Diverse Requirements for Microglial Survival, Specification, and Function Revealed by Defined-Medium Cultures

P2Y6 receptor-dependent microglial phagocytosis of synapses mediates synaptic and memory loss in aging

Aging causes loss of brain synapses and memory, and microglial phagocytosis of synapses may contribute to this loss. Stressed neurons can release the nucleotide UTP, which is rapidly converted into UDP, that in turn activates the P2Y₆ receptor (P2Y₆ R) on the surface of microglia, inducing microglial phagocytosis of neurons. However, whether the activation of P2Y₆ R affects microglial phagocytosis of synapses is unknown. We show here that inactivation of P2Y₆ R decreases microglial phagocytosis of isolated synapses (synaptosomes) and synaptic loss in neuronal-glial co-cultures. In vivo, wild-type mice aged from 4 to 17 months exhibited reduced synaptic density in cortical and hippocampal regions, which correlated with increased internalization of synaptic material within microglia. However, this aging-induced synaptic loss and internalization were absent in P2Y₆ R knockout mice, and these mice also lacked any aging-induced memory loss. Thus, P2Y₆ R appears to mediate aging-induced loss of synapses and memory by increasing microglial phagocytosis of synapses. Consequently, blocking P2Y₆ R has the potential to prevent age-associated memory impairment.

Single-cell microglial transcriptomics during demyelination defines a microglial state required for lytic carcass clearance

BACKGROUND

Microglia regulate the response to injury and disease in the brain and spinal cord. In white matter diseases microglia may cause demyelination. However, how microglia respond and regulate demyelination is not fully understood.

METHODS

To understand how microglia respond during demyelination, we fed mice cuprizone-a potent demyelinating agent-and assessed the dynamics of genetically fate-mapped microglia. We then used single-cell RNA sequencing to identify and track the microglial subpopulations that arise during demyelination. To understand how microglia contribute to the clearance of dead oligodendrocytes, we ablated microglia starting at the peak of cuprizone-induced cell death and used the viability dye acridine orange to monitor apoptotic and lytic cell morphologies after microglial ablation. Lastly, we treated serum-free primary microglial cultures to model distinct aspects of cuprizone-induced demyelination and assessed the response.

RESULTS

The cuprizone diet generated a robust microglial response by week 4 of the diet. Single-cell RNA sequencing at this time point revealed the presence of several cuprizone-associated microglia (CAM) clusters. These clusters expressed a transcriptomic signature indicative of cytokine regulation and reactive oxygen species production with altered lysosomal and metabolic changes consistent with ongoing phagocytosis. Using acridine orange to monitor apoptotic and lytic cell death after microglial ablation, we found that microglia preferentially phagocytose lytic carcasses. In culture, microglia exposed to lytic carcasses partially recapitulated the CAM state, suggesting that phagocytosis contributes to this distinct microglial state during cuprizone demyelination.

CONCLUSIONS

Microglia serve multiple roles during demyelination, yet their transcriptomic state resembles other neurodegenerative conditions. The phagocytosis of cellular debris is likely a universal cause for a common neurodegenerative microglial state.

A primary rodent triculture model to investigate the role of glia-neuron crosstalk in regulation of neuronal activity

Neuroinflammation and hyperexcitability have been implicated in the pathogenesis of neurodegenerative disease, and new models are required to investigate the cellular crosstalk involved in these processes. We developed an approach to generate a quantitative and reproducible triculture system that is suitable for pharmacological studies. While primary rat cells were previously grown in a coculture medium formulated to support only neurons and astrocytes, we now optimised a protocol to generate tricultures containing neurons, astrocytes and microglia by culturing in a medium designed to support all three cell types and adding exogenous microglia to cocultures. Immunocytochemistry was used to confirm the intended cell types were present. The percentage of ramified microglia in the tricultures decreases as the number of microglia present increases. Multi-electrode array recordings indicate that microglia in the triculture model suppress neuronal activity in a dose-dependent manner. Neurons in both cocultures and tricultures are responsive to the potassium channel blocker 4-aminopyridine, suggesting that neurons remained viable and functional in the triculture model. Furthermore, suppressed neuronal activity in tricultures correlates with decreased densities of dendritic spines and of the postsynaptic protein Homer1 along dendrites, indicative of a direct or indirect effect of microglia on synapse function. We thus present a functional triculture model, which, due to its more complete cellular composition, is a more relevant model than standard cocultures. The model can be used to probe glia-neuron interactions and subsequently aid the development of assays for drug discovery, using neuronal excitability as a functional endpoint.

A simplified procedure for isolation of primary murine microglia

There are various approaches in which one can isolate microglia from murine brains, such as immunomagnetic, density gradient, FACS and differential adhesive methods. In this procedure a modified flask-tapping approach was used due to its simplicity and reproducibility. Our protocol requires only a single step to isolate the microglia from the mixed cell population. Once the microglia were isolated, we characterized cell purity, microglial morphology and phagocytic activity. The single-step protocol, without the need for additional astrocyte or oligodendrocyte separation, allows microglial cells to be used immediately for experimental purposes. The protocol is low-cost and can be performed in any lab with standard cell-culture equipment.

The appearance of phagocytic microglia in the postnatal brain of Niemann Pick type C mice is developmentally regulated and underscores shortfalls in fine odor discrimination

The loss of NPC1 or NPC2 function results in cholesterol and sphingolipid dyshomeostasis that impairs developmental trajectories, predisposing the postnatal brain to the appearance of pathological signs, including progressive and stereotyped Purkinje cell loss and microgliosis. Despite increasing evidence reporting the activation of pro-inflammatory microglia as a cardinal event of NPC1 disease progression at symptomatic stages both in patients and preclinical models, how microglia cells respond to altered neurodevelopmental dynamics remains not completely understood. To gain an insight on this issue, we have characterized patterns of microglia activation in the early postnatal cerebellum and young adult olfactory bulb of the hypomorphic Npc1^nmf164 mouse model. Previous evidence has shown that both these areas display a number of anomalies affecting neuron and glial cell proliferation and differentiation, which largely anticipate cellular changes and clinical signs, raising our interest on how microglia interplay to these changes. Even so, to separate the contribution of cues provided by the dysfunctional microenvironment we have also studied microglia isolated from mice of increasing ages and cultured in vitro for 1 week. Our findings show that microglia of both cerebellum and olfactory bulb of Npc1^nmf164 mice adopt an activated phenotype, characterized by increased cell proliferation, enlarged soma size and de-ramified processes, as well as a robust phagocytic activity, in a time- and space-specific manner. Enhanced phagocytosis associates with a profound remodeling of gene expression signatures towards gene products involved in chemotaxis, cell recognition and engulfment, including Cd68 and Trem2. These early changes in microglia morphology and activities are induced by region-specific developmental anomalies that likely anticipate alterations in neuronal connectivity. As a proof of concept, we show that microglia activation within the granule cell layer and glomerular layer of the olfactory bulb of Npc1^nmf164 mice is associated with shortfalls in fine odor discrimination.

