Differential Phagocytic Properties of CD45low Microglia and CD45high Brain Mononuclear Phagocytes-Activation and Age-Related Effects

The Downregulation of ITGAX Exacerbates Amyloid-β Plaque Deposition in Alzheimer's Disease by Increasing Polarization of M1 Microglia

Background

Alzheimer's disease (AD) is the most common sort of neurodegenerative dementia, characterized by its challenging, diverse, and progressive nature. Despite significant progress in neuroscience, the current treatment strategies remain suboptimal.

Objective

Identifying a more accurate molecular target for the involvement of microglia in the pathogenic process of AD and exploring potential mechanisms via which it could influence disease.

Methods

We utilized single-cell RNA sequencing (scRNA-seq) analysis in conjunction with APP/PS1 mouse models to find out the molecular mechanism of AD. With the goal of investigating the cellular heterogeneity of AD, we downloaded the scRNA-seq data from the Gene Expression Omnibus (GEO) database and identified differentially expressed genes (DEGs). Additionally, we evaluated learning and memory capacity using the behavioral experiment. We also examined the expression of proteins associated with memory using western blotting. Immunofluorescence was employed to investigate alterations in amyloid plaques and microglia.

Results

Our findings revealed an upregulation of ITGAX expression in APP/PS1 transgenic mice, which coincided with a downregulation of synaptic plasticity-related proteins, an increase in amyloid-β (Aβ) plaques, and an elevation in the number of M1 microglia. Interestingly, deletion of ITGAX resulted in increased Aβ plaque deposition, a rise in the M1 microglial phenotype, and decreased production of synaptic plasticity-related proteins, all of which contributed to a decline in learning and memory.

Conclusions

This research suggested that ITGAX may have a beneficial impact on the APP/PS1 mice model, as its decreased expression could exacerbate the impairment of synaptic plasticity and worsen cognitive dysfunction.

Dynamics of CD44+ bovine nucleus pulposus cells with inflammation

Intervertebral Disc (IVD) degeneration has been associated with a chronic inflammatory response, but knowledge on the contribution of distinct IVD cells, namely CD44, to the progression of IVD degeneration remains elusive. Here, bovine nucleus pulposus (NP) CD44 cells were sorted and compared by gene expression and proteomics with the negative counterpart. NP cells were then stimulated with IL-1b (10 ng/ml) and dynamics of CD44 gene and protein expression was analyzed upon pro-inflammatory treatment. The results emphasize that CD44 has a multidimensional functional role in IVD metabolism, ECM synthesis and production of neuropermissive factors. CD44 widespread expression in NP was partially associated with CD14 and CD45, resulting in the identification of distinct cell subsets. In conclusion, this study points out CD44 and CD44-based cell subsets as relevant targets in the modulation of the IVD pro-inflammatory/degenerative cascade.

Two-Month Voluntary Ethanol Consumption Promotes Mild Neuroinflammation in the Cerebellum but Not in the Prefrontal Cortex, Hippocampus, or Striatum of Mice

Chronic ethanol exposure often triggers neuroinflammation in the brain's reward system, potentially promoting the drive for ethanol consumption. A main marker of neuroinflammation is the microglia-derived monocyte chemoattractant protein 1 (MCP1) in animal models of alcohol use disorder in which ethanol is forcefully given. However, there are conflicting findings on whether MCP1 is elevated when ethanol is taken voluntarily, which challenges its key role in promoting motivation for ethanol consumption. Here, we studied MCP1 mRNA levels in areas implicated in consumption motivation-specifically, the prefrontal cortex, hippocampus, and striatum-as well as in the cerebellum, a brain area highly sensitive to ethanol, of C57BL/6 mice subjected to intermittent and voluntary ethanol consumption for two months. We found a significant increase in MCP1 mRNA levels in the cerebellum of mice that consumed ethanol compared to controls, whereas no significant changes were observed in the prefrontal cortex, hippocampus, or striatum or in microglia isolated from the hippocampus and striatum. To further characterize cerebellar neuroinflammation, we measured the expression changes in other proinflammatory markers and chemokines, revealing a significant increase in the proinflammatory microRNA miR-155. Notably, other classical proinflammatory markers, such as TNFα, IL6, and IL-1β, remained unaltered, suggesting mild neuroinflammation. These results suggest that the onset of neuroinflammation in motivation-related areas is not required for high voluntary consumption in C57BL/6 mice. In addition, cerebellar susceptibility to neuroinflammation may be a trigger to the cerebellar degeneration that occurs after chronic ethanol consumption in humans.

Established and emerging techniques for the study of microglia: visualization, depletion, and fate mapping

The central nervous system (CNS) is an essential hub for neuronal communication. As a major component of the CNS, glial cells are vital in the maintenance and regulation of neuronal network dynamics. Research on microglia, the resident innate immune cells of the CNS, has advanced considerably in recent years, and our understanding of their diverse functions continues to grow. Microglia play critical roles in the formation and regulation of neuronal synapses, myelination, responses to injury, neurogenesis, inflammation, and many other physiological processes. In parallel with advances in microglial biology, cutting-edge techniques for the characterization of microglial properties have emerged with increasing depth and precision. Labeling tools and reporter models are important for the study of microglial morphology, ultrastructure, and dynamics, but also for microglial isolation, which is required to glean key phenotypic information through single-cell transcriptomics and other emerging approaches. Strategies for selective microglial depletion and modulation can provide novel insights into microglia-targeted treatment strategies in models of neuropsychiatric and neurodegenerative conditions, cancer, and autoimmunity. Finally, fate mapping has emerged as an important tool to answer fundamental questions about microglial biology, including their origin, migration, and proliferation throughout the lifetime of an organism. This review aims to provide a comprehensive discussion of these established and emerging techniques, with applications to the study of microglia in development, homeostasis, and CNS pathologies.

Immune response of BV-2 microglial cells is impacted by peroxisomal beta-oxidation

Microglia are crucial for brain homeostasis, and dysfunction of these cells is a key driver in most neurodegenerative diseases, including peroxisomal leukodystrophies. In X-linked adrenoleukodystrophy (X-ALD), a neuroinflammatory disorder, very long-chain fatty acid (VLCFA) accumulation due to impaired degradation within peroxisomes results in microglial defects, but the underlying mechanisms remain unclear. Using CRISPR/Cas9 gene editing of key genes in peroxisomal VLCFA breakdown (Abcd1, Abcd2, and Acox1), we recently established easily accessible microglial BV-2 cell models to study the impact of dysfunctional peroxisomal β-oxidation and revealed a disease-associated microglial-like signature in these cell lines. Transcriptomic analysis suggested consequences on the immune response. To clarify how impaired lipid degradation impacts the immune function of microglia, we here used RNA-sequencing and functional assays related to the immune response to compare wild-type and mutant BV-2 cell lines under basal conditions and upon pro-inflammatory lipopolysaccharide (LPS) activation. A majority of genes encoding proinflammatory cytokines, as well as genes involved in phagocytosis, antigen presentation, and co-stimulation of T lymphocytes, were found differentially overexpressed. The transcriptomic alterations were reflected by altered phagocytic capacity, inflammasome activation, increased release of inflammatory cytokines, including TNF, and upregulated response of T lymphocytes primed by mutant BV-2 cells presenting peptides. Together, the present study shows that peroxisomal β-oxidation defects resulting in lipid alterations, including VLCFA accumulation, directly reprogram the main cellular functions of microglia. The elucidation of this link between lipid metabolism and the immune response of microglia will help to better understand the pathogenesis of peroxisomal leukodystrophies.

Specificity and efficiency of tamoxifen-mediated Cre induction is equivalent regardless of age

Temporally controlling Cre recombination through tamoxifen (Tam) induction has many advantages for biomedical research. Most studies report early post-natal/juvenile (<2 m.o.) Tam induction, but age-related neurodegeneration and aging studies can require Cre induction in older mice (>12 m.o.). While anecdotally reported as problematic, there are no published comparisons of Tam-mediated Cre induction at early and late ages. Here, microglial-specific Cx3cr1creERT2 mice were crossed to a floxed NuTRAP reporter to compare Cre induction at early (3-6 m.o.) and late (20 m.o.) ages. Specificity and efficiency of microglial labeling at 21-22 m.o. were identical in mice induced with Tam at early and late ages. Age-related microglial translatomic changes were also similar regardless of Tam induction age. Each Cre and flox mouse line should be independently validated, however, these findings demonstrate that Tam-mediated Cre induction can be performed even into older mouse ages and should be generalizable to other inducible Cre models.

Neuroinflammation-Modulating Agent SB1617 Enhances LC3-Associated Phagocytosis to Mitigate Tau Pathology

Tau protein aggregation and propagation in neurons and surrounding microglia are well-known risk factors for neurodegenerative diseases. Therefore, emerging therapeutic strategies that target neuroinflammatory activity in microglia have the potential to prevent tauopathy. Here, we explored the microglia-mediated neuroprotective function of SB1617 against tau aggregation. Our study revealed that SB1617-inactivated pathogenic M1-like microglia, reduced the secretion of pro-inflammatory cytokines via translational regulation, and induced microglial polarization toward the M2 phenotype and phagocytic function. Furthermore, we observed that extracellular pathogenic tau aggregates were eliminated via LC3-associated phagocytosis. The in vivo efficacy of SB1617 was confirmed in mice with traumatic brain injury in which SB1617 exerted neuroprotective effects by reducing pathogenic tau levels through microglia-mediated anti-inflammatory activity. Our results indicated that SB1617-mediated microglial surveillance with LC3-associated phagocytosis is a critical molecular mechanism in the regulation of tau proteostasis. This study provides new insights into tauopathies and directions for developing novel therapies for neurodegenerative diseases.

