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

Transcription factors that define the epigenome structures and transcriptomes in microglia

Microglia, the resident macrophages of the brain, play critical roles in maintaining brain health. Recent genome-wide analyses, including ATAC-seq, ChIP-seq/CUT&RUN, and single-cell RNA-seq, have identified key transcription factors that define the transcriptome programs of microglia. Four transcription factors-PU.1, IRF8, SALL1, and SMAD4-form enhancer complexes and act as lineage-determining factors, shaping microglial identity. These factors co-bind with other lineage-determining transcription factors, directing one towards designated regions that program microglia while inhibiting the other from binding to DNA. Other transcription factors, such as BATF3 and MAFB, contribute to transcriptional cascades in microglia. TGF-β is a crucial cytokine driving these transcription factors to bind DNA and maintain homeostatic microglia. These findings provide insights into the physiological aspects of microglia and their roles in neuroinflammatory and neurodegenerative diseases. TEASER ABSTRACT: eTOC blurb: In this article, we compiled more than 100 transcription factors expressed in microglia. Our analysis illustrates that some transcription factors are under a distinct hierarchical rank and are sequentially activated to achieve microglia specific transcriptome programs. This article offers a new scope on the mechanistic foundation underlying microglia's complex activity.

Intestinal neuron-associated macrophages in health and disease

Neuron-macrophage cross-talk in the intestine plays a crucial role in the maintenance of tissue homeostasis and the modulation of immune responses throughout life. Here, we describe how gut neuron-macrophage interactions shift macrophage phenotype and function from early development to adulthood and how this cross-talk modulates the macrophage function in response to infection and inflammation. We highlight how a neural microenvironment instructs a neuron-associated macrophage phenotype in the gut and show that their phenotype may resemble nerve-associated macrophages in other organs. Finally, we note that the loss of neuron-associated macrophages or a shift in their phenotype can contribute to enteric neurodegeneration in the gastrointestinal tract, causing gut motility disorders.

Microglia transcriptional states and their functional significance: Context drives diversity

In the brain, microglia are continuously exposed to a dynamic microenvironment throughout life, requiring them to adapt accordingly to specific developmental or disease-related demands. The advent of single-cell sequencing technologies has revealed the diversity of microglial transcriptional states. In this review, we explore the various contexts that drive transcriptional diversity in microglia and assess the extent to which non-homeostatic conditions induce context-specific signatures. We discuss our current understanding and knowledge gaps regarding the relationship between transcriptional states and microglial function, review the influence of complex microenvironments and prior experiences on microglial state induction, and highlight strategies to bridge the gap between mouse and human studies to advance microglia-targeting therapeutics.

Refining the interactions between microglia and astrocytes in Alzheimer's disease pathology

Microglia and astrocytes are central to the pathogenesis and progression of Alzheimer's Disease (AD), working both independently and collaboratively to regulate key pathological processes such as β-amyloid protein (Aβ) deposition, tau aggregation, neuroinflammation, and synapse loss. These glial cells interact through complex molecular pathways, including IL-3/IL-3Ra and C3/C3aR, which influence disease progression and cognitive decline. Emerging research suggests that modulating these pathways could offer therapeutic benefits. For instance, recombinant IL-3 administration in mice reduced Aβ plaques and improved cognitive functions, while C3aR inhibition alleviated Aβ and tau pathologies, restored synaptic function, and corrected immune dysregulation. However, the effects of these interactions are context-dependent. Acute C3/C3aR activation enhances microglial Aβ clearance, whereas chronic activation impairs it, highlighting the dual roles of glial signaling in AD. Furthermore, C3/C3aR signaling not only impacts Aβ clearance but also modulates tau pathology and synaptic integrity. Given AD's multifactorial nature, understanding the specific pathological environment is crucial when investigating glial cell contributions. The interplay between microglia and astrocytes can be both neuroprotective and neurotoxic, depending on the disease stage and brain region. This complexity underscores the need for targeted therapies that modulate glial cell activity in a context-specific manner. By elucidating the molecular mechanisms underlying microglia-astrocyte interactions, this research advances our understanding of AD and paves the way for novel therapeutic strategies aimed at mitigating neurodegeneration and cognitive decline in AD and related disorders.

Modulation of ageing mice microglia functions during neuroinflammation using synthetic cannabinoids

Ageing is marked by a gradual rise in systemic inflammation, with neuroinflammation being a key feature. Neuroinflammation, which refers to the immune response within the CNS, is primarily mediated by microglia. These resident macrophages in the CNS parenchyma are essential for maintaining homeostasis and initiating immune responses. Their function depends on the timely activation and deactivation of microglia to regulate these processes effectively. Excessive activation of microglia has been shown to disrupt brain functions and promote a proinflammatory response leading to dysfunctional microglia that are unable to carry out immune response due to various reasons, with major implications in neuroinflammation. Therefore, there is a need to monitor and modulate the functionality of microglia to elicit a healthy immune response. The endocannabinoid system is a negative feedback system that is activated to modulate various mechanisms related to inflammation. In our research, we therefore investigated the functionality of microglia in relation to phagocytosis and oxidative stress during neuroinflammation by stimulating the endocannabinoid receptors Cnr1 and Cnr2 with synthetic cannabinoid compounds in ageing mice. Our results show that the expression of the endocannabinoid system (ECS) increases with age. Activation of CB1 and CB2 receptors reduces reactive oxygen species (ROS) in both young and aged mice, with the effect being more pronounced effect in younger mice. In aged mice, the upregulation of these receptors indicates persistent inflammation, while microglial phagocytosis is modulated through CB1 receptors in both age groups.

Characterizing Microglial Signaling Dynamics During Inflammation Using Single-Cell Mass Cytometry

Microglia play a critical role in maintaining central nervous system (CNS) homeostasis and display remarkable plasticity in their response to inflammatory stimuli. However, the specific signaling profiles that microglia adopt during such challenges remain incompletely understood. Traditional transcriptomic approaches provide valuable insights, but fail to capture dynamic post-translational changes. In this study, we utilized time-resolved single-cell mass cytometry (CyTOF) to measure distinct signaling pathways activated in microglia upon exposure to bacterial and viral mimetics-lipopolysaccharide (LPS) and polyinosinic-polycytidylic acid (Poly(I:C)), respectively. Furthermore, we evaluated the immunomodulatory role of astrocytes on microglial signaling in mixed cultures. Microglia or mixed cultures derived from neonatal mice were treated with LPS or Poly(I:C) for 48 h. Cultures were stained with a panel of 33 metal-conjugated antibodies targeting signaling and identity markers. High-dimensional clustering analysis was used to identify emergent signaling modules. We found that LPS treatment led to more robust early activation of pp38, pERK, pRSK, and pCREB compared to Poly(I:C). Despite these differences, both LPS and Poly(I:C) upregulated the classical reactivity markers CD40 and CD86 at later time points. Strikingly, the presence of astrocytes significantly blunted microglial responses to both stimuli, particularly dampening CD40 upregulation. Our studies demonstrate that single-cell mass cytometry effectively captures the dynamic signaling landscape of microglia under pro-inflammatory conditions. This approach may pave the way for targeted therapeutic investigations of various neuroinflammatory disorders. Moreover, our findings underscore the necessity of considering cellular context, such as astrocyte presence, in interpreting microglial behavior during inflammation.

