Distinguishing features of microglia- and monocyte-derived macrophages after stroke

Monitoring of single-nucleus chromatin landscape of ischemic stroke in mouse cerebral cortex across time

Ischemic stroke constitutes a multifaceted neurological affliction that spans various cellular types. Lack of dynamic chromatin accessibility data after stroke is one of the obstacles to understanding this process. To gain insights into the variations in transcriptional regulation among various cell types subsequent to a stroke, we employed single-nucleus ATAC-seq to curate a chromatin accessibility compendium from the cerebral cortex of mice subjected to middle cerebral artery occlusion/reperfusion (MCAO/R). Tissue samples were collected at various time points including 0, 6, 12, 24 hours, and 7, 14 days post-reperfusion, in addition to Sham control group. We obtained 99,271 high-quality nuclei across nine cell types, thereby establishing the single-nucleus chromatin accessibility atlas. This atlas provides data for interpreting the regulatory mechanisms that pervade the continuum of ischemic stroke. The data presented herein constitutes a valuable resource for the comprehension of regulatory interplays within the pathology-afflicted cerebrum.

Markers of tumor-associated macrophages and microglia exhibit high intratumoral heterogeneity in human glioblastoma tissue

BACKGROUND

Human glioblastoma multiforme (GBM) is a highly aggressive tumor with insufficient therapies available. Especially, novel concepts of immune therapies fail due to a complex immunosuppressive microenvironment, high mutational rates, and inter-patient variations. The intratumoral heterogeneity is currently not sufficiently investigated.

METHODS

Biopsies from six different locations were taken in a cohort of 16 GBM patients who underwent surgery. The tissue slides were analyzed utilizing high-content imaging microscopy and algorithm-based image quantification. Several immune markers for macrophage and microglia subpopulations were investigated. Flow cytometry was used to validate key results. Besides the surface marker, cytokines were measured and categorized based on their heterogenicity and overall expression.

RESULTS

M2-like antigens, including CD204, CD163, Arg1, and CSF1R, showed comparatively higher expression, with GFAP displaying the least intratumoral heterogeneity. In contrast, anti-tumor-macrophage-like antigens, such as PSGL-1, CD16, CD68, and MHC-II, exhibited low overall expression and concurrent high intratumoral heterogeneity. CD16 and PSGL-1 were the most heterogeneous antigens. High expression levels were observed for cytokines IL-6, VEGF, and CCL-2. VILIP-a was revealed to differentiate most in principle component analysis. Cytokines with the lowest overall expression, such as TGF-β1, β-NGF, TNF-α, and TREM1, showed low intratumoral heterogeneity, in contrast to βNGF, TNF-α, and IL-18, which displayed high heterogeneity despite low expression.

CONCLUSION

The study showed high intratumoral heterogeneity in GBM, emphasizing the need for a more detailed understanding of the tumor microenvironment. The described findings could be essential for future personalized treatment strategies and the implementation of reliable diagnostics in GBM.

Crosstalk Among Glial Cells in the Blood-Brain Barrier Injury After Ischemic Stroke

Blood-brain barrier (BBB) is comprised of brain microvascular endothelial cells (ECs), astrocytes, perivascular microglia, pericytes, neuronal processes, and the basal lamina. As a complex and dynamic interface between the blood and the central nervous system (CNS), BBB is responsible for transporting nutrients essential for the normal metabolism of brain cells and hinders many toxic compounds entering into the CNS. The loss of BBB integrity following stroke induces tissue damage, inflammation, edema, and neural dysfunction. Thus, BBB disruption is an important pathophysiological process of acute ischemic stroke. Understanding the mechanism underlying BBB disruption can uncover more promising biological targets for developing treatments for ischemic stroke. Ischemic stroke-induced activation of microglia and astrocytes leads to increased production of inflammatory mediators, containing chemokines, cytokines, matrix metalloproteinases (MMPs), etc., which are important factors in the pathological process of BBB breakdown. In this review, we discussed the current knowledges about the vital and dual roles of astrocytes and microglia on the BBB breakdown during ischemic stroke. Specifically, we provided an updated overview of phenotypic transformation of microglia and astrocytes, as well as uncovered the crosstalk among astrocyte, microglia, and oligodendrocyte in the BBB disruption following ischemic stroke.

CB2 expression in mouse brain: from mapping to regulation in microglia under inflammatory conditions

Since its detection in the brain, the cannabinoid receptor type 2 (CB2) has been considered a promising therapeutic target for various neurological and psychiatric disorders. However, precise brain mapping of its expression is still lacking. Using magnetic cell sorting, calibrated RT-qPCR and single-nucleus RNAseq, we show that CB2 is expressed at a low level in all brain regions studied, mainly by few microglial cells, and by neurons in an even lower proportion. Upon lipopolysaccharide stimulation, modeling neuroinflammation in non-sterile conditions, we demonstrate that the inflammatory response is associated with a transient reduction in CB2 mRNA levels in brain tissue, particularly in microglial cells. This result, confirmed in the BV2 microglial cell line, contrasts with the positive correlation observed between CB2 mRNA levels and the inflammatory response upon stimulation by interferon-gamma, modeling neuroinflammation in sterile condition. Discrete brain CB2 expression might thus be up- or down-regulated depending on the inflammatory context.

Role of microglia in stroke

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

Phenotypic and spatial heterogeneity of brain myeloid cells after stroke is associated with cell ontogeny, tissue damage, and brain connectivity

Acute stroke triggers extensive changes to myeloid immune cell populations in the brain that may be targets for limiting brain damage and enhancing repair. Immunomodulatory approaches will be most effective with precise manipulation of discrete myeloid cell phenotypes in time and space. Here, we investigate how stroke alters mononuclear myeloid cell composition and phenotypes at single-cell resolution and key spatial patterns. Our results show that multiple reactive microglial states and monocyte-derived populations contribute to an extensive myeloid cell repertoire in post-stroke brains. We identify important overlaps and distinctions among different cell types/states that involve ontogeny- and spatial-related properties. Notably, brain connectivity with infarcted tissue underpins the pattern of local and remote altered cell accumulation and reactivity. Our discoveries suggest a global but anatomically governed brain myeloid cell response to stroke that comprises diverse phenotypes arising through intrinsic cell ontogeny factors interacting with exposure to spatially organized brain damage and neuro-axonal cues.

Targeting brain-peripheral immune responses for secondary brain injury after ischemic and hemorrhagic stroke

The notion that the central nervous system is an immunologically immune-exempt organ has changed over the past two decades, with increasing evidence of strong links and interactions between the central nervous system and the peripheral immune system, both in the healthy state and after ischemic and hemorrhagic stroke. Although primary injury after stroke is certainly important, the limited therapeutic efficacy, poor neurological prognosis and high mortality have led researchers to realize that secondary injury and damage may also play important roles in influencing long-term neurological prognosis and mortality and that the neuroinflammatory process in secondary injury is one of the most important influences on disease progression. Here, we summarize the interactions of the central nervous system with the peripheral immune system after ischemic and hemorrhagic stroke, in particular, how the central nervous system activates and recruits peripheral immune components, and we review recent advances in corresponding therapeutic approaches and clinical studies, emphasizing the importance of the role of the peripheral immune system in ischemic and hemorrhagic stroke.

Advanced Nano-Drug Delivery Systems in the Treatment of Ischemic Stroke

In recent years, the frequency of strokes has been on the rise year by year and has become the second leading cause of death around the world, which is characterized by a high mortality rate, high recurrence rate, and high disability rate. Ischemic strokes account for a large percentage of strokes. A reperfusion injury in ischemic strokes is a complex cascade of oxidative stress, neuroinflammation, immune infiltration, and mitochondrial damage. Conventional treatments are ineffective, and the presence of the blood-brain barrier (BBB) leads to inefficient drug delivery utilization, so researchers are turning their attention to nano-drug delivery systems. Functionalized nano-drug delivery systems have been widely studied and applied to the study of cerebral ischemic diseases due to their favorable biocompatibility, high efficiency, strong specificity, and specific targeting ability. In this paper, we briefly describe the pathological process of reperfusion injuries in strokes and focus on the therapeutic research progress of nano-drug delivery systems in ischemic strokes, aiming to provide certain references to understand the progress of research on nano-drug delivery systems (NDDSs).