Prph2 knock-in mice recapitulate human central areolar choroidal dystrophy retinal degeneration and exhibit aberrant synaptic remodeling and microglial activation

Central areolar choroidal dystrophy is an inherited disorder characterized by progressive choriocapillaris atrophy and retinal degeneration and is usually associated with mutations in the PRPH2 gene. We aimed to generate and characterize a mouse model with the p.Arg195Leu mutation previously described in patients. Heterozygous (Prph2WT/KI) and homozygous (Prph2KI/KI) mice were generated using the CRISPR/Cas9 system to introduce the p.Arg195Leu mutation. Retinal function was assessed by electroretinography and optomotor tests at 1, 3, 6, 9, 12, and 20 months of age. The structural integrity of the retinas was evaluated at the same ages using optical coherence tomography. Immunofluorescence and transmission electron microscopy images of the retina were also analyzed. Genetic sequencing confirmed that both Prph2WT/KI and Prph2KI/KI mice presented the p.Arg195Leu mutation. A progressive loss of retinal function was found in both mutant groups, with significantly reduced visual acuity from 3 months of age in Prph2KI/KI mice and from 6 months of age in Prph2WT/KI mice. Decreased amplitudes in the electroretinography responses were observed from 1 month of age in Prph2KI/KI mice and from 6 months of age in Prph2WT/KI mice. Morphological analysis of the retinas correlated with functional findings, showing a progressive decrease in retinal thickness of mutant mice, with earlier and more severe changes in the homozygous mutant mice. We corroborated the alteration of the outer segment structure, and we found changes in the synaptic connectivity in the outer plexiform layer as well as gliosis and signs of microglial activation. The new Prph2WT/KI and Prph2KI/KI murine models show a pattern of retinal degeneration similar to that described in human patients with central areolar choroidal dystrophy and appear to be good models to study the mechanisms involved in the onset and progression of the disease, as well as to test the efficacy of new therapeutic strategies.

Germinal matrix hemorrhage induces immune responses, brain injury, and motor impairment in neonatal rats

Germinal matrix hemorrhage (GMH) is a major complication of prematurity that causes secondary brain injury and is associated with long-term neurological disabilities. This study used a postnatal day 5 rat model of GMH to explore immune response, brain injury, and neurobehavioral changes after hemorrhagic injury. The results showed that CD45high/CD11b+ immune cells increased in the brain after GMH and were accompanied by increased macrophage-related chemokine/cytokines and inflammatory mediators. Hematoma formed as early as 2 h after injection of collagenase VII and white matter injury appeared not only in the external capsule and hippocampus, but also in the thalamus. In addition, GMH caused abnormal motor function as revealed by gait analysis, and locomotor hyperactivity in the elevated plus maze, though no other obvious anxiety or recognition/memory function changes were noted when examined by the open field test and novel object recognition test. The animal model used here partially reproduces the GMH-induced brain injury and motor dysfunction seen in human neonates and therefore can be used as a valid tool in experimental studies for the development of effective therapeutic strategies for GMH-induced brain injury.

Alcohol and the Brain-Gut Axis: The Involvement of Microglia and Enteric Glia in the Process of Neuro-Enteric Inflammation

Binge or chronic alcohol consumption causes neuroinflammation and leads to alcohol use disorder (AUD). AUD not only affects the central nervous system (CNS) but also leads to pathologies in the peripheral and enteric nervous systems (ENS). Thus, understanding the mechanism of the immune signaling to target the effector molecules in the signaling pathway is necessary to alleviate AUD. Growing evidence shows that excessive alcohol consumption can activate neuroimmune cells, including microglia, and change the status of neurotransmitters, affecting the neuroimmune system. Microglia, like peripheral macrophages, are an integral part of the immune defense and represent the reticuloendothelial system in the CNS. Microglia constantly survey the CNS to scavenge the neuronal debris. These cells also protect parenchymal cells in the brain and spinal cord by repairing nerve circuits to keep the nervous system healthy against infectious and stress-derived agents. In an activated state, they become highly dynamic and mobile and can modulate the levels of neurotransmitters in the CNS. In several ways, microglia, enteric glial cells, and macrophages are similar in terms of causing inflammation. Microglia also express most of the receptors that are constitutively present in macrophages. Several receptors on microglia respond to the inflammatory signals that arise from danger-associated molecular patterns (DAMPs), pathogen-associated molecular patterns (PAMPs), endotoxins (e.g., lipopolysaccharides), and stress-causing molecules (e.g., alcohol). Therefore, this review article presents the latest findings, describing the roles of microglia and enteric glial cells in the brain and gut, respectively, and their association with neurotransmitters, neurotrophic factors, and receptors under the influence of binge and chronic alcohol use, and AUD.

Consistent specificity and efficiency of tamoxifen-mediated cre induction across ages

Temporally controlling cre recombination through tamoxifen (Tam) induction has many advantages for biomedical research. Most studies report Tam induction at early post-natal/juvenile (<2 m.o.) mouse ages, but age-related neurodegeneration and aging studies can require cre induction in older mice (>12 m.o.). While anecdotally reported as problematic, there are no published comparisons of Tam mediated cre induction at early and late ages. Here, microglial-specific Cx3cr1 creERT 2 mice were crossed to a floxed NuTRAP reporter to compare cre induction at early (3-6 m.o.) and late (20 m.o.) ages. Specificity and efficiency of microglial labeling at 21-22 m.o. were identical in mice induced with Tam at 3-6 m.o. or 20 m.o. of age. Age-related microglial translatomic changes were also similar regardless of Tam induction age. Each cre and flox mouse line should be validated independently, however, these findings demonstrate that Tam-mediated cre induction can be performed even into older mouse ages.

Interrogation of human microglial phagocytosis by CRISPR genome editing

Background

Microglia are an integral part of central nervous system, but our understanding of microglial biology is limited due to the challenges in obtaining and culturing primary human microglia. HMC3 is an important cell line for studying human microglia because it is readily accessible and straightforward to maintain in standard laboratories. Although HMC3 is widely used for microglial research, a robust genetic method has not been described. Here, we report a CRISPR genome editing platform, by the electroporation of Cas9 ribonucleoproteins (Cas9 RNP) and synthetic DNA repair templates, to enable rapid and precise genetic modifications of HMC3. For proof-of-concept demonstrations, we targeted the genes implicated in the regulation of amyloid beta (Aβ) and glioblastoma phagocytosis in microglia. We showed that CRISPR genome editing could enhance the phagocytic activities of HMC3.

Methods

We performed CRISPR gene knockout (KO) in HMC3 by the electroporation of pre-assembled Cas9 RNP. Co-introduction of DNA repair templates allowed site-specific knock-in (KI) of an epitope tag, a synthetic promoter and a fluorescent reporter gene. The editing efficiencies were determined genotypically by DNA sequencing and phenotypically by immunofluorescent staining and flow cytometry. The gene-edited HMC3 cells were examined in vitro by fluorescent Aβ and glioblastoma phagocytosis assays.

Results

Our platform enabled robust single (>90%) and double (>70%) KO without detectable off-target editing by high throughput DNA sequencing. We also inserted a synthetic SFFV promoter to efficiently upregulate the expression of endogenous CD14 and TREM2 genes associated with microglial phagocytosis. The CRISPR-edited HMC3 showed stable phenotypes and enhanced phagocytosis of fluorescence-labeled Aβ1-42 peptides. Confocal microscopy further confirmed the localization of Aβ_1-42 aggregates in the acidified lysosomes. HMC3 mutants also changed the phagocytic characteristic toward apoptotic glioblastoma cells.

Conclusion

CRISPR genome editing by Cas9 RNP electroporation is a robust approach to genetically modify HMC3 for functional studies such as the interrogation of Aβ and tumor phagocytosis, and is readily adoptable to investigate other aspects of microglial biology.

The role of signaling crosstalk of microglia in hippocampus on progression of ageing and Alzheimer's disease

Based on single-cell sequencing of the hippocampi of 5× familiar Alzheimer's disease (5× FAD) and wild type mice at 2-, 12-, and 24-month of age, we found an increased percentage of microglia in aging and Alzheimer's disease (AD) mice. Blood brain barrier injury may also have contributed to this increase. Immune regulation by microglia plays a major role in the progression of aging and AD, according to the functions of 41 intersecting differentially expressed genes in microglia. Signaling crosstalk between C-C motif chemokine ligand (CCL) and major histocompatibility complex-1 bridges intercellular communication in the hippocampus during aging and AD. The amyloid precursor protein (APP) and colony stimulating factor (CSF) signals drive 5× FAD to deviate from aging track to AD occurrence among intercellular communication in hippocampus. Microglia are involved in the progression of aging and AD can be divided into 10 functional types. The strength of the interaction among microglial subtypes weakened with aging, and the CCL and CSF signaling pathways were the fundamental bridge of communication among microglial subtypes.

