P2Y12R-Dependent Translocation Mechanisms Gate the Changing Microglial Landscape

In vivo imaging in mouse spinal cord reveals that microglia prevent degeneration of injured axons

Microglia, the primary immune cells in the central nervous system, play a critical role in regulating neuronal function and fate through their interaction with neurons. Despite extensive research, the specific functions and mechanisms of microglia-neuron interactions remain incompletely understood. In this study, we demonstrate that microglia establish direct contact with myelinated axons at Nodes of Ranvier in the spinal cord of mice. The contact associated with neuronal activity occurs in a random scanning pattern. In response to axonal injury, microglia rapidly transform their contact into a robust wrapping form, preventing acute axonal degeneration from extending beyond the nodes. This wrapping behavior is dependent on the function of microglial P2Y12 receptors, which may be activated by ATP released through axonal volume-activated anion channels at the nodes. Additionally, voltage-gated sodium channels (NaV) and two-pore-domain potassium (K2P) channels contribute to the interaction between nodes and glial cells following injury, and inhibition of NaV delays axonal degeneration. Through in vivo imaging, our findings reveal a neuroprotective role of microglia during the acute phase of single spinal cord axon injury, achieved through neuron-glia interaction.

The effects of P2Y12 loss on microglial gene expression, dynamics, and injury response in the cerebellum and cerebral cortex

Despite the emerging consensus that microglia are critical to physiological and pathological brain function, it is unclear how microglial roles and their underlying mechanisms differ between brain regions. Microglia throughout the brain express common markers, such as the purinergic receptor P2Y12, that delineate them from peripheral macrophages. P2Y12 is a critical sensor of injury but also contributes to the sensing of neuronal activity and remodeling of synapses, with microglial loss of P2Y12 resulting in behavioral deficits. P2Y12 has largely been studied in cortical microglia, despite the fact that a growing body of evidence suggests that microglia exhibit a high degree of regional specialization. Cerebellar microglia, in particular, exhibit transcriptional, epigenetic, and functional profiles that set them apart from their better studied cortical and hippocampal counterparts. Here, we demonstrate that P2Y12 deficiency does not alter the morphology, distribution, or dynamics of microglia in the cerebellum. In fact, loss of P2Y12 does little to disturb the distinct transcriptomic profiles of cortical and cerebellar microglia. However, unlike in cortex, P2Y12 is not required for a full microglial response to focal injury, suggesting that cerebellar and cortical microglia use different cues to respond to injury. Finally, we show that P2Y12 deficiency impairs cerebellar learning in a delay eyeblink conditioning task, a common test of cerebellar plasticity and circuit function. Our findings suggest not only region-specific roles of microglial P2Y12 signaling in the focal injury response, but also indicate a conserved role for P2Y12 in microglial modulation of plasticity across regions.

Co-localization and co-expression of Olfml3 with Iba1 in brain of mice

Olfml3 is a microglia-specific protein whose role in neuroinflammation is elusive. In silico analysis was conducted to characterize the Olfml3 protein, followed by molecular docking and MD simulation to check possible interaction with Iba1. Further, expression and co-localization analysis was performed in the LPS-induced neuroinflammatory mice brains. Results suggest that Olfml3 physically interacts with Iba1. Olfml3 and Iba1 expression increases during neuroinflammation in mice brains. Olfml3 was observed to co-localize with Iba1, and the number of Olfml3 and Iba1 dual-positive cells increased in the brain of the neuroinflammatory mice model. Thus, Olfml3 could potentially participate in microglia functions by interacting with Iba1.

Synaptic Pruning by Microglia: Lessons from Genetic Studies in Mice

BACKGROUND

Neural circuits are subjected to refinement throughout life. The dynamic addition and elimination (pruning) of synapses are necessary for maturation of neural circuits and synaptic plasticity. Due to their phagocytic nature, microglia have been considered as the primary mediators of synaptic pruning. Synaptic pruning can strengthen an active synapse by removing excess weaker synapses during development. Inappropriate synaptic pruning can often influence a disease outcome or an injury response.

SUMMARY

This review offers a focused discussion on microglial roles in synaptic pruning, based on the evidence gathered from genetic manipulations in mice. Genetically labeled microglia and synapses often allow assessment of their interactions in real time. Further manipulations involving synaptically localized molecules, neuronally or glial-derived diffusible factors, and their respective cognate receptors in microglia provide critical evidence in support of a direct role of microglia in synaptic pruning.

KEY MESSAGE

We discuss microglial contact-dependent "eat-me," "don't-eat-me," and "find-me" signals, as well as recently identified noncontact pruning, under the contexts of neural circuit, brain region, developmental window, and an injury or a disease state.

Hematopoietic stem cell gene therapy to halt neurodegeneration

Microglia play fundamental roles in multiple pathological primary and secondary processes affecting the central nervous system that ultimately result in neurodegeneration and for this reason they are considered as a key therapeutic target in several neurodegenerative diseases. Microglia-targeted therapies are directed at either restoring or modulating microglia function, to redirect their functional features toward neuroprotection. Among these strategies, hematopoietic stem cell gene therapy have proven to be endowed with a unique potential for replacing diseased microglia with engineered, transplant progeny cells that can integrate and exert relevant beneficial effects in the central nervous system of patients affected by inherited and acquired neurodegenerative conditions.

Drug repurposing for neurodegenerative diseases

Neurodegenerative diseases (NDDs) are neuronal problems that include the brain and spinal cord and result in loss of sensory and motor dysfunction. Common NDDs include Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), Multiple Sclerosis (MS), and Amyotrophic Lateral Sclerosis (ALS) etc. The occurrence of these diseases increases with age and is one of the challenging problems among elderly people. Though, several scientific research has demonstrated the key pathologies associated with NDDs still the underlying mechanisms and molecular details are not well understood and need to be explored and this poses a lack of effective treatments for NDDs. Several lines of evidence have shown that NDDs have a high prevalence and affect more than a billion individuals globally but still, researchers need to work forward in identifying the best therapeutic target for NDDs. Thus, several researchers are working in the directions to find potential therapeutic targets to alter the disease pathology and treat the diseases. Several steps have been taken to identify the early detection of the disease and drug repurposing for effective treatment of NDDs. Moreover, it is logical that current medications are being evaluated for their efficacy in treating such disorders; therefore, drug repurposing would be an efficient, safe, and cost-effective way in finding out better medication. In the current manuscript we discussed the utilization of drugs that have been repurposed for the treatment of AD, PD, HD, MS, and ALS.

TREM2 mediates MHCII-associated CD4+ T-cell response against gliomas

BACKGROUND

Myeloid cells comprise up to 50% of the total tumor mass in glioblastoma (GBM) and have been implicated in promoting tumor progression and immunosuppression. Modulating the response of myeloid cells to the tumor has emerged as a promising new approach for cancer treatment. In this regard, we focus on the Triggering Receptor Expressed on Myeloid Cells 2 (TREM2), which has recently emerged as a novel immune modulator in peripheral tumors.

METHODS

We studied the TREM2 expression profile in various patient tumor samples and conducted single-cell transcriptomic analysis in both GBM patients and the GL261 mouse glioma model. We utilized multiple mouse glioma models and employed state-of-the-art techniques such as invivo 2-photon imaging, spectrum flow cytometry, and in vitro co-culture assays to study TREM2 function in myeloid cell-mediated phagocytosis of tumor cells, antigen presentation, and response of CD4+ T cells within the tumor hemispheres.