Protective effects of fecal microbiota transplantation against ischemic stroke and other neurological disorders: an update

The bidirectional communication between the gut and brain or gut-brain axis is regulated by several gut microbes and microbial derived metabolites, such as short-chain fatty acids, trimethylamine N-oxide, and lipopolysaccharides. The Gut microbiota (GM) produce neuroactives, specifically neurotransmitters that modulates local and central neuronal brain functions. An imbalance between intestinal commensals and pathobionts leads to a disruption in the gut microbiota or dysbiosis, which affects intestinal barrier integrity and gut-immune and neuroimmune systems. Currently, fecal microbiota transplantation (FMT) is recommended for the treatment of recurrent Clostridioides difficile infection. FMT elicits its action by ameliorating inflammatory responses through the restoration of microbial composition and functionality. Thus, FMT may be a potential therapeutic option in suppressing neuroinflammation in post-stroke conditions and other neurological disorders involving the neuroimmune axis. Specifically, FMT protects against ischemic injury by decreasing IL-17, IFN-γ, Bax, and increasing Bcl-2 expression. Interestingly, FMT improves cognitive function by lowering amyloid-β accumulation and upregulating synaptic marker (PSD-95, synapsin-1) expression in Alzheimer's disease. In Parkinson's disease, FMT was shown to inhibit the expression of TLR4 and NF-κB. In this review article, we have summarized the potential sources and methods of administration of FMT and its impact on neuroimmune and cognitive functions. We also provide a comprehensive update on the beneficial effects of FMT in various neurological disorders by undertaking a detailed interrogation of the preclinical and clinical published literature.

Perspective from single-cell sequencing: Is inflammation in acute ischemic stroke beneficial or detrimental?

BACKGROUND

Acute ischemic stroke (AIS) is a common cerebrovascular event associated with high incidence, disability, and poor prognosis. Studies have shown that various cell types, including microglia, astrocytes, oligodendrocytes, neurons, and neutrophils, play complex roles in the early stages of AIS and significantly affect its prognosis. Thus, a comprehensive understanding of the mechanisms of action of these cells will be beneficial for improving stroke prognosis. With the rapid development of single-cell sequencing technology, researchers have explored the pathophysiological mechanisms underlying AIS at the single-cell level.

METHOD

We systematically summarize the latest research on single-cell sequencing in AIS.

RESULT

In this review, we summarize the phenotypes and functions of microglia, astrocytes, oligodendrocytes, neurons, neutrophils, monocytes, and lymphocytes, as well as their respective subtypes, at different time points following AIS. In particular, we focused on the crosstalk between microglia and astrocytes, oligodendrocytes, and neurons. Our findings reveal diverse and sometimes opposing roles within the same cell type, with the possibility of interconversion between different subclusters.

CONCLUSION

This review offers a pioneering exploration of the functions of various glial cells and cell subclusters after AIS, shedding light on their regulatory mechanisms that facilitate the transformation of detrimental cell subclusters towards those that are beneficial for improving the prognosis of AIS. This approach has the potential to advance the discovery of new specific targets and the development of drugs, thus representing a significant breakthrough in addressing the challenges in AIS treatment.

Research progress on the roles of neurovascular unit in stroke-induced immunosuppression

A complex pathophysiological mechanism is involved in brain injury following cerebral infarction. The neurovascular unit (NVU) is a complex multi-cellular structure consisting of neurons, endothelial cells, pericyte, astrocyte, microglia and extracellular matrix, etc. The dyshomeostasis of NVU directly participates in the regulation of inflammatory immune process. The components of NVU promote inflammatory overreaction and synergize with the overactivation of autonomic nervous system to initiate stroke-induced immunodepression (SIID). SIID can alleviate the damage caused by inflammation, however, it also makes stroke patients more susceptible to infection, leading to systemic damage. This article reviews the mechanism of SIID and the roles of NVU in SIID, to provide a perspective for reperfusion, prognosis and immunomodulatory therapy of cerebral infarction.

Ablation of microglia following infection of the central nervous system with a neurotropic murine coronavirus infection leads to increased demyelination and impaired remyelination

Intracranial inoculation of susceptible mice with a glial-tropic strain of mouse hepatitis virus (JHMV), a murine coronavirus, results in an acute encephalomyelitis followed by viral persistence in white matter tracts accompanied by chronic neuroinflammation and demyelination. Microglia are the resident immune cell of the central nervous system (CNS) and are considered important in regulating events associated with neuroinflammation as well as influencing both white matter damage and remyelination. To better understand mechanisms by which microglia contribute to these immune-mediated events, JHMV-infected mice with established demyelination were treated with the small molecular inhibitor of colony stimulating factor 1 receptor (CSF1R), PLX5622, to deplete microglia. Treatment with PLX5622 did not affect viral replication within the CNS yet the severity of demyelination was increased and remyelination impaired compared to control mice. Gene expression analysis revealed that targeting microglia resulted in altered expression of genes associated with immune cell activation and phagocytosis of myelin debris. These findings indicate that microglia are not critical in viral surveillance in persistently JHMV-infected mice yet restrict white matter damage and remyelination, in part, by influencing phagocytosis of myelin debris.

Translatome analysis reveals microglia and astrocytes to be distinct regulators of inflammation in the hyperacute and acute phases after stroke

Neuroinflammation is a hallmark of ischemic stroke, which is a leading cause of death and long-term disability. Understanding the exact cellular signaling pathways that initiate and propagate neuroinflammation after stroke will be critical for developing immunomodulatory stroke therapies. In particular, the precise mechanisms of inflammatory signaling in the clinically relevant hyperacute period, hours after stroke, have not been elucidated. We used the RiboTag technique to obtain microglia and astrocyte-derived mRNA transcripts in a hyperacute (4 h) and acute (3 days) period after stroke, as these two cell types are key modulators of acute neuroinflammation. Microglia initiated a rapid response to stroke at 4 h by adopting an inflammatory profile associated with the recruitment of immune cells. The hyperacute astrocyte profile was marked by stress response genes and transcription factors, such as Fos and Jun, involved in pro-inflammatory pathways such as TNF-α. By 3 days, microglia shift to a proliferative state and astrocytes strengthen their inflammatory response. The astrocyte pro-inflammatory response at 3 days is partially driven by the upregulation of the transcription factors C/EBPβ, Spi1, and Rel, which comprise 25% of upregulated transcription factor-target interactions. Surprisingly, few sex differences across all groups were observed. Expression and log2 fold data for all sequenced genes are available on a user-friendly website for researchers to examine gene changes and generate hypotheses for stroke targets. Taken together, our data comprehensively describe the microglia and astrocyte-specific translatome response in the hyperacute and acute period after stroke and identify pathways critical for initiating neuroinflammation.

Friends or foes: The mononuclear phagocyte system in ischemic stroke

Ischemic stroke (IS) is a major cause of disability and death in adults, and the immune response plays an indispensable role in its pathological process. After the onset of IS, an inflammatory storm, with the infiltration and mobilization of the mononuclear phagocyte system (MPS), is triggered in the brain. Microglia are rapidly activated in situ, followed by waves of circulating monocytes into the ischemic area. Activated microglia and monocytes/macrophages are mainly distributed in the peri-infarct area. These cells have similar morphology and functions, such as secreting cytokines and phagocytosis. Previously, the presence of the MPS was considered a marker of an exacerbated inflammatory response that contributes to brain damage. However, recent studies have suggested a rather complicated role of the MPS in IS. Here, we reviewed articles focusing on various functions of the MPS among different phases of IS, including recruitment, polarization, phagocytosis, angiogenesis, and interaction with other types of cells. Moreover, due to the characteristics of the MPS, we also noted clinical research addressing alterations in the MPS as potential biomarkers for IS patients for the purposes of predicting prognosis and developing novel therapeutic strategies.