Advances in proteomic phenotyping of microglia in neurodegeneration

Microglia are dynamic resident immune cells of the central nervous system (CNS) that sense, survey, and respond to changes in their environment. In disease states, microglia transform from homeostatic to diverse molecular phenotypic states that play complex and causal roles in neurologic disease pathogenesis, as evidenced by the identification of microglial genes as genetic risk factors for neurodegenerative disease. While advances in transcriptomic profiling of microglia from the CNS of humans and animal models have provided transformative insights, the transcriptome is only modestly reflective of the proteome. Proteomic profiling of microglia is therefore more likely to provide functionally and therapeutically relevant targets. In this review, we discuss molecular insights gained from transcriptomic studies of microglia in the context of Alzheimer's disease as a prototypic neurodegenerative disease, and highlight existing and emerging approaches for proteomic profiling of microglia derived from in vivo model systems and human brain.

Ceftriaxone ameliorates hippocampal synapse loss by inhibiting microglial/macrophages activation in glial glutamate transporter-1 dependent manner in the APP/PS1 mouse model of Alzheimer's disease

Synapse loss is a major contributor to cognitive dysfunction in Alzheimer's disease (AD). Impairments in the expression and/or glutamate uptake activity of glia glutamate transporter-1 (GLT-1) contribute to synapse loss in AD. Hence, targeting the restoration of GLT-1 activity may have potential for alleviating synapse loss in AD. Ceftriaxone (Cef) can upregulate the expression and glutamate uptake activity of GLT-1 in many disease models, including those for AD. The present study investigated the effects of Cef on synapse loss and the role of GLT-1 using APP/PS1 transgenic and GLT-1 knockdown APP/PS1 AD mice. Furthermore, the involvement of microglia in the process was investigated due to its important role in synapse loss in AD. We found that Cef treatment significantly ameliorated synapse loss and dendritic degeneration in APP/PS1 AD mice, evidenced by an increased dendritic spine density, decreased dendritic beading density, and upregulated levels of postsynaptic density protein 95 (PSD95) and synaptophysin. The effects of Cef were suppressed by GLT-1 knockdown in GLT-1^+/-/APP/PS1 AD mice. Simultaneously, Cef treatment inhibited ionized calcium binding adapter molecule 1 (Iba1) expression, decreased the proportion of CD11b⁺CD45^hi cells, declined interleukin-6 (IL-6) content, and reduced the co-expression of Iba1 with PSD95 or synaptophysin in APP/PS1 AD mice. In conclusion, Cef treatment ameliorated synapse loss and dendritic degeneration in APP/PS1 AD mice in a GLT-1-dependent manner, and the inhibitory effect of Cef on the activation of microglia/macrophages and their phagocytosis for synaptic elements contributed to the mechanism.

Cellular Proteomic Profiling Using Proximity Labeling by TurboID-NES in Microglial and Neuronal Cell Lines

Different brain cell types play distinct roles in brain development and disease. Molecular characterization of cell-specific mechanisms using cell type-specific approaches at the protein (proteomic) level can provide biological and therapeutic insights. To overcome the barriers of conventional isolation-based methods for cell type-specific proteomics, in vivo proteomic labeling with proximity-dependent biotinylation of cytosolic proteins using biotin ligase TurboID, coupled with mass spectrometry (MS) of labeled proteins, emerged as a powerful strategy for cell type-specific proteomics in the native state of cells without the need for cellular isolation. To complement in vivo proximity labeling approaches, in vitro studies are needed to ensure that cellular proteomes using the TurboID approach are representative of the whole-cell proteome and capture cellular responses to stimuli without disruption of cellular processes. To address this, we generated murine neuroblastoma (N2A) and microglial (BV2) lines stably expressing cytosolic TurboID to biotinylate the cellular proteome for downstream purification and analysis using MS. TurboID-mediated biotinylation captured 59% of BV2 and 65% of N2A proteomes under homeostatic conditions. TurboID labeled endolysosome, translation, vesicle, and signaling proteins in BV2 microglia and synaptic, neuron projection, and microtubule proteins in N2A neurons. TurboID expression and biotinylation minimally impacted homeostatic cellular proteomes of BV2 and N2A cells and did not affect lipopolysaccharide-mediated cytokine production or resting cellular respiration in BV2 cells. MS analysis of the microglial biotin-labeled proteins captured the impact of lipopolysaccharide treatment (>500 differentially abundant proteins) including increased canonical proinflammatory proteins (Il1a, Irg1, and Oasl1) and decreased anti-inflammatory proteins (Arg1 and Mgl2).

Recent Progress in Multiple Sclerosis Treatment Using Immune Cells as Targets

Multiple sclerosis (MS) is an autoimmune-mediated demyelinating disease of the central nervous system. The main pathological features are inflammatory reaction, demyelination, axonal disintegration, reactive gliosis, etc. The etiology and pathogenesis of the disease have not been clarified. The initial studies believed that T cell-mediated cellular immunity is the key to the pathogenesis of MS. In recent years, more and more evidence has shown that B cells and their mediated humoral immune and innate immune cells (such as microglia, dendritic cells, macrophages, etc.) also play an important role in the pathogenesis of MS. This article mainly reviews the research progress of MS by targeting different immune cells and analyzes the action pathways of drugs. The types and mechanisms of immune cells related to the pathogenesis are introduced in detail, and the mechanisms of drugs targeting different immune cells are discussed in depth. This article aims to clarify the pathogenesis and immunotherapy pathway of MS, hoping to find new targets and strategies for the development of therapeutic drugs for MS.

Mitochondrial control of microglial phagocytosis by the translocator protein and hexokinase 2 in Alzheimer's disease

Microglial phagocytosis is an energetically demanding process that plays a critical role in the removal of toxic protein aggregates in Alzheimer's disease (AD). Recent evidence indicates that a switch in energy production from mitochondrial respiration to glycolysis disrupts this important protective microglial function and may provide therapeutic targets for AD. Here, we demonstrate that the translocator protein (TSPO) and a member of its mitochondrial complex, hexokinase-2 (HK), play critical roles in microglial respiratory-glycolytic metabolism and phagocytosis. Pharmacological and genetic loss-of-function experiments showed that TSPO is critical for microglial respiratory metabolism and energy supply for phagocytosis, and its expression is enriched in phagocytic microglia of AD mice. Meanwhile, HK controlled glycolytic metabolism and phagocytosis via mitochondrial binding or displacement. In cultured microglia, TSPO deletion impaired mitochondrial respiration and increased mitochondrial recruitment of HK, inducing a switch to glycolysis and reducing phagocytosis. To determine the functional significance of mitochondrial HK recruitment, we developed an optogenetic tool for reversible control of HK localization. Displacement of mitochondrial HK inhibited glycolysis and improved phagocytosis in TSPO-knockout microglia. Mitochondrial HK recruitment also coordinated the inflammatory switch to glycolysis that occurs in response to lipopolysaccharide in normal microglia. Interestingly, cytosolic HK increased phagocytosis independent of its metabolic activity, indicating an immune signaling function. Alzheimer's beta amyloid drastically stimulated mitochondrial HK recruitment in cultured microglia, which may contribute to microglial dysfunction in AD. Thus, targeting mitochondrial HK may offer an immunotherapeutic approach to promote phagocytic microglial function in AD.

The choroid plexus links innate immunity to CSF dysregulation in hydrocephalus

The choroid plexus (ChP) is the blood-cerebrospinal fluid (CSF) barrier and the primary source of CSF. Acquired hydrocephalus, caused by brain infection or hemorrhage, lacks drug treatments due to obscure pathobiology. Our integrated, multi-omic investigation of post-infectious hydrocephalus (PIH) and post-hemorrhagic hydrocephalus (PHH) models revealed that lipopolysaccharide and blood breakdown products trigger highly similar TLR4-dependent immune responses at the ChP-CSF interface. The resulting CSF "cytokine storm", elicited from peripherally derived and border-associated ChP macrophages, causes increased CSF production from ChP epithelial cells via phospho-activation of the TNF-receptor-associated kinase SPAK, which serves as a regulatory scaffold of a multi-ion transporter protein complex. Genetic or pharmacological immunomodulation prevents PIH and PHH by antagonizing SPAK-dependent CSF hypersecretion. These results reveal the ChP as a dynamic, cellularly heterogeneous tissue with highly regulated immune-secretory capacity, expand our understanding of ChP immune-epithelial cell cross talk, and reframe PIH and PHH as related neuroimmune disorders vulnerable to small molecule pharmacotherapy.

Myeloid cell subpopulations compensate each other for Ccr2-deficiency in glioblastoma

AIMS

Glioblastomas are high-grade brain tumours that are characterised by the accumulation of brain-resident microglia and peripheral macrophages. Recruitment of these myeloid cells can be facilitated by CCR2/CCL2 signalling. Besides the well-known CCR2⁺ macrophages, we have identified microglia expressing CCR2 in glioma tissues. Thus, we investigated how Ccr2-deficiency of one of the myeloid cell populations affects the other population and tumour biology.