RESULTS

Our research revealed significantly elevated levels of TREM2 expression in brain tumors compared to other types of tumors in patients. TREM2 was predominantly localized in tumor-associated myeloid cells and was highly expressed in nearly all microglia, as well as various subtypes of macrophages. Surprisingly, in preclinical glioma models, TREM2 deficiency did not confer a beneficial effect; instead, it accelerated glioma progression. Through detailed investigations, we determined that TREM2 deficiency impaired the ability of tumor-myeloid cells to phagocytose tumor cells and led to reduced expression of MHCII. This deficiency further significantly decreased the presence of CD4+ T cells within the tumor hemispheres.

CONCLUSIONS

Our study unveiled a previously unrecognized protective role of tumor-myeloid TREM2. Specifically, we found that TREM2 enhances the phagocytosis of tumor cells and promotes an immune response by facilitating MHCII-associated CD4+ T-cell responses against gliomas.

Cranial irradiation disrupts homeostatic microglial dynamic behavior

Cranial irradiation causes cognitive deficits that are in part mediated by microglia, the resident immune cells of the brain. Microglia are highly reactive, exhibiting changes in shape and morphology depending on the function they are performing. Additionally, microglia processes make dynamic, physical contacts with different components of their environment to monitor the functional state of the brain and promote plasticity. Though evidence suggests radiation perturbs homeostatic microglia functions, it is unknown how cranial irradiation impacts the dynamic behavior of microglia over time. Here, we paired in vivo two-photon microscopy with a transgenic mouse model that labels cortical microglia to follow these cells and determine how they change over time in cranial irradiated mice and their control littermates. We show that a single dose of 10 Gy cranial irradiation disrupts homeostatic cortical microglia dynamics during a 1-month time course. We found a lasting loss of microglial cells following cranial irradiation, coupled with a modest dysregulation of microglial soma displacement at earlier timepoints. The homogeneous distribution of microglia was maintained, suggesting microglia rearrange themselves to account for cell loss and maintain territorial organization following cranial irradiation. Furthermore, we found cranial irradiation reduced microglia coverage of the parenchyma and their surveillance capacity, without overtly changing morphology. Our results demonstrate that a single dose of radiation can induce changes in microglial behavior and function that could influence neurological health. These results set the foundation for future work examining how cranial irradiation impacts complex cellular dynamics in the brain which could contribute to the manifestation of cognitive deficits.

Understanding immune microenvironment alterations in the brain to improve the diagnosis and treatment of diverse brain diseases

Abnormal inflammatory states in the brain are associated with a variety of brain diseases. The dynamic changes in the number and function of immune cells in cerebrospinal fluid (CSF) are advantageous for the early prediction and diagnosis of immune diseases affecting the brain. The aggregated factors and cells in inflamed CSF may represent candidate targets for therapy. The physiological barriers in the brain, such as the blood‒brain barrier (BBB), establish a stable environment for the distribution of resident immune cells. However, the underlying mechanism by which peripheral immune cells migrate into the brain and their role in maintaining immune homeostasis in CSF are still unclear. To advance our understanding of the causal link between brain diseases and immune cell status, we investigated the characteristics of immune cell changes in CSF and the molecular mechanisms involved in common brain diseases. Furthermore, we summarized the diagnostic and treatment methods for brain diseases in which immune cells and related cytokines in CSF are used as targets. Further investigations of the new immune cell subtypes and their contributions to the development of brain diseases are needed to improve diagnostic specificity and therapy.

Discrete class I molecules on brain endothelium differentially regulate neuropathology in experimental cerebral malaria

Cerebral malaria is the deadliest complication that can arise from Plasmodium infection. CD8 T-cell engagement of brain vasculature is a putative mechanism of neuropathology in cerebral malaria. To define contributions of brain endothelial cell major histocompatibility complex (MHC) class I antigen-presentation to CD8 T cells in establishing cerebral malaria pathology, we developed novel H-2Kb LoxP and H-2Db LoxP mice crossed with Cdh5-Cre mice to achieve targeted deletion of discrete class I molecules, specifically from brain endothelium. This strategy allowed us to avoid off-target effects on iron homeostasis and class I-like molecules, which are known to perturb Plasmodium infection. This is the first endothelial-specific ablation of individual class-I molecules enabling us to interrogate these molecular interactions. In these studies, we interrogated human and mouse transcriptomics data to compare antigen presentation capacity during cerebral malaria. Using the Plasmodium berghei ANKA model of experimental cerebral malaria (ECM), we observed that H-2Kb and H-2Db class I molecules regulate distinct patterns of disease onset, CD8 T-cell infiltration, targeted cell death and regional blood-brain barrier disruption. Strikingly, ablation of either molecule from brain endothelial cells resulted in reduced CD8 T-cell activation, attenuated T-cell interaction with brain vasculature, lessened targeted cell death, preserved blood-brain barrier integrity and prevention of ECM and the death of the animal. We were able to show that these events were brain-specific through the use of parabiosis and created the novel technique of dual small animal MRI to simultaneously scan conjoined parabionts during infection. These data demonstrate that interactions of CD8 T cells with discrete MHC class I molecules on brain endothelium differentially regulate development of ECM neuropathology. Therefore, targeting MHC class I interactions therapeutically may hold potential for treatment of cases of severe malaria.

Contactomics of Microglia and Intercellular Communication

Microglia represent the main immunocompetent cell type in the parenchyma of the brain and the spinal cord, with roles extending way beyond their immune functions. While emerging data show the pivotal role of microglia in brain development, brain health and brain diseases, the exact mechanisms through which microglia contribute to complex neuroimmune interactions are still largely unclear. Understanding the communication between microglia and other cells represents an important cornerstone of these interactions, which may provide novel opportunities for therapeutic interventions in neurological or psychiatric disorders. As such, in line with studying the effects of the numerous soluble mediators that influence neuroimmune processes, attention on physical interactions between microglia and other cells in the CNS has increased substantially in recent years. In this chapter, we briefly summarize the latest literature on "microglial contactomics" and its functional implications in health and disease.

Cortical microglia dynamics are conserved during voluntary wheel running

We present the first demonstration of chronic in vivo imaging of microglia in mice undergoing voluntary wheel running. We find that healthy mice undergoing voluntary wheel running have similar microglia dynamics, morphologies, and responses to injury when compared to sedentary mice. This suggests that exercise over a period of 1 mo does not grossly alter cortical microglial phenotypes and that exercise may exert its beneficial effects on the brain through other mechanisms. Future work examining how microglia dynamics may be altered during exercise in disease or injury models could provide further insights into the therapeutic benefit of exercise.NEW & NOTEWORTHY We demonstrate the first use of chronic in vivo imaging of microglia over time during physical exercise. We found that microglia movement, morphology, and process motility were remarkably stable during voluntary wheel running (VWR). Additionally, microglia in running mice respond similarly to laser ablation injury compared to sedentary mice. These findings indicate that VWR does not induce changes in microglia dynamics in healthy adults. Exercise may elicit positive effects on the brain through other mechanisms.

