhibited microglia M1-type polarization and pro-inflammatory factor expression. In contrast, inhibition of STAT6 phosphorylation exacerbated neural damage, inhibited STAT6/PPARγ pathway activity, promoted microglia M1-type polarization and exacerbated neuroinflammation, resulting in an attenuated positive effect of EA intervention. Therefore, we concluded that EA intervention could attenuate microglia-associated neuroinflammation by enhancing the expression and activity of STAT6/PPARγ pathway, thereby reducing CIRI in MCAO rats.

Signaling pathways in brain ischemia: Mechanisms and therapeutic implications

Ischemic stroke occurs when the arteries supplying blood to the brain are narrowed or blocked, inducing damage to brain tissue due to a lack of blood supply. One effective way to reduce brain damage and alleviate symptoms is to reopen blocked blood vessels in a timely manner and reduce neuronal damage. To achieve this, researchers have focused on identifying key cellular signaling pathways that can be targeted with drugs. These pathways include oxidative/nitrosative stress, excitatory amino acids and their receptors, inflammatory signaling molecules, metabolic pathways, ion channels, and other molecular events involved in stroke pathology. However, evidence suggests that solely focusing on protecting neurons may not yield satisfactory clinical results. Instead, researchers should consider the multifactorial and complex mechanisms underlying stroke pathology, including the interactions between different components of the neurovascular unit. Such an approach is more representative of the actual pathological process observed in clinical settings. This review summarizes recent research on the multiple molecular mechanisms and drug targets in ischemic stroke, as well as recent advances in novel therapeutic strategies. Finally, we discuss the challenges and future prospects of new strategies based on the biological characteristics of stroke.

Genome-Wide Epistasis Study of Cerebrospinal Fluid Hyperphosphorylated Tau in ADNI Cohort

Alzheimer's disease (AD) is the main cause of dementia worldwide, and the genetic mechanism of which is not yet fully understood. Much evidence has accumulated over the past decade to suggest that after the first large-scale genome-wide association studies (GWAS) were conducted, the problem of "missing heritability" in AD is still a great challenge. Epistasis has been considered as one of the main causes of "missing heritability" in AD, which has been largely ignored in human genetics. The focus of current genome-wide epistasis studies is usually on single nucleotide polymorphisms (SNPs) that have significant individual effects, and the amount of heritability explained by which was very low. Moreover, AD is characterized by progressive cognitive decline and neuronal damage, and some studies have suggested that hyperphosphorylated tau (P-tau) mediates neuronal death by inducing necroptosis and inflammation in AD. Therefore, this study focused on identifying epistasis between two-marker interactions at marginal main effects across the whole genome using cerebrospinal fluid (CSF) P-tau as quantitative trait (QT). We sought to detect interactions between SNPs in a multi-GPU based linear regression method by using age, gender, and clinical diagnostic status (cds) as covariates. We then used the STRING online tool to perform the PPI network and identify two-marker epistasis at the level of gene-gene interaction. A total of 758 SNP pairs were found to be statistically significant. Particularly, between the marginal main effect SNP pairs, highly significant SNP-SNP interactions were identified, which explained a relatively high variance at the P-tau level. In addition, 331 AD-related genes were identified, 10 gene-gene interaction pairs were replicated in the PPI network. The identified gene-gene interactions and genes showed associations with AD in terms of neuroinflammation and neurodegeneration, neuronal cells activation and brain development, thereby leading to cognitive decline in AD, which is indirectly associated with the P-tau pathological feature of AD and in turn supports the results of this study. Thus, the results of our study might be beneficial for explaining part of the "missing heritability" of AD.

Immune Regulatory Functions of Macrophages and Microglia in Central Nervous System Diseases

Macrophages can be characterized as a very multifunctional cell type with a spectrum of phenotypes and functions being observed spatially and temporally in various disease states. Ample studies have now demonstrated a possible causal link between macrophage activation and the development of autoimmune disorders. How these cells may be contributing to the adaptive immune response and potentially perpetuating the progression of neurodegenerative diseases and neural injuries is not fully understood. Within this review, we hope to illustrate the role that macrophages and microglia play as initiators of adaptive immune response in various CNS diseases by offering evidence of: (1) the types of immune responses and the processes of antigen presentation in each disease, (2) receptors involved in macrophage/microglial phagocytosis of disease-related cell debris or molecules, and, finally, (3) the implications of macrophages/microglia on the pathogenesis of the diseases.