METHODS

We generated four chimeric groups to analyse single and combined Ccr2-deficiency of microglia and macrophages. On day 21 after tumour cell implantation (GL261), we conducted flow cytometry, immunofluorescence and real-time polymerase chain reaction analyses. Tumour volume and metabolism were determined by magnetic resonance imaging and magnetic resonance spectroscopy. Moreover, in vitro studies were performed with primary microglia and bone marrow-derived macrophages.

RESULTS

We demonstrated reduced infiltration of macrophages and microglia depending on the lack of Ccr2. However, the total number of myeloid cells remained constant except for the animals with dual Ccr2-knockout. Both microglia and macrophages with Ccr2-deficiency showed impaired expression of proinflammatory molecules and altered phagocytic activity. Despite the altered immunologic phenotype caused by Ccr2-deficiency, glioma progression and metabolism were hardly affected. Alterations were detected solely in apoptosis and proliferation of tumours from animals with specific Ccr2-deficient microglia, whereas vessel stability was increased in mice with Ccr2-knockout in both cell populations.

CONCLUSION

These results indicate that microglia and macrophages provide a homoeostatic balance within glioma tissue and compensate for the lack of the corresponding counterpart. Moreover, we identified that the CCR2/CCL2 axis is involved in the immunologic function of microglia and macrophages beyond its relevance for migration.

Alterations in immune cell heterogeneities in the brain of aged zebrafish using single-cell resolution

Immunocytes, including the microglia, are crucial in the neurodegenerative process in old people. However, the understanding of regarding microglia heterogeneity and other involved immunocytes remains elusive. We analyzed 26,456 immunocytes from 12-and 26-month-old zebrafish brains at single-cell resolution. Microglia and T lymphocytes were detected in the brain at both time points. Two types of microglia were annotated, namely, ac⁺ microglia and xr⁺ microglia, which were clustered into subsets 1, 2, 3, 4, 5, and subsets 6, 7, 8, 9, respectively. Diversified microglia predominated the adult brains and cooperated with T cells to perform the functions of immune response and neuronal nutrition. We validated the specific microglia markers. The novel transgenic lines, Tg(lgals3bpb:eGFP) and Tg(apoc1:eGFP), were created, which faithfully labeled ac⁺ microglia and served as valuable labeling tools. However, the microglia population reduced while T cells of six subtypes intriguingly increased to serve as the primary immune cells in aged brains. Unlike in 12-month-old brains, T cells, together with microglia, exhibited a coordinated signature of inflammation in the 26-month-old brains. Our findings revealed the immunocytes atlas in aged zebrafish brains. It implied the involvement of microglia and T cells in the progression of neurodegeneration in aging.

Exploring molecular signatures related to the mechanism of aging in different brain regions by integrated bioinformatics

The mechanism of brain aging is not fully understood. Few studies have attempted to identify molecular changes using bioinformatics at the subregional level in the aging brain. This study aimed to identify the molecular signatures and key genes involved in aging, depending on the brain region. Differentially expressed genes (DEGs) associated with aging of the cerebral cortex (CX), hippocampus (HC), and cerebellum (CB) were identified based on five datasets from the Gene Expression Omnibus (GEO). The molecular signatures of aging were explored using functional and pathway analyses. Hub genes of each brain region were determined by protein-protein interaction network analysis, and commonly expressed DEGs (co-DEGs) were also found. Gene-microRNAs (miRNAs) and gene-disease interactions were constructed using online databases. The expression levels and regional specificity of the hub genes and co-DEGs were validated using animal experiments. In total, 32, 293, and 141 DEGs were identified in aging CX, HC, and CB, respectively. Enrichment analysis indicated molecular changes related to leukocyte invasion, abnormal neurotransmission, and impaired neurogenesis due to inflammation as the major signatures of the CX, HC, and CB. Itgax is a hub gene of cortical aging. Zfp51 and Zfp62 were identified as hub genes involved in hippocampal aging. Itgax and Cxcl10 were identified as hub genes involved in cerebellar aging. S100a8 was the only co-DEG in all three regions. In addition, a series of molecular changes associated with inflammation was observed in all three brain regions. Several miRNAs interact with hub genes and S100a8. The change in gene levels was further validated in an animal experiment. Only the upregulation of Zfp51 and Zfp62 was restricted to the HC. The molecular signatures of aging exhibit regional differences in the brain and seem to be closely related to neuroinflammation. Itgax, Zfp51, Zfp62, Cxcl10, and S100a8 may be key genes and potential targets for the prevention of brain aging.

The Structure, Function and Regulation of Protein Tyrosine Phosphatase Receptor Type J and Its Role in Diseases

Protein tyrosine phosphatase receptor type J (PTPRJ), also known as DEP-1, HPTPη, or CD148, belongs to the R3 subfamily of receptor protein tyrosine phosphatases (RPTPs). It was first identified as an antioncogene due to its protein level being significantly downregulated in most epithelial tumors and cancer cell lines (e.g., colon, lung, thyroid, breast, and pancreas). PTPRJ regulates mouse optic nerve projection by inhibiting the phosphorylation of the erythropoietin-producing hepatocellular carcinoma (Eph) receptor and abelson murine leukemia viral oncogene homolog 1 (c-Abl). PTPRJ is crucial for metabolism. Recent studies have demonstrated that PTPRJ dephosphorylates JAK2 at positions Y813 and Y868 to inhibit leptin signaling. Akt is more phosphorylated at the Ser473 and Thr308 sites in Ptprj^-/- mice, suggesting that PTPRJ may be a novel negative regulator of insulin signaling. PTPRJ also plays an important role in balancing the pro- and anti-osteoclastogenic activity of the M-CSF receptor (M-CSFR), and in maintaining NFATc1 expression during the late stages of osteoclastogenesis to promote bone-resorbing osteoclast (OCL) maturation. Furthermore, multiple receptor tyrosine kinases (RTKs) as substrates of PTPRJ are probably a potential therapeutic target for many types of diseases, such as cancer, neurodegenerative diseases, and metabolic diseases, by inhibiting their phosphorylation activity. In light of the important roles that PTPRJ plays in many diseases, this review summarizes the structural features of the protein, its expression pattern, and the physiological and pathological functions of PTPRJ, to provide new ideas for treating PTPRJ as a potential therapeutic target for related metabolic diseases and cancer.

Prophylactic effects of cucurbitacin B in the EAE Model of multiple sclerosis by adjustment of STAT3/IL-23/IL-17 axis and improvement of neuropsychological symptoms

Multiple sclerosis (MS) is an autoimmune disease that affects the central nervous system. Although remarkable progress has been made in treating MS, current therapies are less effective in protecting against the progression of the disease. Since cucurbitacins have shown an extreme range of pharmacological properties, in this study, we aimed to investigate the prophylactic effect of cucurbitacin B (CuB) in the experimental MS model. Experimental autoimmune encephalomyelitis (EAE) induced by subcutaneous immunization of MOG35-55 in C57BL/6 mice. CuB interventions (0.5 and 1 mg/kg, i.p.) were performed every other day from the first day of EAE induction. Assessment of clinical scores and motor function, inflammatory responses, and microglial activation were assessed by qRT-PCR, western blotting, and immunohistochemical (IHC) analyses. CuB (1 mg/kg) significantly decreased the population of CD45⁺ (P < 0.01), CD11b⁺ (P < 0.01) and CD45⁺/CD11b⁺ (P < 0.05) cells in cortical lesions of EAE mice. In addition, activation of STAT3 (P < 0.001), expression of IL-17 A and IL-23 A (both mRNA and protein), and transcription of Iba-1 significantly decreased. On the contrary, CuB (1 mg/kg) significantly increased the transcription of MBP and Olig-2. Furthermore, a significant decrease in the severity of EAE (P < 0.05), and an improvement in motor function (P < 0.05) and coordination (P < 0.05) were observed after treatment with a high dose of CuB. Our results suggest that CuB may have a wide-ranging effect on autoimmune responses in MS via a reduction in STAT3 activation, microgliosis, and adaptation of the IL-23/IL-17 axis. Further studies are needed to investigate the exact effect of CuB in glial cells and its efficiency and bioavailability in other neuroinflammatory diseases.

New models of Parkinson's like neuroinflammation in human microglia clone 3: Activation profiles induced by INF-γ plus high glucose and mitochondrial inhibitors

Microglia activation and neuroinflammation have been extensively studied in murine models of neurodegenerative diseases; however, to overcome the genetic differences between species, a human cell model of microglia able to recapitulate the activation profiles described in patients is needed. Here we developed human models of Parkinson's like neuroinflammation by using the human microglia clone 3 (HMC3) cells, whose activation profile in response to classic inflammatory stimuli has been controversial and reported only at mRNA levels so far. In fact, we showed the increased expression of the pro-inflammatory markers iNOS, Caspase 1, IL-1β, in response to IFN-γ plus high glucose, a non-specific disease stimulus that emphasized the dynamic polarization and heterogenicity of the microglial population. More specifically, we demonstrated the polarization of HMC3 cells through the upregulation of iNOS expression and nitrite production in response to the Parkinson's like stimuli, 6-hydroxidopamine (6-OHDA) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the latter depending on the NF-κB pathway. Furthermore, we identified inflammatory mediators that promote the pro-inflammatory activation of human microglia as function of different pathways that can simulate the phenotypic transition according to the stage of the pathology. In conclusion, we established and characterized different systems of HMC3 cells activation as in vitro models of Parkinson's like neuroinflammation.