Papain Affects the Percentage and Morphology of Microglia in Hippocampal Neuron-Glial Cultures

Background. Microglia, accounting for 5-15% of total brain cells, represent an essential population of glial cells in the cultures used for modeling neuroinflammation in vitro. However, microglia proliferation is poor in neuron-glial cultures. Here, we studied the population composition of rat hippocampal neuron-glial cell cultures prepared utilizing papain (PAP cultures) and trypsin (TRY cultures) as proteolytic enzymes for cell isolation. Methods. To evaluate the percentage and morphology of microglia in TRY and PAP cultures and cultures incubated in the presence of TGFβ+MCSF+cholesterol, which should enhance microglia proliferation, we used an immunostaining and calcium imaging approach in combination with staining using the recently developed vital microglia fluorescent probe CDr20. Results. We have shown that the microglia percentage in PAP cultures was higher than in TRY cultures. Microglia in PAP cultures are predominantly polarized, while bushy morphology was more characteristic of TRY cultures. We have also demonstrated that the TGFβ+MCSF+cholesterol combination increases the microglia number both in PAP and TRY cultures (up to 25-30%) and promotes the appearance of ameboid microglia characterized by high mobility. However, the significant appearance of ameboid microglia was observed already at the early stages of cultivation (2 DIV) in TRY cultures, while in PAP cultures, the described transformation was observed at 7 DIV. Based on the absence of the ATP-induced Ca2+ response, round shape, significant proliferation, and high mobility, we have suggested that ameboid microglia are reactive. Conclusions. Thus, our results demonstrate that papain is a more suitable proteolytic enzyme for preparing mixed hippocampal neuron-glial cultures with a higher percentage of heterogeneous microglia and functional neurons and astrocytes (tricultures).

Expression of anti-amyloid CARs in microglia promotes efficient and selective phagocytosis of Aβ1‒42

Genetic engineering of microglial cells is a promising therapeutic avenue emerging with advancements in gene delivery techniques. Using a recently developed AAV capsid for efficient in vitro transduction we report the engineering of microglia with CARs (CAR-Mic) targeting phagocytosis of amyloid beta 1‒42 (Aβ42). Functional screening of seven CAR constructs in human iPSC-derived microglia revealed up to 6-fold increases in internalized Aβ relative to viral control. CAR-driven phagocytic enhancement was selective for Aβ, dependent on intracellular domain signaling, and was confirmed in primary mouse microglia. These findings highlight the potential of using this approach to target dysfunctional microglia in Alzheimer's disease and other CNS disorders.

Spatial transcriptomics of the aging mouse brain reveals origins of inflammation in the white matter

To systematically understand age-induced molecular changes, we performed spatial transcriptomics of young, middle-aged, and old mouse brains and identified seven transcriptionally distinct regions. All regions exhibited age-associated upregulation of inflammatory mRNAs and downregulation of mRNAs related to synaptic function. Notably, aging white matter fiber tracts showed the most prominent changes with pronounced effects in females. The inflammatory signatures indicated major ongoing events: microglia activation, astrogliosis, complement activation, and myeloid cell infiltration. Immunofluorescence and quantitative MRI analyses confirmed physical interaction of activated microglia with fiber tracts and concomitant reduction of myelin in old mice. In silico analyses identified potential transcription factors influencing these changes. Our study provides a resourceful dataset of spatially resolved transcriptomic features in the naturally aging murine brain encompassing three age groups and both sexes. The results link previous disjointed findings and provide a comprehensive overview of brain aging identifying fiber tracts as a focal point of inflammation.

Expanding the neuroimmune research toolkit with in vivo brain organoid technologies

Human pluripotent stem cell-derived microglia-like cells (MLCs) and brain organoid systems have revolutionized the study of neuroimmune interactions, providing new opportunities to model human-specific brain development and disease. Over the past decade, advances in protocol design have improved the fidelity, reproducibility and scalability of MLC and brain organoid generation. Co-culturing of MLCs and brain organoids have enabled direct investigations of human microglial interactions in vitro, although opportunities remain to improve microglial maturation and long-term survival. To address these limitations, innovative xenotransplantation approaches have introduced MLCs, organoids or neuroimmune organoids into the rodent brain, providing a vascularized environment that supports prolonged development and potential behavioral readouts. These expanding in vitro and in vivo toolkits offer complementary strategies to study neuroimmune interactions in health and disease. In this Perspective, we discuss the strengths, limitations and synergies of these models, highlighting important considerations for their future applications.

Lipid metabolism, remodelling and intercellular transfer in the CNS

Lipid metabolism encompasses the catabolism and anabolism of lipids, and is fundamental for the maintenance of cellular homeostasis, particularly within the lipid-rich CNS. Increasing evidence further underscores the importance of lipid remodelling and transfer within and between glial cells and neurons as key orchestrators of CNS lipid homeostasis. In this Review, we summarize and discuss the complex landscape of processes involved in lipid metabolism, remodelling and intercellular transfer in the CNS. Highlighted are key pathways, including those mediating lipid (and lipid droplet) biogenesis and breakdown, lipid oxidation and phospholipid metabolism, as well as cell-cell lipid transfer mediated via lipoproteins, extracellular vesicles and tunnelling nanotubes. We further explore how the dysregulation of these pathways contributes to the onset and progression of neurodegenerative diseases, and examine the homeostatic and pathogenic impacts of environment, diet and lifestyle on CNS lipid metabolism.

The A53T Mutation in α-Synuclein Enhances Proinflammatory Activation in Human Microglia Upon Inflammatory Stimulus

BACKGROUND

Parkinson's disease (PD) is the second most common neurodegenerative disease, following Alzheimer's. It is characterized by the aggregation of α-synuclein into Lewy bodies and Lewy neurites in the brain. Microglia-driven neuroinflammation may contribute to neuronal death in PD; however, the exact role of microglia remains unclear and has been understudied. The A53T mutation in the gene coding for α-synuclein has been linked to early-onset PD, and exposure to A53T mutant human α-synuclein increases the potential for inflammation of murine microglia. To date, its effect has not been studied in human microglia.

METHODS

Here, we used 2-dimensional cultures of human pluripotent stem cell-derived microglia and transplantation of these cells into the mouse brain to assess the cell autonomous effects of the A53T mutation on human microglia.

RESULTS

We found that A53T mutant human microglia had an intrinsically increased propensity toward proinflammatory activation upon inflammatory stimulus. Additionally, transplanted A53T mutant microglia showed a strong decrease in catalase expression in noninflammatory conditions and increased oxidative stress.

CONCLUSIONS

Our results indicate that A53T mutant human microglia display cell autonomous phenotypes that may worsen neuronal damage in early-onset PD.

Reactive Astrocytes in Glioma: Emerging Opportunities and Challenges

Gliomas are the most prevalent malignant tumors in the adult central nervous system (CNS). Glioblastoma (GBM) accounts for approximately 60-70% of primary gliomas. It is a great challenge to human health because of its high degree of malignancy, rapid progression, short survival time, and susceptibility to recurrence. Owing to the specificity of the CNS, the glioma microenvironment often contains numerous glial cells. Astrocytes are most widely distributed in the human brain and form reactive astrocyte proliferation regions around glioma tissue. In addition, astrocytes are activated under pathological conditions and regulate tumor and microenvironmental cells through cell-to-cell contact or the secretion of active substances. Therefore, astrocytes have attracted attention as important components of the glioma microenvironment. Here, we focus on the mechanisms of reactive astrocyte activation under glioma conditions, their contribution to the mechanisms of glioma genesis and progression, and their potential value as targets for clinical intervention in gliomas.