Atrazine induces phagocytotic dysfunction of microglia depends on nucleocytoplasmic translocation of acetylated HMGB1

Atrazine (ATR) is a widely applied herbicide which was named an environmental endocrine disrupting chemical (EDC). Increasing evidence indicates ATR causes neurotoxic effects resulting in central nervous system (CNS) disease. As the primary immunocytes in the CNS, microglia cells carry out their phagocytosis to maintain the CNS microenvironment by preventing damage from healthy cells. However, the mechanism in which ATR affects the phagocytic function of microglia remains unclear. The present study was designed to investigate the effect of ATR on the phagocytosis of microglia. BV-2 cells and primary microglia selected as microglial models in which BV-2 cells were administrated by ATR at different concentrations (0, 4, 8, 16 μM) for 24 h. Results demonstrated ATR dose-dependently increased the expression of ionized calcium binding adapter molecule 1 (Iba-1), indicating that microglia were activated. Microglial phagocytotic activity induced by ATR fluctuated at the different time points, accompanied by fluctuations in membrane receptor MERTK and cytoplasmic lysosomal marker LAMP1 (two markers related to cell phagocytosis). In this period, the expression of iNOS gradually increased. A mechanistic study further demonstrated that the translocation of High Mobility Group Protein-B1 (HMGB1) from nucleus to cytoplasm in the BV-2 and primary microglial cells induced by ATR, and the process showed a positive correlation with phagocytosis activity of BV-2 cells induced by ATR (r = 0.8030, P = 0.05; α = 0.1). ATR was also shown to spur the acetylation of HMGB1 by breaking the balance between acetylase P300 and deacetylase SIRT1. Unexpectedly, the inhibition of acetylating HMGB1 by resveratrol (Res) was effectively retained by HMGB1 in the nucleus, reversed the SIRT1 and MERTK expression, and enhanced the phagocytosis activity in BV-2 cells. Our results suggested that ATR exposure influenced microglial phagocytosis by acetylating HMGB1 further translocated it in the nucleoplasm.

The roles of microglia and astrocytes in myelin phagocytosis in the central nervous system

Myelination is an important process in the central nervous system (CNS). Oligodendrocytes (OLs) extend multiple layers to densely sheath on axons, composing the myelin to achieve efficient electrical signal conduction. The myelination during developmental stage maintains a balanced state. However, numerous CNS diseases including neurodegenerative and cerebrovascular diseases cause demyelination and disrupt the homeostasis, resulting in inflammation and white matter deficits. Effective clearance of myelin debris is needed in the region of demyelination, which is a key step for remyelination and tissue regeneration. Microglia and astrocytes are the major resident phagocytic cells in the brain, which may play different or collaborative roles in myelination. Microglia and astrocytes participate in developmental myelination through engulfing excessive unneeded myelin. They are also involved in the clearance of degenerated myelin debris for accelerating remyelination, or engulfing healthy myelin sheath for inhibiting remyelination. This review focuses on the roles of microglia and astrocytes in phagocytosing myelin in the developmental brain and diseased brain. In addition, the interaction between microglia and astrocytes to mediate myelin engulfment is also summarized.

Use of metal-based contrast agents for in vivo MR and CT imaging of phagocytic cells in neurological pathologies

The activation of phagocytic cells is a hallmark of many neurological diseases. Imaging them in their 3-dimensional cerebral environment over time is crucial to better understand their role in disease pathogenesis and to monitor their potential therapeutic effects. Phagocytic cells have the ability to internalize metal-based contrast agents both in vitro and in vivo and can thus be tracked by magnetic resonance imaging (MRI) or computed tomography (CT). In this review article, we summarize the different labelling strategies, contrast agents, and in vivo imaging modalities that can be used to monitor cells with phagocytic activity in the central nervous system using MRI and CT, with a focus on clinical applications. Metal-based nanoparticle contrast agents such as gadolinium, gold and iron are ideal candidates for these applications as they have favourable magnetic and/or radiopaque properties and can be fine-tuned for optimal uptake by phagocytic cells. However, they also come with downsides due to their potential toxicity, especially in the brain where they might accumulate. We therefore conclude our review by discussing the pitfalls, safety and potential for clinical translation of these metal-based neuroimaging techniques. Early results in patients with neuropathologies such as multiple sclerosis, stroke, trauma, cerebral aneurysm and glioblastoma are promising. If the challenges represented by safety issues are overcome, phagocytic cells imaging will be a very valuable tool for studying and understanding the inflammatory response and evaluating treatments that aim at mitigating this response in patients with neurological diseases.