Translatome analysis reveals microglia and astrocytes to be distinct regulators of inflammation in the hyperacute and acute phases after stroke

Neuroinflammation is a hallmark of ischemic stroke, which is a leading cause of death and long-term disability. Understanding the exact cellular signaling pathways that initiate and propagate neuroinflammation after stroke will be critical for developing immunomodulatory stroke therapies. In particular, the precise mechanisms of inflammatory signaling in the clinically relevant hyperacute period, hours after stroke, have not been elucidated. We used the RiboTag technique to obtain astrocyte and microglia-derived mRNA transcripts in a hyperacute (4 hours) and acute (3 days) period after stroke, as these two cell types are key modulators of acute neuroinflammation. Microglia initiated a rapid response to stroke at 4 hours by adopting an inflammatory profile associated with the recruitment of immune cells. The hyperacute astrocyte profile was marked by stress response genes and transcription factors, such as Fos and Jun , involved in pro-inflammatory pathways such as TNF-α. By 3 days, microglia shift to a proliferative state and astrocytes strengthen their inflammatory response. The astrocyte pro-inflammatory response at 3 days is partially driven by the upregulation of the transcription factors C/EBPβ, Spi1 , and Rel , which comprise 25% of upregulated transcription factor-target interactions. Surprisingly, few sex differences across all groups were observed. Expression and log ₂ fold data for all sequenced genes are available on a user-friendly website for researchers to examine gene changes and generate hypotheses for stroke targets. Taken together our data comprehensively describe the astrocyte and microglia-specific translatome response in the hyperacute and acute period after stroke and identify pathways critical for initiating neuroinflammation.

Tumor Necrosis Factor (TNF)-α-Stimulated Gene 6 (TSG-6): A Promising Immunomodulatory Target in Acute Neurodegenerative Diseases

Tumor necrosis factor (TNF)-α-stimulated gene 6 (TSG-6), the first soluble chemokine-binding protein to be identified in mammals, inhibits chemotaxis and transendothelial migration of neutrophils and attenuates the inflammatory response of dendritic cells, macrophages, monocytes, and T cells. This immunoregulatory protein is a pivotal mediator of the therapeutic efficacy of mesenchymal stem/stromal cells (MSC) in diverse pathological conditions, including neuroinflammation. However, TSG-6 is also constitutively expressed in some tissues, such as the brain and spinal cord, and is generally upregulated in response to inflammation in monocytes/macrophages, dendritic cells, astrocytes, vascular smooth muscle cells and fibroblasts. Due to its ability to modulate sterile inflammation, TSG-6 exerts protective effects in diverse degenerative and inflammatory diseases, including brain disorders. Emerging evidence provides insights into the potential use of TSG-6 as a peripheral diagnostic and/or prognostic biomarker, especially in the context of ischemic stroke, whereby the pathobiological relevance of this protein has also been demonstrated in patients. Thus, in this review, we will discuss the most recent data on the involvement of TSG-6 in neurodegenerative diseases, particularly focusing on relevant anti-inflammatory and immunomodulatory functions. Furthermore, we will examine evidence suggesting novel therapeutic opportunities that can be afforded by modulating TSG-6-related pathways in neuropathological contexts and, most notably, in stroke.

Regulation of microglia related neuroinflammation contributes to the protective effect of Gelsevirine on ischemic stroke

Stroke, especially ischemic stroke, is an important cause of neurological morbidity and mortality worldwide. Growing evidence suggests that the immune system plays an intricate function in the pathophysiology of stroke. Gelsevirine (Gs), an alkaloid from Gelsemium elegans, has been proven to decrease inflammation and neuralgia in osteoarthritis previously, but its role in stroke is unknown. In this study, the middle cerebral artery occlusion (MCAO) mice model was used to evaluate the protective effect of Gs on stroke, and the administration of Gs significantly improved infarct volume, Bederson score, neurobiological function, apoptosis of neurons, and inflammation state in vivo. According to the data in vivo and the conditioned medium (CM) stimulated model in vitro, the beneficial effect of Gs came from the downregulation of the over-activity of microglia, such as the generation of inflammatory factors, dysfunction of mitochondria, production of ROS and so on. By RNA-seq analysis and Western-blot analysis, the JAK-STAT signal pathway plays a critical role in the anti-inflammatory effect of Gs. According to the results of molecular docking, inhibition assay, and thermal shift assay, the binding of Gs on JAK2 inhibited the activity of JAK2 which inhibited the over-activity of JAK2 and downregulated the phosphorylation of STAT3. Over-expression of a gain-of-function STAT3 mutation (K392R) abolished the beneficial effects of Gs. So, the downregulation of JAK2-STAT3 signaling pathway by Gs contributed to its anti-inflammatory effect on microglia in stroke. Our study revealed that Gs was benefit to stroke treatment by decreasing neuroinflammation in stroke as a potential drug candidate regulating the JAK2-STAT3 signal pathway.

The therapeutic potential of bone marrow-derived macrophages in neurological diseases

Circulating monocytes are precursors of both tissue macrophages and dendritic cells, and they can infiltrate the central nervous system (CNS) where they transform into bone marrow-derived macrophages (BMDMs). BMDMs play essential roles in various CNS diseases, thus modulating BMDMs might be a way to treat these disorders because there are currently no efficient therapeutic methods available for most of these neurological diseases. Moreover, BMDMs can serve as promising gene delivery vehicles following bone marrow transplantation for otherwise incurable genetic CNS diseases. Understanding the distinct roles that BMDMs play in CNS diseases and their potential as gene delivery vehicles may provide new insights and opportunities for using BMDMs as therapeutic targets or delivery vehicles. This review attempts to comprehensively summarize the neurological diseases that might be treated by modulating BMDMs or by delivering gene therapies via BMDMs after bone marrow transplantation.

Molecular Mechanism of Epimedium Extract against Ischemic Stroke Based on Network Pharmacology and Experimental Validation

Ischemic stroke exhibits high morbidity, disability, and mortality, and treatments for ischemic stroke are limited despite intensive research. The potent neuroprotective benefits of Epimedium against ischemic stroke have gained lots of interest. Nevertheless, systematic research on the direct role and mechanisms of Epimedium in ischemic stroke is still lacking. Network pharmacology analysis coupled with experimental verification was utilized to systematically evaluate the potential pharmacological mechanism of Epimedium against ischemic stroke. The TCMSP database was used to mine the bioactive ingredients and Epimedium's targets. The DrugBank, OMIM, and GeneCards databases were employed to identify potential targets of ischemic stroke. GO and KEGG pathway analyses were also carried out. The interaction between active components and hub targets was confirmed via molecular docking. An experimental ischemic stroke model was used to evaluate the possible therapeutic mechanism of Epimedium. As a result, 23 bioactive compounds of Epimedium were selected, and 30 hub targets of Epimedium in its function against ischemic stroke were identified, and molecular docking results demonstrated good binding. The IL-17 signaling pathway was revealed as a potentially significant pathway, with the NF-κB and MAPK/ERK signaling pathways being involved. Furthermore, in vivo experiments demonstrated that Epimedium treatment could improve neurological function and reduce infarct volume. Additionally, Epimedium reduced the activation of microglia and astrocytes in both the ischemic penumbra of the hippocampus and cerebral cortex following ischemic stroke. Western blot and RT-qPCR analyses demonstrated that Epimedium not only depressed the expression of IL-1β, TNF-α, IL-6, and IL-4 but also inhibited the NF-κB and MAPK/ERK signaling pathways. This study applied network pharmacology and in vivo experiment to explore possible mechanism of Epimedium's role against ischemic stroke, which provides insight into the treatment of ischemic stroke.

LncRNA Tug1 Contributes Post-stroke NLRP3 Inflammasome-Dependent Pyroptosis via miR-145a-5p/Tlr4 Axis

Pyroptosis, a type of programmed cell death illuminated by inflammasomes and active caspases, is implicated in post-stroke inflammation. Our previous study showed that lncRNA taurine upregulated gene 1 (Tug1) sponging miR-145a-5p modulated microglial activation after oxygen-glucose deprivation (OGD). However, the role and mechanism of Tug1 on post-stroke pyroptosis is not fully clear. Photo-thrombosis stroke mice and OGD-treated BV-2 microglia were established respectively. Tug1 knockdown or overexpression was achieved by intraventricular infusion of AAV-shTug1 in vivo, or transfection of siTug1 and pcDNA3.1-Tug1 in vitro. Neurological function and infarction volume were evaluated. Meanwhile, pyroptosis-associated proteins (IL-1β, IL-18, NLRP3, ASC, cleaved-caspase-1, and GSDMD-N), TLR4, and p-p65/p65 as well as Tug1 and miR-145a-5p were detected 24 h after photo-thrombosis or 4 h after OGD by qRT-PCR, western blot, and ELISA. The correlation between Tug1/miR-145a-5p/Tlr4 axis and pyroptosis was explored by dual-luciferase reporter assay and functional gain-and-loss experiments. Photo-thrombosis or OGD caused neural injury and upregulated pyroptosis-associated proteins, Tug1, TLR4, and p-p65 as well as downregulated miR-145a-5p, which was prevented by Tug1 knockdown in vivo and in vitro. Tlr4 gene, putatively binding with miR-145a-5p by bioinformatics analysis, was found to be a direct target of miR-145a-5p with negative interactions. Furthermore, miR-145a-5p inhibitor abolished the inhibitive effects of siTug1 on TLR4 and p-p65 as well as pyroptosis-associated proteins, whereas miR-145a-5p mimics abrogated the enhanced effects of pcDNA3.1-Tug1 on that, suggesting an involvement of Tug1/miR-145a-5p/Tlr4 axis on pyroptosis. Tug1 contributes NLRP3 inflammasome-dependent pyroptosis through miR-145a-5p/Tlr4 axis post-stroke, providing a promising therapeutic strategy against inflammatory injury.