Imaging microglia surveillance during sleep-wake cycles in freely behaving mice

Microglia surveillance manifests itself as dynamic changes in cell morphology and functional remodeling. Whether and how microglia surveillance is coupled to brain state switches during natural sleep-wake cycles remains unclear. To address this question, we used miniature two-photon microscopy (mTPM) to acquire time-lapse high-resolution microglia images of the somatosensory cortex, along with EEG/EMG recordings and behavioral video, in freely-behaving mice. We uncovered fast and robust brain state-dependent changes in microglia surveillance, occurring in parallel with sleep dynamics and early-onset phagocytic microglial contraction during sleep deprivation stress. We also detected local norepinephrine fluctuation occurring in a sleep state-dependent manner. We showed that the locus coeruleus-norepinephrine system, which is crucial to sleep homeostasis, is required for both sleep state-dependent and stress-induced microglial responses and β2-adrenergic receptor signaling plays a significant role in this process. These results provide direct evidence that microglial surveillance is exquisitely tuned to signals and stressors that regulate sleep dynamics and homeostasis so as to adjust its varied roles to complement those of neurons in the brain. In vivo imaging with mTPM in freely behaving animals, as demonstrated here, opens a new avenue for future investigation of microglia dynamics and sleep biology in freely behaving animals.

Using focused ultrasound to modulate microglial structure and function

Transcranial focused ultrasound (FUS) has the unique ability to target regions of the brain with high spatial precision, in a minimally invasive manner. Neuromodulation studies have shown that FUS can excite or inhibit neuronal activity, demonstrating its tremendous potential to improve the outcome of neurological diseases. Recent evidence has also shed light on the emerging promise that FUS has, with and without the use of intravenously injected microbubbles, in modulating the blood-brain barrier and the immune cells of the brain. As the resident immune cells of the central nervous system, microglia are at the forefront of the brain's maintenance and immune defense. Notably, microglia are highly dynamic and continuously survey the brain parenchyma by extending and retracting their processes. This surveillance activity aids microglia in performing key physiological functions required for brain activity and plasticity. In response to stressors, microglia rapidly alter their cellular and molecular profile to help facilitate a return to homeostasis. While the underlying mechanisms by which both FUS and FUS + microbubbles modify microglial structure and function remain largely unknown, several studies in adult mice have reported changes in the expression of the microglia/macrophage marker ionized calcium binding adaptor molecule 1, and in their phagocytosis, notably of protein aggregates, such as amyloid beta. In this review, we discuss the demonstrated and putative biological effects of FUS and FUS + microbubbles in modulating microglial activities, with an emphasis on the key cellular and molecular changes observed in vitro and in vivo across models of brain health and disease. Understanding how this innovative technology can modulate microglia paves the way for future therapeutic strategies aimed to promote beneficial physiological microglial roles, and prevent or treat maladaptive responses.

Plaque vicinity as a hotspot of microglial turnover in a mouse model of Alzheimer's disease

Microglia, the major immune cells of the brain, are functionally heterogeneous but in vivo functional properties of these cells are rarely studied at single-cell resolution. By using microRNA-9 regulated viral vectors for multicolor labeling and longitudinal in vivo monitoring of individual microglia, we followed their fate in the cortex of healthy adult mice and at the onset of amyloidosis in a mouse model of Alzheimer's disease. In wild-type mice, microglia were rather mobile (16% of the cells migrated at least once in 10-20 days) but had a low turnover as documented by low division and death rates. Half of the migratory events were tightly associated with blood vessels. Surprisingly, basic migration properties of microglia (i.e., fraction of migrating cells, saltatory migration pattern, speed of migration, translocation distance, and strong association with blood vessels) were preserved in amyloid-depositing brains, despite amyloid plaques becoming the major destination of migration. Besides, amyloid deposition significantly increased microglial division and death rates. Moreover, the plaque vicinity became a hotspot of microglial turnover, harboring 33% of all migration, 70% of death and 54% of division events.

Brain macrophage development, diversity and dysregulation in health and disease

Brain macrophages include microglia in the parenchyma, border-associated macrophages in the meningeal-choroid plexus-perivascular space, and monocyte-derived macrophages that infiltrate the brain under various disease conditions. The vast heterogeneity of these cells has been elucidated over the last decade using revolutionary multiomics technologies. As such, we can now start to define these various macrophage populations according to their ontogeny and their diverse functional programs during brain development, homeostasis and disease pathogenesis. In this review, we first outline the critical roles played by brain macrophages during development and healthy aging. We then discuss how brain macrophages might undergo reprogramming and contribute to neurodegenerative disorders, autoimmune diseases, and glioma. Finally, we speculate about the most recent and ongoing discoveries that are prompting translational attempts to leverage brain macrophages as prognostic markers or therapeutic targets for diseases that affect the brain.

Noteworthy perspectives on microglia in neuropsychiatric disorders

Microglia are so versatile that they not only provide immune surveillance for central nervous system, but participate in neural circuitry development, brain blood vessels formation, blood-brain barrier architecture, and intriguingly, the regulation of emotions and behaviors. Microglia have a profound impact on neuronal survival, brain wiring and synaptic plasticity. As professional phagocytic cells in the brain, they remove dead cell debris and neurotoxic agents via an elaborate mechanism. The functional profile of microglia varies considerately depending on age, gender, disease context and other internal or external environmental factors. Numerous studies have demonstrated a pivotal involvement of microglia in neuropsychiatric disorders, including negative affection, social deficit, compulsive behavior, fear memory, pain and other symptoms associated with major depression disorder, anxiety disorder, autism spectrum disorder and schizophrenia. In this review, we summarized the latest discoveries regarding microglial ontogeny, cell subtypes or state spectrum, biological functions and mechanistic underpinnings of emotional and behavioral disorders. Furthermore, we highlight the potential of microglia-targeted therapies of neuropsychiatric disorders, and propose outstanding questions to be addressed in future research of human microglia.

Microglial P2Y12 mediates chronic stress-induced synapse loss in the prefrontal cortex and associated behavioral consequences

Chronic unpredictable stress (CUS) drives microglia-mediated neuronal remodeling and synapse loss in the prefrontal cortex (PFC), contributing to deficits in cognition and behavior. However, it remains unclear what mechanisms guide microglia-neuron interactions in stress. Evidence indicates that neuronal activity-dependent purinergic signaling directs microglial processes and synaptic engagement via P2Y12, a purinergic receptor exclusively expressed by microglia in the brain. Stress alters excitatory neurotransmission in the PFC, thus we aimed to determine if P2Y12 signaling promotes functional changes in microglia in chronic stress. Here we used genetic ablation of P2Y12 (P2ry12-/-) or pharmacological blockade (clopidogrel, ticagrelor) to examine the role of purinergic signaling in stress-induced microglia-neuron interaction. Multiple behavioral, physiological, and cytometric endpoints were analyzed. Deletion of P2Y12 led to a number of fundamental alterations in the PFC, including the heightened microglial number and increased dendritic spine density. Flow cytometry revealed that microglia in P2ry12-/- mice had shifts in surface levels of CX3CR1, CSF1R, and CD11b, suggesting changes in synaptic engagement and phagocytosis in the PFC. In line with this, pharmacological blockade of P2Y12 prevented CUS-induced increases in the proportion of microglia with neuronal inclusions, limited dendritic spine loss in the PFC, and attenuated alterations in stress coping behavior and working memory function. Overall, these findings indicate that microglial P2Y12 is a critical mediator of stress-induced synapse loss in the PFC and subsequent behavioral deficits.