Advanced in vitro models: Microglia in action

In the central nervous system (CNS), microglia carry out multiple tasks related to brain development, maintenance of brain homeostasis, and function of the CNS. Recent advanced in vitro model systems allow us to perform more detailed and specific analyses of microglial functions in the CNS. The development of human pluripotent stem cells (hPSCs)-based 2D and 3D cell culture methods, particularly advancements in brain organoid models, offers a better platform to dissect microglial function in various contexts. Despite the improvement of these methods, there are still definite restrictions. Understanding their drawbacks and benefits ensures their proper use. In this primer, we review current developments regarding in vitro microglial production and characterization and their use to address fundamental questions about microglial function in healthy and diseased states, and we discuss potential future improvements with a particular emphasis on brain organoid models.

Advancing basic and translational research to deepen understanding of the molecular immune-mediated mechanisms regulating long-term persistence of HIV-1 in microglia in the adult human brain

Knowledge about the diversity microglia (MG) type and function in the rodent and human brain has advanced significantly in the last few years. Nevertheless, we have known for 40 years that MG, monocytes, and macrophages in the brain play crucial roles in the pathogenesis of the HIV-1 in all tissues. HIV enters and spreads in the brain early, long before the initiation of antiviral therapy. As a result, many people with HIV continue to experience neurologic and neuropsychiatric comorbid conditions collectively known as HIV-associated neurocognitive disorder (HAND). HIV pathogenic sequelae in the CNS pose a challenge for cure strategies. Detailed understanding at a mechanistic level of how low-level and latent HIV-1 infection in MG negatively impacts neuroglial function has remained somewhat elusive. Direct rigorous in vivo experimental validation that the virus can integrate into MG and assume a latent but reactivatable state has remained constrained. However, there is much excitement that human in vitro models for MG can now help close the gap. This review will provide a brief background to place the role of MG in the ongoing neurologic complications of HIV infection of the CNS, then focus on the use and refinement of human postmitotic monocyte-derived MG-like cells and how they are being applied to advance research on HIV persistence and proinflammatory signaling in the CNS. Critically, an understanding of myeloid plasticity and heterogeneity and rigorous attention to all aspects of cell handling is essential for reproducibility. Summary Sentence: This review focuses on human postmitotic monocyte-derived microglia-like cells as tools to advance research on HIV persistence and neuroinflammatory signaling.

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.

Role of TREM2 in the Development of Neurodegenerative Diseases After Traumatic Brain Injury

Traumatic brain injury (TBI) has been found as the primary cause of morbidity and disability worldwide, which has posed a significant social and economic burden. The first stage of TBI produces brain edema, axonal damage, and hypoxia, thus having an effect on the blood-brain barrier function, promoting inflammatory responses, and increasing oxidative stress. Patients with TBI are more likely to develop post-traumatic epilepsy, behavioral issues, as well as mental illnesses. The long-term effects arising from TBI have aroused rising attention over the past few years. Microglia in the brain can express the triggering receptor expressed on myeloid cells 2 (TREM2), which is a single transmembrane receptor pertaining to the immunoglobulin superfamily. The receptor has been correlated with a number of neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, and other relevant diseases. In this review, it is demonstrated that TREM2 is promising to serve as a neuroprotective factor for neurodegenerative disorders following TBI by modulating the function of microglial cells. Accordingly, it has potential avenues for TREM2-related therapies to improve long-term recovery after TBI.

Brain milieu induces early microglial maturation through the BAX-Notch axis

Microglia are derived from primitive myeloid cells and gain their early identity in the embryonic brains. However, the mechanism by which the brain milieu confers microglial maturation signature remains elusive. Here, we demonstrate that the baxcq55 zebrafish and Baxtm1Sjk mouse embryos exhibit similarly defective early microglial maturation. BAX, a typical pro-apoptotic factor, is highly enriched in neuronal cells and regulates microglial maturation through both pro-apoptotic and non-apoptotic mechanisms. BAX regulates dlb via the CaMKII-CREB axis calcium-dependently in living neurons while ensuring the efficient Notch activation in the immigrated pre-microglia by apoptotic neurons. Notch signaling is conserved in supporting embryonic microglia maturation. Compromised microglial development occurred in the Cx3cr1Cre/+Rbpjfl/fl embryonic mice; however, microglia acquire their appropriate signature when incubated with DLL3 in vitro. Thus, our findings elucidate a BAX-CaMKII-CREB-Notch network triggered by the neuronal milieu in microglial development, which may provide innovative insights for targeting microglia in neuronal disorder treatment.

Brain Energy Metabolism: Astrocytes in Neurodegenerative Diseases

Astrocytes are the most abundant cells in the brain. They have many important functions in the central nervous system (CNS), including the maintenance of glutamate and ion homeostasis, the elimination of oxidative stress, energy storage in glycogen, tissue repair, regulating synaptic activity by releasing neurotransmitters, and participating in synaptic formation. Astrocytes have special highly ramified structure. Their branches contact with synapses of neurons inwardly, with fine structure and wrapping synapses; their feet contact with blood vessels of brain parenchyma outward, almost wrapping the whole brain. The adjacent astrocytes rarely overlap and communicate with each other through gap junction channels. The ideal location of astrocytes enables them to sense the weak changes of their surroundings and provide the structural basis for the energy supply of neurons. Neurons and astrocytes are closely coupled units of energy metabolism in the brain. Neurons consume a lot of ATPs in the process of neurotransmission. Astrocytes provide metabolic substrates for neurons, maintain high activity of neuron, and facilitate information transmission of neurons. This article reviews the characteristics of glucose metabolism, lipid metabolism, and amino acid metabolism of astrocytes. The metabolic interactions between astrocytes and neurons, astrocytes and microglia were also detailed discussed. Finally, we classified analyzed the role of metabolic disorder of astrocytes in the occurrence and development of neurodegenerative diseases.

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

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

Microglial STAT1-sufficiency is required for resistance to toxoplasmic encephalitis

Toxoplasma gondii is a ubiquitous intracellular protozoan parasite that establishes a life-long chronic infection largely restricted to the central nervous system (CNS). Constant immune pressure, notably IFN-γ-STAT1 signaling, is required for preventing fatal pathology during T. gondii infection. Here, we report that abrogation of STAT1 signaling in microglia, the resident immune cells of the CNS, is sufficient to induce a loss of parasite control in the CNS and susceptibility to toxoplasmic encephalitis during the early stages of chronic infection. Using a microglia-specific genetic labeling and targeting system that discriminates microglia from blood-derived myeloid cells that infiltrate the brain during infection, we find that, contrary to previous in vitro reports, microglia do not express inducible nitric-oxide synthase (iNOS) during T. gondii infection in vivo. Instead, transcriptomic analyses of microglia reveal that STAT1 regulates both (i) a transcriptional shift from homeostatic to “disease-associated microglia” (DAM) phenotype conserved across several neuroinflammatory models, including T. gondii infection, and (ii) the expression of anti-parasitic cytosolic molecules that are required for eliminating T. gondii in a cell-intrinsic manner. Further, genetic deletion of Stat1 from microglia during T. gondii challenge leads to fatal pathology despite largely equivalent or enhanced immune effector functions displayed by brain-infiltrating immune populations. Finally, we show that microglial STAT1-deficiency results in the overrepresentation of the highly replicative, lytic tachyzoite form of T. gondii, relative to its quiescent, semi-dormant bradyzoite form typical of chronic CNS infection. Our data suggest an overall protective role of CNS-resident microglia against T. gondii infection, illuminating (i) general mechanisms of CNS-specific immunity to infection (ii) and a clear role for IFN-STAT1 signaling in regulating a microglial activation phenotype observed across diverse neuroinflammatory disease states.