Resolution of inflammation is disturbed in acute ischemic stroke with diabetes mellitus and rescued by resolvin D2 treatment

BACKGROUND

Inflammation plays an important role in diabetes mellitus (DM)-related acute ischemic stroke (AIS). The mechanisms of un-resolved inflammation in DM-related AIS are not fully understood. Specialized pro-resolving mediators (SPMs) are key regulators that promote resolution of inflammation. We aimed to examine resolution function in patients with AIS complicated with DM, and explore potential treatment effects of one of the SPMs, resolvin D2 (RvD2) ex vivo and in vivo.

METHODS

Cultured human macrophages, which were derived from peripheral blood mononuclear cells of AIS and none-AIS patients with or without DM, were stimulated with oxidized-low density lipoprotein (ox-LDL). Levels of SPMs and inflammatory markers were analysed, and RvD2 treatment effects were evaluated in these cells. For experiments in vivo, challenges with high fat diet and low-dose streptozotocin (STZ) were used to induce DM in C57BL/6J mice. AIS model was established by permanent middle cerebral artery occlusion (pMCAO) followed by intra-cerebroventricular injection of RvD2.

RESULTS

Compared with macrophages of AIS patients without DM, the ratios of SPMs to leukotriene B4 (LTB₄) were decreased in AIS patients with DM, accompanied by reduced expression of SPM synthesis enzyme, 15-lipoxygenase-1. Moreover, the levels of pro-inflammatory pathway markers were increased, and the macrophages were skewed to M1 polarization in AIS patients with DM. In mice, treatment with RvD2 ameliorated pMCAO-induced brain injury, neurological dysfunction, and inflammatory response. Furthermore, RvD2 rescued resolution of inflammation by promoting macrophage/microglia polarization to pro-resolving M2 phenotype ex vivo and in vivo.

CONCLUSIONS

Our data demonstrate resolution of inflammation is impaired by DM in AIS patients, implicating a novel mechanism of un-resolved inflammation in DM-related AIS. Furthermore, RvD2 promotes inflammation resolution in macrophages/microglia and protects DM-related AIS, and may thus serve as a novel therapeutic target.

CD11bhigh B Cells Increase after Stroke and Regulate Microglia

Recent studies have highlighted the deleterious contributions of B cells to post-stroke recovery and cognitive decline. Different B cell subsets have been proposed on the basis of expression levels of transcription factors (e.g., T-bet) as well as specific surface proteins. CD11b (α-chain of integrin) is expressed by several immune cell types and is involved in regulation of cell motility, phagocytosis, and other essential functions of host immunity. Although B cells express CD11b, the CD11b^high subset of B cells has not been well characterized, especially in immune dysregulation seen with aging and after stroke. Here, we investigate the role of CD11b^high B cells in immune responses after stroke in young and aged mice. We evaluated the ability of CD11b^high B cells to influence pro- and anti-inflammatory phenotypes of young and aged microglia (MG). We hypothesized that CD11b^high B cells accumulate in the brain and contribute to neuroinflammation in aging and after stroke. We found that CD11b^high B cells are a heterogeneous subpopulation of B cells predominantly present in naive aged mice. Their frequency increases in the brain after stroke in young and aged mice. Importantly, CD11b^high B cells regulate MG phenotype and increase MG phagocytosis in both ex vivo and in vivo settings, likely by production of regulatory cytokines (e.g., TNF-α). As both APCs and adaptive immune cells with long-term memory function, B cells are uniquely positioned to regulate acute and chronic phases of the post-stroke immune response, and their influence is subset specific.

CNS Antigen-Specific Neuroinflammation Attenuates Ischemic Stroke With Involvement of Polarized Myeloid Cells

Background and Objectives

Experimental studies indicate shared molecular pathomechanisms in cerebral hypoxia-ischemia and autoimmune neuroinflammation. This has led to clinical studies investigating the effects of immunomodulatory therapies approved in multiple sclerosis on inflammatory damage in stroke. So far, mutual and combined interactions of autoimmune, CNS antigen-specific inflammatory reactions and cerebral ischemia have not been investigated so far.

Methods

Active MOG_35-55 experimental autoimmune encephalomyelitis (EAE) was induced in male C57Bl/6J mice. During different phases of EAE, transient middle cerebral artery occlusion (tMCAO, 60 minutes) was induced. Brain tissue was analyzed for infarct size and immune cell infiltration. Multiplex gene expression analysis was performed for 186 genes associated with neuroinflammation and hypoxic-ischemic damage.

Results

Mice with severe EAE disease showed a substantial reduction in infarct size after tMCAO. Histopathologic analysis showed less infiltration of CD45⁺ hematopoietic cells in the infarct core of severely diseased acute EAE mice; this was accompanied by an accumulation of Arginase1-positive/Iba1-positive cells. Gene expression analysis indicated an involvement of myeloid cell-driven anti-inflammatory mechanisms in the attenuation of ischemic injury in severely diseased mice exposed to tMCAO in the acute EAE phase.

Discussion

CNS autoantigen-specific autoimmunity has a protective influence on primary tissue damage after experimental stroke, indicating a very early involvement of CNS antigen-specific, myeloid cell-associated anti-inflammatory immune mechanisms that mitigate ischemic injury in the acute EAE phase.

Circadian Regulator CLOCK Drives Immunosuppression in Glioblastoma

The symbiotic interactions between cancer stem cells and the tumor microenvironment (TME) are critical for tumor progression. However, the molecular mechanism underlying this symbiosis in glioblastoma (GBM) remains enigmatic. Here, we show that circadian locomotor output cycles kaput (CLOCK) and its heterodimeric partner brain and muscle ARNT-like 1 (BMAL1) in glioma stem cells (GSC) drive immunosuppression in GBM. Integrated analyses of the data from transcriptome profiling, single-cell RNA sequencing, and TCGA datasets, coupled with functional studies, identified legumain (LGMN) as a direct transcriptional target of the CLOCK-BMAL1 complex in GSCs. Moreover, CLOCK-directed olfactomedin-like 3 (OLFML3) upregulates LGMN in GSCs via hypoxia-inducible factor 1-alpha (HIF1α) signaling. Consequently, LGMN promotes microglial infiltration into the GBM TME via upregulating CD162 and polarizes infiltrating microglia toward an immune-suppressive phenotype. In GBM mouse models, inhibition of the CLOCK-OLFML3-HIF1α-LGMN-CD162 axis reduces intratumoral immune-suppressive microglia, increases CD8+ T-cell infiltration, activation, and cytotoxicity, and synergizes with anti-programmed cell death protein 1 (anti-PD-1 therapy). In human GBM, the CLOCK-regulated LGMN signaling correlates positively with microglial abundance and poor prognosis. Together, these findings uncover the CLOCK-OLFML3-HIF1α-LGMN axis as a molecular switch that controls microglial biology and immunosuppression, thus revealing potential new therapeutic targets for patients with GBM.