A versatile mouse model to advance human microglia transplantation research in neurodegenerative diseases

BACKGROUND

Recent studies highlight the critical role of microglia in neurodegenerative disorders, and emphasize the need for humanized models to accurately study microglial responses. Human-mouse microglia xenotransplantation models are a valuable platform for functional studies and for testing therapeutic approaches, yet currently those models are only available for academic research. This hampers their implementation for the development and testing of medication that targets human microglia.

METHODS

We developed the hCSF1Bdes mouse line, which is suitable as a new transplantation model and available to be crossed to any disease model of interest. The hCSF1Bdes model created by CRISPR gene editing is RAG2 deficient and expresses human CSF1. Additionally, we crossed this model with two humanized App KI mice, the AppHu and the AppSAA. Flow cytometry, immunohistochemistry and bulk sequencing was used to study the response of microglia in the context of Alzheimer's disease.

RESULTS

Our results demonstrate the successful transplantation of iPSC-derived human microglia into the brains of hCSF1Bdes mice without triggering a NK-driven immune response. Furthermore, we confirmed the multipronged response of microglia in the context of Alzheimer's disease. The hCSF1Bdes and the crosses with the Alzheimer's disease knock-in model AppSAA and the humanized App knock-in control mice, AppHu are deposited with EMMA and fully accessible to the research community.

CONCLUSION

The hCSF1Bdes mouse is available for both non-profit and for-profit organisations, facilitating the use of the xenotransplantation paradigm for human microglia to study complex human disease.

Serum starvation induces cytosolic DNA trafficking via exosome and autophagy-lysosome pathway in microglia

The imbalance of microglial homeostasis is highly associated with age-related neurological diseases, where cytosolic endogenous DNA is also likely to be found. As the main medium for storing biological information, endogenous DNA could be localized to cellular compartments normally free of DNA when cells are stimulated. However, the intracellular trafficking of endogenous DNA remains unidentified. In this study, we demonstrated that nuclear DNA (nDNA) and mitochondrial DNA (mtDNA), as the components of endogenous DNA, undergo different intracellular trafficking under conditions of microglial homeostasis imbalance induced by serum starvation. Upon detecting various components of endogenous DNA in the cytoplasmic and extracellular microglia, we found that cytosolic nDNA primarily exists in a free form and undergoes degradation through the autophagy-lysosome pathway. In contrast, cytosolic mtDNA predominantly exists in a membrane-wrapped form and is trafficked through both exosome and autophagy-lysosome pathways, with the exosome pathway serving as the primary one. When the autophagy-lysosome pathway was inhibited, there was an increase in exosomes. More importantly, the inhibition of the autophagy-lysosome pathway resulted in enhanced trafficking of mtDNA through the exosome pathway. These findings unveiled the crosstalk between these two pathways in the trafficking of microglial cytosolic DNA and thus provide new insights into intervening in age-related neurological diseases.

Microglia-like cells from patient monocytes demonstrate increased phagocytic activity in probable Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder that is characterized by the accumulation of amyloid plaques, phosphorylated tau tangles and microglia toxicity, resulting in neuronal death and cognitive decline. Since microglia are recognized as one of the key players in the disease, it is crucial to understand how microglia operate in disease conditions and incorporate them into models. The studies on human microglia functions are thought to reflect the post-symptomatic stage of the disease. Recently developed methods involve induced microglia-like cells (iMGs) generated from patients' blood monocytes or induced pluripotent stem cells (iPSCs) as an alternative to studying the microglia cells in vitro. In this research, we aimed to investigate the phenotype and inflammatory responses of iMGs from AD patients. Monocytes derived from blood using density gradient centrifugation were differentiated into iMGs using a cytokine cocktail, including granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-34 (IL-34). After differentiation, cells were assessed by morphological analysis and a microglia surface marker, TMEM119. We used stimulants, lipopolysaccharide (LPS) and beta-amyloid, to examine iMGs' functions. Results showed that iMGs derived from AD patients exhibited increased secretion of pro-inflammatory cytokines upon LPS stimulation. Furthermore, their phagocytic ability was also heightened in stimulated and unstimulated conditions, with cells derived from patients showing increased phagocytic activity compared to healthy controls. Overall, these findings suggest that iMGs derived from patients using the direct conversion method possess characteristics of human microglia, making them an easy and promising model for studying microglia function in AD.

Microglial cell proliferation is regulated, in part, by reactive astrocyte ETBR signaling after ischemic stroke

Reciprocal communication between reactive astrocytes and microglial cells provides local, coordinated control over critical processes such as neuroinflammation, neuroprotection, and scar formation after CNS injury, but is poorly understood. The vasoactive peptide hormone endothelin (ET) is released and/or secreted by endothelial cells, microglial cells and astrocytes early after ischemic stroke and other forms of brain injury. To better understand glial cell communication after stroke, we sought to identify paracrine effectors produced and secreted downstream of astroglial endothelin receptor B (ETBR) signaling. Using a genetic loss-of-function screen, we identified angiopoietin-2 (Ang-2) as a factor produced by reactive astrocytes in response to ET. In experiments with primary adult astrocytes stimulated by IRL1620, a specific ETBR agonist, we found that ERK1/2 and NFkB mediated the effects of ET on Ang-2 production. To determine astroglial Ang-2 levels in vivo, reactive astrocytes expressing the high affinity glutamate transporter (GLAST, EAAT1) were isolated by magnetic-activated cell sorting 3 days after stroke. Astrocytes obtained from the ipsilateral hemisphere expressed significantly more Ang-2 compared with astrocytes isolated from the contralateral hemisphere, or from cortices of sham-operated (control) mice. Notably, analysis of microglia sorted from CX3CR1-eGFP mice demonstrated increased cell surface expression of Tie-2, the Ang-2 receptor, on cells obtained from ipsilateral versus contralateral tissue. Addition of recombinant Ang-2 to astrocyte-conditioned medium significantly increased the number of SIM-A9 murine microglial cells cultured under hypoxic conditions (1 % oxygen for 48 h). In transgenic GFAP-CreER™-EDNRB-fl/fl mice with stroke, conditional knockout of astroglial ETBR significantly decreased the number of proliferating cells in the peri-infarct area with a microglial phenotype (Ki67+/CD11b+). Our results indicate that Ang-2, and possibly other paracrine effectors functioning downstream of astroglial ETBR signaling, are important mediators of microglial cell dynamics after stroke.

Lipids: Emerging Players of Microglial Biology

Lipids are small molecule immunomodulators that play critical roles in maintaining cellular health and function. Microglia, the resident immune cells of the central nervous system, regulate lipid metabolism both in the extracellular environment and within intracellular compartments through various mechanisms. For instance, glycerophospholipids and fatty acids interact with protein receptors on the microglial surface, such as the Triggering Receptor Expressed on Myeloid Cells 2, influencing cellular functions like phagocytosis and migration. Moreover, cholesterol is essential not only for microglial survival but, along with other lipids such as fatty acids, is crucial for the formation, function, and accumulation of lipid droplets, which modulate microglial activity in inflammatory diseases. Other lipids, including acylcarnitines and ceramides, participate in various signaling pathways within microglia. Despite the complexity of the microglial lipidome, only a few studies have investigated the effects of specific lipid classes on microglial biology. In this review, we focus on major lipid classes and their roles in modulating microglial function. We also discuss novel analytical techniques for characterizing the microglial lipidome and highlight gaps in current knowledge, suggesting new directions for future research on microglial lipid biology.