The brain, an immune-privileged organ, can be invaded and colonized by pathogens such as the opportunistic parasite, Toxoplasma gondii. How microglia, the resident immune cells of the brain, provide resistance to infection is an active area of investigation. In this study, we used a genetic approach to generate and study mice with microglia that lack STAT1, a critical transcription factor that confers protection against intracellular pathogens in both humans and mice. We find that despite robust activation and recruitment of immune cells from the blood to the brain during infection, STAT1 deficiency in microglia leads to increased brain parasite burden and uniform lethality in mice when challenged with T. gondii. Our bioinformatic analyses also indicate that STAT1 in microglia regulates (i) the expression of large families of genes associated with parasite killing and (ii) a microglial activation state that has been classically seen in neurodegeneration. Our findings identify mechanisms by which microglia contribute to parasite control and contribute to a greater understanding of their cellular physiology during neuroinflammation.

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.

A lysosomal regulatory circuit essential for the development and function of microglia

As the primary phagocytic cells of the central nervous system, microglia exquisitely regulate their lysosomal activity to facilitate brain development and homeostasis. However, mechanisms that coordinate lysosomal activity with microglia development, chemotaxis, and function remain unclear. Here, we show that embryonic macrophages require the lysosomal guanosine triphosphatase (GTPase) RagA and the GTPase-activating protein Folliculin to colonize the brain in zebrafish. We demonstrate that embryonic macrophages in rraga mutants show increased expression of lysosomal genes but display significant down-regulation of immune- and chemotaxis-related genes. Furthermore, we find that RagA and Folliculin repress the key lysosomal transcription factor Tfeb and its homologs Tfe3a and Tfe3b in the macrophage lineage. Using RNA sequencing, we establish that Tfeb and Tfe3 are required for activation of lysosomal target genes under conditions of stress but not for basal expression of lysosomal pathways. Collectively, our data define a lysosomal regulatory circuit essential for macrophage development and function in vivo.

Molecular and histological correlates of cognitive decline across age in male C57BL/6J mice

INTRODUCTION

Increasing age is the number one risk factor for developing cognitive decline and neurodegenerative disease. Aged humans and mice exhibit numerous molecular changes that contribute to a decline in cognitive function and increased risk of developing age-associated diseases. Here, we characterize multiple age-associated changes in male C57BL/6J mice to understand the translational utility of mouse aging.

METHODS

Male C57BL/6J mice from various ages between 2 and 24 months of age were used to assess behavioral, as well as, histological and molecular changes across three modalities: neuronal, microgliosis/neuroinflammation, and the neurovascular unit (NVU). Additionally, a cohort of 4- and 22-month-old mice was used to assess blood-brain barrier (BBB) breakdown. Mice in this cohort were treated with a high, acute dose of lipopolysaccharide (LPS, 10 mg/kg) or saline control 6 h prior to sacrifice followed by tail vein injection of 0.4 kDa sodium fluorescein (100 mg/kg) 2 h later.

RESULTS

Aged mice showed a decline in cognitive and motor abilities alongside decreased neurogenesis, proliferation, and synapse density. Further, neuroinflammation and circulating proinflammatory cytokines were increased in aged mice. Additionally, we found changes at the BBB, including increased T cell infiltration in multiple brain regions and an exacerbation in BBB leakiness following chemical insult with age. There were also a number of readouts that were unchanged with age and have limited utility as markers of aging in male C57BL/6J mice.

CONCLUSIONS

Here we propose that these changes may be used as molecular and histological readouts that correspond to aging-related behavioral decline. These comprehensive findings, in the context of the published literature, are an important resource toward deepening our understanding of normal aging and provide an important tool for studying aging in mice.

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

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

Oxytocin and microglia in the development of social behaviour

Oxytocin is a well-established regulator of social behaviour. Microglia, the resident immune cells of the central nervous system, regulate brain development and maintenance in health and disease. Oxytocin and microglia interact: microglia appear to regulate the oxytocin system and are, in turn, regulated by oxytocin, which appears to have anti-inflammatory effects. Both microglia and oxytocin are regulated in sex-specific ways. Oxytocin and microglia may work together to promote experience-dependent circuit refinement through multiple developmental-sensitive periods contributing to individual differences in social behaviour. This article is part of the theme issue 'Interplays between oxytocin and other neuromodulators in shaping complex social behaviours'.

Primary Cortical Cell Tri-Culture-Based Screening of Neuroinflammatory Response in Toll-like Receptor Activation

The activation of toll-like receptors (TLRs) in the central nervous system (CNS) can lead to neuroinflammation and contribute to many neurological disorders, including autoimmune diseases. Cell culture models are powerful tools for studying specific molecular and cellular mechanisms that contribute to these disease states and identifying potential therapeutics. However, most cell culture models have limitations in capturing biologically relevant phenomena, due in part to the non-inclusion of necessary cell types. Neurons, astrocytes, and microglia (critical cell types that play a role in neuroinflammation) all express at least a subset of TLRs. However, the response of each of these cell types to various TLR activation, along with their relative contribution to neuroinflammatory processes, is far from clear. In this study, we demonstrate the screening capabilities of a primary cortical cell tri-culture of neuron, astrocyte, and microglia from neonatal rats. Specifically, we compare the neuroinflammatory response of tri-cultures to that of primary neuron-astrocyte co-cultures to a suite of known TLR agonists. We demonstrate that microglia are required for observation of neurotoxic neuroinflammatory responses, such as increased cell death and apoptosis, in response to TLR2, 3, 4, and 7/8 activation. Additionally, we show that following TLR3 agonist treatment, microglia and astrocytes play opposing roles in the neuroinflammatory response, and that the observed response is dictated by the degree of TLR3 activation. Overall, we demonstrate that microglia play a significant role in the neuroinflammatory response to TLR activation in vitro and, hence, the tri-culture has the potential to serve as a screening platform that better replicates the in vivo responses.

An improved organotypic cell culture system to study tissue-resident macrophages ex vivo

Tissue-resident macrophages (TRMs) perform organ-specific functions that are dependent on factors such as hematopoietic origin, local environment, and biological influences. A diverse range of in vitro culture systems have been developed to decipher TRM functions, including bone marrow-derived macrophages (BMDMs), induced pluripotent stem cell (iPSC)-derived TRMs, or immortalized cell lines. However, despite the usefulness of such systems, there are notable limitations. Attempts to culture primary macrophages often require purification of cells and lack a high cell yield and consistent phenotype. Here, we aimed to address these limitations by establishing an organotypic primary cell culture protocol. We obtained long-term monocultures of macrophages derived from distinct organs without prior purification using specific growth factors and tissue normoxic conditions that largely conserved a TRM-like identity in vitro. Thus, this organotypic system offers an ideal screening platform for primary macrophages from different organs that can be used for a wide range of assays and readouts.