A Critical Role of δ-Opioid Receptor in Anti-microglial Activation Under Stress

Microglia are involved in the regulation of cerebral homeostasis and pathogen confrontation. There is, however, evidence showing that excessive microglia activation is implicated in various age-related cerebral diseases. On the other hand, microglia may experience complex changes of polarization in pathological insults, i.e., from a proinflammatory M1 to an anti-inflammatory M2 phenotype, which differentially contribute to the exacerbation or alleviation of cellular injury. Remolding the phenotype of microglia or inhibiting the excessive activation of microglia seems to be a promising approach against neurodegenerative pathologies. Since δ-opioid receptor (DOR) activation exhibits a strong protective capacity against various neuronal injuries, especially the hypoxic/ischemic injury, we asked if the DOR-induced neuroprotection is associated with its effect on microglia. We explored this fundamental issue by using pharmacological and genetic approaches in the BV2 cell line, a general type of microglial cells. The results showed that DOR expression significantly increased in the activated microglial M2 phenotype, but slightly decreased in the microglial M1 phenotype. Hypoxia induced dual polarizations of BV2 cells with an increase in DOR expression. Administration of a specific DOR agonist, UFP-512, largely inhibited lipopolysaccharide (LPS) or hypoxia-induced microglial M1 activation and inflammatory activity with high concentrations of UFP-512 being effective to reverse the interleukin-4 (IL4)-induced microglial activation. Consistent with these observations, inhibiting DOR or knocking-down DOR promoted the excessive activation of BV2 cells in both M1 and M2 directions, while DOR overexpression did the opposite. Furthermore, the PC12 cells exposed to the conditioned medium of BV2 cells treated by UFP-512 grew better than those treated directly with UFP-512 under LPS or hypoxic insults. DOR inhibitor naltrindole could block all the effects of DOR activation. The medium from the BV2 cells with DOR knock-down decreased the viability of PC12 cell, while the medium from the BV2 cells with DOR overexpression largely attenuated LPS or hypoxic injury in the PC12 cells. These first data suggest a close linkage between DOR expression/function and microglial polarization and a critical role of DOR in negative controlling microglial activation. Our work provides a novel clue for new protective strategies against neurodegenerative pathophysiology through DOR-mediated regulation of microglia.

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.

Thromboinflammation in Brain Ischemia: Recent Updates and Future Perspectives

Despite decades of promising preclinical validation and clinical translation, ischemic stroke still remains as one of the leading causes of death and disability worldwide. Within its complex pathophysiological signatures, thrombosis and inflammation, that is, thromboinflammation, are highly interconnected processes leading to cerebral vessel occlusion, inflammatory responses, and severe neuronal damage following the ischemic event. Hence, we here review the most recent updates on thromboinflammatory-dependent mediators relevant after stroke focusing on recent discoveries on platelet modulation, a potential regulation of the innate and adaptive immune system in thromboinflammation, utterly providing a thorough up-to-date overview of all therapeutic approaches currently undergoing clinical trial.

Microglia and monocytes in inflammatory CNS disease: integrating phenotype and function

In neurological diseases, the actions of microglia, the resident myeloid cells of the CNS parenchyma, may diverge from, or intersect with, those of recruited monocytes to drive immune-mediated pathology. However, defining the precise roles of each cell type has historically been impeded by the lack of discriminating markers and experimental systems capable of accurately identifying them. Our ability to distinguish microglia from monocytes in neuroinflammation has advanced with single-cell technologies, new markers and drugs that identify and deplete them, respectively. Nevertheless, the focus of individual studies on particular cell types, diseases or experimental approaches has limited our ability to connect phenotype and function more widely and across diverse CNS pathologies. Here, we critically review, tabulate and integrate the disease-specific functions and immune profiles of microglia and monocytes to provide a comprehensive atlas of myeloid responses in viral encephalitis, demyelination, neurodegeneration and ischemic injury. In emphasizing the differential roles of microglia and monocytes in the severe neuroinflammatory disease of viral encephalitis, we connect inflammatory pathways common to equally incapacitating diseases with less severe inflammation. We examine these findings in the context of human studies and highlight the benefits and inherent limitations of animal models that may impede or facilitate clinical translation. This enables us to highlight common and contrasting, non-redundant and often opposing roles of microglia and monocytes in disease that could be targeted therapeutically.

Comprehensive Landscape of Immune Infiltration and Aberrant Pathway Activation in Ischemic Stroke

Ischemic stroke (IS) is a multifactorial disease caused by the interaction of multiple environmental and genetic risk factors, and it is the most common cause of disability. The immune microenvironment and inflammatory response participate in the whole process of IS occurrence and development. Therefore, the rational use of relevant markers or characteristic pathways in the immune microenvironment will become one of the important therapeutic strategies for the treatment of IS. We collected peripheral blood samples from 10 patients diagnosed with IS at the First Affiliated Hospital of Gannan Medical University and First Affiliated Hospital, Jinan" University, and from 10 normal people. The GSE16561 dataset was downloaded from the Gene Expression Omnibus (GEO) database. xCell, gene set enrichment analysis (GSEA), single-sample GSEA (ssGSEA) and immune-related gene analysis were used to evaluate the differences in the immune microenvironment and characteristic pathways between the IS and control groups of the two datasets. xCell analysis showed that the IS-24h group had significantly reduced central memory CD8+ T cell, effector memory CD8+ T cell, B cell and Th1 cell scores and significantly increased M1 macrophage and macrophage scores. GSEA showed that the IS-24h group had significantly increased inflammation-related pathway activity(myeloid leukocyte activation, positive regulation of tumor necrosis factor biosynthetic process, myeloid leukocyte migration and leukocyte chemotaxis), platelet-related pathway activity(platelet activation, signaling and aggregation; protein polymerization; platelet degranulation; cell-cell contact zone) and pathology-related pathway activity (ERBB signaling pathway, positive regulation of ERK1 and ERK2 cascade, vascular endothelial growth factor receptor signaling pathway, and regulation of MAP kinase activity). Immune-related signature analysis showed that the macrophage signature, antigen presentation-related signature, cytotoxicity-related signature, B cell-related signature and inflammation-related signature were significantly lower in the IS-24h group than in the control group. In this study, we found that there were significant differences in the immune microenvironment between the peripheral blood of IS patients and control patients, as shown by the IS group having significantly reduced CD8+ Tcm, CD8+ Tem, B cell and Th1 cell scores and significantly increased macrophage and M1 macrophage scores. Additionally, inflammation-related, pathological, and platelet-related pathway activities were significantly higher in the IS group than in the control group.

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.

Single-cell RNA-seq reveals the transcriptional landscape in ischemic stroke

Ischemic stroke (IS) is a detrimental neurological disease with limited treatments options. It has been challenging to define the roles of brain cell subsets in IS onset and progression due to cellular heterogeneity in the CNS. Here, we employed single-cell RNA sequencing (scRNA-seq) to comprehensively map the cell populations in the mouse model of MCAO (middle cerebral artery occlusion). We identified 17 principal brain clusters with cell-type specific gene expression patterns as well as specific cell subpopulations and their functions in various pathways. The CNS inflammation triggered upregulation of key cell type-specific genes unpublished before. Notably, microglia displayed a cell differentiation diversity after stroke among its five distinct subtypes. Importantly, we found the potential trajectory branches of the monocytes/macrophage's subsets. Finally, we also identified distinct subclusters among brain vasculature cells, ependymal cells and other glia cells. Overall, scRNA-seq revealed the precise transcriptional changes during neuroinflammation at the single-cell level, opening up a new field for exploration of the disease mechanisms and drug discovery in stroke based on the cell-subtype specific molecules.

δ-opioid Receptor, Microglia and Neuroinflammation

Neuroinflammation underlies the pathophysiology of multiple age-related neurological disorders. Microglia, the resident immune cells of the central nervous system, are critically involved in neuroinflammatory regulation and neural survival. Modulating microglial activation is thus a promising approach to alleviate neuronal injury. Our serial studies have revealed a neuroprotective role of the δ-opioid receptor (DOR) in several acute and chronic cerebral injuries by regulating neuroinflammation and cellular oxidative stress. More recently, we found an endogenous mechanism for the inhibition of neuroinflammation is closely related to DOR's modulation of microglia. Our recent studies showed that DOR activation could strongly protect neurons from hypoxia- and lipopolysaccharide (LPS)-induced injury by inhibiting microglial pro-inflammatory transformation, while knocking-down DOR or restraining DOR activity promoted microglia activation and the relevant inflammatory events with an aggravation of cell injury. This novel finding highlights a therapeutic potential of DOR in numerous age-related neurological disorders through the modulation of neuroinflammation by targeting microglia. This review summarized the current data regarding the role of microglia in neuroinflammation, oxidative stress, and age-related neurological diseases focusing on the pharmacological effects and signaling transduction of DOR in microglia.