Mechanisms of astrocyte aging in reactivity and disease

Normal aging alters brain functions and phenotypes. However, it is not well understood how astrocytes are impacted by aging, nor how they contribute to neuronal dysfunction and disease risk as organisms age. Here, we examine the transcriptional, cell biology, and functional differences in astrocytes across normal aging. Astrocytes at baseline are heterogenous, responsive to their environments, and critical regulators of brain microenvironments and neuronal function. With increasing age, astrocytes adopt different immune-related and senescence-associated states, which relate to organelle dysfunction and loss of homeostasis maintenance, both cell autonomously and non-cell autonomously. These perturbed states are increasingly associated with age-related dysfunction and the onset of neurodegeneration, suggesting that astrocyte aging is a compelling target for future manipulation in the prevention of disease.

Phenotypic and Functional Alterations in Peripheral Blood Mononuclear Cell-Derived Microglia in a Primate Model of Chronic Alcohol Consumption

Alcohol-induced dysregulation of microglial activity is associated with neuroinflammation, cognitive decline, heightened risk for neurodegenerative diseases, alcohol dependence, and escalation of alcohol drinking. Given the challenge of longitudinally sampling primary microglia, we optimized an in vitro method to differentiate peripheral blood mononuclear cells (PBMC) from non-human primates (NHP) into microglia-like cells (induced-microglia; iMGL). The iMGLs displayed transcriptional profiles distinct from those of monocyte progenitors and closely resembling those of primary microglia. Notably, morphological features showed that differentiated iMGLs derived from NHPs with chronic alcohol consumption (CAC) possessed a more mature-like microglial morphology. Additionally, dysregulation in key inflammatory and regulatory markers alongside increased baseline phagocytic activity was observed in CAC-derived IMGLs in the resting state. Phenotypic and functional assessments following LPS stimulation indicated the presence of an immune-tolerant phenotype and enrichment of a CD86+ hyper-inflammatory subpopulation in iMGLs derived from ethanol-exposed animals. Collectively, these findings demonstrate that in vitro differentiation of PBMC offers a minimally invasive approach to studying the impact of CAC on microglial function revealing that CAC reshapes both functional and transcriptional profiles of microglia.

Mouse Adrenal Macrophages Are Associated with Pre- and Postsynaptic Neuronal Elements and Respond to Multiple Neuromodulators

The adrenal medulla is packed with chromaffin cells, modified postganglionic sympathetic neurons that secrete the catecholamines, epinephrine and norepinephrine, during the fight-or-flight response. Sometimes overlooked is a population of immune cells that also resides within the gland but whose distribution and function are not clear. Here I examine the location of CD45+ hematopoietic cells in the mouse adrenal medulla and show the majority are F4/80+/Lyz2+ macrophages. These cells are present from early postnatal development and widely distributed. Anatomically they are associated with chromaffin cells, found aligned alongside synapsin-IR neuronal varicosities and juxtaposed to CD31-IR blood vessels. Using Lyz2cre-GCaMP6f mice to quantify calcium signaling in macrophages revealed these cells respond directly and indirectly to a wide variety of neuromodulators, including pre- and postganglionic transmitters and systemic hormones. Purinergic agonists, histamine, acetylcholine, and bradykinin rapidly and reversibly increased intracellular calcium. These results are consistent with a substantial resident population of innate immune cells in the adrenal medulla. Their close association with chromaffin cells and the preganglionic input suggests they may regulate sympatho-adrenal activity and thus the strength of the fight-or-flight response.

Microglia internalize tau monomers and fibrils using distinct receptors but similar mechanisms

INTRODUCTION

Alzheimer's disease (AD) and other tauopathies are characterized by intracellular aggregates of microtubule-associated protein tau that are actively released and promote proteopathic spread. Microglia engulf pathological proteins, but how they endocytose tau is unknown.

METHODS

We measured endocytosis of different tau species by microglia after pharmacological modulation of macropinocytosis or clathrin-mediated endocytosis (CME) or antagonism/genetic depletion of known tau receptors heparan-sulfate proteoglycans (HSPGs) and low-density lipoprotein receptor-related protein 1 (LRP1).

RESULTS

Dynamin inhibition decreased microglial endocytosis of all tested tau species. Meanwhile, HSPG antagonism blocked only fibril uptake, and LRP1 antagonism or genetic depletion inconsistently inhibited the endocytosis of fibrils and monomers. Cre recombinase robustly enhanced tau uptake with partial selectivity for fibrils.

DISCUSSION

These data show that microglia take up both tau monomers and aggregates via a dynamin-dependent form of endocytosis (eg, CME) but may differ in using HSPGs for entry depending on species.

HIGHLIGHTS

Microglial endocytosis of tau monomers and fibrils is dynamin-dependent. HSPG antagonism blocks microglial uptake of tau fibrils but not monomers. LRP1 antagonism or knockdown inconsistently inhibits tau uptake. TAT-Cre stimulates semi-selective uptake of fibrils over monomers.

Gene expression of psychiatric disorder-related kinesin superfamily proteins (Kifs) is potentiated in alternatively activated primary cultured microglia

OBJECTIVE

Reactivity of microglia, the resident cells of the brain, underlies innate immune mechanisms (e.g., injury repair), and disruption of microglial reactivity has been shown to facilitate psychiatric disorder dysfunctions. Although cellular analyses based on cultured microglia have been conducted, the molecular mechanism regulating microglial polarization remains elusive. We established a primary microglia culture that enabled manipulation of the substate of cells. This allowed us to investigate the expression levels of psychiatric disorder-related Kifs messenger RNA (mRNA) in each condition. Kifs encode molecular motor proteins that transport cargo along microtubules, which are thought to dynamically reorganize during a substate change.

RESULTS

As a candidate for a crucial Kifs gene that is associated with microglia polarization, we selected psychiatric disorder-related Kifs including Kif17. We found that the relative amounts of Kif3a, Kif17, and Kif13a mRNA were potentiated in alternatively activated microglia, whereas there were no significant changes in activated microglia. Furthermore, the microglia derived from a mouse line which possesses a mutation inducing truncated KIF17 indicated disrupted morphological phenotype of alternatively activated microglia. These results suggest that the potentiation of specific molecular motor expression is required to maintain the function of alternatively activated microglia.

Development and characterization of in vitro inducible immortalization of a murine microglia cell line for high throughput studies

There are few in vitro models available to study microglial physiology in a homeostatic context. Recent approaches include the human induced pluripotent stem cell model, but these can be challenging for large-scale assays and may lead to batch variability. To advance our understanding of microglial biology while enabling scalability for high-throughput assays, we developed an inducible immortalized murine microglial cell line using a tetracycline expression system. The addition of doxycycline facilitates rapid cell proliferation, allowing for population expansion. Upon withdrawal of doxycycline, this monoclonal microglial cell line differentiates, resembling in vivo microglial physiology as demonstrated by the expression of microglial genes, innate immune responses, chemotaxis, and phagocytic abilities. We utilized live imaging and various molecular techniques to functionally characterize the clonal 2E11murine microglial cell line. Transcriptomic analysis showed that the 2E11 line exhibited characteristics of immature, proliferative microglia during doxycycline induction, and further differentiation led to a more homeostatic phenotype. Treatment with transforming growth factor-β modified the transcriptome of the 2E11 cell line, affecting cellular immune pathways. Our findings indicate that the 2E11 inducible immortalized cell line is a practical and convenient tool for studying microglial biology in vitro.