Clopidogrel Administration Impairs Post-Stroke Learning and Memory Recovery in Mice

Clopidogrel, which is one of the most prescribed antiplatelet medications in the world, is given to stroke survivors for the prevention of secondary cardiovascular events. Clopidogrel exerts its antiplatelet activity via antagonism of the P2Y12 receptor (P2RY12). Although not widely known or considered during the initial clinical trials for clopidogrel, P2RY12 is also expressed on microglia, which are the brain's immune cells, where the receptor facilitates chemotactic migration toward sites of cellular damage. If microglial P2RY12 is blocked, microglia lose the ability to migrate to damaged sites and carry out essential repair processes. We aimed to investigate whether administering clopidogrel to mice post-stroke was associated with (i) impaired motor skills and cognitive recovery; (ii) physiological changes, such as survival rate and body weight; (iii) changes in the neurovascular unit, including blood vessels, microglia, and neurons; and (iv) changes in immune cells. Photothrombotic stroke (or sham surgery) was induced in adult male mice. From 24 h post-stroke, mice were treated daily for 14 days with either clopidogrel or a control. Cognitive performance (memory and learning) was assessed using a mouse touchscreen platform (paired associated learning task), while motor impairment was assessed using the cylinder task for paw asymmetry. On day 15, the mice were euthanized and their brains were collected for immunohistochemistry analysis. Clopidogrel administration significantly impaired learning and memory recovery, reduced mouse survival rates, and reduced body weight post-stroke. Furthermore, clopidogrel significantly increased vascular leakage, significantly increased the number and appearance of microglia, and significantly reduced the number of T cells within the peri-infarct region post-stroke. These data suggest that clopidogrel hampers cognitive performance post-stroke. This effect is potentially mediated by an increase in vascular permeability post-stroke, providing a pathway for clopidogrel to access the central nervous system, and thus, interfere in repair and recovery processes.

Sex and interferon gamma signaling regulate microglia migration in the adult mouse cortex in vivo

Although microglia possess the unique ability to migrate, whether mobility is evident in all microglia, is sex dependent, and what molecular mechanisms drive this, is not well understood in the adult brain. Using longitudinal in vivo two-photon imaging of sparsely labeled microglia, we find a relatively small population of microglia (~5%) are mobile under normal conditions. Following injury (microbleed), the fraction of mobile microglia increased in a sex-dependent manner, with male microglia migrating significantly greater distances toward the microbleed relative to their female counterparts. To understand the signaling pathways involved, we interrogated the role of interferon gamma (IFNγ). Our data show that in male mice, stimulating microglia with IFNγ promotes migration whereas inhibiting IFNγ receptor 1 signaling inhibits them. By contrast, female microglia were generally unaffected by these manipulations. These findings highlight the diversity of microglia migratory responses to injury, its dependence on sex and the signaling mechanisms that modulate this behavior.

Movers and shakers: Microglial dynamics and modulation of neural networks

Microglia are multifaceted cells that act as immune sentinels, with important roles in pathological events, but also as integral contributors to the normal development and function of neural circuits. In the last decade, our understanding of the contributions these cells make to synaptic health and dysfunction has expanded at a dizzying pace. Here we review the known mechanisms that govern the dynamics of microglia allowing these motile cells to interact with synapses, and recruit microglia to specific sites on neurons. We then review the molecular signals that may underlie the function of microglia in synaptic remodeling. The emerging picture from the literature suggests that microglia are highly sensitive cells, reacting to neuronal signals with dynamic and specific actions tuned to the need of specific synapses and networks.

Diagnostic and therapeutic value of P2Y12R in epilepsy

There lacks biomarkers in current epilepsy diagnosis, and epilepsy is thus exposed to inadequate treatment, making it necessarily important to conduct search on new biomarkers and drug targets. The P2Y12 receptor is primarily expressed on microglia in the central nervous system, and acts as intrinsic immune cells in the central nervous system mediating neuroinflammation. In previous studies, P2Y12R in epilepsy has been found capable of controlling neuroinflammation and regulating neurogenesis as well as immature neuronal projections, and its expression is altered. P2Y12R is involved in microglia inhibition of neuronal activity and timely termination of seizures in acute seizures. In status epilepticus, the failure of P2Y12R in the process of "brake buffering" may not terminate the neuronal hyperexcitability timely. In chronic epilepsy, neuroinflammation causes seizures, which can in turn induce neuroinflammation, while on the other hand, neuroinflammation leads to neurogenesis, thereby causing abnormal neuronal discharges that give rise to seizures. In this case, targeting P2Y12R may be a novel strategy for the treatment of epilepsy. The detection of P2Y12R and its expression changes can contribute to the diagnosis of epilepsy. Meanwhile, the P2Y12R single-nucleotide polymorphism is associated with epilepsy susceptibility and endowed with the potential to individualize epilepsy diagnosis. To this end, functions of P2Y12R in the central nervous system were hereby reviewed, the effects of P2Y12R in epilepsy were explored, and the potential of P2Y12R in the diagnosis and treatment of epilepsy was further demonstrated.

Microglia modulate general anesthesia through P2Y12 receptor

General anesthesia (GA) is an unconscious state produced by anesthetic drugs, which act on neurons to cause overall suppression of neuronal activity in the brain. Recent studies have revealed that GA also substantially enhances the dynamics of microglia, the primary brain immune cells, with increased process motility and territory surveillance. However, whether microglia are actively involved in GA modulation remains unknown. Here, we report a previously unrecognized role for microglia engaging in multiple GA processes. We found that microglial ablation reduced the sensitivity of mice to anesthetics and substantially shortened duration of loss of righting reflex (LORR) or unconsciousness induced by multiple anesthetics, thereby promoting earlier emergence from GA. Microglial repopulation restored the regular anesthetic recovery, and chemogenetic activation of microglia prolonged the duration of LORR. In addition, anesthesia-accompanying analgesia and hypothermia were also attenuated after microglial depletion. Single-cell RNA sequencing analyses showed that anesthesia prominently affected the transcriptional levels of chemotaxis and migration-related genes in microglia. By pharmacologically targeting different microglial motility pathways, we found that blocking P2Y₁₂ receptor (P2Y₁₂R) reduced the duration of LORR of mice. Moreover, genetic ablation of P2Y₁₂R in microglia also promoted quicker recovery in mice from anesthesia, verifying the importance of microglial P2Y₁₂R in anesthetic regulation. Our work presents the first evidence that microglia actively participate in multiple processes of GA through P2Y₁₂R-mediated signaling and expands the non-immune roles of microglia in the brain.

TREM2 mediates MHCII-associated CD4+ T cell response against gliomas

Triggering receptor expressed on myeloid cells 2 (TREM2) was recently highlighted as a novel immune suppressive marker in peripheral tumors. The aim of this study was to characterize TREM2 expression in gliomas and investigate its contribution in glioma progression by using Trem2-/- mouse line. Our results showed that higher TREM2 expression was correlated with poor prognosis in glioma patients. Unexpectedly, TREM2 deficiency did not have a beneficial effect in a pre-clinical model of glioma. The increased TREM2 expression in glioma was likely due to increased myeloid cell infiltration, as evidenced by our single-cell analysis showing that almost all microglia and macrophages in gliomas were TREM2+. Furthermore, we found that deficiency of TREM2 impaired tumor-myeloid phagocytosis and MHCII presentation, and significantly reduced CD4+ T cells in tumor hemispheres. Our results revealed a previously unrecognized protective role of tumor-myeloid TREM2 in promoting MHCII-associated CD4+ T cell response against gliomas.