Naturally-aged microglia exhibit phagocytic dysfunction accompanied by gene expression changes reflective of underlying neurologic disease

Age-associated microglial dysfunction contributes to the accumulation of amyloid-β (Aβ) plaques in Alzheimer's disease. Although several studies have shown age-related declines in the phagocytic capacity of myeloid cells, relatively few have examined phagocytosis of normally aged microglia. Furthermore, much of the existing data on aging microglial function have been generated in accelerated genetic models of Alzheimer's disease. Here we found that naturally aged microglia phagocytosed less Aβ over time. To gain a better understanding of such dysfunction, we assessed differences in gene expression between young and old microglia that either did or did not phagocytose Aβ. Young microglia had both phagocytic and neuronal maintenance signatures indicative of normal microglial responses, whereas, old microglia, regardless of phagocytic status, exhibit signs of broad dysfunction reflective of underlying neurologic disease states. We also found downregulation of many phagocytic receptors on old microglia, including TREM2, an Aβ phagocytic receptor. TREM2 protein expression was diminished in old microglia and loss of TREM2+ microglia was correlated with impaired Aβ uptake, suggesting a mechanism for phagocytic dysfunction in old microglia. Combined, our work reveals that normally aged microglia have broad changes in gene expression, including defects in Aβ phagocytosis that likely underlies the progression to neurologic disease.

Assaying Microglia Functions In Vitro

Microglia, the main immune modulators of the central nervous system, have key roles in both the developing and adult brain. These functions include shaping healthy neuronal networks, carrying out immune surveillance, mediating inflammatory responses, and disposing of unwanted material. A wide variety of pathological conditions present with microglia dysregulation, highlighting the importance of these cells in both normal brain function and disease. Studies into microglial function in the context of both health and disease thus have the potential to provide tremendous insight across a broad range of research areas. In vitro culture of microglia, using primary cells, cell lines, or induced pluripotent stem cell derived microglia, allows researchers to generate reproducible, robust, and quantifiable data regarding microglia function. A broad range of assays have been successfully developed and optimised for characterizing microglial morphology, mediation of inflammation, endocytosis, phagocytosis, chemotaxis and random motility, and mediation of immunometabolism. This review describes the main functions of microglia, compares existing protocols for measuring these functions in vitro, and highlights common pitfalls and future areas for development. We aim to provide a comprehensive methodological guide for researchers planning to characterise microglial functions within a range of contexts and in vitro models.

GATA1 controls numbers of hematopoietic progenitors and their response to autoimmune neuroinflammation

GATA-binding factor 1 (GATA1) is a transcription factor that governs the development and function of multiple hematopoietic cell lineages. GATA1 is expressed in hematopoietic stem and progenitor cells (HSPCs) and is essential for erythroid lineage commitment; however, whether it plays a role in hematopoietic stem cell (HSC) biology and the development of myeloid cells, and what that role might be, remains unclear. We initially set out to test the role of eosinophils in experimental autoimmune encephalomyelitis (EAE), a model of central nervous system autoimmunity, using mice lacking a double GATA-site (ΔdblGATA), which lacks eosinophils due to the deletion of the dblGATA enhancer to Gata1, which alters its expression. ΔdblGATA mice were resistant to EAE, but not because of a lack of eosinophils, suggesting that these mice have an additional defect. ΔdblGATA mice with EAE had fewer inflammatory myeloid cells than the control mice, suggesting that resistance to EAE is caused by a defect in myeloid cells. Naïve ΔdblGATA mice also showed reduced frequency of CD11b+ myeloid cells in the blood, indicating a defect in myeloid cell production. Examination of HSPCs revealed fewer HSCs and myeloid cell progenitors in the ΔdblGATA bone marrow (BM), and competitive BM chimera experiments showed a reduced capacity of the ΔdblGATA BM to reconstitute immune cells, suggesting that reduced numbers of ΔdblGATA HSPCs cause a functional deficit during inflammation. Taken together, our data show that GATA1 regulates the number of HSPCs and that reduced GATA1 expression due to dblGATA deletion results in a diminished immune response following the inflammatory challenge.

Immunoproteasomes control activation of innate immune signaling and microglial function

Microglia are the resident immune cells of the central nervous system (CNS) and play a major role in the regulation of brain homeostasis. To maintain their cellular protein homeostasis, microglia express standard proteasomes and immunoproteasomes (IP), a proteasome isoform that preserves protein homeostasis also in non-immune cells under challenging conditions. The impact of IP on microglia function in innate immunity of the CNS is however not well described. Here, we establish that IP impairment leads to proteotoxic stress and triggers the unfolded and integrated stress responses in mouse and human microglia models. Using proteomic analysis, we demonstrate that IP deficiency in microglia results in profound alterations of the ubiquitin-modified proteome among which proteins involved in the regulation of stress and immune responses. In line with this, molecular analysis revealed chronic activation of NF-κB signaling in IP-deficient microglia without further stimulus. In addition, we show that IP impairment alters microglial function based on markers for phagocytosis and motility. At the molecular level IP impairment activates interferon signaling promoted by the activation of the cytosolic stress response protein kinase R. The presented data highlight the importance of IP function for the proteostatic potential as well as for precision proteolysis to control stress and immune signaling in microglia function.

Herpesvirus Infections in the Human Brain: A Neural Cell Model of the Complement System Derived from Induced Pluripotent Stem Cells

BACKGROUND

Herpesviruses alter cognitive functions in humans following acute infections; progressive cognitive decline and dementia have also been suggested. It is important to understand the pathogenic mechanisms of such infections. The complement system - comprising functionally related proteins integral for systemic innate and adaptive immunity - is an important component of host responses. The complement system has specialized functions in the brain. Still, the dynamics of the brain complement system are still poorly understood. Many complement proteins have limited access to the brain from plasma, necessitating synthesis and specific regulation of expression in the brain; thus, complement protein synthesis, activation, regulation, and signaling should be investigated in human brain-relevant cellular models. Cells derived from human-induced pluripotent stem cells (hiPSCs) could enable tractable models.

METHODS

Human-induced pluripotent stem cells were differentiated into neuronal (hi-N) and microglial (hi-M) cells that were cultured with primary culture human astrocyte-like cells (ha-D). Gene expression analyses and complement protein levels were analyzed in mono- and co-cultures.

RESULTS

Transcript levels of complement proteins differ by cell type and co-culture conditions, with evidence for cellular crosstalk in co-cultures. Hi-N and hi-M cells have distinct patterns of expression of complement receptors, soluble factors, and regulatory proteins. hi-N cells produce complement factor 4 (C4) and factor B (FB), whereas hi-M cells produce complement factor 2 (C2) and complement factor 3 (C3). Thus, neither hi-N nor hi-M cells can form either of the C3-convertases - C4bC2a and C3bBb. However, when hi-N and hi-M cells are combined in co-cultures, both types of functional C3 convertase are produced, indicated by elevated levels of the cleaved C3 protein, C3a.

CONCLUSIONS

hiPSC-derived co-culture models can be used to study viral infection in the brain, particularly complement receptor and function in relation to cellular "crosstalk." The models could be refined to further investigate pathogenic mechanisms.

Immune recognition of syngeneic, allogeneic and xenogeneic stromal cell transplants in healthy retinas

Background

Advanced therapies using adult mesenchymal stromal cells (MSCs) for neurodegenerative diseases are not effectively translated into the clinic. The cross talk between the transplanted cells and the host tissue is something that, despite its importance, is not being systematically investigated.

Methods

We have compared the response of the mouse healthy retina to the intravitreal transplantation of MSCs derived from the bone marrow in four modalities: syngeneic, allogeneic, xenogeneic and allogeneic with immunosuppression using functional analysis in vivo and histology, cytometry and protein measurement post-mortem. Data were considered significant (p < 0.05) after nonparametric suitable statistical tests.

Results

Transplanted cells remain in the vitreous and are cleared by microglial cells a process that is quicker in allotransplants regardless of immunosuppression. All transplants cause anatomical remodelling which is more severe after xenotransplants. Xeno- and allotransplants with or without immunosuppression cause macro- and microglial activation and retinal functional impairment, being xenotransplants the most detrimental and the only ones that recruit CD45⁺Iba1⁻cells. The profile of proinflammatory cytokines changes in all transplantation settings. However, none of these changes affect the retinal ganglion cell population.

Conclusions

We show here a specific functional and anatomical retinal response depending on the MSC transplantation modality, an aspect that should be taken into consideration when conducting preclinical studies if we intend a more realistic translation into clinical practice.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03129-y.

Reduction in the Migration Activity of Microglia Treated with Silica-Coated Magnetic Nanoparticles and their Recovery Using Citrate

Nanoparticles have garnered significant interest in neurological research in recent years owing to their efficient penetration of the blood–brain barrier (BBB). However, significant concerns are associated with their harmful effects, including those related to the immune response mediated by microglia, the resident immune cells in the brain, which are exposed to nanoparticles. We analysed the cytotoxic effects of silica-coated magnetic nanoparticles containing rhodamine B isothiocyanate dye [MNPs@SiO2(RITC)] in a BV2 microglial cell line using systems toxicological analysis. We performed the invasion assay and the exocytosis assay and transcriptomics, proteomics, metabolomics, and integrated triple-omics analysis, generating a single network using a machine learning algorithm. The results highlight alteration in the mechanisms of the nanotoxic effects of nanoparticles using integrated omics analysis.