Mechanisms and environmental factors shaping the ecosystem of brain macrophages

Brain macrophages encompass two major populations: microglia in the parenchyma and border-associated macrophages (BAMs) in the extra-parenchymal compartments. These cells play crucial roles in maintaining brain homeostasis and immune surveillance. Microglia and BAMs are phenotypically and epigenetically distinct and exhibit highly specialized functions tailored to their environmental niches. Intriguingly, recent studies have shown that both microglia and BAMs originate from the same myeloid progenitor during yolk sac hematopoiesis, but their developmental fates diverge within the brain. Several works have partially unveiled the mechanisms orchestrating the development of microglia and BAMs in both mice and humans; however, many questions remain unanswered. Defining the molecular underpinnings controlling the transcriptional and epigenetic programs of microglia and BAMs is one of the upcoming challenges for the field. In this review, we outline current knowledge on ontogeny, phenotypic diversity, and the factors shaping the ecosystem of brain macrophages. We discuss insights garnered from human studies, highlighting similarities and differences compared to mice. Lastly, we address current research gaps and potential future directions in the field. Understanding how brain macrophages communicate with their local environment and how the tissue instructs their developmental trajectories and functional features is essential to fully comprehend brain physiology in homeostasis and disease.

Microglia regulate nucleus accumbens synaptic development and circuit function underlying threat avoidance behaviors

While CNS microglia have well-established roles in synapse pruning during neurodevelopment, only a few studies have identified roles for microglia in synapse formation. These studies focused on the cortex and primary sensory circuits during restricted developmental time periods, leaving substantial gaps in our understanding of the early developmental functions of microglia. Here we investigated how the absence of microglia impacts synaptic development in the nucleus accumbens (NAc), a region critical for emotional regulation and motivated behaviors and where dysfunction is implicated in psychiatric disorders that arise early in life. Using a genetically modified mouse that lacks microglia (Csf1r ΔFIRE/ΔFIRE), we found blunted excitatory synapse formation in the NAc. This effect was most prominent during the second and third postnatal weeks, when we previously found microglia to be overproduced, and was accompanied by an increase in presynaptic release probability and alterations in postsynaptic kinetics. Tissue-level NAc proteomics confirmed that microglial absence impacted numerous proteins involved in synapse structure, trans-synaptic signaling, and pre-synaptic function. However, microglial absence did not perturb levels of astrocyte-derived cues and adhesive proteins that promote synaptogenesis, suggesting that reduced synapse number may be caused by absence of a microglial-derived synaptogenic cue. Although observed electrophysiological synaptic changes were largely normalized by adulthood, we identified lasting effects of microglial absence on threat avoidance behavior, and these behavioral effects were directly associated with alterations of NAc neuronal activity. Together, these results indicate a critical role for microglia in regulating the synaptic landscape of the developing NAc and in establishing functional circuits underlying adult behavioral repertoires.

Microglia regulate nucleus accumbens synaptic development and circuit function underlying threat avoidance behaviors

While CNS microglia have well-established roles in synapse pruning during neurodevelopment, only a few studies have identified roles for microglia in synapse formation. These studies focused on the cortex and primary sensory circuits during restricted developmental time periods, leaving substantial gaps in our understanding of the early developmental functions of microglia. Here we investigated how the absence of microglia impacts synaptic development in the nucleus accumbens (NAc), a region critical for emotional regulation and motivated behaviors and where dysfunction is implicated in psychiatric disorders that arise early in life. Using a genetically modified mouse that lacks microglia (Csf1r ΔFIRE/ΔFIRE), we found blunted excitatory synapse formation in the NAc. This effect was most prominent during the second and third postnatal weeks, when we previously found microglia to be overproduced, and was accompanied by an increase in presynaptic release probability and alterations in postsynaptic kinetics. Tissue-level NAc proteomics confirmed that microglial absence impacted numerous proteins involved in synapse structure, trans-synaptic signaling, and pre-synaptic function. However, microglial absence did not perturb levels of astrocyte-derived cues and adhesive proteins that promote synaptogenesis, suggesting that reduced synapse number may be caused by absence of a microglial-derived synaptogenic cue. Although observed electrophysiological synaptic changes were largely normalized by adulthood, we identified lasting effects of microglial absence on threat avoidance behavior, and these behavioral effects were directly associated with alterations of NAc neuronal activity. Together, these results indicate a critical role for microglia in regulating the synaptic landscape of the developing NAc and in establishing functional circuits underlying adult behavioral repertoires.

A primary rat neuron-astrocyte-microglia tri-culture model for studying mechanisms of neurotoxicity

Primary cell cultures from rodent brain are widely used to investigate molecular and cellular mechanisms of neurotoxicity. To date, however, it has been challenging to reliably culture endogenous microglia in dissociated mixed cultures. This is a significant limitation of most in vitro neural cell models given the growing awareness of the importance of interactions between neurons, astrocytes and microglia in defining responses to neurotoxic exposures. We recently developed a tri-culture model consisting of neurons, astrocytes and microglia dissociated from the developing rat neocortex and demonstrated that this tri-culture model more faithfully mimics in vivo neuroinflammatory responses then standard neuron-only or neuron-astrocyte co-cultures. Here, we describe our protocol for generating tri-cultures of rat cortical neurons, astrocytes and microglia in which all 3 cell types can be maintained for up to 1 month in culture at the same relative ratio observed in the developing rat neocortex. We also discuss applications of this model for neurotoxicity testing, as well as the potential of this model to fill a current gap for assessing neuroinflammation in the in vitro testing battery for developmental neurotoxicity.

Human iPSC-derived microglial cells protect neurons from neurodegeneration in long-term cultured adhesion brain organoids

Brain organoid models have greatly facilitated our understanding of human brain development and disease. However, key brain cell types, such as microglia, are lacking in most brain organoid models. Because microglia have been shown to play important roles in brain development and pathologies, attempts have been made to add microglia to brain organoids through co-culture. However, only short-term microglia-organoid co-cultures can be established, and it remains challenging to have long-lasting survival of microglia in organoids to mimic long-term residency of microglia in the brain. In this study, we developed an adhesion brain organoid (ABO) platform that allows prolonged culture of brain organoids (greater than a year). Moreover, the long-term (LT)-ABO system contains abundant astrocytes and can support prolonged survival and ramification of microglia. Furthermore, we showed that microglia in the LT-ABO could protect neurons from neurodegeneration by increasing synaptic density and reducing p-Tau level and cell death in the LT-ABO. Therefore, the microglia-containing LT-ABO platform generated in this study provides a promising human cellular model for studying neuron-glia and glia-glia interactions in brain development and the pathogenesis of neurodegenerative diseases such as Alzheimer's disease.

Special Issue: Recent Advances in Microglia Research

This Editorial introduces the new Special Issue, published in the International Journal of Molecular Sciences and entitled "Recent Advances in Microglia Research", which covers this important topic with a collection of five valuable contributions: three Original Research Articles and two Reviews [...].

Neurological impact of HIV/AIDS and substance use alters brain function and structure

Human immunodeficiency virus (HIV) infection is the cause of acquired immunodeficiency syndrome (AIDS). Combination antiretroviral therapy (cART) has successfully controlled AIDS, but HIV-associated neurocognitive disorders (HANDs) remain prevalent among people with HIV. HIV infection is often associated with substance use, which promotes HIV transmission and viral replication and exacerbates HANDs even in the era of cART. Thus, the comorbid effects of substance use exacerbate the neuropathogenesis of HANDs. Unraveling the mechanism(s) of this comorbid exacerbation at the molecular, cell-type, and brain region levels may provide a better understanding of HAND persistence. This review aims to highlight the comorbid effects of HIV and substance use in specific brain regions and cell types involved in the persistence of HANDs. This review includes an overview of post-translational modifications, alterations in microglia-specific biomarkers, and possible mechanistic pathways that may link epigenomic modifications to functional protein alterations in microglia. The impairment of the microglial proteins that are involved in neural circuit function appears to contribute to the breakdown of cellular communication and neurodegeneration in HANDs. The epigenetic modification of N-terminal acetylation is currently understudied, which is discussed in brief to demonstrate the important role of this epigenetic modification in infected microglia within specific brain regions. The discussion also explores whether combined antiretroviral therapy is effective in preventing HIV infection or substance-use-mediated post-translational modifications and protein alterations in the persistence of neuropathogenesis in HANDs.