Purinergic signaling in cognitive impairment and neuropsychiatric symptoms of Alzheimer's disease

About 10 million new cases of dementia develop worldwide each year, of which up to 70% are attributable to Alzheimer's disease (AD). In addition to the widely known symptoms of memory loss and cognitive impairment, AD patients frequently develop non-cognitive symptoms, referred to as behavioral and psychological symptoms of dementia (BPSDs). Sleep disorders are often associated with AD, but mood alterations, notably depression and apathy, comprise the most frequent class of BPSDs. BPSDs negatively affect the lives of AD patients and their caregivers, and have a significant impact on public health systems and the economy. Because treatments currently available for AD are not disease-modifying and mainly aim to ameliorate some of the cognitive symptoms, elucidating the mechanisms underlying mood alterations and other BPSDs in AD may reveal novel avenues for progress in AD therapy. Purinergic signaling is implicated in the pathophysiology of several central nervous system (CNS) disorders, such as AD, depression and sleep disorders. Here, we review recent findings indicating that purinergic receptors, mainly the A₁, A_2A, and P2X7 subtypes, are associated with the development/progression of AD. Current evidence suggests that targeting purinergic signaling may represent a promising therapeutic approach in AD and related conditions. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Depletion of microglial BDNF increases susceptibility to the behavioral and synaptic effects of chronic unpredictable stress

In the medial prefrontal cortex (PFC), chronic stress reduces synaptic expression of glutamate receptors, leading to decreased excitatory signaling from layer V pyramidal neurons and working memory deficits. One key element driving these changes is a reduction in brain-derived neurotrophic factor (BDNF) signaling. BDNF is a potent mediator of synaptic growth and deficient BDNF signaling has been linked to stress susceptibility. Prior studies indicated that neurons are the primary source of BDNF, but more recent work suggests that microglia are also an important source of BDNF. Adding to this, our work showed that 14 days of chronic unpredictable stress (CUS) reduced Bdnf transcript in PFC microglia, evincing its relevance in the effects of stress. To explore this further, we utilized transgenic mice with microglia-specific depletion of BDNF (Cx3cr1Cre/+:Bdnffl/fl) and genotype controls (Cx3cr1Cre/+:Bdnf+/+). In the following experiments, mice were exposed to a shortened CUS paradigm (7 days) to determine if microglial Bdnf depletion promotes stress susceptibility. Analyses of PFC microglia revealed that Cx3cr1Cre/+:Bdnffl/fl mice had shifts in phenotypic markers and gene expression. In a separate cohort, synaptoneurosomes were collected from the PFC and western blotting was performed for synaptic markers. These experiments showed that Cx3cr1Cre/+:Bdnffl/fl mice had baseline deficits in GluN2B, and that 7 days of CUS additionally reduced GluN2A levels in Cx3cr1Cre/+:Bdnffl/fl mice, but not genotype controls. Behavioral and cognitive testing showed that this coincided with exacerbated stress effects on temporal object recognition in Cx3cr1Cre/+:Bdnffl/fl mice. These results indicate that microglial BDNF promotes glutamate receptor expression in the PFC. As such, mice with deficient microglial BDNF had increased susceptibility to the behavioral and cognitive consequences of stress.

Postoperative delirium, neuroinflammation, and influencing factors of postoperative delirium: A review

Postoperative delirium (POD) is an acute cognitive dysfunction that is mainly characterized by memory impairment and disturbances in consciousness. POD can prolong the hospital stay and increase the 1-month mortality rate of patients. The overall incidence of POD is approximately 23%, and its prevalence can go up to 50% in high-risk surgeries. Neuroinflammation is an important pathogenic mechanism of POD that mediates microglial activation and leads to synaptic remodeling. Neuroinflammation, as an indispensable pathogenesis of POD, can occur due to a variety of factors, including aseptic inflammation caused by surgery, effects of anesthetic drugs, disruption of the blood-brain barrier, and epigenetics. Understanding these factors and avoiding the occurrence of risk factors may help prevent POD in time. This review provides a brief overview of POD and neuroinflammation and summarizes various factors affecting POD development mediated by neuroinflammation, which may serve as future targets for the prevention and treatment of POD.

Elevating microglia TREM2 reduces amyloid seeding and suppresses disease-associated microglia

TREM2 is exclusively expressed by microglia in the brain and is strongly linked to the risk for Alzheimer's disease (AD). As microglial responses modulated by TREM2 are central to AD pathogenesis, enhancing TREM2 signaling has been explored as an AD therapeutic strategy. However, the effective therapeutic window targeting TREM2 is unclear. Here, by using microglia-specific inducible mouse models overexpressing human wild-type TREM2 (TREM2-WT) or R47H risk variant (TREM2-R47H), we show that TREM2-WT expression reduces amyloid deposition and neuritic dystrophy only during the early amyloid seeding stage, whereas TREM2-R47H exacerbates amyloid burden during the middle amyloid rapid growth stage. Single-cell RNA sequencing reveals suppressed disease-associated microglia (DAM) signature and reduced DAM population upon TREM2-WT expression in the early stage, whereas upregulated antigen presentation pathway is detected with TREM2-R47H expression in the middle stage. Together, our findings highlight the dynamic effects of TREM2 in modulating AD pathogenesis and emphasize the beneficial effect of enhancing TREM2 function in the early stage of AD development.

Spinal microglia contribute to sustained inflammatory pain via amplifying neuronal activity

Microglia are highly dynamic immune cells of the central nervous system (CNS). Microglial processes interact with neuronal elements constantly on the order of minutes. The functional significance of this acute microglia-neuron interaction and its potential role in the context of pain is still largely unknown. Here, we found that spinal microglia increased their process motility and electrophysiological reactivity within an hour after the insult in a mouse model of formalin-induced acute, sustained, inflammatory pain. Using an ablation strategy to specifically deplete resident microglia in the CNS, we demonstrate that microglia participate in formalin-induced acute sustained pain behaviors by amplifying neuronal activity in the spinal dorsal horn. Moreover, we identified that the P2Y12 receptor, which is specifically expressed in microglia in the CNS, was required for microglial function in formalin-induced pain. Taken together, our study provides a novel insight into the contribution of microglia and the P2Y12 receptor in inflammatory pain that could be used for potential therapeutic strategies.

Microglial debris is cleared by astrocytes via C4b-facilitated phagocytosis and degraded via RUBICON-dependent noncanonical autophagy in mice

Microglia are important immune cells in the central nervous system (CNS) that undergo turnover throughout the lifespan. If microglial debris is not removed in a timely manner, accumulated debris may influence CNS function. Clearance of microglial debris is crucial for CNS homeostasis. However, underlying mechanisms remain obscure. We here investigate how dead microglia are removed. We find that although microglia can phagocytose microglial debris in vitro, the territory-dependent competition hinders the microglia-to-microglial debris engulfment in vivo. In contrast, microglial debris is mainly phagocytosed by astrocytes in the brain, facilitated by C4b opsonization. The engulfed microglial fragments are then degraded in astrocytes via RUBICON-dependent LC3-associated phagocytosis (LAP), a form of noncanonical autophagy. Interference with C4b-mediated engulfment and subsequent LAP disrupt the removal and degradation of microglial debris, respectively. Together, we elucidate the cellular and molecular mechanisms of microglial debris removal in mice, extending the knowledge on the maintenance of CNS homeostasis.