Phagocytosis converts infiltrated monocytes to microglia-like phenotype in experimental brain ischemia

BACKGROUND

Monocyte-derived macrophages (MDMs) and microglia elicit neural inflammation and clear debris for subsequent tissue repair and remodeling. The role of infiltrating MDMs in the injured brain, however, has been controversial due to overlapping antigen expression with microglia. In this study, we define the origin and function of MDMs in cerebral ischemia.

METHODS

Using adoptive transfer of GFP+ splenocytes into adult asplenic mice subjected to transient middle cerebral artery occlusion, we compared the role of CD11b+/CD45+/NK1.1-/Ly6G- MDMs and microglia in the ischemic brain. The phagocytic activities of MDMs and microglia were measured by the uptake of fluorescent beads both in vivo with mice infused with GFP+ splenocytes and ex vivo with cultures of isolated brain immune cells.

RESULTS

Stroke induced an infiltration of MDMs [GFP+] into the ipsilateral hemisphere at acute (3 days) and sub-acute phases (7 days) of post-stroke. At 7 days, the infiltrating MDMs contained both CD45High and CD45Low subsets. The CD45High MDMs in the injured hemisphere exhibited a significantly higher proliferation capacity (Ki-67 expression levels) as well as higher expression levels of CD11c when compared to CD45Low MDMs. The CD45High and CD45Low MDM subsets in the injured hemisphere were approximately equal populations, indicating that CD45High MDMs infiltrating the ischemic brain changes their phenotype to CD45Low microglia-like phenotype. Studies with fluorescent beads reveal high levels of MDM phagocytic activity in the post-stroke brain, but this phagocytic activity was exclusive to post-ischemic brain tissue and was not detected in circulating monocytes. By contrast, CD45Low microglia-like cells had low levels of phagocytic activity when compared to CD45High cells. Both in vivo and ex vivo studies also show that the phagocytic activity in CD45High MDMs is associated with an increase in the CD45Low/CD45High ratio, indicating that phagocytosis promotes MDM phenotype conversion.

CONCLUSIONS

This study demonstrates that MDMs are the predominant phagocytes in the post-ischemic brain, with the CD45High subset having the highest phagocytic activity levels. Upon phagocytosis, CD45High MDMs in the post-ischemic brain adopt a CD45Low phenotype that is microglia-like. Together, these studies reveal key roles for MDMs and their phagocytic function in tissue repair and remodeling following cerebral ischemia.

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.

The changing landscape of immune cells in the fetal mouse testis

Fetal testis growth involves cell influx and extensive remodeling. Immediately after sex determination in mouse, macrophages enable normal cord formation and removal of inappropriately positioned cells. This study provides new information about macrophages and other immune cells after cord formation in fetal testes, including their density, distribution, and close cellular contacts. C57BL6J mouse testes from embryonic day (E) 13.5 to birth (post-natal day 0; PND0), were examined using immunofluorescence, immunohistochemistry, and RT-qPCR to identify macrophages (F4/80, CD206, MHCII), T cells (CD3), granulocytes/neutrophils (Ly6G), and germ cells (DDX4). F4/80⁺ cells were the most abundant, comprising 90% of CD45⁺ cells at E13.5 and declining to 65% at PND0. Changes in size, shape, and markers (CD206 and MHCII) documented during this interval align with the understanding that F4/80⁺ cells have different origins during embryonic life. CD3⁺ cells and F4/80⁻/MHCII⁺ were absent to rare until PND0. Ly6G⁺ cells were scarce at E13.5 but increased robustly by PND0 to represent half of the CD45⁺ cells. These immunofluorescence data were in accord with transcript analysis, which showed that immune marker mRNAs increased with testis age. F4/80⁺ and Ly6G⁺ cells were frequently inside cords adjacent to germ cells at E13.5 and E15.5. F4/80⁺ cells were often in clusters next to other immune cells. Macrophages inside cords at E13.5 and E15.5 (F4/80^Hi/CD206⁺) were different from macrophages at PND0 (F4/80^Dim/CD206⁻), indicating that they have distinct origins. This histological quantification coupled with transcript information identifies new cellular interactions for immune cells in fetal testis morphogenesis, and highlights new avenues for studies of their functional significance.

SARS-CoV-2 Infection of Microglia Elicits Proinflammatory Activation and Apoptotic Cell Death

Accumulating evidence suggests that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection causes various neurological symptoms in patients with coronavirus disease 2019 (COVID-19). The most dominant immune cells in the brain are microglia. Yet, the relationship between neurological manifestations, neuroinflammation, and host immune response of microglia to SARS-CoV-2 has not been well characterized. Here, we reported that SARS-CoV-2 can directly infect human microglia, eliciting M1-like proinflammatory responses, followed by cytopathic effects. Specifically, SARS-CoV-2 infected human microglial clone 3 (HMC3), leading to inflammatory activation and cell death. RNA sequencing (RNA-seq) analysis also revealed that endoplasmic reticulum (ER) stress and immune responses were induced in the early, and apoptotic processes in the late phases of viral infection. SARS-CoV-2-infected HMC3 showed the M1 phenotype and produced proinflammatory cytokines, such as interleukin (IL)-1β, IL-6, and tumor necrosis factor α (TNF-α), but not the anti-inflammatory cytokine IL-10. After this proinflammatory activation, SARS-CoV-2 infection promoted both intrinsic and extrinsic death receptor-mediated apoptosis in HMC3. Using K18-hACE2 transgenic mice, murine microglia were also infected by intranasal inoculation of SARS-CoV-2. This infection induced the acute production of proinflammatory microglial IL-6 and TNF-α and provoked a chronic loss of microglia. Our findings suggest that microglia are potential mediators of SARS-CoV-2-induced neurological problems and, consequently, can be targets of therapeutic strategies against neurological diseases in patients with COVID-19. IMPORTANCE Recent studies reported neurological and cognitive sequelae in patients with COVID-19 months after the viral infection with several symptoms, including ageusia, anosmia, asthenia, headache, and brain fog. Our conclusions raise awareness of COVID-19-related microglia-mediated neurological disorders to develop treatment strategies for the affected patients. We also indicated that HMC3 was a novel human cell line susceptible to SARS-CoV-2 infection that exhibited cytopathic effects, which could be further used to investigate cellular and molecular mechanisms of neurological manifestations of patients with COVID-19.

BIN1 is a key regulator of proinflammatory and neurodegeneration-related activation in microglia

BACKGROUND

The BIN1 locus contains the second-most significant genetic risk factor for late-onset Alzheimer's disease. BIN1 undergoes alternate splicing to generate tissue- and cell-type-specific BIN1 isoforms, which regulate membrane dynamics in a range of crucial cellular processes. Whilst the expression of BIN1 in the brain has been characterized in neurons and oligodendrocytes in detail, information regarding microglial BIN1 expression is mainly limited to large-scale transcriptomic and proteomic data. Notably, BIN1 protein expression and its functional roles in microglia, a cell type most relevant to Alzheimer's disease, have not been examined in depth.

METHODS

Microglial BIN1 expression was analyzed by immunostaining mouse and human brain, as well as by immunoblot and RT-PCR assays of isolated microglia or human iPSC-derived microglial cells. Bin1 expression was ablated by siRNA knockdown in primary microglial cultures in vitro and Cre-lox mediated conditional deletion in adult mouse brain microglia in vivo. Regulation of neuroinflammatory microglial signatures by BIN1 in vitro and in vivo was characterized using NanoString gene panels and flow cytometry methods. The transcriptome data was explored by in silico pathway analysis and validated by complementary molecular approaches.

RESULTS

Here, we characterized microglial BIN1 expression in vitro and in vivo and ascertained microglia expressed BIN1 isoforms. By silencing Bin1 expression in primary microglial cultures, we demonstrate that BIN1 regulates the activation of proinflammatory and disease-associated responses in microglia as measured by gene expression and cytokine production. Our transcriptomic profiling revealed key homeostatic and lipopolysaccharide (LPS)-induced inflammatory response pathways, as well as transcription factors PU.1 and IRF1 that are regulated by BIN1. Microglia-specific Bin1 conditional knockout in vivo revealed novel roles of BIN1 in regulating the expression of disease-associated genes while counteracting CX3CR1 signaling. The consensus from in vitro and in vivo findings showed that loss of Bin1 impaired the ability of microglia to mount type 1 interferon responses to proinflammatory challenge, particularly the upregulation of a critical type 1 immune response gene, Ifitm3.

CONCLUSIONS

Our convergent findings provide novel insights into microglial BIN1 function and demonstrate an essential role of microglial BIN1 in regulating brain inflammatory response and microglial phenotypic changes. Moreover, for the first time, our study shows a regulatory relationship between Bin1 and Ifitm3, two Alzheimer's disease-related genes in microglia. The requirement for BIN1 to regulate Ifitm3 upregulation during inflammation has important implications for inflammatory responses during the pathogenesis and progression of many neurodegenerative diseases.