Contextual fear conditioning regulates synapse-related gene transcription in mouse microglia

Microglia have been suggested to be involved in the underlying mechanism of conditional fear memory formation by regulating inflammatory cytokines. However, the mechanism linking microglia and neuronal activity related to fear conditioning remains unclear. This study characterized the transcription profile of microglia in a fear memory conditional mouse model. Compared with those in control mice microglia, the most significantly induced genes were synapse-related, whereas immune-related genes were reduced due to fear memory consolidation. Whilst the increased expression of synapse-related genes was reversed after fear memory extinction, that of immunological genes was not, strongly suggesting a connection between microglia, neurons, and a dysregulated immune response following contextual fear conditioning. Furthermore, in the hippocampal microglia, we found that the expression of neurotransmitter release regulators, γ-aminobutyric acid (GABA) receptor GABRB3 and synapsin 1/2, increased under fear memory consolidation and restored (decreased) after extinction. In addition, compared with the transcription profile in peripheral monocytes, few overlapping genes were not enriched in biological processes. Taken together, the identified conditional fear stress-induced changes in mouse microglial transcription profiles suggest that microglia-neuron communication mediates contextual fear conditioning.

Chimeric GPCRs mimic distinct signaling pathways and modulate microglia responses

G protein-coupled receptors (GPCRs) regulate processes ranging from immune responses to neuronal signaling. However, ligands for many GPCRs remain unknown, suffer from off-target effects or have poor bioavailability. Additionally, dissecting cell type-specific responses is challenging when the same GPCR is expressed on different cells within a tissue. Here, we overcome these limitations by engineering DREADD-based GPCR chimeras that bind clozapine-N-oxide and mimic a GPCR-of-interest. We show that chimeric DREADD-β2AR triggers responses comparable to β2AR on second messenger and kinase activity, post-translational modifications, and protein-protein interactions. Moreover, we successfully recapitulate β2AR-mediated filopodia formation in microglia, an immune cell capable of driving central nervous system inflammation. When dissecting microglial inflammation, we included two additional DREADD-based chimeras mimicking microglia-enriched GPR65 and GPR109A. DREADD-β2AR and DREADD-GPR65 modulate the inflammatory response with high similarity to endogenous β2AR, while DREADD-GPR109A shows no impact. Our DREADD-based approach allows investigation of cell type-dependent pathways without known endogenous ligands.

Understanding the function of GPCRs requires stimulation with their specific ligands. Here, the authors design chemogenetic G-protein coupled receptors that allows for the study of receptors without knowing the immediate ligand, and demonstrate its use for the β2-adrenergic receptor in microglia.

A CRISPRi/a platform in human iPSC-derived microglia uncovers regulators of disease states

Microglia are emerging as key drivers of neurological diseases. However, we lack a systematic understanding of the underlying mechanisms. Here, we present a screening platform to systematically elucidate functional consequences of genetic perturbations in human induced pluripotent stem cell-derived microglia. We developed an efficient 8-day protocol for the generation of microglia-like cells based on the inducible expression of six transcription factors. We established inducible CRISPR interference and activation in this system and conducted three screens targeting the ‘druggable genome’. These screens uncovered genes controlling microglia survival, activation and phagocytosis, including neurodegeneration-associated genes. A screen with single-cell RNA sequencing as the readout revealed that these microglia adopt a spectrum of states mirroring those observed in human brains and identified regulators of these states. A disease-associated state characterized by osteopontin (SPP1) expression was selectively depleted by colony-stimulating factor-1 (CSF1R) inhibition. Thus, our platform can systematically uncover regulators of microglial states, enabling their functional characterization and therapeutic targeting.

Dräger et al. establish a rapid, scalable platform for iPSC-derived microglia. CRISPRi/a screens uncover roles of disease-associated genes in phagocytosis, and regulators of disease-relevant microglial states that can be targeted pharmacologically.

Anti-inflammatory clearance of amyloid-β by a chimeric Gas6 fusion protein

Clearing amyloid-β (Aβ) through immunotherapy is one of the most promising therapeutic approaches to Alzheimer's disease (AD). Although several monoclonal antibodies against Aβ have been shown to substantially reduce Aβ burden in patients with AD, their effects on improving cognitive function remain marginal. In addition, a significant portion of patients treated with Aβ-targeting antibodies experience brain edema and microhemorrhage associated with antibody-mediated Fc receptor activation in the brain. Here, we develop a phagocytosis inducer for Aβ consisting of a single-chain variable fragment of an Aβ-targeting monoclonal antibody fused with a truncated receptor binding domain of growth arrest-specific 6 (Gas6), a bridging molecule for the clearance of dead cells via TAM (TYRO3, AXL, and MERTK) receptors. This chimeric fusion protein (αAβ-Gas6) selectively eliminates Aβ plaques through TAM receptor-dependent phagocytosis without inducing NF-kB-mediated inflammatory responses or reactive gliosis. Furthermore, αAβ-Gas6 can induce synergistic clearance of Aβ by activating both microglial and astrocytic phagocytosis, resulting in better behavioral outcomes with substantially reduced synapse elimination and microhemorrhage in AD and cerebral amyloid angiopathy model mice compared with Aβ antibody treatment. Our results suggest that αAβ-Gas6 could be a novel immunotherapeutic agent for AD that overcomes the side effects of conventional antibody therapy.

Directed evolution of adeno-associated virus for efficient gene delivery to microglia

As the resident immune cells in the central nervous system (CNS), microglia orchestrate immune responses and dynamically sculpt neural circuits in the CNS. Microglial dysfunction and mutations of microglia-specific genes have been implicated in many diseases of the CNS. Developing effective and safe vehicles for transgene delivery into microglia will facilitate the studies of microglia biology and microglia-associated disease mechanisms. Here, we report the discovery of adeno-associated virus (AAV) variants that mediate efficient in vitro and in vivo microglial transduction via directed evolution of the AAV capsid protein. These AAV-cMG and AAV-MG variants are capable of delivering various genetic payloads into microglia with high efficiency, and enable sufficient transgene expression to support fluorescent labeling, Ca²⁺ and neurotransmitter imaging and genome editing in microglia in vivo. Furthermore, single-cell RNA sequencing shows that the AAV-MG variants mediate in vivo transgene delivery without inducing microglia immune activation. These AAV variants should facilitate the use of various genetically encoded sensors and effectors in the study of microglia-related biology.

Multiomics reveal the central role of pentose phosphate pathway in resident thymic macrophages to cope with efferocytosis-associated stress

Tissue-resident macrophages (TRMs) are heterogeneous cell populations found throughout the body. Depending on their location, they perform diverse functions maintaining tissue homeostasis and providing immune surveillance. To survive and function within, TRMs adapt metabolically to the distinct microenvironments. However, little is known about the metabolic signatures of TRMs. The thymus provides a nurturing milieu for developing thymocytes yet efficiently removes those that fail the selection, relying on the resident thymic macrophages (TMφs). This study harnesses multiomics analyses to characterize TMφs and unveils their metabolic features. We find that the pentose phosphate pathway (PPP) is preferentially activated in TMφs, responding to the reduction-oxidation demands associated with the efferocytosis of dying thymocytes. The blockade of PPP in Mφs leads to decreased efferocytosis, which can be rescued by reactive oxygen species (ROS) scavengers. Our study reveals the key role of the PPP in TMφs and underscores the importance of metabolic adaptation in supporting Mφ efferocytosis.