Cell Heterogeneity Uncovered by Single-Cell RNA Sequencing Offers Potential Therapeutic Targets for Ischemic Stroke

Ischemic stroke is a detrimental neurological disease characterized by an irreversible infarct core surrounded by an ischemic penumbra, a salvageable region of brain tissue. Unique roles of distinct brain cell subpopulations within the neurovascular unit and peripheral immune cells during ischemic stroke remain elusive due to the heterogeneity of cells in the brain. Single-cell RNA sequencing (scRNA-seq) allows for an unbiased determination of cellular heterogeneity at high-resolution and identification of cell markers, thereby unveiling the principal brain clusters within the cell-type-specific gene expression patterns as well as cell-specific subclusters and their functions in different pathways underlying ischemic stroke. In this review, we have summarized the changes in differentiation trajectories of distinct cell types and highlighted the specific pathways and genes in brain cells that are impacted by stroke. This review is expected to inspire new research and provide directions for investigating the potential pathological mechanisms and novel treatment strategies for ischemic stroke at the level of a single cell.

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

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

Microglial M2 Polarization Mediated the Neuroprotective Effect of Morroniside in Transient MCAO-Induced Mice

Morroniside, a secoiridoid glycoside from Cornus officinalis, is a class of small molecule non-peptide glucagon-like peptide-1 receptor (GLP-1R) agonists and possess many important biomedical functions. Our previous studies reported that GLP-1R agonist exenatide promoted M2 polarization and the expression of cell-specific anti-inflammatory factor interleukin-10 in neuropathological pain model. In this study, we proved that morroniside not only induced M2 polarization and stimulated interleukin-10 expression specifically in cortical primary microglia by p38β mitogen-activated protein kinases pathway but also protected nerve cells against H₂O₂-induced cell oxidative damage and prohibited ischemic injury by reducing infarct size, which is at least in part mediated by enhanced expression of microglial interleukin-10. In the cortical penumbra area in middle cerebral artery occlusion (MCAO) mice. In general, our results indicated that GLP-1R agonist morroniside might play a neuroprotective effect by inducing M2 polarization, and cyclic-AMP/protein kinase A/p38β pathway might mediate morroniside-induced expression of interleukin-10 protein in M2 microglia.

Reparative System Arising from CCR2(+) Monocyte Conversion Attenuates Neuroinflammation Following Ischemic Stroke

Monocytes recruitment from the blood to inflamed tissues following ischemic stroke is an important immune response to wound healing and tissue repair. Mouse monocytes can be endogenously divided into two distinct populations: pro-inflammatory or classical monocytes that express CCR2highCX3CR1low and circulate in blood, and anti-inflammatory or non-classical monocytes that express CCR2lowCX3CR1high and patrol locally. In this study of transgenic mice with functional CX3CR1GFP/+ or CX3CR1GFP/+-CCR2RFP/+, we found that CCR2highCX3CR1low monocytes recruited to the injured brain were cytokine-dependently converted into CCR2lowCX3CR1high macrophages, especially under the influence of IL-4 and IL-13, thereby attenuating the neuroinflammation following sterile ischemic stroke. The overall data suggest that (1) the regulation of monocyte-switching is one of the ultimate reparative strategies in ischemic stroke, and (2) the adaptation of monocytes in a locally inflamed milieu is vital to alleviating the effects of ischemic stroke through innate immunity.

Tumor-Associated Microglia and Macrophages in the Glioblastoma Microenvironment and Their Implications for Therapy

Glioblastoma is the most frequent and malignant primary brain tumor. Standard of care includes surgery followed by radiation and temozolomide chemotherapy. Despite treatment, patients have a poor prognosis with a median survival of less than 15 months. The poor prognosis is associated with an increased abundance of tumor-associated microglia and macrophages (TAMs), which are known to play a role in creating a pro-tumorigenic environment and aiding tumor progression. Most treatment strategies are directed against glioblastoma cells; however, accumulating evidence suggests targeting of TAMs as a promising therapeutic strategy. While TAMs are typically dichotomously classified as M1 and M2 phenotypes, recent studies utilizing single cell technologies have identified expression pattern differences, which is beginning to give a deeper understanding of the heterogeneous subpopulations of TAMs in glioblastomas. In this review, we evaluate the role of TAMs in the glioblastoma microenvironment and discuss how their interactions with cancer cells have an extensive impact on glioblastoma progression and treatment resistance. Finally, we summarize the effects and challenges of therapeutic strategies, which specifically aim to target TAMs.

The emerging roles of absent in melanoma 2 (AIM2) inflammasome in central nervous system disorders

As a double-stranded DNA (dsDNA) sensor, the PYHIN family member absent in melanoma 2 (AIM2) is an essential component of the inflammasome families. Activation of AIM2 by dsDNA leads to the assembly of cytosolic multimolecular complexes termed the AIM2 inflammasome, resulting in activation of caspase-1, the maturation and secretion of pro-inflammatory cytokines interleukin (IL)-1β and IL-18, and pyroptosis. Multiple central nervous system (CNS) diseases are accompanied by immune responses and inflammatory cascade. As the resident macrophage cells, microglia cells act as the first and main form of active immune defense in the CNS. AIM2 is highly expressed in microglia as well as astrocytes and neurons and is essential in neurodevelopment. In this review, we highlight the recent progress on the role of AIM2 inflammasome in CNS disorders, including cerebral stroke, brain injury, neuropsychiatric disease, neurodegenerative diseases, and glioblastoma.

Immune Cells in the BBB Disruption After Acute Ischemic Stroke: Targets for Immune Therapy?

Blood-Brain Barrier (BBB) disruption is an important pathophysiological process of acute ischemic stroke (AIS), resulting in devastating malignant brain edema and hemorrhagic transformation. The rapid activation of immune cells plays a critical role in BBB disruption after ischemic stroke. Infiltrating blood-borne immune cells (neutrophils, monocytes, and T lymphocytes) increase BBB permeability, as they cause microvascular disorder and secrete inflammation-associated molecules. In contrast, they promote BBB repair and angiogenesis in the latter phase of ischemic stroke. The profound immunological effects of cerebral immune cells (microglia, astrocytes, and pericytes) on BBB disruption have been underestimated in ischemic stroke. Post-stroke microglia and astrocytes can adopt both an M1/A1 or M2/A2 phenotype, which influence BBB integrity differently. However, whether pericytes acquire microglia phenotype and exert immunological effects on the BBB remains controversial. Thus, better understanding the inflammatory mechanism underlying BBB disruption can lead to the identification of more promising biological targets to develop treatments that minimize the onset of life-threatening complications and to improve existing treatments in patients. However, early attempts to inhibit the infiltration of circulating immune cells into the brain by blocking adhesion molecules, that were successful in experimental stroke failed in clinical trials. Therefore, new immunoregulatory therapeutic strategies for acute ischemic stroke are desperately warranted. Herein, we highlight the role of circulating and cerebral immune cells in BBB disruption and the crosstalk between them following acute ischemic stroke. Using a robust theoretical background, we discuss potential and effective immunotherapeutic targets to regulate BBB permeability after acute ischemic stroke.

Remote ischemic conditioning enhances oxygen supply to ischemic brain tissue in a mouse model of stroke: Role of elevated 2,3-biphosphoglycerate in erythrocytes

Oxygen supply for ischemic brain tissue during stroke is critical to neuroprotection. Remote ischemic conditioning (RIC) treatment is effective for stroke. However, it is not known whether RIC can improve brain tissue oxygen supply. In current study, we employed a mouse model of stroke created by middle cerebral artery occlusion (MCAO) to investigate the effect of RIC on oxygen supply to the ischemic brain tissue using a hypoxyprobe system. Erythrocyte oxygen-carrying capacity and tissue oxygen exchange were assessed by measuring oxygenated hemoglobin and oxygen dissociation curve. We found that RIC significantly mitigated hypoxic signals and decreased neural cell death, thereby preserving neurological functions. The tissue oxygen exchange was markedly enhanced, along with the elevated hemoglobin P50 and right-shifted oxygen dissociation curve. Intriguingly, RIC markedly elevated 2,3-biphosphoglycerate (2,3-BPG) levels in erythrocyte, and the erythrocyte 2,3-BPG levels were highly negatively correlated with the hypoxia in the ischemic brain tissue. Further, adoptive transfusion of 2,3-BPG-rich erythrocytes prepared from RIC-treated mice significantly enhanced the oxygen supply to the ischemic tissue in MCAO mouse model. Collectively, RIC protects against ischemic stroke through improving oxygen supply to the ischemic brain tissue where the enhanced tissue oxygen delivery and exchange by RIC-induced 2,3-BPG-rich erythrocytes may play a role.