Selective brain hypothermia attenuates focal cerebral ischemic injury and improves long-term neurological outcome in aged female mice

AIMS

This study aimed to investigate the effects of mild selective brain hypothermia on aged female ischemic mice.

METHODS

A distal middle cerebral artery occlusion (dMCAO) model was established in aged female mice, who were then subjected to mild selective brain hypothermia immediately after the dMCAO procedure. Neurological behavioral examinations were conducted prior to and up to 35 days post-ischemia. Infarct volume, brain atrophy, pro-inflammation, and anti-inflammation microglia/macrophages phenotype and white matter injury were evaluated by immunofluorescence staining. Correlations between neurological behaviors and histological parameters were evaluated by Pearson product linear regression analysis.

RESULTS

Sensorimotor and cognitive function tests confirmed the protective effect of mild selective brain hypothermia in elderly female ischemic mice. In addition, hypothermia decreased the infarct volume and brain atrophy induced by focal cerebral ischemia. Furthermore, hypothermia alleviated ischemia-induced short-term and long-term white matter injury, which was correlated with behavioral deficits. Finally, hypothermia suppressed the harmful immunological response by promoting the transformation of pro-inflammatory microglia/macrophages to anti-inflammatory phenotype. This polarization was negatively correlated with neuronal loss and white matter injury.

CONCLUSION

Mild selective brain hypothermia promoted long-term functional recovery by alleviating white matter damage in an aged female mouse model of ischemia.

Microglia subtypes show substrate- and time-dependent phagocytosis preferences and phenotype plasticity

Microglia are phagocytosis-competent CNS cells comprising a spectrum of subtypes with beneficial and/or detrimental functions in acute and chronic neurodegenerative disorders. The heterogeneity of microglia suggests differences in phagocytic activity and phenotype plasticity between microglia subtypes. To study these issues, primary murine glial cultures were cultivated in the presence of serum, different growth factors and cytokines to obtain M0-like, M1-like, and M2-like microglia as confirmed by morphology, M1/M2 gene marker expression, and nitric oxide assay. Single-cell analysis after 3 hours of phagocytosis of E.coli particles or IgG-opsonized beads showed equal internalization by M0-like microglia, whereas M1-like microglia preferably internalized E.coli particles and M2-like microglia preferably internalized IgG beads, suggesting subtype-specific preferences for different phagocytosis substrates. Time-lapse live-cells imaging over 16 hours revealed further differences between microglia subtypes in phagocytosis preference and internalization dynamics. M0- and, more efficiently, M1-like microglia continuously internalized E.coli particles for 16 hours, whereas M2-like microglia discontinued internalization after approximately 8 hours. IgG beads were continuously internalized by M0- and M1-like microglia but strikingly less by M2-like microglia. M2-like microglia initially showed continuous internalization similar to M0-like microglia but again discontinuation of internalization after 8 hours suggesting that the time of substrate exposure differently affect microglia subtypes. After prolonged exposure to E.coli particles or IgG beads for 5 days all microglia subtypes showed increased internalization of E.coli particles compared to IgG beads, increased nitric oxide release and up-regulation of M1 gene markers, irrespectively of the phagocytosis substrate, suggesting phenotype plasticity. In summary, microglia subtypes show substrate- and time-dependent phagocytosis preferences and phenotype plasticity. The results suggest that prolonged phagocytosis substrate exposure enhances M1-like profiles and M2-M1 repolarization of microglia. Similar processes may also take place in conditions of acute and chronic brain insults when microglia encounter different types of phagocytic substrates.