Human Microglia Extracellular Vesicles Derived from Different Microglia Cell Lines: Similarities and Differences

Microglial cells are a component of the innate immune system in the brain that support cell-to-cell communication via secreted molecules and extracellular vesicles (EVs). EVs can be divided into two major populations: large (LEVs) and small (SEVs) EVs, carrying different mediators, such as proteins, lipids, and miRNAs. The microglia EVs cargo crucially reflects the status of parental cells and can lead to both beneficial and detrimental effects in many physiopathological states. Herein, a workflow for the extraction and characterization of SEVs and LEVs from human C20 and HMC3 microglia cell lines derived, respectively, from adult and embryonic microglia is reported. EVs were gathered from the culture media of the two cell lines by sequential ultracentrifugation steps and their biochemical and biophysical properties were analyzed by Western blot, transmission electron microscopy, and dynamic light scattering. Although the C20- and HMC3-derived EVs shared several common features, C20-derived EVs were slightly lower in number and more polydispersed. Interestingly, C20- but not HMC3-SEVs were able to interfere with the proliferation of U87 glioblastoma cells. This correlated with the different relative levels of eight miRNAs involved in neuroinflammation and tumor progression in the C20- and HMC3-derived EVs, which in turn reflected a different basal activation state of the two cell types. Our data fill a gap in the community of microglia EVs, in which the preparations from human cells have been poorly characterized so far. Furthermore, these results shed light on both the differences and similarities of EVs extracted from different human microglia cell models, underlining the need to better characterize the features and biological effects of EVs for therein useful and correct application.

Acute inhibition of transient receptor potential vanilloid-type 4 cation channel halts cytoskeletal dynamism in microglia

Microglia, the resident macrophages of the central nervous system, are highly motile cells that support brain development, provision neuronal signaling, and protect brain cells against damage. Proper microglial functioning requires constant cell movement and morphological changes. Interestingly, the transient receptor potential vanilloid 4 (TRPV4) channel, a calcium-permeable channel, is involved in hypoosmotic morphological changes of retinal microglia and regulates temperature-dependent movement of microglial cells both in vitro and in vivo. Despite the broad functions of TRPV4 and the recent findings stating a role for TRPV4 in microglial movement, little is known about how TRPV4 modulates cytoskeletal remodeling to promote changes of microglial motility. Here we show that acute inhibition of TRPV4, but not its constitutive absence in the Trpv4 KO cells, affects the morphology and motility of microglia in vitro. Using high-end confocal imaging techniques, we show a decrease in actin-rich filopodia and tubulin dynamics upon acute inhibition of TRPV4 in vitro. Furthermore, using acute brain slices we demonstrate that Trpv4 knockout microglia display lower ramification complexity, slower process extension speed and consequently smaller surveyed area. We conclude that TRPV4 inhibition triggers a shift in cytoskeleton remodeling of microglia influencing their migration and morphology.

Microglia and Neurodevelopmental Disorders

Mounting evidence indicates that microglia, which are the resident immune cells of the brain, play critical roles in a diverse array of neurodevelopmental processes required for proper brain maturation and function. This evidence has ultimately led to growing speculation that microglial dysfunction may play a role in neurodevelopmental disorder (NDD) pathoetiology. In this review, we first provide an overview of how microglia mechanistically contribute to the sculpting of the developing brain and neuronal circuits. To provide an example of how disruption of microglial biology impacts NDD development, we also highlight emerging evidence that has linked microglial dysregulation to autism spectrum disorder pathogenesis. In recent years, there has been increasing interest in how the gut microbiome shapes microglial biology. In the last section of this review, we put a spotlight on this burgeoning area of microglial research and discuss how microbiota-dependent modulation of microglial biology is currently thought to influence NDD progression.

Activin A Secreted From Peripheral Nerve Fibroblasts Promotes Proliferation and Migration of Schwann Cells

The peripheral nervous system has remarkable regenerative capabilities. Schwann cells and fibroblasts are known to play crucial roles in these processes. In this study, we delineated the differential effects of peripheral nerve fibroblasts and cardiac fibroblasts on Schwann cells. We found that peripheral nerve fibroblasts significantly promoted Schwann cell proliferation and migration compared with cardiac fibroblasts. The cytokine array results identified 32 of 67 proteins that were considered differentially expressed in peripheral nerve fibroblasts versus cardiac fibroblasts. Among them, 25 were significantly upregulated in peripheral nerve fibroblasts compared with cardiac fibroblasts. Activin A, the protein with the greatest differential expression, clearly co-localized with fibroblasts in the in vivo sciatic never injury rat model. In vitro experiments proved that activin A secreted from nerve fibroblasts is the key factor responsible for boosting proliferation and migration of Schwann cells through ALK4, ALK5, and ALK7. Overall, these findings suggest that peripheral nerve fibroblasts and cardiac fibroblasts exhibit different patterns of cytokine secretion and activin A secreted from peripheral nerve fibroblasts can promote the proliferation and migration of Schwann cells.

Bacteria reduce flagellin synthesis to evade microglia-astrocyte-driven immunity in the brain

The immune response of brain cells to invading bacteria in vivo and the mechanism used by pathogenic bacteria to escape brain immune surveillance remain largely unknown. It is believed that microglia eliminate bacteria by phagocytosis based on in vitro data. Here we find that a small percentage of microglia in the brain engulf neonatal meningitis-causing Escherichia coli (NMEC), but more microglia are activated to produce tumor necrosis factor alpha (TNFα), which activates astrocytes to secrete complement component 3 (C3) involved in anti-bacterial activity. To evade anti-bacterial activity of the immune system, NMEC senses low concentration of threonine in cerebrospinal fluid (CSF) to down-modulate the expression of flagellin and reduce microglial TNFα and astrocyte C3 production. Our findings may help develop strategies for bacterial meningitis treatment.

Neuronal hyperexcitability in Alzheimer's disease: what are the drivers behind this aberrant phenotype?

Alzheimer's disease (AD) is a progressive neurodegenerative disorder leading to loss of cognitive abilities and ultimately, death. With no cure available, limited treatments mostly focus on symptom management. Identifying early changes in the disease course may provide new therapeutic targets to halt or reverse disease progression. Clinical studies have shown that cortical and hippocampal hyperactivity are a feature shared by patients in the early stages of disease, progressing to hypoactivity during later stages of neurodegeneration. The exact mechanisms causing neuronal excitability changes are not fully characterized; however, animal and cell models have provided insights into some of the factors involved in this phenotype. In this review, we summarize the evidence for neuronal excitability changes over the course of AD onset and progression and the molecular mechanisms underpinning these differences. Specifically, we discuss contributors to aberrant neuronal excitability, including abnormal levels of intracellular Ca²⁺ and glutamate, pathological amyloid β (Aβ) and tau, genetic risk factors, including APOE, and impaired inhibitory interneuron and glial function. In light of recent research indicating hyperexcitability could be a predictive marker of cognitive dysfunction, we further argue that the hyperexcitability phenotype could be leveraged to improve the diagnosis and treatment of AD, and present potential targets for future AD treatment development.