From Physiology to Pathology of Astrocytes: Highlighting Their Potential as Therapeutic Targets for CNS Injury

In the mammalian central nervous system (CNS), astrocytes are the ubiquitous glial cells that have complex morphological and molecular characteristics. These fascinating cells play essential neurosupportive and homeostatic roles in the healthy CNS and undergo morphological, molecular, and functional changes to adopt so-called 'reactive' states in response to CNS injury or disease. In recent years, interest in astrocyte research has increased dramatically and some new biological features and roles of astrocytes in physiological and pathological conditions have been discovered thanks to technological advances. Here, we will review and discuss the well-established and emerging astroglial biology and functions, with emphasis on their potential as therapeutic targets for CNS injury, including traumatic and ischemic injury. This review article will highlight the importance of astrocytes in the neuropathological process and repair of CNS injury.

Orchestrating the neuroglial compartment: Ontogeny and developmental interaction of astrocytes, oligodendrocytes, and microglia

Neuroglial cells serve as the master regulators of the central nervous system, making it imperative for glial development to be tightly regulated both spatially and temporally to ensure optimal brain function. In this chapter, we will discuss the origin and development of the three major glia cells such as astrocytes, oligodendrocytes, and microglia in the central nervous system. While much of our understanding of neuroglia development stems from studies using animal models, we will also explore recent insights into human glial development and potential differences from rodent models. Finally, the extensive crosstalk between glia cells will be highlighted, discussing how interactions among astrocyte, oligodendrocyte, and microglial influence their respective developmental pathways.

Human Microglia-Like Cells Differentiated from Monocytes with GM-CSF and IL-34 Show Phagocytosis of α-Synuclein Aggregates and C/EBPβ-Dependent Proinflammatory Activation

Microglia, the main resident immune cells in the central nervous system, are implicated in the pathogenesis of various neurological disorders. Much of our knowledge on microglial biology was obtained using rodent microglial cultures. To understand the role of microglia in human disease, reliable in vitro models of human microglia are necessary. Monocyte-derived microglia-like cells (MDMi) are a promising approach. This study aimed to characterize MDMi cells generated from adult human monocytes using granulocyte-macrophage colony-stimulating factor and interleukin-34. To this end, 49 independent cultures of MDMI were prepared, and various methodological and functional studies were performed. We show that with this protocol, adult human monocytes develop into microglia-like cells, a coating is unnecessary, and high cell density seeding is preferable. When compared to monocytes, MDMi upregulate the expression of many, but not all, microglial markers, indicating that, although these cells display a microglia-like phenotype, they cannot be considered bona fide human microglia. At the functional level, MDMi phagocytose α-synuclein aggregates and responds to lipopolysaccharide (LPS) by nuclear translocation of the transcription factor nuclear factor-kappaB (NFkappaB) and the upregulation of proinflammatory genes. Finally, a long-lasting silencing of the transcription factor CCAAT/enhancer protein β (C/EBPβ) was achieved by small interfering RNA, resulting in the subsequent downregulation of proinflammatory genes. This supports the hypothesis that C/EBPβ plays a key role in proinflammatory gene program activation in human microglia. Altogether, this study sheds new light on the properties of MDMi cells and supports these cells as a promising in vitro model for studying adult human microglia-like cells.

Advancements in Immunity and Dementia Research: Highlights from the 2023 AAIC Advancements: Immunity Conference

The immune system is a key player in the onset and progression of neurodegenerative disorders. While brain resident immune cell-mediated neuroinflammation and peripheral immune cell (eg, T cell) infiltration into the brain have been shown to significantly contribute to Alzheimer's disease (AD) pathology, the nature and extent of immune responses in the brain in the context of AD and related dementias (ADRD) remain unclear. Furthermore, the roles of the peripheral immune system in driving ADRD pathology remain incompletely elucidated. In March of 2023, the Alzheimer's Association convened the Alzheimer's Association International Conference (AAIC), Advancements: Immunity, to discuss the roles of the immune system in ADRD. A wide range of topics were discussed, such as animal models that replicate human pathology, immune-related biomarkers and clinical trials, and lessons from other fields describing immune responses in neurodegeneration. This manuscript presents highlights from the conference and outlines avenues for future research on the roles of immunity in neurodegenerative disorders. HIGHLIGHTS: The immune system plays a central role in the pathogenesis of Alzheimer's disease. The immune system exerts numerous effects throughout the brain on amyloid-beta, tau, and other pathways. The 2023 AAIC, Advancements: Immunity, encouraged discussions and collaborations on understanding the role of the immune system.

Microglial Lyzl4 Facilitates β-Amyloid Clearance in Alzheimer's Disease

Alzheimer's Disease (AD) is a neurodegenerative condition characterized by the accumulation and deposition of amyloid-β (Aβ) aggregates in the brain. Despite a wealth of research on the toxicity of Aβ and its role in synaptic damage, the mechanisms facilitating Aβ clearance are not yet fully understood. However, microglia, the primary immune cells of the central nervous system, are known to maintain homeostasis through the phagocytic clearance of protein aggregates and cellular debris. In this study, RNA sequencing analysis and live cell functional screens are employed to uncover microglial genetic modifiers related to AD. Lyzl4 is identified, which encodes a c-type lysozyme-like enzyme primarily localized to microglial lysosomes, as a gene significantly upregulated in AD microglia with aging and propose that Lyzl4 upregulation acts as a positive regulator of Aβ clearance. Furthermore, it is found that Lyzl4 overexpression boosts Aβ clearance both in vitro and in vivo, underscoring its potential for mitigating Aβ burden. These novel insights position Lyzl4 as a promising therapeutic target for Alzheimer's disease, paving the way for further exploration into potential AD treatments.

Animal-based approaches to understanding neuroglia physiology in vitro and in vivo

This chapter describes the pivotal role of animal models for unraveling the physiology of neuroglial cells in the central nervous system (CNS). The two rodent species Mus musculus (mice) and Rattus norvegicus (rats) have been indispensable in scientific research due to their remarkable resemblance to humans anatomically, physiologically, and genetically. Their ease of maintenance, short gestation times, and rapid development make them ideal candidates for studying the physiology of astrocytes, oligodendrocyte-lineage cells, and microglia. Moreover, their genetic similarity to humans facilitates the investigation of molecular mechanisms governing neural physiology. Mice are largely the predominant model of neuroglial research, owing to advanced genetic manipulation techniques, whereas rats remain invaluable for applications requiring larger CNS structures for surgical manipulations. Next to rodents, other animal models, namely, Danio rerio (zebrafish) and Drosophila melanogaster (fruit fly), will be discussed to emphasize their critical role in advancing our understanding of glial physiology. Each animal model provides distinct advantages and disadvantages. By combining the strengths of each of them, researchers can gain comprehensive insights into glial function across species, ultimately promoting the understanding of glial physiology in the human CNS and driving the development of novel therapeutic interventions for CNS disorders.