Microglial phagocytosis and regulatory mechanisms after stroke

Stroke, including ischemic stroke and hemorrhagic stroke can cause massive neuronal death and disruption of brain structure, which is followed by secondary inflammatory injury initiated by pro-inflammatory molecules and cellular debris. Phagocytic clearance of cellular debris by microglia, the brain's scavenger cells, is pivotal for neuroinflammation resolution and neurorestoration. However, microglia can also exacerbate neuronal loss by phagocytosing stressed-but-viable neurons in the penumbra, thereby expanding the injury area and hindering neurofunctional recovery. Microglia constantly patrol the central nervous system using their processes to scour the cellular environment and start or cease the phagocytosis progress depending on the "eat me" or "don't eat me'' signals on cellular surface. An optimal immune response requires a delicate balance between different phenotypic states to regulate neuro-inflammation and facilitate reconstruction after stroke. Here, we examine the literature and discuss the molecular mechanisms and cellular pathways regulating microglial phagocytosis, their resulting effects in brain injury and neural regeneration, as well as the potential therapeutic targets that might modulate microglial phagocytic activity to improve neurological function after stroke.

Microglia subtypes show substrate- and time-dependent phagocytosis preferences and phenotype plasticity

Microglia are phagocytosis-competent CNS cells comprising a spectrum of subtypes with beneficial and/or detrimental functions in acute and chronic neurodegenerative disorders. The heterogeneity of microglia suggests differences in phagocytic activity and phenotype plasticity between microglia subtypes. To study these issues, primary murine glial cultures were cultivated in the presence of serum, different growth factors and cytokines to obtain M0-like, M1-like, and M2-like microglia as confirmed by morphology, M1/M2 gene marker expression, and nitric oxide assay. Single-cell analysis after 3 hours of phagocytosis of E.coli particles or IgG-opsonized beads showed equal internalization by M0-like microglia, whereas M1-like microglia preferably internalized E.coli particles and M2-like microglia preferably internalized IgG beads, suggesting subtype-specific preferences for different phagocytosis substrates. Time-lapse live-cells imaging over 16 hours revealed further differences between microglia subtypes in phagocytosis preference and internalization dynamics. M0- and, more efficiently, M1-like microglia continuously internalized E.coli particles for 16 hours, whereas M2-like microglia discontinued internalization after approximately 8 hours. IgG beads were continuously internalized by M0- and M1-like microglia but strikingly less by M2-like microglia. M2-like microglia initially showed continuous internalization similar to M0-like microglia but again discontinuation of internalization after 8 hours suggesting that the time of substrate exposure differently affect microglia subtypes. After prolonged exposure to E.coli particles or IgG beads for 5 days all microglia subtypes showed increased internalization of E.coli particles compared to IgG beads, increased nitric oxide release and up-regulation of M1 gene markers, irrespectively of the phagocytosis substrate, suggesting phenotype plasticity. In summary, microglia subtypes show substrate- and time-dependent phagocytosis preferences and phenotype plasticity. The results suggest that prolonged phagocytosis substrate exposure enhances M1-like profiles and M2-M1 repolarization of microglia. Similar processes may also take place in conditions of acute and chronic brain insults when microglia encounter different types of phagocytic substrates.

Microglia as Therapeutic Target for Radiation-Induced Brain Injury

Radiation-induced brain injury (RIBI) after radiotherapy has become an increasingly important factor affecting the prognosis of patients with head and neck tumor. With the delivery of high doses of radiation to brain tissue, microglia rapidly transit to a pro-inflammatory phenotype, upregulate phagocytic machinery, and reduce the release of neurotrophic factors. Persistently activated microglia mediate the progression of chronic neuroinflammation, which may inhibit brain neurogenesis leading to the occurrence of neurocognitive disorders at the advanced stage of RIBI. Fully understanding the microglial pathophysiology and cellular and molecular mechanisms after irradiation may facilitate the development of novel therapy by targeting microglia to prevent RIBI and subsequent neurological and neuropsychiatric disorders.

The role of microglial/macrophagic salt-inducible kinase 3 on normal and excessive phagocytosis after transient focal cerebral ischemia