Astrocyte Reaction to Catechol-Induced Cytotoxicity Relies on the Contact with Microglia Before Isolation

Astrocytes preserve the brain microenvironment homeostasis in order to protect other brain cells, mainly neurons, against damages. Glial cells have specific functions that are important in the context of neuronal survival in different models of central nervous system (CNS) diseases. Microglia are among these cells, secreting several molecules that can modulate astrocyte functions. Although 1,2-dihydroxybenzene (catechol) is a neurotoxic monoaromatic compound of exogenous origin, several endogenous molecules also present the catechol group. This study compared two methods to obtain astrocyte-enriched cultures from newborn Wistar rats of both sexes. In the first technique (P1), microglial cells began to be removed early 48 h after primary mixed glial cultures were plated. In the second one (P2), microglial cells were late removed 7 to 10 days after plating. Both cultures were exposed to catechol for 72 h. Catechol was more cytotoxic to P1 cultures than to P2, decreasing cellularity and changing the cell morphology. Microglial-conditioned medium (MCM) protected P1 cultures and inhibited the catechol autoxidation. P2 cultures, as well as P1 in the presence of 20% MCM, presented long, dense, and fibrillary processes positive for glial fibrillary acidic protein, which retracted the cytoplasm when exposed to catechol. The Ngf and Il1beta transcription increased in P1, meanwhile astrocytes expressed more Il10 in P2. Catechol decreased Bdnf and Il10 in P2 cultures, and it decreased the expression of Il1beta in both conditions. A prolonged contact with microglia before isolation of astrocyte-enriched cultures modifies astrocyte functions and morphology, protecting these cells against catechol-induced cytotoxicity.

Microglial amyloid beta clearance is driven by PIEZO1 channels

Background

Microglia are the endogenous immune cells of the brain and act as sensors of pathology to maintain brain homeostasis and eliminate potential threats. In Alzheimer's disease (AD), toxic amyloid beta (Aβ) accumulates in the brain and forms stiff plaques. In late-onset AD accounting for 95% of all cases, this is thought to be due to reduced clearance of Aβ. Human genome-wide association studies and animal models suggest that reduced clearance results from aberrant function of microglia. While the impact of neurochemical pathways on microglia had been broadly studied, mechanical receptors regulating microglial functions remain largely unexplored.

Methods

Here we showed that a mechanotransduction ion channel, PIEZO1, is expressed and functional in human and mouse microglia. We used a small molecule agonist, Yoda1, to study how activation of PIEZO1 affects AD-related functions in human induced pluripotent stem cell (iPSC)-derived microglia-like cells (iMGL) under controlled laboratory experiments. Cell survival, metabolism, phagocytosis and lysosomal activity were assessed using real-time functional assays. To evaluate the effect of activation of PIEZO1 in vivo, 5-month-old 5xFAD male mice were infused daily with Yoda1 for two weeks through intracranial cannulas. Microglial Iba1 expression and Aβ pathology were quantified with immunohistochemistry and confocal microscopy. Published human and mouse AD datasets were used for in-depth analysis of PIEZO1 gene expression and related pathways in microglial subpopulations.

Results

We show that PIEZO1 orchestrates Aβ clearance by enhancing microglial survival, phagocytosis, and lysosomal activity. Aβ inhibited PIEZO1-mediated calcium transients, whereas activation of PIEZO1 with a selective agonist, Yoda1, improved microglial phagocytosis resulting in Aβ clearance both in human and mouse models of AD. Moreover, PIEZO1 expression was associated with a unique microglial transcriptional phenotype in AD as indicated by assessment of cellular metabolism, and human and mouse single-cell datasets.

Conclusion

These results indicate that the compromised function of microglia in AD could be improved by controlled activation of PIEZO1 channels resulting in alleviated Aβ burden. Pharmacological regulation of these mechanoreceptors in microglia could represent a novel therapeutic paradigm for AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02486-y.

Collaborative Action of Microglia and Astrocytes Mediates Neutrophil Recruitment to the CNS to Defend against Escherichia coli K1 Infection

Escherichia coli K1 is a leading cause of neonatal bacterial meningitis. Recruitment of neutrophils to the central nervous system (CNS) via local immune response plays a critical role in defense against E. coli K1 infection; however, the mechanism underlying this recruitment remains unclear. In this study, we report that microglia and astrocytes are activated in response to stimulation by E. coli K1 and/or E. coli K1-derived outer membrane vesicles (OMVs) and work collaboratively to drive neutrophil recruitment to the CNS. Microglial activation results in the release of the pro-inflammatory cytokine TNF-α, which activates astrocytes, resulting in the production of CXCL1, a chemokine critical for recruiting neutrophils. Mice lacking either microglia or TNF-α exhibit impaired production of CXCL1, impaired neutrophil recruitment, and an increased CNS bacterial burden. C-X-C chemokine receptor 2 (CXCR2)-expressing neutrophils primarily respond to CXCL1 released by astrocytes. This study provides further insights into how immune responses drive neutrophil recruitment to the brain to combat E. coli K1 infection. In addition, we show that direct recognition of E. coli K1 by microglia is prevented by the K1 capsule. This study also reveals that OMVs are sufficient to induce microglial activation.

The nonclassical MHC class I Qa-1 expressed in layer 6 neurons regulates activity-dependent plasticity via microglial CD94/NKG2 in the cortex

During developmental critical periods, circuits are sculpted by a process of activity-dependent competition. The molecular machinery involved in regulating the complex process of responding to different levels of activity is now beginning to be identified. Here, we show that the nonclassical major histocompatibility class I (MHCI) molecule Qa-1 is expressed in the healthy brain in layer 6 corticothalamic neurons. In the visual cortex, Qa-1 expression begins during the critical period for ocular dominance (OD) plasticity and is regulated by neuronal activity, suggesting a role in regulating activity-dependent competition. Indeed, in mice lacking Qa-1, OD plasticity is perturbed. Moreover, signaling through CD94/NKG2, a known cognate Qa-1 heterodimeric receptor in the immune system, is implicated: selectively targeting this interaction phenocopies the plasticity perturbation observed in Qa-1 knockouts. In the cortex, CD94/NKG2 is expressed by microglial cells, which undergo activity-dependent changes in their morphology in a Qa-1–dependent manner. Our study thus reveals a neuron–microglial interaction dependent upon a nonclassical MHCI molecule expressed in L6 neurons, which regulates plasticity in the visual cortex. These results also point to an unexpected function for the Qa-1/HLA-E (ligand) and CD94/NKG2 (receptor) interaction in the nervous system, in addition to that described in the immune system.

Cladribine Treatment for MS Preserves the Differentiative Capacity of Subsequently Generated Monocytes, Whereas Its Administration In Vitro Acutely Influences Monocyte Differentiation but Not Microglial Activation

Cladribine (2-chlorodeoxyadenosine, 2CdA) is one of the most effective disease-modifying drugs for multiple sclerosis (MS). Cladribine is a synthetic purine nucleoside analog that induces cell death of lymphocytes and oral cladribine treatment leads to a long-lasting disease stabilization, potentially attributable to immune reconstitution. In addition to its effects on lymphocytes, cladribine has been shown to have immunomodulatory effects on innate immune cells, including dendritic cells and monocytes, which could also contribute to its therapeutic efficacy. However, whether cladribine can modulate human macrophage/microglial activation or monocyte differentiation is currently unknown. The aim of this study was to determine the immunomodulatory effects of cladribine upon monocytes, monocyte-derived macrophages (MDMs) and microglia. We analyzed the phenotype and differentiation of monocytes from MS patients receiving their first course of oral cladribine both before and three weeks after the start of treatment. Flow cytometric analysis of monocytes from MS patients undergoing cladribine treatment revealed that the number and composition of CD14/CD16 monocyte subsets remained unchanged after treatment. Furthermore, after differentiation with M-CSF, such MDMs from treated MS patients showed no difference in gene expression of the inflammatory markers compared to baseline. We further investigated the direct effects of cladribine in vitro using human adult primary MDMs and microglia. GM-CSF-derived MDMs were more sensitive to cell death than M-CSF-derived MDMs. In addition, MDMs treated with cladribine showed increased expression of costimulatory molecules CD80 and CD40, as well as expression of anti-inflammatory, pro-trophic genes IL10 and MERTK, depending on the differentiation condition. Cladribine treatment in vitro did not modulate the expression of activation markers in human microglia. Our study shows that cladribine treatment in vitro affects the differentiation of monocytes into macrophages by modulating the expression of activation markers, which might occur similarly in tissue after their infiltration in the CNS during MS.