The Potential of Induced Pluripotent Stem Cells to Treat and Model Alzheimer's Disease

An estimated 6.2 million Americans aged 65 or older are currently living with Alzheimer's disease (AD), a neurodegenerative disease that disrupts an individual's ability to function independently through the degeneration of key regions in the brain, including but not limited to the hippocampus, the prefrontal cortex, and the motor cortex. The cause of this degeneration is not known, but research has found two proteins that undergo posttranslational modifications: tau, a protein concentrated in the axons of neurons, and amyloid precursor protein (APP), a protein concentrated near the synapse. Through mechanisms that have yet to be elucidated, the accumulation of these two proteins in their abnormal aggregate forms leads to the neurodegeneration that is characteristic of AD. Until the invention of induced pluripotent stem cells (iPSCs) in 2006, the bulk of research was carried out using transgenic animal models that offered little promise in their ability to translate well from benchtop to bedside, creating a bottleneck in the development of therapeutics. However, with iPSC, patient-specific cell cultures can be utilized to create models based on human cells. These human cells have the potential to avoid issues in translatability that have plagued animal models by providing researchers with a model that closely resembles and mimics the neurons found in humans. By using human iPSC technology, researchers can create more accurate models of AD ex vivo while also focusing on regenerative medicine using iPSC in vivo. The following review focuses on the current uses of iPSC and how they have the potential to regenerate damaged neuronal tissue, in the hopes that these technologies can assist in getting through the bottleneck of AD therapeutic research.

Microglia Function on Precursor Cells in the Adult Hippocampus and Their Responsiveness to Serotonin Signaling

Microglia are the resident immune cells of the adult brain that become activated in response to pathogen- or damage-associated stimuli. The acute inflammatory response to injury, stress, or infection comprises the release of cytokines and phagocytosis of damaged cells. Accumulating evidence indicates chronic microglia-mediated inflammation in diseases of the central nervous system, most notably neurodegenerative disorders, that is associated with disease progression. To understand microglia function in pathology, knowledge of microglia communication with their surroundings during normal state and the release of neurotrophins and growth factors in order to maintain homeostasis of neural circuits is of importance. Recent evidence shows that microglia interact with serotonin, the neurotransmitter crucially involved in adult neurogenesis, and known for its role in antidepressant action. In this chapter, we illustrate how microglia contribute to neuroplasticity of the hippocampus and interact with local factors, e.g., BDNF, and external stimuli that promote neurogenesis. We summarize the recent findings on the role of various receptors in microglia-mediated neurotransmission and particularly focus on microglia’s response to serotonin signaling. We review microglia function in neuroinflammation and neurodegeneration and discuss their novel role in antidepressant mechanisms. This synopsis sheds light on microglia in healthy brain and pathology that involves serotonin and may be a potential therapeutic model by which microglia play a crucial role in the maintenance of mood.

Myeloid Arginase 1 Insufficiency Exacerbates Amyloid-β Associated Neurodegenerative Pathways and Glial Signatures in a Mouse Model of Alzheimer's Disease: A Targeted Transcriptome Analysis

Brain myeloid cells, include infiltrating macrophages and resident microglia, play an essential role in responding to and inducing neurodegenerative diseases, such as Alzheimer's disease (AD). Genome-wide association studies (GWAS) implicate many AD casual and risk genes enriched in brain myeloid cells. Coordinated arginine metabolism through arginase 1 (Arg1) is critical for brain myeloid cells to perform biological functions, whereas dysregulated arginine metabolism disrupts them. Altered arginine metabolism is proposed as a new biomarker pathway for AD. We previously reported Arg1 deficiency in myeloid biased cells using lysozyme M (LysM) promoter-driven deletion worsened amyloidosis-related neuropathology and behavioral impairment. However, it remains unclear how Arg1 deficiency in these cells impacts the whole brain to promote amyloidosis. Herein, we aim to determine how Arg1 deficiency driven by LysM restriction during amyloidosis affects fundamental neurodegenerative pathways at the transcriptome level. By applying several bioinformatic tools and analyses, we found that amyloid-β (Aβ) stimulated transcriptomic signatures in autophagy-related pathways and myeloid cells' inflammatory response. At the same time, myeloid Arg1 deficiency during amyloidosis promoted gene signatures of lipid metabolism, myelination, and migration of myeloid cells. Focusing on Aβ associated glial transcriptomic signatures, we found myeloid Arg1 deficiency up-regulated glial gene transcripts that positively correlated with Aβ plaque burden. We also observed that Aβ preferentially activated disease-associated microglial signatures to increase phagocytic response, whereas myeloid Arg1 deficiency selectively promoted homeostatic microglial signature that is non-phagocytic. These transcriptomic findings suggest a critical role for proper Arg1 function during normal and pathological challenges associated with amyloidosis. Furthermore, understanding pathways that govern Arg1 metabolism may provide new therapeutic opportunities to rebalance immune function and improve microglia/macrophage fitness.

Can quantifying morphology and TMEM119 expression distinguish between microglia and infiltrating macrophages after ischemic stroke and reperfusion in male and female mice?

Background

Ischemic stroke is an acquired brain injury with gender-dependent outcomes. A persistent obstacle in understanding the sex-specific neuroinflammatory contributions to ischemic brain injury is distinguishing between resident microglia and infiltrating macrophages—both phagocytes—and determining cell population-specific contributions to injury evolution and recovery processes. Our purpose was to identify microglial and macrophage populations regulated by ischemic stroke using morphology analysis and the presence of microglia transmembrane protein 119 (TMEM119). Second, we examined sex and menopause differences in microglia/macrophage cell populations after an ischemic stroke.

Methods

Male and female, premenopausal and postmenopausal, mice underwent either 60 min of middle cerebral artery occlusion and 24 h of reperfusion or sham surgery. The accelerated ovarian failure model was used to model postmenopause. Brain tissue was collected to quantify the infarct area and for immunohistochemistry and western blot methods. Ionized calcium-binding adapter molecule, TMEM119, and confocal microscopy were used to analyze the microglia morphology and TMEM119 area in the ipsilateral brain regions. Western blot was used to quantify protein quantity.

Results

Post-stroke injury is increased in male and postmenopause female mice vs. premenopause female mice (p < 0.05) with differences primarily occurring in the caudal sections. After stroke, the microglia underwent a region, but not sex group, dependent transformation into less ramified cells (p < 0.0001). However, the number of phagocytic microglia was increased in distal ipsilateral regions of postmenopausal mice vs. the other sex groups (p < 0.05). The number of TMEM119-positive cells was decreased in proximity to the infarct (p < 0.0001) but without a sex group effect. Two key findings prevented distinguishing microglia from systemic macrophages. First, morphological data were not congruent with TMEM119 immunofluorescence data. Cells with severely decreased TMEM119 immunofluorescence were ramified, a distinguishing microglia characteristic. Second, whereas the TMEM119 immunofluorescence area decreased in proximity to the infarcted area, the TMEM119 protein quantity was unchanged in the ipsilateral hemisphere regions using western blot methods.

Conclusions

Our findings suggest that TMEM119 is not a stable microglia marker in male and female mice in the context of ischemic stroke. Until TMEM119 function in the brain is elucidated, its use to distinguish between cell populations following brain injury with cell infiltration is cautioned.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02105-2.

Microglia as target for anti-inflammatory approaches to prevent secondary brain injury after subarachnoid hemorrhage (SAH)

Background

Microglia-driven cerebral spreading inflammation is a key contributor to secondary brain injury after SAH. Genetic depletion or deactivation of microglia has been shown to ameliorate neuronal cell death. Therefore, clinically feasible anti-inflammatory approaches counteracting microglia accumulation or activation are interesting targets for SAH treatment. Here, we tested two different methods of interference with microglia-driven cerebral inflammation in a murine SAH model: (i) inflammatory preconditioning and (ii) pharmacological deactivation.

Methods

7T-MRI-controlled SAH was induced by endovascular perforation in four groups of C57Bl/6 mice: (i) Sham-operation, (ii) SAH naïve, (iii) SAH followed by inflammatory preconditioning (LPS intraperitoneally), and (iv) SAH followed by pharmacological microglia deactivation (colony-stimulating factor-1 receptor-antagonist PLX3397 intraperitoneally). Microglia accumulation and neuronal cell death (immuno-fluorescence), as well as activation status (RT-PCR for inflammation-associated molecules from isolated microglia) were recorded at day 4 and 14. Toll-like receptor4 (TLR4) status was analyzed using FACS.