Inhibiting the Cholesterol Storage Enzyme ACAT1/SOAT1 in Aging Apolipoprotein E4 Mice Alters Their Brains' Inflammatory Profiles

Aging and apolipoprotein E4 (APOE4) are the two most significant risk factors for late-onset Alzheimer's disease (LOAD). Compared to APOE3, APOE4 disrupts cholesterol homeostasis, increases cholesteryl esters (CEs), and exacerbates neuroinflammation in brain cells, including microglia. Targeting CEs and neuroinflammation could be a novel strategy to ameliorate APOE4-dependent phenotypes. Toll-like receptor 4 (TLR4) is a key macromolecule in inflammation, and its regulation is associated with the cholesterol content of lipid rafts in cell membranes. We previously demonstrated that in normal microglia expressing APOE3, inhibiting the cholesterol storage enzyme acyl-CoA:cholesterol acyltransferase 1 (ACAT1/SOAT1) reduces CEs, dampened neuroinflammation via modulating the fate of TLR4. We also showed that treating myelin debris-loaded normal microglia with ACAT inhibitor F12511 reduced cellular CEs and activated ABC transporter 1 (ABCA1) for cholesterol efflux. This study found that treating primary microglia expressing APOE4 with F12511 also reduces CEs, activates ABCA1, and dampens LPS-dependent NFκB activation. In vivo, two-week injections of nanoparticle F12511, which consists of DSPE-PEG2000, phosphatidylcholine, and F12511, to aged female APOE4 mice reduced TLR4 protein content and decreased proinflammatory cytokines, including IL-1β in mice brains. Overall, our work suggests nanoparticle F12511 is a novel agent to ameliorate LOAD.

Plasticity of Human Microglia and Brain Perivascular Macrophages in Aging and Alzheimer's Disease

The complex roles of myeloid cells, including microglia and perivascular macrophages, are central to the neurobiology of Alzheimer's disease (AD), yet they remain incompletely understood. Here, we profiled 832,505 human myeloid cells from the prefrontal cortex of 1,607 unique donors covering the human lifespan and varying degrees of AD neuropathology. We delineated 13 transcriptionally distinct myeloid subtypes organized into 6 subclasses and identified AD-associated adaptive changes in myeloid cells over aging and disease progression. The GPNMB subtype, linked to phagocytosis, increased significantly with AD burden and correlated with polygenic AD risk scores. By organizing AD-risk genes into a regulatory hierarchy, we identified and validated MITF as an upstream transcriptional activator of GPNMB, critical for maintaining phagocytosis. Through cell-to-cell interaction networks, we prioritized APOE-SORL1 and APOE-TREM2 ligand-receptor pairs, associated with AD progression. In both human and mouse models, TREM2 deficiency disrupted GPNMB expansion and reduced phagocytic function, suggesting that GPNMB's role in neuroprotection was TREM2-dependent. Our findings clarify myeloid subtypes implicated in aging and AD, advancing the mechanistic understanding of their role in AD and aiding therapeutic discovery.

Primary cortical cell tri-culture to study effects of amyloid-β on microglia function and neuroinflammatory response

BACKGROUND

Microglia play a critical role in neurodegenerative disorders, such as Alzheimer's disease, where alterations in microglial function may result in pathogenic amyloid-β (Aβ) accumulation, chronic neuroinflammation, and deleterious effects on neuronal function. However, studying these complex factors in vivo, where numerous confounding processes exist, is challenging, and until recently, in vitro models have not allowed sustained culture of critical cell types in the same culture.

OBJECTIVE

We employed a rat primary tri-culture (neurons, astrocytes, and microglia) model and compared it to co-culture (neurons and astrocytes) and mono-culture (microglia) to study microglial function (i.e., motility and Aβ clearance) and proteomic response to exogenous Aβ.

METHODS

The cultures were exposed to fluorescently-labeled Aβ (FITC-Aβ) particles for varying durations. Epifluorescence microscopy images were analyzed to quantify the number of FITC-Aβ particles and assess cytomorphological features. Cytokine profiles from conditioned media were obtained. Live-cell imaging was employed to extract microglia motility parameters.

RESULTS

FITC-Aβ particles were more effectively cleared in the tri-culture compared to the co-culture. This was attributed to microglia engulfing FITC-Aβ particles, as confirmed via epifluorescence and confocal microscopy. FITC-Aβ treatment significantly increased microglia size, but had no significant effect on neuronal surface coverage or astrocyte size. Upon FITC-Aβ treatment, there was a significant increase in proinflammatory cytokines in tri-culture, but not in co-culture. Aβ treatment altered microglia motility evident as a swarming-like motion.

CONCLUSIONS

The results suggest that neuron-astrocyte-microglia interactions influence microglia function and highlight the utility of the tri-culture model for studies of neuroinflammation, neurodegeneration, and cell-cell communication.

The role of astrocytes response triggered by hyperglycaemia during spinal cord injury

OBJECTIVE

This manuscript aimed to provide a comprehensive overview of the physiological, molecular, and cellular mechanisms triggered by reactive astrocytes (RA) in the context of spinal cord injury (SCI), with a particular focus on cases involving hyperglycaemia.

METHODS

The compilation of articles related to astrocyte responses in neuropathological conditions, with a specific emphasis on those related to SCI and hyperglycaemia, was conducted by searching through databases including Science Direct, Web of Science, and PubMed.

RESULTS AND CONCLUSIONS

This article explores the dual role of astrocytes in both neurophysiological and neurodegenerative conditions within the central nervous system (CNS). In the aftermath of SCI and hyperglycaemia, astrocytes undergo a transformation into RA, adopting a distinct phenotype. While there are currently no approved therapies for SCI, various therapeutic strategies have been proposed to alleviate the detrimental effects of RAs following SCI and hyperglycemia. These strategies show promising potential in the treatment of SCI and its likely comorbidities.

Brain organoid models for studying the function of iPSC-derived microglia in neurodegeneration and brain tumours

Microglia represent the main resident immune cells of the brain. The interplay between microglia and other cells in the central nervous system, such as neurons or other glial cells, influences the function and ability of microglia to respond to various stimuli. These cellular communications, when disrupted, can affect the structure and function of the brain, and the initiation and progression of neurodegenerative diseases including Alzheimer's disease and Parkinson's disease, as well as the progression of other brain diseases like glioblastoma. Due to the difficult access to patient brain tissue and the differences reported in the murine models, the available models to study the role of microglia in disease progression are limited. Pluripotent stem cell technology has facilitated the generation of highly complex models, allowing the study of control and patient-derived microglia in vitro. Moreover, the ability to generate brain organoids that can mimic the 3D tissue environment and intercellular interactions in the brain provide powerful tools to study cellular pathways under homeostatic conditions and various disease pathologies. In this review, we summarise the most recent developments in modelling degenerative diseases and glioblastoma, with a focus on brain organoids with integrated microglia. We provide an overview of the most relevant research on intercellular interactions of microglia to evaluate their potential to study brain pathologies.