Microglia in a Dish—Which Techniques Are on the Menu for Functional Studies?

Microglia build the first line of defense in the central nervous system (CNS) and play central roles during development and homeostasis. Indeed, they serve a plethora of diverse functions in the CNS of which many are not yet fully described and more are still to be discovered. Research of the last decades unraveled an implication of microglia in nearly every neurodegenerative and neuroinflammatory disease, making it even more challenging to elucidate molecular mechanisms behind microglial functions and to modulate aberrant microglial behavior. To understand microglial functions and the underlying signaling machinery, many attempts were made to employ functional in vitro studies of microglia. However, the range of available cell culture models is wide and they come with different advantages and disadvantages for functional assays. Here we aim to provide a condensed summary of common microglia in vitro systems and discuss their potentials and shortcomings for functional studies in vitro.

Imaging Neuroinflammation in Neurodegenerative Disorders

Neuroinflammation plays a major role in the etiopathology of neurodegenerative diseases, including Alzheimer and Parkinson diseases, frontotemporal lobar degeneration, and amyotrophic lateral sclerosis. In vivo monitoring of neuroinflammation using PET is critical to understand this process, and data are accumulating in this regard, thus a review is useful. From PubMed, we retrieved publications using any of the available PET tracers to image neuroinflammation in humans as well as selected articles dealing with experimental animal models or the chemistry of currently used or potential radiotracers. We reviewed 280 articles. The most common PET neuroinflammation target, translocator protein (TSPO), has limitations, lacking cellular specificity and the ability to separate neuroprotective from neurotoxic inflammation. However, TSPO PET is useful to define the amount and location of inflammation in the brain of people with neurodegenerative disorders. We describe the characteristics of TSPO and other potential PET neuroinflammation targets and PET tracers available or in development. Despite target and tracer limitations, in recent years there has been a sharp increase in the number of reports of neuroinflammation PET in humans. The most studied has been Alzheimer disease, in which neuroinflammation seems initially neuroprotective and neurotoxic later in the progression of the disease. We describe the findings in all the major neurodegenerative disorders. Neuroinflammation PET is an indispensable tool to understand the process of neurodegeneration, particularly in humans, as well as to validate target engagement in therapeutic clinical trials.

Cell type specific isolation of primary astrocytes and microglia from adult mouse spinal cord

BACKGROUND

Astrocytes and microglia are essential cellular elements of the CNS that are critical for normal development, function, and injury responses. Both cell types are highly pleiotropic and respond rapidly to environmental changes, making them challenging to characterize. One approach is to develop efficient isolation paradigms of distinct cell populations, allowing for characterization of their roles in distinct CNS regions and in pathological states.

NEW METHOD

We have developed an efficient and reliable protocol for isolation of astrocytes and microglia from the adult mouse spinal cord, which can be easily manipulated for immediate or future analyses. This method involves (1) rapid tissue dissociation; (2) cell release after myelin debris removal; (3) magnetic-activated cell sorting; and (4) optional downstream molecular and functional analyses.

RESULTS

High levels of viability and purity of the cells were confirmed after isolation. More importantly, characterization of cells verified their ability to proliferate and respond to external stimuli for potential use in downstream molecular and functional assays.

COMPARISON WITH EXISTING METHOD(S)

Long-term culture of cells isolated from neonatal animals and cell type specific isolation from the brain have been successful; however, isolation of spinal cord cells from adult mice has been challenging due to the large amount of myelin and limited size of the tissue compared to the brain. Our method allows for efficient isolation of astrocytes and microglia from spinal cord alone and includes simple modifications to allow for various downstream applications.

CONCLUSIONS

This technique will be a valuable tool to better understand the functions of astrocytes and microglia in spinal cord function and pathology.

Regulation of activated microglia and macrophages by systemically administered DNA/RNA heteroduplex oligonucleotides

Microglial activation followed by recruitment of blood-borne macrophages into the central nervous system (CNS) aggravates neuroinflammation. Specifically, in multiple sclerosis (MS) as well as in experimental autoimmune encephalomyelitis (EAE), a rodent model of MS, activated microglia and macrophages (Mg/Mφ) promote proinflammatory responses and expand demyelination in the CNS. However, a potent therapeutic approach through the systemic route for regulating their functions has not yet been developed. Here, we demonstrate that a systemically injected DNA/RNA heteroduplex oligonucleotide (HDO), composed of an antisense oligonucleotide (ASO) and its complementary RNA, conjugated to cholesterol (Chol-HDO) distributed more efficiently to demyelinating lesions of the spinal cord in EAE mice with significant gene silencing than the parent ASO. Importantly, systemic administration of Cd40-targeting Chol-HDO improved clinical signs of EAE with significant downregulation of Cd40 in Mg/Mφ. Furthermore, we successfully identify that macrophage scavenger receptor 1 (MSR1) is responsible for the uptake of Chol-HDO by Mg/Mφ of EAE mice. Overall, our findings demonstrate the therapeutic potency of systemically administered Chol-HDO to regulate activated Mg/Mφ in neuroinflammation.

Characterization of Ex Vivo and In Vitro Wnt Transcriptome Induced by Spinal Cord Injury in Rat Microglial Cells

It is well known that inflammation is crucial in the onset and progression of neurodegenerative diseases and traumatic central nervous system (CNS) injuries, and that microglia and monocyte-derived macrophages (MDMs) play a pivotal role in neuroinflammation. Therefore, the exploration of molecular signaling pathways that are involved in the microglia/macrophage response might help us to shed light on their eventual therapeutic modulation. Interestingly, there is growing evidence showing that the Wnt family of proteins is involved in different neuropathologies that are characterized by a dysregulated neuroinflammatory response, including spinal cord injury (SCI). Here, we aimed to validate a methodology with competence to assess the physiologically relevant Wnt expression patterns of active microglia and MDMs in a rat model of SCI. For that purpose, we have selected and adapted an in vitro system of primary microglia culture that were stimulated with a lesioned spinal cord extract (SCE), together with an ex vivo protocol of flow cytometry sorting of rat microglia/MDMs at different time-points after contusive SCI. Our study demonstrates that the expression profile of Wnt-related genes in microglia/MDM cells exhibit important differences between these particular scenarios which would be in line with previous studies where similar discrepancies have been described for other molecules. Moreover, our results provide for a first experimental report of the Wnt transcriptome in rat microglia and MDMs after SCI which, together with the research platform that was used in the study, and considering its limitations, we expect might contribute to foster the research on Wnt-driven immunomodulatory therapies.