Results

Following SAH, significant cerebral spreading inflammation occurred. Microglia accumulation and pro-inflammatory gene expression were accompanied by neuronal cell death with a maximum on day 14 after SAH. Inflammatory preconditioning as well as PLX3397-treatment resulted in significantly reduced microglia accumulation and activation as well as neuronal cell death. TLR4 surface expression in preconditioned animals was diminished as a sign for receptor activation and internalization.

Conclusions

Microglia-driven cerebral spreading inflammation following SAH contributes to secondary brain injury. Two microglia-focused treatment strategies, (i) inflammatory preconditioning with LPS and (ii) pharmacological deactivation with PLX3397, led to significant reduction of neuronal cell death. Increased internalization of inflammation-driving TLR4 after preconditioning leaves less receptor molecules on the cell surface, providing a probable explanation for significantly reduced microglia activation. Our findings support microglia-focused treatment strategies to overcome secondary brain injury after SAH. Delayed inflammation onset provides a valuable clinical window of opportunity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02085-3.

New Mechanistic Insights, Novel Treatment Paradigms, and Clinical Progress in Cerebrovascular Diseases

The past decade has brought tremendous progress in diagnostic and therapeutic options for cerebrovascular diseases as exemplified by the advent of thrombectomy in ischemic stroke, benefitting a steeply increasing number of stroke patients and potentially paving the way for a renaissance of neuroprotectants. Progress in basic science has been equally impressive. Based on a deeper understanding of pathomechanisms underlying cerebrovascular diseases, new therapeutic targets have been identified and novel treatment strategies such as pre- and post-conditioning methods were developed. Moreover, translationally relevant aspects are increasingly recognized in basic science studies, which is believed to increase their predictive value and the relevance of obtained findings for clinical application.This review reports key results from some of the most remarkable and encouraging achievements in neurovascular research that have been reported at the 10th International Symposium on Neuroprotection and Neurorepair. Basic science topics discussed herein focus on aspects such as neuroinflammation, extracellular vesicles, and the role of sex and age on stroke recovery. Translational reports highlighted endovascular techniques and targeted delivery methods, neurorehabilitation, advanced functional testing approaches for experimental studies, pre-and post-conditioning approaches as well as novel imaging and treatment strategies. Beyond ischemic stroke, particular emphasis was given on activities in the fields of traumatic brain injury and cerebral hemorrhage in which promising preclinical and clinical results have been reported. Although the number of neutral outcomes in clinical trials is still remarkably high when targeting cerebrovascular diseases, we begin to evidence stepwise but continuous progress towards novel treatment options. Advances in preclinical and translational research as reported herein are believed to have formed a solid foundation for this progress.

Neuronal extracellular vesicle derived miR-98 prevents salvageable neurons from microglial phagocytosis in acute ischemic stroke

Extracellular vesicles (EVs), as a novel intercellular communication carrier transferring cargo microRNAs (miRNAs), could play important roles in the brain remodeling process after ischemic stroke. However, the detailed mechanisms involved in EVs derived miRNAs-mediated cellular interactions in the brain remain unclear. Several studies indicated that microRNA-98 (miR-98) might participate in the pathogenesis of ischemic stroke. Here, we showed that expression of miR-98 in penumbra field kept up on the first day but dropped sharply on the 3rd day after ischemic stroke in rats, indicating that miR-98 could function as an endogenous protective factor post-ischemia. Overexpression of miR-98 targeted inhibiting platelet activating factor receptor-mediated microglial phagocytosis to attenuate neuronal death. Furthermore, we showed that neurons transferred miR-98 to microglia via EVs secretion after ischemic stroke, to prevent the stress-but-viable neurons from microglial phagocytosis. Therefore, we reveal that EVs derived miR-98 act as an intercellular signal mediating neurons and microglia communication during the brain remodeling after ischemic stroke. The present work provides a novel insight into the roles of EVs in the stroke pathogenesis and a new EVs-miRNAs-based therapeutic strategy for stroke.

Deletion of muscarinic acetylcholine receptor 3 in microglia impacts brain ischemic injury

Microglia are the immune cells of the brain and become activated during any type of brain injury. In the middle cerebral artery occlusion (MCAo) model, a mouse model for ischemic stroke, we have previously shown that microglia and invaded monocytes upregulate the expression of the muscarinic acetylcholine receptor 3 (M3R) in the ischemic lesion. Here we tested whether this upregulation has an impact on the pathogenesis of MCAo. We depleted the m3R receptor in microglia, but not in circulating monocytes by giving tamoxifen to CX3CR1-CreERT^+/+M3R^flox/flox (M3RKO^mi) animals 3 weeks prior to MCAo. We found that M3RKO^mi male mice had bigger lesions, more pronounced motor deficits after one week and cognitive deficits after about one month compared to control males. The density of Iba1⁺ cells was lower in the lesions of M3RKO male mice in the early, but not in the late disease phase. In females, these differences were not significant. By giving tamoxifen 1 week prior to MCAo, we depleted m3R in microglia and in circulating monocytes (M3RKO^mi/mo). Male M3RKO^mi/mo did not differ in lesion size, but had a lower survival rate, showed motor deficits and a reduced accumulation of Iba1⁺ positive cells into the lesion site. In conclusion, our data suggest that the upregulation of m3R in microglia and monocytes in stroke has a beneficial effect on the clinical outcome in male mice.

Microglial vesicles improve post-stroke recovery by preventing immune cell senescence and favoring oligodendrogenesis

Contrasting myelin damage through the generation of new myelinating oligodendrocytes represents a promising approach to promote functional recovery after stroke. Here, we asked whether activation of microglia and monocyte-derived macrophages affects the regenerative process sustained by G protein-coupled receptor 17 (GPR17)-expressing oligodendrocyte precursor cells (OPCs), a subpopulation of OPCs specifically reacting to ischemic injury. GPR17-iCreER^T2:CAG-eGFP reporter mice were employed to trace the fate of GPR17-expressing OPCs, labeled by the green fluorescent protein (GFP), after permanent middle cerebral artery occlusion. By microglia/macrophages pharmacological depletion studies, we show that innate immune cells favor GFP⁺ OPC reaction and limit myelin damage early after injury, whereas they lose their pro-resolving capacity and acquire a dystrophic "senescent-like" phenotype at later stages. Intracerebral infusion of regenerative microglia-derived extracellular vesicles (EVs) restores protective microglia/macrophages functions, limiting their senescence during the post-stroke phase, and enhances the maturation of GFP⁺ OPCs at lesion borders, resulting in ameliorated neurological functionality. In vitro experiments show that EV-carried transmembrane tumor necrosis factor (tmTNF) mediates the pro-differentiating effects on OPCs, with future implications for regenerative therapies.

Functional consequences of a close encounter between microglia and brain-infiltrating monocytes during CNS pathology and repair

Neuroinflammation is recognized as an important factor contributing to the development and progression of several central nervous system (CNS) disorders. Upon CNS trauma or disease, parenchymal microglia highly proliferate and accumulate in and around the lesion site. In addition, blood-derived monocytes can infiltrate the inflamed CNS in response to cellular damage and/or a compromised blood-brain barrier. Both microglia and infiltrating monocytes are characterized by multiple functional states and can either display highly proinflammatory properties or promote resolution of inflammation and tissue regeneration. Despite sharing some basic immunologic functions, microglia and monocytes display many distinctive features, which ultimately define their contribution to neuropathology. Understanding how the innate immune system participates to brain disease is imperative to identify novel treatment options for CNS inflammatory disorders. In this context, existing and newly developed in vitro platforms for disease modeling are fundamental tools to investigate and modulate microglia and monocyte immune functions within a specific neuropathologic context. In this review, we first briefly summarize the current knowledge on microglia and monocyte ontogenesis, as well as their complex and interconnected contributions to the development of various CNS pathologies. Following the well-recognized concept that both microglia and monocytes can either exert neuroprotective functions or exacerbate tissue damage, we provide a comprehensive overview of cellular models currently available for in vitro study of neuroinflammatory responses. In this context, we highlight how simplified single-cell models may not always correctly recapitulate in vivo biology, hence future research should move toward novel models with higher and multicellular complexity.