Microglia-dependent remodeling of neuronal circuits

Microglia are tissue-resident macrophages responsible for the surveillance, neuronal support, and immune defense of the brain parenchyma. Recently, the role played by microglia in the formation and function of neuronal circuits has garnered substantial attention. During development, microglia have been shown to engulf neuronal precursors and participate in pruning mechanisms while, in the mature brain, they influence synaptic signaling, provide trophic support and shape synaptic plasticity. Recently, studies have unveiled different microglial characteristics associated with specific brain regions. This emerging view suggests that the maturation and function of distinct neuronal circuits may be potentially associated with the molecular identity microglia adopts across the brain. Here, we review and summarize the known role of these cells in the thalamus, hippocampus, cortex, and cerebellum. We focus on in vivo studies to highlight the characteristics of microglia that may be important in the remodeling of these neuronal circuits and in relation to neurodevelopmental and neuropsychiatric disorders.

P2X7/P2X4 Receptors Mediate Proliferation and Migration of Retinal Microglia in Experimental Glaucoma in Mice

Microglia are involved in the inflammatory response and retinal ganglion cell damage in glaucoma. Here, we investigated how microglia proliferate and migrate in a mouse model of chronic ocular hypertension (COH). In COH retinas, the microglial proliferation that occurred was inhibited by the P2X7 receptor (P2X7R) blocker BBG or P2X7R knockout, but not by the P2X4R blocker 5-BDBD. Treatment of primary cultured microglia with BzATP, a P2X7R agonist, mimicked the effects of cell proliferation and migration in COH retinas through the intracellular MEK/ERK signaling pathway. Transwell migration assays showed that the P2X4R agonist CTP induced microglial migration, which was completely blocked by 5-BDBD. In vivo and in vitro experiments demonstrated that ATP, released from activated Müller cells through connexin43 hemichannels, acted on P2X7R to induce microglial proliferation, and acted on P2X4R/P2X7R (mainly P2X4R) to induce microglial migration. Our results suggest that inhibiting the interaction of Müller cells and microglia may attenuate microglial proliferation and migration in glaucoma.

Glial Tiling in the Insect Nervous System

The Drosophila nervous system comprises a small number of well characterized glial cell classes. The outer surface of the central nervous system (CNS) is protected by a glial derived blood-brain barrier generated by perineurial and subperineurial glia. All neural stem cells and all neurons are engulfed by cortex glial cells. The inner neuropil region, that harbors all synapses and dendrites, is covered by ensheathing glia and infiltrated by astrocyte-like glial cells. All these glial cells show a tiled organization with an often remarkable plasticity where glial cells of one cell type invade the territory of the neighboring glial cell type upon its ablation. Here, we summarize the different glial tiling patterns and based on the different modes of cell-cell contacts we hypothesize that different molecular mechanisms underlie tiling of the different glial cell types.

Central nervous system macrophages in progressive multiple sclerosis: relationship to neurodegeneration and therapeutics

There are over 15 disease-modifying drugs that have been approved over the last 20 years for the treatment of relapsing–remitting multiple sclerosis (MS), but there are limited treatment options available for progressive MS. The development of new drugs for the treatment of progressive MS remains challenging as the pathophysiology of progressive MS is poorly understood.

The progressive phase of MS is dominated by neurodegeneration and a heightened innate immune response with trapped immune cells behind a closed blood–brain barrier in the central nervous system. Here we review microglia and border-associated macrophages, which include perivascular, meningeal, and choroid plexus macrophages, during the progressive phase of MS. These cells are vital and are largely the basis to define lesion types in MS. We will review the evidence that reactive microglia and macrophages upregulate pro-inflammatory genes and downregulate homeostatic genes, that may promote neurodegeneration in progressive MS. We will also review the factors that regulate microglia and macrophage function during progressive MS, as well as potential toxic functions of these cells. Disease-modifying drugs that solely target microglia and macrophage in progressive MS are lacking. The recent treatment successes for progressive MS include include B-cell depletion therapies and sphingosine-1-phosphate receptor modulators. We will describe several therapies being evaluated as a potential treatment option for progressive MS, such as immunomodulatory therapies that can target myeloid cells or as a potential neuroprotective agent.

Microglial TREM2 in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is an aggressive motor neuron degenerative disease characterized by selective loss of both upper and lower motor neurons. The mechanisms underlying disease initiation and progression are poorly understood. The involvement of nonmotor neuraxis emphasizes the contribution of glial cells in disease progress. Microglia comprise a unique subset of glial cells and are the principal immune cells in the central nervous system (CNS). Triggering receptor expressed on myeloid cell 2 (TREM2) is a surface receptor that, within the CNS, is exclusively expressed on microglia and plays crucial roles in microglial proliferation, migration, activation, metabolism, and phagocytosis. Genetic evidence has linked TREM2 to neurodegenerative diseases including ALS, but its function in ALS pathogenesis is largely unknown. In this review, we summarize how microglial activation, with a specific focus on TREM2 function, affects ALS progression clinically and experimentally. Understanding microglial TREM2 function will help pinpoint the molecular target for ALS treatment.

TREM2 interacts with TDP-43 and mediates microglial neuroprotection against TDP-43-related neurodegeneration

Triggering receptor expressed on myeloid cell 2 (TREM2) is linked to neurodegenerative disease risk. However, the function of TREM2 in neurodegeneration is still not fully understood. Here we investigated the role of microglial TREM2 in TAR-DNA binding protein 43 kDa (TDP-43)-related neurodegeneration using viral-mediated and transgenic mouse models. We found that TREM2 deficiency impaired phagocytic clearance of pathological TDP-43 by microglia, and enhanced neuronal damage and motor impairments. Mass cytometry analysis revealed that hTDP-43 induced a TREM2-dependent subpopulation of microglia with high CD11c expression and phagocytic ability. Using mass spectrometry and surface plasmon resonance analysis, we further demonstrated an interaction between TDP-43 and TREM2 in vitro and in vivo as well as in ALS patient tissues. We computationally identified regions within hTDP-43 that interact with TREM2. Our data highlights that TDP-43 is a possible ligand for microglial TREM2 and that this interaction mediates neuroprotection of microglia in TDP-43-related neurodegeneration.

Altered synaptic connectivity and brain function in mice lacking microglial adapter protein Iba1

Growing evidence indicates that microglia impact brain function by regulating synaptic pruning and formation as well as synaptic transmission and plasticity. Iba1 (ionized Ca⁺²-binding adapter protein 1), encoded by the Allograft inflammatory factor 1 (Aif1) gene, is an actin-interacting protein in microglia. Although Iba1 has long been used as a cellular marker for microglia, its functional role remains unknown. Here, we used global, Iba1-deficient (Aif1 ^-/-) mice to characterize microglial activity, synaptic function, and behavior. Microglial imaging in acute hippocampal slices and fixed tissues from juvenile mice revealed that Aif1 ^-/- microglia display reductions in ATP-induced motility and ramification, respectively. Biochemical assays further demonstrated that Aif1 ^-/- brain tissues exhibit an altered expression of microglial-enriched proteins associated with synaptic pruning. Consistent with these changes, juvenile Aif1 ^-/- mice displayed deficits in the excitatory synapse number and synaptic drive assessed by neuronal labeling and whole-cell patch-clamp recording in acute hippocampal slices. Unexpectedly, microglial synaptic engulfment capacity was diminished in juvenile Aif1 ^-/- mice. During early postnatal development, when synapse formation is a predominant event in the hippocampus, the excitatory synapse number was still reduced in Aif1 ^-/- mice. Together, these findings support an overall role of Iba1 in excitatory synaptic growth in juvenile mice. Lastly, postnatal synaptic deficits persisted in adulthood and correlated with significant behavioral changes in adult Aif1 ^-/- mice, which exhibited impairments in object recognition memory and social interaction. These results suggest that Iba1 critically contributes to microglial activity underlying essential neuroglia developmental processes that may deeply influence behavior.