Protein Expression of the Microglial Marker Tmem119 Decreases in Association With Morphological Changes and Location in a Mouse Model of Traumatic Brain Injury

The activation of microglia and the infiltration of macrophages are hallmarks of neuroinflammation after acute brain injuries, including traumatic brain injury (TBI). The two myeloid populations share many features in the post-injury inflammatory response, thus, being antigenically indistinguishable. Recently Tmem119, a type I transmembrane protein specifically expressed by microglia under physiological conditions, was proposed as a tool to differentiate resident microglia from blood-borne macrophages, not expressing it. However, the validity of Tmem119 as a specific marker of resident microglia in the context of acute brain injury, where microglia are activated and macrophages are recruited, needs validation. Our purpose was to investigate Tmem119 expression and distribution in relation to the morphology of brain myeloid cells present in the injured area after TBI. Mice underwent sham surgery or TBI by controlled cortical impact (CCI). Brains from sham-operated, or TBI mice, were analyzed by in situ hybridization to identify the cells expressing Tmem119, and by Western blot and quantitative immunofluorescence to measure Tmem119 protein levels in the entire brain regions and single cells. The morphology of Iba1+ myeloid cells was analyzed at different times (4 and 7 days after TBI) and several distances from the contused edge in order to associate Tmem119 expression with morphological evolution of active microglia. In situ hybridization indicated an increased Tmem119 RNA along with increased microglial complement C1q activation in the contused area and surrounding regions. On the contrary, the biochemical evaluation showed a drop in Tmem119 protein levels in the same areas. The Tmem119 immunoreactivity decreased in Iba1+ myeloid cells found in the contused cortex at both time points, with the cells showing the hypertrophic ameboid morphology having no Tmem119 expression. The Tmem119 was present on ramifications of resident microglia and its presence was decreased as a consequence of microglial activation in cortical areas close to contusion. Based on the data, we conclude that the decrease of Tmem119 in reactive microglia may depend on the process of microglial activation, which involves the retracting of their branchings to acquire an ameboid shape. The Tmem119 immunoreactivity decreases in reactive microglia to similar levels than the blood-borne macrophages, thus, failing to discriminate the two myeloid populations after TBI.

Decoding the temporal and regional specification of microglia in the developing human brain

Region-related heterogeneity and state transitions of microglia are important for brain development and neurological pathogenesis. However, regional specialization and state transition in microglia during early human CNS development remain unclear. Here, we profile single-cell transcriptomes of microglia from distinct regions of the developing human brain, and combined with experimental verification, we define and characterize early microglial fate determinations related to regional specification and state transition. We identified several subclasses of neuronal gene-enriched microglia with regional specification that dynamically and transiently appeared as early brain regions formed. In contrast, immune-related microglia were regionally specialized at later stages of CNS development. Surprisingly, we discovered that region-specialized immune-related microglia exit from a relative resting state and transition into distinct active states. In addition, we experimentally verified the microglial state transition. Finally, we showed that the state transition is conserved but that there are molecular differences in developing microglia in humans and mice.

Novel protocol for the isolation of highly purified neonatal murine microglia and astrocytes

BACKGROUND

The crosstalk and reactivity of the cell type glia, especially microglia and astrocytes, have progressively gathered research attention in understanding proper brain function regulated by the innate immune response. Therefore, methods to isolate highly viable and pure glia for the analysis on a cell-specific level are indispensable.

NEW METHOD

We modified previously established techniques: Animal numbers were reduced by multiple microglial harvests from the same mixed glial culture, thereby maximizing microglial yields following the principles of the 3Rs (replacement, reduction, and refinement). We optimized Magnetic-activated cell sorting (MACS®) of microglia and astrocytes by applying cultivated primary glial cell suspensions instead of directly sorting dissociated single cell suspension.

RESULTS

We generated highly viable and pure microglia and astrocytes derived from a single mixed culture with a purity of ~99%, as confirmed by FACS analysis. Field emission scanning electron microscopy (FESEM) demonstrated integrity of the MACS-purified glial cells. Tumor necrosis factor (TNF) and Interleukin-10 (IL-10) ELISA confirmed pro- and anti-inflammatory responses to be functional in purified glia, but significantly weakened compared to non-purified cells, further highlighting the importance of cellular crosstalk for proper immune activation.

COMPARISON WITH EXISTING METHOD(S)

Unlike previous studies that either isolated a single type of glia or displayed a substantial proportion of contamination with other cell types, we achieved isolation of both microglia and astrocytes at an increased purity (99-100%).

CONCLUSIONS

We have created an optimized protocol for the efficient purification of both primary microglia and astrocytes. Our results clearly demonstrate the importance of purity in glial cell cultivation in order to examine immune responses, which particularly holds true for astrocytes. We propose the novel protocol as a tool to investigate the cell type-specific crosstalk between microglia and astrocytes in the frame of CNS diseases.

Detection and Functional Evaluation of the P2X7 Receptor in hiPSC Derived Neurons and Microglia-Like Cells

A large body of evidence suggests the involvement of the ATP-gated purinergic receptor P2X7 (P2X7R) in neurodegenerative diseases, including Alzheimer's disease. While it is well-described to be present and functional on microglia cells contributing to inflammatory responses, some reports suggest a neuronal expression of the receptor as well. Here, we present experimental results showing P2X7 receptors to be expressed on human hiPSC-derived microglia-like cells, hiPSC-derived neuronal progenitors and hiPSC-derived matured neuronal cells. By applying cell surface protein detection assays, we show that P2X7R is not localized on the cell membrane, despite being detected in neuronal cells and thus may not be available for directly mediating neurotoxicity. On hiPSC-derived microglia-like cells, a clear membranous expression was detected. Additionally, we have not observed differences in P2X7R functions between control and familial Alzheimer's disease patient-derived neuronal cells. Functional assays employing a P2X7R antagonist JNJ 47965567 confirm these findings by showing P2X7R-dependent modulation of microglia-like cells viability upon treatment with P2X7R agonists ATP and BzATP, while the same effect was absent from neuronal cells. Since the majority of P2X7R research was done on rodent models, our work on human hiPSC-derived cells presents a valuable contribution to the field, extending the work on animal models to the human cellular system and toward clinical translation.

Method of Microglial DNA-RNA Purification from a Single Brain of an Adult Mouse

Microglia, the resident brain immune effectors cells, show dynamic activation level changes for most neuropsychiatric diseases, reflecting their complex regulatory function and potential as a therapeutic target. Emerging single-cell molecular biology studies are used to investigate the genetic modification of individual cells to better understand complex gene regulatory pathways. Although multiple protocols for microglia isolation from adult mice are available, it is always challenging to get sufficient purified microglia from a single brain for simultaneous DNA and RNA extraction for subsequent downstream analysis. Moreover, for data comparison between treated and untreated groups, standardized cell isolation techniques are essential to decrease variability. Here, we present a combined method of microglia isolation from a single adult mouse brain, using a magnetic bead-based column separation technique, and a column-based extraction of purified DNA-RNA from the isolated microglia for downstream application. Our current method provides step-by-step instructions accompanied by visual explanations of important steps for isolating DNA-RNA simultaneously from a highly purified microglia population.

The effects of okadaic acid-treated SH-SY5Y cells on microglia activation and phagocytosis

The activation of microglia is found to be associated with neurodegenerative disorders including Alzheimer's disease (AD). Several studies have shown that okadaic acid (OA) induced deposition of tau hyperphosphorylation, and subsequent neuronal degeneration, loss of synapses, and memory impairment, all of which resemble the pathology of AD. Although OA is a powerful tool available for mechanisms of the neurotoxicity associated with AD, the exact mechanism underlying the activation of microglial cells remains unrevealed. The aim of this study was to determine the effect of both OA and OA-treated neuroblastoma SH-SY5Y cells on microglial HAPI cell viability, activation, and phagocytosis. The results showed that both OA and OA-treated neurons did not induce any detectable cytotoxicity of microglial cells. Furthermore, incubation with OA-treated SH-SY5Y cells could increase the expression of ionized calcium-binding adapter molecule 1 (Iba1) on microglial HAPI cells. This result indicated that OA may induce microglial activation through the toxicity of neurons. Moreover, we also demonstrated that OA-treated SH-SY5Y cells were engulfed by CD11b/c-labeled microglial HAPI cells, which were abolished after treatment with 10 mM O-phospho- l-serine ( L-SOP) for 30 min before co-culture with OA-treated SH-SY5Y cells, indicating cells experiencing phagocytic activity. We also confirmed that OA treatment for 24 h significantly increased tau hyperphosphorylation at S396 in SH-SY5Y cells. In conclusion, our findings indicate that OA is a potential toxic inducer underlying the role of microglia in AD pathogenesis.

d-mannose suppresses oxidative response and blocks phagocytosis in experimental neuroinflammation