Previous studies suggested that anti-inflammatory microglia/macrophages (Mi/MΦ) play a role in “normal phagocytosis,” which promoted the rapid clearance of necrotic substances and apoptotic cells. More recently, a few studies have found that Mi/MΦ also play a role in “pathological phagocytosis” in the form of excessive or reduced phagocytosis, thereby worsening damage induced by CNS diseases. However, the underlying mechanisms and the Mi/MΦ subtypes related to this pathological phagocytosis are still unknown. Salt-inducible kinase 3 (SIK3), a member of the 5’ adenosine monophosphate-activated protein kinase (AMPK) family, has been shown to regulate inflammation in several peripheral diseases. Whether SIK3 also regulates the inflammatory response in CNS diseases is currently unknown. Therefore, in this study, we created a transgenic tamoxifen-induced Mi/MΦ-specific SIK3 conditional knockout (SIK3-cKO) mouse to examine SIK3’s role in phagocytotic function induced by transient focal cerebral ischemia (tFCI). By single-cell RNA-seq, we found the pro-inflammatory Mi/MΦ phenotype performed an excessive phagocytotic function, but the anti-inflammatory Mi/MΦ phenotype performed a normal phagocytotic function. We found that SIK3-cKO caused Mi/MΦ heterogenization from the transitional phenotype to the anti-inflammatory phenotype after tFCI. This phenotypic shift corresponded with enhanced phagocytosis of both apoptotic and live neurons. Interestingly, SIK3-cKO enhanced normal phagocytosis of myelin debris but attenuating excessive phagocytosis of non-damaged myelin sheath, thereby protecting white matter integrity after tFCI. CD16, a pro-inflammation marker, was decreased significantly by SIK3-cKO and correlated with “excessive phagocytosis.” SIK3-cKO promoted long-term recovery of white matter function and neurological function as assessed with electrophysiological compound action potential (CAPs) and behavioral analysis. This study is the first to show a role of SIK3 in Mi/MΦ phagocytosis in CNS diseases, and reveals that promoting Mi/MΦ anti-inflammatory heterogenization inhibits “excessive phagocytosis” of live cells and facilitates “normal phagocytosis” of apoptotic cells. Therefore, inhibition of SIK3 in Mi/MΦ may be a potential therapeutic target in stroke and other CNS diseases with accompanying white matter destruction.

Signaling pathways involved in ischemic stroke: molecular mechanisms and therapeutic interventions

Ischemic stroke is caused primarily by an interruption in cerebral blood flow, which induces severe neural injuries, and is one of the leading causes of death and disability worldwide. Thus, it is of great necessity to further detailly elucidate the mechanisms of ischemic stroke and find out new therapies against the disease. In recent years, efforts have been made to understand the pathophysiology of ischemic stroke, including cellular excitotoxicity, oxidative stress, cell death processes, and neuroinflammation. In the meantime, a plethora of signaling pathways, either detrimental or neuroprotective, are also highly involved in the forementioned pathophysiology. These pathways are closely intertwined and form a complex signaling network. Also, these signaling pathways reveal therapeutic potential, as targeting these signaling pathways could possibly serve as therapeutic approaches against ischemic stroke. In this review, we describe the signaling pathways involved in ischemic stroke and categorize them based on the pathophysiological processes they participate in. Therapeutic approaches targeting these signaling pathways, which are associated with the pathophysiology mentioned above, are also discussed. Meanwhile, clinical trials regarding ischemic stroke, which potentially target the pathophysiology and the signaling pathways involved, are summarized in details. Conclusively, this review elucidated potential molecular mechanisms and related signaling pathways underlying ischemic stroke, and summarize the therapeutic approaches targeted various pathophysiology, with particular reference to clinical trials and future prospects for treating ischemic stroke.

Microglia: The Hub of Intercellular Communication in Ischemic Stroke

Communication between microglia and other cells has recently been at the forefront of research in central nervous system (CNS) disease. In this review, we provide an overview of the neuroinflammation mediated by microglia, highlight recent studies of crosstalk between microglia and CNS resident and infiltrating cells in the context of ischemic stroke (IS), and discuss how these interactions affect the course of IS. The in-depth exploration of microglia-intercellular communication will be beneficial for therapeutic tools development and clinical translation for stroke control.

Knockout of the P2Y6 Receptor Prevents Peri-Infarct Neuronal Loss after Transient, Focal Ischemia in Mouse Brain

After stroke, there is a delayed neuronal loss in brain areas surrounding the infarct, which may in part be mediated by microglial phagocytosis of stressed neurons. Microglial phagocytosis of stressed or damaged neurons can be mediated by UDP released from stressed neurons activating the P2Y6 receptor on microglia, inducing microglial phagocytosis of such neurons. We show evidence here from a small trial that the knockout of the P2Y6 receptor, required for microglial phagocytosis of neurons, prevents the delayed neuronal loss after transient, focal brain ischemia induced by endothelin-1 injection in mice. Wild-type mice had neuronal loss and neuronal nuclear material within microglia in peri-infarct areas. P2Y6 receptor knockout mice had no significant neuronal loss in peri-infarct brain areas seven days after brain ischemia. Thus, delayed neuronal loss after stroke may in part be mediated by microglial phagocytosis of stressed neurons, and the P2Y6 receptor is a potential treatment target to prevent peri-infarct neuronal loss.