Key protein-coding genes related to microglia in immune regulation and inflammatory response induced by epilepsy

Several studies have shown a link between immunity, inflammatory processes, and epilepsy. Active neuroinflammation and marked immune cell infiltration occur in epilepsy of diverse etiologies. Microglia, as the first line of defense in the central nervous system, are the main effectors of neuroinflammatory processes. Discovery of new biomarkers associated with microglia activation after epileptogenesis indicates that targeting specific molecules may help control seizures. In this research, we used a combination of several bioinformatics approaches, including RNA sequencing, to explore differentially expressed genes (DEGs) in epileptic lesions and control samples, and to construct a protein-protein interaction (PPI) network for DEGs, which was examined utilizing plug-ins in Cytoscape software. Finally, we aimed to identify 10 hub genes in immune and inflammation-related sub-networks, which were subsequently validated in real-time quantitative polymerase chain reaction analysis in a mouse model of kainic acid-induced epilepsy. The expression patterns of nine genes were consistent with sequencing outcomes. Meanwhile, several genes, including CX3CR1, CX3CL1, GPR183, FPR1, P2RY13, P2RY12 and LPAR5, were associated with microglial activation and migration, providing novel candidate targets for immunotherapy in epilepsy and laying the foundation for further research.

Uncovering microglial pathways driving sex-specific neurobiological effects in stress and depression

Women suffer from major depressive disorder (MDD) more often than men and report greater MDD symptom severity. Mounting evidence suggests that sex differences in MDD may be driven, in part, by sex-specific neurobiological mechanisms. Chronic stress is a significant risk factor in MDD, and preclinical rodent models show differential patterns of stress-induced neural remodeling and cognitive-behavioral dysfunction in males and females. For instance, chronic stress leads to synapse loss in the medial prefrontal cortex in male rodents yet has either no effect on- or increases-synapse number in females. Recent reports have implicated microglia, the immune cells of the brain, in MDD, and findings demonstrate sex-specific microglial signatures in both preclinical stress models and MDD patients. Given that microglia can remodel neural architecture, modulate synaptic transmission, and affect subsequent changes in behavior, it is plausible that microglial pathways contribute to differential stress effects on neuroplasticity and function in males and females. As such, this review examines the evidence for sex-specific microglia-neuron interactions in preclinical stress models and in patients with MDD. Discoveries highlighted herein demonstrate divergent microglial contributions in males and females and suggest that future studies investigating stress-linked disorders should be guided by sex-dependent neurobiological and behavioral findings. Examining these pathways represents a clear avenue toward both a richer understanding of brain, behavior, and immunity, and innovative psychoneuroimmunology-based applications in personalized medicine.

Tetris in the Nervous System: What Principles of Neuronal Tiling Can Tell Us About How Glia Play the Game

The precise organization and arrangement of neural cells is essential for nervous system functionality. Cellular tiling is an evolutionarily conserved phenomenon that organizes neural cells, ensuring non-redundant coverage of receptive fields in the nervous system. First recorded in the drawings of Ramon y Cajal more than a century ago, we now have extensive knowledge of the biochemical and molecular mechanisms that mediate tiling of neurons. The advent of live imaging techniques in both invertebrate and vertebrate model organisms has enhanced our understanding of these processes. Despite advancements in our understanding of neuronal tiling, we know relatively little about how glia, an essential non-neuronal component of the nervous system, tile and contribute to the overall spatial arrangement of the nervous system. Here, we discuss lessons learned from neurons and apply them to potential mechanisms that glial cells may use to tile, including cell diversity, contact-dependent repulsion, and chemical signaling. We also discuss open questions in the field of tiling and what new technologies need to be developed in order to better understand glial tiling.

Capillary-associated microglia regulate vascular structure and function through PANX1-P2RY12 coupling in mice

Microglia are brain-resident immune cells with a repertoire of functions in the brain. However, the extent of their interactions with the vasculature and potential regulation of vascular physiology has been insufficiently explored. Here, we document interactions between ramified CX3CR1 + myeloid cell somata and brain capillaries. We confirm that these cells are bona fide microglia by molecular, morphological and ultrastructural approaches. Then, we give a detailed spatio-temporal characterization of these capillary-associated microglia (CAMs) comparing them with parenchymal microglia (PCMs) in their morphological activities including during microglial depletion and repopulation. Molecularly, we identify P2RY12 receptors as a regulator of CAM interactions under the control of released purines from pannexin 1 (PANX1) channels. Furthermore, microglial elimination triggered capillary dilation, blood flow increase, and impaired vasodilation that were recapitulated in P2RY12-/- and PANX1-/- mice suggesting purines released through PANX1 channels play important roles in activating microglial P2RY12 receptors to regulate neurovascular structure and function.

Microglia and Sensitive Periods in Brain Development

From embryonic neuronal migration to adolescent circuit refinement, the immune system plays an essential role throughout central nervous system (CNS) development. Immune signaling molecules serve as a common language between the immune system and CNS, allowing them to work together to modulate brain function both in health and disease. As the resident CNS macrophage, microglia comprise the majority of immune cells in the brain. Much like their peripheral counterparts, microglia survey their environment for pathology, clean up debris, and propagate inflammatory responses when necessary. Beyond this, recent studies have highlighted that microglia perform a number of complex tasks during neural development, from directing neuronal and axonal positioning to pruning synapses, receptors, and even whole cells. In this chapter, we discuss this literature within the framework that immune activation during discrete windows of neural development can profoundly impact brain function long-term, and thus the risk of neurodevelopmental and neuropsychiatric disorders. In this chapter, we review three sensitive developmental periods - embryonic wiring, early postnatal synaptic pruning, and adolescent circuit refinement - in order to highlight the diversity of functions that microglia perform in building a brain. In reviewing this literature, it becomes obvious that timing matters, perhaps more so than the nature of the immune activation itself; largely conserved patterns of microglial response to diverse insults result in different functional impacts depending on the stage of brain maturation at the time of the challenge.

Electrified microglia: Impact of direct current stimulation on diverse properties of the most versatile brain cell

BACKGROUND

Transcranial direct current stimulation [(t)DCS], modulates cortical excitability and promotes neuroplasticity. Microglia has been identified to respond to electrical currents as well as neuronal activity, but its response to DCS is mostly unknown.

OBJECTIVE

This study addresses effects of DCS applied in vivo to the sensorimotor cortex on physiological microglia properties and neuron-microglia communication.

METHODS

Time lapse in vivo 2-photon microscopy in anaesthetized mice was timely coupled with DCS of the sensorimotor cortex to observe microglia dynamics on a population-based and single cell level. Neuron-microglia communication during DCS was investigated in mice with a functional knock out of the fractalkine receptor CX3CR1. Moreover, the role of voltage gated microglial channels and DCS effects on phagocytosis were studied.

RESULTS

DCS promoted several physiological microglia properties, depending on the glial activation state and stimulation intensity. On a single cell level, process motility was predominantly enhanced in ramified cells whereas horizontal soma movement and galvanotaxis was pronounced in reactive microglia. Blockage of voltage sensitive microglial channels suppressed DCS effects in vivo and in vitro. Microglial motility changes were partially driven by the fractalkine signaling pathway. Moreover, phagocytosis increased after DCS in vitro.