Inflammation drives the pathology of many neurological diseases. d-mannose has been found to exert an antiinflammatory effect in peripheral diseases, but its effects on neuroinflammation and inflammatory cells in the central nervous system have not been studied. We aimed to determine the effects of d-mannose on key macrophage/microglial functions-oxidative stress and phagocytosis. In murine experimental autoimmune encephalomyelitis (EAE), we found d-mannose improved EAE symptoms compared to phosphate-buffered saline (PBS)-control mice, while other monosaccharides did not. Multiagent molecular MRI performed to assess oxidative stress (targeting myeloperoxidase [MPO] using MPO-bis-5-hydroxytryptamide diethylenetriaminepentaacetate gadolinium [Gd]) and phagocytosis (using cross-linked iron oxide [CLIO] nanoparticles) in vivo revealed that d-mannose-treated mice had smaller total MPO-Gd+ areas than those of PBS-control mice, consistent with decreased MPO-mediated oxidative stress. Interestingly, d-mannose-treated mice exhibited markedly smaller CLIO+ areas and much less T2 shortening effect in the CLIO+ lesions compared to PBS-control mice, revealing that d-mannose partially blocked phagocytosis. In vitro experiments with different monosaccharides further confirmed that only d-mannose treatment blocked macrophage phagocytosis in a dose-dependent manner. As phagocytosis of myelin debris has been known to increase inflammation, decreasing phagocytosis could result in decreased activation of proinflammatory macrophages. Indeed, compared to PBS-control EAE mice, d-mannose-treated EAE mice exhibited significantly fewer infiltrating macrophages/activated microglia, among which proinflammatory macrophages/microglia were greatly reduced while antiinflammatory macrophages/microglia increased. By uncovering that d-mannose diminishes the proinflammatory response and boosts the antiinflammatory response, our findings suggest that d-mannose, an over-the-counter supplement with a high safety profile, may be a low-cost treatment option for neuroinflammatory diseases such as multiple sclerosis.

Topological Dissection of Proteomic Changes Linked to the Limbic Stage of Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disorder and the most common cause of dementia worldwide. In AD, neurodegeneration spreads throughout different areas of the central nervous system (CNS) in a gradual and predictable pattern, causing progressive memory decline and cognitive impairment. Deposition of neurofibrillary tangles (NFTs) in specific CNS regions correlates with the severity of AD and constitutes the basis for disease classification into different Braak stages (I-VI). Early clinical symptoms are typically associated with stages III-IV (i.e., limbic stages) when the involvement of the hippocampus begins. Histopathological changes in AD have been linked to brain proteome alterations, including aberrant posttranslational modifications (PTMs) such as the hyperphosphorylation of Tau. Most proteomic studies to date have focused on AD progression across different stages of the disease, by targeting one specific brain area at a time. However, in AD vulnerable regions, stage-specific proteomic alterations, including changes in PTM status occur in parallel and remain poorly characterized. Here, we conducted proteomic, phosphoproteomic, and acetylomic analyses of human postmortem tissue samples from AD (Braak stage III-IV, n=11) and control brains (n=12), covering all anatomical areas affected during the limbic stage of the disease (total hippocampus, CA1, entorhinal and perirhinal cortices). Overall, ~6000 proteins, ~9000 unique phosphopeptides and 221 acetylated peptides were accurately quantified across all tissues. Our results reveal significant proteome changes in AD brains compared to controls. Among others, we have observed the dysregulation of pathways related to the adaptive and innate immune responses, including several altered antimicrobial peptides (AMPs). Notably, some of these changes were restricted to specific anatomical areas, while others altered according to disease progression across the regions studied. Our data highlights the molecular heterogeneity of AD and the relevance of neuroinflammation as a major player in AD pathology. Data are available via ProteomeXchange with identifier PXD027173.

Unique molecular characteristics and microglial origin of Kv1.3 channel-positive brain myeloid cells in Alzheimer's disease

Kv1.3 potassium channels, expressed by proinflammatory central nervous system mononuclear phagocytes (CNS-MPs), are promising therapeutic targets for modulating neuroinflammation in Alzheimer's disease (AD). The molecular characteristics of Kv1.3-high CNS-MPs and their cellular origin from microglia or CNS-infiltrating monocytes are unclear. While Kv1.3 blockade reduces amyloid beta (Aβ) burden in mouse models, the downstream immune effects on molecular profiles of CNS-MPs remain unknown. We show that functional Kv1.3 channels are selectively expressed by a subset of CD11b⁺CD45⁺ CNS-MPs acutely isolated from an Aβ mouse model (5xFAD) as well as fresh postmortem human AD brain. Transcriptomic profiling of purified CD11b⁺Kv1.3⁺ CNS-MPs, CD11b⁺CD45^int Kv1.3^neg microglia, and peripheral monocytes from 5xFAD mice revealed that Kv1.3-high CNS-MPs highly express canonical microglial markers (Tmem119, P2ry12) and are distinct from peripheral Ly6c^high/Ly6c^low monocytes. Unlike homeostatic microglia, Kv1.3-high CNS-MPs express relatively lower levels of homeostatic genes, higher levels of CD11c, and increased levels of glutamatergic transcripts, potentially representing phagocytic uptake of neuronal elements. Using irradiation bone marrow CD45.1/CD45.2 chimerism in 5xFAD mice, we show that Kv1.3⁺ CNS-MPs originate from microglia and not blood-derived monocytes. We show that Kv1.3 channels regulate membrane potential and early signaling events in microglia. Finally, in vivo blockade of Kv1.3 channels in 5xFAD mice by ShK-223 reduced Aβ burden, increased CD11c⁺ CNS-MPs, and expression of phagocytic genes while suppressing proinflammatory genes (IL1b). Our results confirm the microglial origin and identify unique molecular features of Kv1.3-expressing CNS-MPs. In addition, we provide evidence for CNS immunomodulation by Kv1.3 blockers in AD mouse models resulting in a prophagocytic phenotype.

Divergent Functions of Tissue-Resident and Blood-Derived Macrophages in the Hemorrhagic Brain

[Figure: see text].

Efficient conversion of human induced pluripotent stem cells into microglia by defined transcription factors

Microglia, the immune cells of the central nervous system, play critical roles in brain physiology and pathology. We report a novel approach that produces, within 10 days, the differentiation of human induced pluripotent stem cells (hiPSCs) into microglia (iMG) by forced expression of both SPI1 and CEBPA. High-level expression of the main microglial markers and the purity of the iMG cells were confirmed by RT-qPCR, immunostaining, and flow cytometry analyses. Whole-transcriptome analysis demonstrated that these iMGs resemble human fetal/adult microglia but not human monocytes. Moreover, these iMGs exhibited appropriate physiological functions, including various inflammatory responses, ADP/ATP-evoked migration, and phagocytic ability. When co-cultured with hiPSC-derived neurons, the iMGs respond and migrate toward injured neurons. This study has established a protocol for the rapid conversion of hiPSCs into functional iMGs, which should facilitate functional studies of human microglia using different disease models and also help with drug discovery.

•

Efficient generation of human iMGs from iPSCs by forced expression of SPI1 and CEBPA

•

The transcriptome profile of iMGs resembles that of human primary microglia

•

The iMG cells possess appropriate physiological functioning

•

An iN-iMG co-culture model is established for studying neuron-microglia interactions

Tamoxifen suppresses brain metastasis of estrogen receptor-deficient breast cancer by skewing microglia polarization and enhancing their immune functions

Background

Brain metastasis of breast cancer exhibits exceedingly poor prognosis, and both triple negative (TN) and Her2⁺ subtypes have the highest incidence of brain metastasis. Although estrogen blockers are considered to be ineffective for their treatment, recent evidence indicates that estrogen blockade using tamoxifen showed certain efficacy. However, how estrogen affects brain metastasis of triple negative breast cancer (TNBC) remains elusive.

Methods

To examine the effect of estrogen on brain metastasis progression, nude mice were implanted with brain metastatic cells and treated with either estrogen supplement, tamoxifen, or ovariectomy for estrogen depletion. For clinical validation study, brain metastasis specimens from pre- and post-menopause breast cancer patients were examined for microglia polarization by immunohistochemistry. To examine the estrogen-induced M2 microglia polarization, microglia cells were treated with estrogen, and the M1/M2 microglia polarization was detected by qRT-PCR and FACS. The estrogen receptor-deficient brain metastatic cells, SkBrM and 231BrM, were treated with conditioned medium (CM) derived from microglia that were treated with estrogen in the presence or absence of tamoxifen. The effect of microglia-derived CM on tumor cells was examined by colony formation assay and sphere forming ability.

Results

We found that M2 microglia were abundantly infiltrated in brain metastasis of pre-menopausal breast cancer patients. A similar observation was made in vivo, when we treated mice systemically with estrogen. Blocking of estrogen signaling either by tamoxifen treatment or surgical resection of mice ovaries suppressed M2 microglial polarization and decreased the secretion of C-C motif chemokine ligand 5, resulting in suppression of brain metastasis. The estrogen modulation also suppressed stemness in TNBC cells in vitro. Importantly, estrogen enhanced the expression of signal regulatory protein α on microglia and restricted their phagocytic ability.

Conclusions

Our results indicate that estrogen promotes brain metastasis by skewing polarity of M2 microglia and inhibiting their phagocytic ability, while tamoxifen suppresses brain metastasis by blocking the M2 polarization of microglia and increasing their anti-tumor phagocytic ability. Our results also highlight a potential therapeutic utility of tamoxifen for treating brain metastasis of hormone receptor-deficient breast cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01412-z.