The Role of Microglial Phagocytosis in Ischemic Stroke

Microglia are the resident immune cells of the central nervous system that exert diverse roles in the pathogenesis of ischemic stroke. During the past decades, microglial polarization and chemotactic properties have been well-studied, whereas less attention has been paid to phagocytic phenotypes of microglia in stroke. Generally, whether phagocytosis mediated by microglia plays a beneficial or detrimental role in stroke remains controversial, which calls for further investigations. Most researchers are in favor of the former proposal currently since efficient clearance of tissue debris promotes tissue reconstruction and neuronal network reorganization in part. Other scholars propose that excessively activated microglia engulf live or stressed neuronal cells, which results in neurological deficits and brain atrophy. Upon ischemia challenge, the microglia infiltrate injured brain tissue and engulf live/dead neurons, myelin debris, apoptotic cell debris, endothelial cells, and leukocytes. Cell phagocytosis is provoked by the exposure of "eat-me" signals or the loss of "don^'t eat-me" signals. We supposed that microglial phagocytosis could be initiated by the specific "eat-me" signal and its corresponding receptor on the specific cell type under pathological circumstances. In this review, we will summarize phagocytic characterizations of microglia after stroke and the potential receptors responsible for this programmed biological progress. Understanding these questions precisely may help to develop appropriate phagocytic regulatory molecules, which are promoting self-limiting inflammation without damaging functional cells.

Neuronal Loss after Stroke Due to Microglial Phagocytosis of Stressed Neurons

After stroke, there is a rapid necrosis of all cells in the infarct, followed by a delayed loss of neurons both in brain areas surrounding the infarct, known as 'selective neuronal loss', and in brain areas remote from, but connected to, the infarct, known as 'secondary neurodegeneration'. Here we review evidence indicating that this delayed loss of neurons after stroke is mediated by the microglial phagocytosis of stressed neurons. After a stroke, neurons are stressed by ongoing ischemia, excitotoxicity and/or inflammation and are known to: (i) release "find-me" signals such as ATP, (ii) expose "eat-me" signals such as phosphatidylserine, and (iii) bind to opsonins, such as complement components C1q and C3b, inducing microglia to phagocytose such neurons. Blocking these factors on neurons, or their phagocytic receptors on microglia, can prevent delayed neuronal loss and behavioral deficits in rodent models of ischemic stroke. Phagocytic receptors on microglia may be attractive treatment targets to prevent delayed neuronal loss after stroke due to the microglial phagocytosis of stressed neurons.

The role of hypoxia in stem cell regulation of the central nervous system: From embryonic development to adult proliferation

Hypoxia is involved in the regulation of various cell functions in the body, including the regulation of stem cells. The hypoxic microenvironment is indispensable from embryonic development to the regeneration and repair of adult cells. In addition to embryonic stem cells, which need to maintain their self-renewal properties and pluripotency in a hypoxic environment, adult stem cells, including neural stem cells (NSCs), also exist in a hypoxic microenvironment. The subventricular zone (SVZ) and hippocampal dentate gyrus (DG) are the main sites of adult neurogenesis in the brain. Hypoxia can promote the proliferation, migration, and maturation of NSCs in these regions. Also, because most neurons in the brain are non-regenerative, stem cell transplantation is considered as a promising strategy for treating central nervous system (CNS) diseases. Hypoxic treatment also increases the effectiveness of stem cell therapy. In this review, we firstly describe the role of hypoxia in different stem cells, such as embryonic stem cells, NSCs, and induced pluripotent stem cells, and discuss the role of hypoxia-treated stem cells in CNS diseases treatment. Furthermore, we highlight the role and mechanisms of hypoxia in regulating adult neurogenesis in the SVZ and DG and adult proliferation of other cells in the CNS.