Microglia and macrophages in glioblastoma: landscapes and treatment directions

Glioblastoma is the most common primary malignant tumour of the central nervous system and remains uniformly and rapidly fatal. The tumour-associated macrophage (TAM) compartment comprises brain-resident microglia and bone marrow-derived macrophages (BMDMs) recruited from the periphery. Immune-suppressive and tumour-supportive TAM cell states predominate in glioblastoma, and immunotherapies, which have achieved striking success in other solid tumours have consistently failed to improve survival in this 'immune-cold' niche context. Hypoxic and necrotic regions in the tumour core are found to enrich, especially in anti-inflammatory and immune-suppressive TAM cell states. Microglia predominate at the invasive tumour margin and express pro-inflammatory and interferon TAM cell signatures. Depletion of TAMs, or repolarisation towards a pro-inflammatory state, are appealing therapeutic strategies and will depend on effective understanding and classification of TAM cell ontogeny and state based on new single-cell and spatial multi-omic in situ profiling. Here, we explore the application of these datasets to expand and refine TAM characterisation, to inform improved modelling approaches, and ultimately underpin the effective manipulation of function.

Deep proteomic analysis of microglia reveals fundamental biological differences between model systems

Using high-resolution quantitative mass spectrometry, we present comprehensive human and mouse microglia proteomic datasets consisting of over 11,000 proteins across six microglia groups. Microglia share a core protein signature of over 5,600 proteins, yet fundamental differences are observed between species and culture conditions. Mouse microglia demonstrate proteome differences in inflammation- and Alzheimer's disease-associated proteins. We identify differences in the protein content of ex vivo and in vitro cells and significant proteome differences associated with protein synthesis, metabolism, microglia marker expression, and environmental sensors. Culturing microglia induces rapidly increased growth, protein content, and inflammatory protein expression. These changes are restored by engrafting in vitro cells into the brain, with xenografted human embryonic stem cell (hESC)-derived microglia closely resembling microglia from the human brain. These data provide an important resource for the field and highlight important considerations needed when using model systems to study human physiology and pathology of microglia.

BTK regulates microglial function and neuroinflammation in human stem cell models and mouse models of multiple sclerosis

Neuroinflammation in the central nervous system (CNS), driven largely by resident phagocytes, has been proposed as a significant contributor to disability accumulation in multiple sclerosis (MS) but has not been addressed therapeutically. Bruton's tyrosine kinase (BTK) is expressed in both B-lymphocytes and innate immune cells, including microglia, where its role is poorly understood. BTK inhibition may provide therapeutic benefit within the CNS by targeting adaptive and innate immunity-mediated disease progression in MS. Using a CNS-penetrant BTK inhibitor (BTKi), we demonstrate robust in vivo effects in mouse models of MS. We further identify a BTK-dependent transcriptional signature in vitro, using the BTKi tolebrutinib, in mouse microglia, human induced pluripotent stem cell (hiPSC)-derived microglia, and a complex hiPSC-derived tri-culture system composed of neurons, astrocytes, and microglia, revealing modulation of neuroinflammatory pathways relevant to MS. Finally, we demonstrate that in MS tissue BTK is expressed in B-cells and microglia, with increased levels in lesions. Our data provide rationale for targeting BTK in the CNS to diminish neuroinflammation and disability accumulation.

Microglial depletion rescues spatial memory impairment caused by LPS administration in adult mice

Recent studies have highlighted the importance of microglia, the resident macrophages in the brain, in regulating cognitive functions such as learning and memory in both healthy and diseased states. However, there are conflicting results and the underlying mechanisms are not fully understood. In this study, we examined the effect of depleting adult microglia on spatial learning and memory under both physiological conditions and lipopolysaccharide (LPS)-induced neuroinflammation. Our results revealed that microglial depletion by PLX5622 caused mild spatial memory impairment in mice under physiological conditions; however, it prevented memory deficits induced by systemic LPS insult. Inactivating microglia through minocycline administration replicated the protective effect of microglial depletion on LPS-induced memory impairment. Furthermore, our study showed that PLX5622 treatment suppressed LPS-induced neuroinflammation, microglial activation, and synaptic dysfunction. These results strengthen the evidence for the involvement of microglial immunoactivation in LPS-induced synaptic and cognitive malfunctions. They also suggest that targeting microglia may be a potential approach to treating neuroinflammation-associated cognitive dysfunction seen in neurodegenerative diseases.

Human-induced pluripotent stem cell-derived microglia integrate into mouse retina and recapitulate features of endogenous microglia

Microglia exhibit both maladaptive and adaptive roles in the pathogenesis of neurodegenerative diseases and have emerged as a cellular target for central nervous system (CNS) disorders, including those affecting the retina. Replacing maladaptive microglia, such as those impacted by aging or over-activation, with exogenous microglia that can enable adaptive functions has been proposed as a potential therapeutic strategy for neurodegenerative diseases. To investigate microglia replacement as an approach for retinal diseases, we first employed a protocol to efficiently generate human-induced pluripotent stem cell (hiPSC)-derived microglia in quantities sufficient for in vivo transplantation. These cells demonstrated expression of microglia-enriched genes and showed typical microglial functions such as LPS-induced responses and phagocytosis. We then performed xenotransplantation of these hiPSC-derived microglia into the subretinal space of adult mice whose endogenous retinal microglia have been pharmacologically depleted. Long-term analysis post-transplantation demonstrated that transplanted hiPSC-derived microglia successfully integrated into the neuroretina as ramified cells, occupying positions previously filled by the endogenous microglia and expressed microglia homeostatic markers such as P2ry12 and Tmem119. Furthermore, these cells were found juxtaposed alongside residual endogenous murine microglia for up to 8 months in the retina, indicating their ability to establish a stable homeostatic state in vivo. Following retinal pigment epithelial cell injury, transplanted microglia demonstrated responses typical of endogenous microglia, including migration, proliferation, and phagocytosis. Our findings indicate the feasibility of microglial transplantation and integration in the retina and suggest that modulating microglia through replacement may be a therapeutic strategy for treating neurodegenerative retinal diseases.

Assembloid models of cell-cell interaction to study tissue and disease biology

Neurodevelopment involves the migration, projection, and integration of various cell types across different regions of the nervous system. Assembloids are self-organizing systems formed by the integration of multiple organoids or cell types. Here, we outline the generation and application of assembloids. We illustrate how assembloids recapitulate critical neurodevelopmental steps, like migration, axon projection, and circuit formation, and how they are starting to provide biological insights into neuropsychiatric disorders. Additionally, we review how assembloids can be used to study properties emerging from cell-cell interactions within non-neural tissues. Overall, assembloid platforms represent a powerful tool for discovering human biology and developing therapeutics.

ER and SOCE Ca2+ signals are not required for directed cell migration in human iPSC-derived microglia

The central nervous system (CNS) is constantly surveilled by microglia, highly motile and dynamic cells deputed to act as the first line of immune defense in the brain and spinal cord. Alterations in the homeostasis of the CNS are detected by microglia that respond by extending their processes or - following major injuries - by migrating toward the affected area. Understanding the mechanisms controlling directed cell migration of microglia is crucial to dissect their responses to neuroinflammation and injury. We used a combination of pharmacological and genetic approaches to explore the involvement of calcium (Ca2+) signaling in the directed migration of human induced pluripotent stem cell (iPSC)-derived microglia challenged with a purinergic stimulus. This approach mimics cues originating from injury of the CNS. Unexpectedly, simultaneous imaging of microglia migration and intracellular Ca2+ changes revealed that this phenomenon does not require Ca2+ signals generated from the endoplasmic reticulum (ER) and store-operated Ca2+ entry (SOCE) pathways. Instead, we find evidence that human microglial chemotaxis to purinergic signals is mediated by cyclic AMP in a Ca2+-independent manner. These results challenge prevailing notions, with important implications in neurological conditions characterized by perturbation in Ca2+ homeostasis.