Neonatal Rat Glia Cultured in Physiological Normoxia for Modeling Neuropathological Conditions In Vitro

Cell culture conditions were proven to highly affect crucial biological processes like proliferation, differentiation, intercellular crosstalk, and senescence. Oxygen tension is one of the major factors influencing cell metabolism and thus, modulating cellular response to pathophysiological conditions. In this context, the presented study aimed at the development of a protocol for efficient culture of rat neonatal glial cells (microglia, astrocytes, and oligodendrocytes) in oxygen concentrations relevant to the nervous tissue. The protocol allows for obtaining three major cell populations, which play crucial roles in sustaining tissue homeostasis and are known to be activated in response to a wide spectrum of external stimuli. The cells are cultured in media without supplement addition to avoid potential modulation of cell processes. The application of active biomolecules for coating culturing surfaces might be useful for mirroring physiological cell interactions with extracellular matrix components. The cell fractions can be assembled as cocultures to further evaluate investigated mechanisms, intercellular crosstalk, or cell response to tested pharmacological compounds. Applying additional procedures, like transient oxygen and glucose deprivation, allows to mimic in vitro the selected pathophysiological conditions. The presented culture system for neonatal rat glial cells is a highly useful tool for in vitro modeling selected neuropathological conditions.

Spi1 -14 Kb upstream regulatory element (URE) is not required for maintenance of PU.1 expression in macrophages

Background: Previous work suggested an upstream regulatory element (URE) of Spi1 was required to maintain constant expression of the PU.1 transcription factor in bone marrow and foetal liver cells. PU.1, encoded by Spi1, is essential for development and maintenance of myeloid and B-lymphocyte populations in mice. Deletion of this (-14 Kb) URE potentially reduces expression of PU.1 and therefore provides a way to investigate its role in myeloid populations in development and disease. This study aimed to examine the impact of removal of the -14 Kb Spi1 URE in Cx3cr1+ cells on the myeloid lineage formation and maintenance. Methods: B6;129-Spi1tm1.2Dgt/J mice, whose -14 Kb Spi1 URE mice is flanked by LoxP sites (‘floxed’), were bred to a strain with constitutively active Cre expressed under the Cx3cr1 promoter (B6J.B6N(Cg)-Cx3cr1tm1.1(cre)Jung/J) to delete the Spi1 URE in myeloid cells. The floxed mice were also bred to mice with a tamoxifen-inducible Cre expressed under the Cx3cr1 promoter (B6.129P2(C)-Cx3cr1tm2.1(cre/ERT2)Jung/J) to be used as URE intact controls and to permit temporally-controlled deletion of the URE if required. PU.1 protein expression was measured in the peritoneal macrophages and microglia by flow cytometry. Additionally, a Cre-encoding lentiviral vector was used to assess the impact on PU.1 expression in bone-marrow derived macrophages from these mice in vitro. Results: Expression of the PU.1 transcription factor was not significantly altered in the peritoneal macrophages or microglia in mice lacking the -14 Kb Spi1 URE. Moreover, initial experiments utilising Cre encoding lentivirus did not reduce PU.1 protein in bone-marrow derived macrophages differentiated from the -14 Kb Spi1 URE floxed mice. Conclusions: These observations suggest that the -14 Kb URE does not play a major role in PU.1 protein expression in either mature peritoneal macrophages or microglia.

URMC-099 prophylaxis prevents hippocampal vascular vulnerability and synaptic damage in an orthopedic model of delirium superimposed on dementia

Systemic perturbations can drive a neuroimmune cascade after surgical trauma, including affecting the blood-brain barrier (BBB), activating microglia, and contributing to cognitive deficits such as delirium. Delirium superimposed on dementia (DSD) is a particularly debilitating complication that renders the brain further vulnerable to neuroinflammation and neurodegeneration, albeit these molecular mechanisms remain poorly understood. Here, we have used an orthopedic model of tibial fracture/fixation in APPSwDI/mNos2^-/- AD (CVN-AD) mice to investigate relevant pathogenetic mechanisms underlying DSD. We conducted the present study in 6-month-old CVN-AD mice, an age at which we speculated amyloid-β pathology had not saturated BBB and neuroimmune functioning. We found that URMC-099, our brain-penetrant anti-inflammatory neuroprotective drug, prevented inflammatory endothelial activation, breakdown of the BBB, synapse loss, and microglial activation in our DSD model. Taken together, our data link post-surgical endothelial activation, microglial MafB immunoreactivity, and synapse loss as key substrates for DSD, all of which can be prevented by URMC-099.

Neuronal apoptosis drives remodeling states of microglia and shifts in survival pathway dependence

Microglia serve critical remodeling roles that shape the developing nervous system, responding to the changing neural environment with phagocytosis or soluble factor secretion. Recent single-cell sequencing (scRNAseq) studies have revealed the context-dependent diversity in microglial properties and gene expression, but the cues promoting this diversity are not well defined. Here, we ask how interactions with apoptotic neurons shape microglial state, including lysosomal and lipid metabolism gene expression and dependence on Colony-stimulating factor 1 receptor (CSF1R) for survival. Using early postnatal mouse retina, a CNS region undergoing significant developmental remodeling, we performed scRNAseq on microglia from mice that are wild-type, lack neuronal apoptosis (Bax KO), or are treated with CSF1R inhibitor (PLX3397). We find that interactions with apoptotic neurons drive multiple microglial remodeling states, subsets of which are resistant to CSF1R inhibition. We find that TAM receptor Mer and complement receptor 3 are required for clearance of apoptotic neurons, but that Mer does not drive expression of remodeling genes. We show TAM receptor Axl is negligible for phagocytosis or remodeling gene expression but is consequential for microglial survival in the absence of CSF1R signaling. Thus, interactions with apoptotic neurons shift microglia toward distinct remodeling states and through Axl, alter microglial dependence on survival pathway, CSF1R.

Altered Cholesterol Homeostasis in Huntington’s Disease

Huntington’s disease (HD) is an autosomal dominant genetic disorder caused by an expansion of the CAG repeat in the first exon of Huntingtin’s gene. The associated neurodegeneration mainly affects the striatum and the cortex at early stages and progressively spreads to other brain structures. Targeting HD at its earlier stages is under intense investigation. Numerous drugs were tested, with a rate of success of only 3.5% approved molecules used as symptomatic treatment. The restoration of cholesterol metabolism, which is central to the brain homeostasis and strongly altered in HD, could be an interesting disease-modifying strategy. Cholesterol is an essential membrane component in the central nervous system (CNS); alterations of its homeostasis have deleterious consequences on neuronal functions. The levels of several sterols, upstream of cholesterol, are markedly decreased within the striatum of HD mouse model. Transcription of cholesterol biosynthetic genes is reduced in HD cell and mouse models as well as post-mortem striatal and cortical tissues from HD patients. Since the dynamic of brain cholesterol metabolism is complex, it is essential to establish the best method to target it in HD. Cholesterol, which does not cross the blood-brain-barrier, is locally synthesized and renewed within the brain. All cell types in the CNS synthesize cholesterol during development but as they progress through adulthood, neurons down-regulate their cholesterol synthesis and turn to astrocytes for their full supply. Cellular levels of cholesterol reflect the dynamic balance between synthesis, uptake and export, all integrated into the context of the cross talk between neurons and glial cells. In this review, we describe the latest advances regarding the role of cholesterol deregulation in neuronal functions and how this could be a determinant factor in neuronal degeneration and HD progression. The pathways and major mechanisms by which cholesterol and sterols are regulated in the CNS will be described. From this overview, we discuss the main clinical strategies for manipulating cholesterol metabolism in the CNS, and how to reinstate a proper balance in HD.