Microglial ASD-related genes are involved in oligodendrocyte differentiation

Autism spectrum disorders (ASD) are associated with mutations of chromodomain-helicase DNA-binding protein 8 (Chd8) and tuberous sclerosis complex 2 (Tsc2). Although these ASD-related genes are detected in glial cells such as microglia, the effect of Chd8 or Tsc2 deficiency on microglial functions and microglia-mediated brain development remains unclear. In this study, we investigated the role of microglial Chd8 and Tsc2 in cytokine expression, phagocytosis activity, and neuro/gliogenesis from neural stem cells (NSCs) in vitro. Chd8 or Tsc2 knockdown in microglia reduced insulin-like growth factor-1(Igf1) expression under lipopolysaccharide (LPS) stimulation. In addition, phagocytosis activity was inhibited by Tsc2 deficiency, microglia-mediated oligodendrocyte development was inhibited, in particular, the differentiation of oligodendrocyte precursor cells to oligodendrocytes was prevented by Chd8 or Tsc2 deficiency. These results suggest that ASD-related gene expression in microglia is involved in oligodendrocyte differentiation, which may contribute to the white matter pathology relating to ASD.

Microglia in Neurodegenerative Events-An Initiator or a Significant Other?

A change in microglia structure, signaling, or function is commonly associated with neurodegeneration. This is evident in the patient population, animal models, and targeted in vitro assays. While there is a clear association, it is not evident that microglia serve as an initiator of neurodegeneration. Rather, the dynamics imply a close interaction between the various cell types and structures in the brain that orchestrate the injury and repair responses. Communication between microglia and neurons contributes to the physiological phenotype of microglia maintaining cells in a surveillance state and allows the cells to respond to events occurring in their environment. Interactions between microglia and astrocytes is not as well characterized, nor are interactions with other members of the neurovascular unit; however, given the influence of systemic factors on neuroinflammation and disease progression, such interactions likely represent significant contributes to any neurodegenerative process. In addition, they offer multiple target sites/processes by which environmental exposures could contribute to neurodegenerative disease. Thus, microglia at least play a role as a significant other with an equal partnership; however, claiming a role as an initiator of neurodegeneration remains somewhat controversial.

Urinary Trypsin Inhibitor Protects Tight Junctions of Septic Pulmonary Capillary Endothelial Cells by Regulating the Functions of Macrophages

BACKGROUND

Our previous study found that urinary trypsin inhibitor (ulinastatin, UTI) protected tight junctions (TJs) of lung endothelia via TNF-α inhibition, thereby alleviating pulmonary capillary permeability in septic rats. As the activated macrophage is the main source of TNF-α in sepsis, we speculate that UTI may exert the above effects by regulating the functions of macrophages.

METHODS

Bone-marrow derived macrophages (BMDM) were divided into control, lipopolysaccharide (LPS), UTI+LPS and UTI groups. TNF-α, TGF-β, IL-10, CD86, CD206 and MCP-1 expression were assessed by Western blot. The phagocytosis and migration of BMDM were detected. Pulmonary microvascular endothelial cells (PMVECs) were cultured with the conditioned medium (CM) from each group of BMDM above. Sprague-Dawley rats were divided into sham, cecal ligation and puncture (CLP), and UTI+CLP groups. Western blot and immunofluorescence were used to detected zonula occludens-1 (ZO-1), occludin and claudin-5 expression in PMVECs, as well as TNF-α, TGF-β, iNOS, CD86 and CD206 expression in lungs. Pulmonary capillary permeability was assessed by extravasated Evans blue, lung injury score (LIS), wet-to-dry weight ratio and electron microscope.

RESULTS

TNF-α and CD86 expression were increased in LPS-treated BMDM, but were reversed by UTI pretreatment. TGF-β, IL-10 and CD206 expression were the opposite. UTI markedly decreased phagocytosis and migration of LPS-treated BMDM. ZO-1, occludin and claudin-5 expression were markedly decreased in PMVECs of the CM-LPS group, but significantly increased in the CM-UTI+LPS group. TNF-α, iNOS and CD86 expression were increased in the lungs of CLP-rats but decreased with UTI pretreatment, while TGF-β and CD206 expression were the opposite. UTI markedly ameliorated the lung EB leakage, improved LIS, reduced the wet-to-dry ratio and revised the damaged TJs of PMVECs in CLP-rats.

CONCLUSION

UTI effectively inhibits the conversion of M1 macrophage but increases M2, reduces the phagocytosis and migration, which helps to protect endothelia TJs and reduce pulmonary capillary permeability during sepsis.