CONCLUSION

Microglia dynamics are rapidly influenced by DCS. This is the first in vivo demonstration of a direct effect of electrical currents on microglia and indirect effects potentially driven by neuronal activity via the fractalkine pathway.

Evolving Models and Tools for Microglial Studies in the Central Nervous System

Microglia play multiple roles in such processes as brain development, homeostasis, and pathology. Due to their diverse mechanisms of functions, the complex sub-classifications, and the large differences between different species, especially compared with humans, very different or even opposite conclusions can be drawn from studies with different research models. The choice of appropriate research models and the associated tools are thus key ingredients of studies on microglia. Mice are the most commonly used animal models. In this review, we summarize in vitro and in vivo models of mouse and human-derived microglial research models, including microglial cell lines, primary microglia, induced microglia-like cells, transgenic mice, human-mouse chimeric models, and microglial replacement models. We also summarize recent developments in novel single-cell and in vivo imaging technologies. We hope our review can serve as an efficient reference for the future study of microglia.

In Vivo Imaging of the Microglial Landscape After Whole Brain Radiation Therapy

PURPOSE

Whole brain radiation therapy (WBRT) is an important treatment for patients with multiple brain metastases, but can also cause cognitive deterioration. Microglia, the resident immune cells of the brain, promote a proinflammatory environment and likely contribute to cognitive decline after WBRT. To investigate the temporal dynamics of the microglial reaction in individual mice to WBRT, we developed a novel in vivo experimental model using cranial window implants and longitudinal imaging.

METHODS AND MATERIALS

Chronic cranial windows were surgically implanted over the somatosensory cortex of transgenic Cx3cr1-enhanced green fluorescent protein (EGFP)/+ C57BL/6 mice, where microglia were fluorescently tagged with EGFP. Cx3cr1-EGFP/+ mice were also crossed with Thy1-YFP mice to fluorescently dual label microglia and subsets of neurons throughout the brain. Three weeks after window implantation and recovery, computed tomography image guided WBRT was delivered (single dose 10 Gy using two 5 Gy parallel-opposed lateral beams). Radiation dosing was confirmed using radiochromic film. Then, in vivo 2-photon microscopy was used to longitudinally image the microglial landscape and microglial motility at 7 days and 16 days after irradiation in the same mice.

RESULTS

Film dosimetry confirmed the average delivered dose per beam at midpoint was accurate within 2%, with no attenuation from the window frame. By 7 days after WBRT, significant changes in the microglial landscape were seen, characterized by apparent loss of microglial cells (20%) and significant rearrangements of microglial location with time after irradiation (36% of cells not found in original location).

CONCLUSIONS

Using longitudinal in vivo 2-photon imaging, this study demonstrated the feasibility of imaging microglia-neuron interactions and defining how microglia react to WBRT in the same mouse. Having demonstrated utility of the model, this experimental paradigm can be used to investigate the dynamic changes of many different brain cell types and their interactions after WBRT and uncover the underlying cellular mechanisms of WBRT-induced cognitive decline.

The role of P2Y12 in the kinetics of microglial self-renewal and maturation in the adult visual cortex in vivo

Microglia are the brain’s resident immune cells with a tremendous capacity to autonomously self-renew. Because microglial self-renewal has largely been studied using static tools, its mechanisms and kinetics are not well understood. Using chronic in vivo two-photon imaging in awake mice, we confirm that cortical microglia show limited turnover and migration under basal conditions. Following depletion, however, microglial repopulation is remarkably rapid and is sustained by the dynamic division of remaining microglia, in a manner that is largely independent of signaling through the P2Y12 receptor. Mathematical modeling of microglial division demonstrates that the observed division rates can account for the rapid repopulation observed in vivo. Additionally, newly born microglia resemble mature microglia within days of repopulation, although morphological maturation is different in newly born microglia in P2Y12 knock out mice. Our work suggests that microglia rapidly locally and that newly born microglia do not recapitulate the slow maturation seen in development but instead take on mature roles in the CNS.

Little cells of the little brain: microglia in cerebellar development and function

Microglia are long-lived resident macrophages of the brain with diverse roles that span development, adulthood, and aging. Once thought to be a relatively homogeneous population, there is a growing recognition that microglia are highly specialized to suit their specific brain region. Cerebellar microglia represent an example of such specialization, exhibiting a dynamical, transcriptional, and immunological profile that differs from that of other microglial populations. Here we review the evidence that cerebellar microglia shape the cerebellar environment and are in turn shaped by it. We examine the roles microglia play in cerebellar function, development, and aging. The emerging findings on cerebellar microglia may also provide insights into disease processes involving cerebellar dysfunction.

Regulation of microglia population dynamics throughout development, health, and disease

The dynamic expansions and contractions of the microglia population in the central nervous system (CNS) to achieve homeostasis are likely vital for their function. Microglia respond to injury or disease but also help guide neurodevelopment, modulate neural circuitry throughout life, and direct regeneration. Throughout these processes, microglia density changes, as does the volume of area that each microglia surveys. Given that microglia are responsible for sensing subtle alterations to their environment, a change in their density could affect their capacity to mobilize rapidly. In this review, we attempt to synthesize the current literature on the ligands and conditions that promote microglial proliferation across development, adulthood, and neurodegenerative conditions. Microglia display an impressive proliferative capacity during development and in neurodegenerative diseases that is almost completely absent at homeostasis. However, the appropriate function of microglia in each state is critically dependent on density fluctuations that are primarily induced by proliferation. Proliferation is a natural microglial response to insult and often serves neuroprotective functions. In contrast, inappropriate microglial proliferation, whether too much or too little, often precipitates undesirable consequences for nervous system health. Thus, fluctuations in the microglia population are tightly regulated to ensure these immune cells can execute their diverse functions.

An overview of microglia ontogeny and maturation in the homeostatic and pathological brain

Microglia are the resident immune cells of the central nervous system (CNS) and are increasingly recognized as critical players in development, brain homeostasis, and disease pathogenesis. The lifespan, maintenance, proliferation, and turnover of microglia are important factors that regulate microglial behavior and affect their roles in the CNS. However, emerging evidence suggests that microglia are morphologically and phenotypically distinct in different brain areas, at different ages, and during disease. Ongoing research focuses on understanding how microglia acquire specific phenotypes in response to extrinsic cues in the environment and how phenotypes are specified by intrinsic properties of different populations of microglia. With the development of pharmacological and genetic tools that allow the investigation of microglia in vivo, there have been considerable advances in understanding molecular signatures of both homeostatic microglia and those reacting to injury and disease. Here, we review the master gene regulators that define microglia as well as discuss the evidence that microglia are heterogeneous and fall into distinct clusters that display specific intrinsic properties and perform unique tasks in different settings. Taken together, the information presented supports the idea that microglia morphology and transcriptional heterogeneity should be considered when studying the complex nature of microglia and their roles in brain health and disease.

Microglial Displacement of GABAergic Synapses Is a Protective Event during Complex Febrile Seizures

Complex febrile seizures (FSs) lead to a high risk of intractable temporal lobe epilepsy during adulthood, yet the pathological process of complex FSs is largely unknown. Here, we demonstrate that activated microglia extensively associated with glutamatergic neuronal soma displace surrounding GABAergic presynapses in complex FSs. Patch-clamp electrophysiology establishes that the microglial displacement of GABAergic presynapses abrogates a complex-FS-induced increase in GABAergic neurotransmission and neuronal excitability, whereas GABA exerts an excitatory action in this immature stage. Pharmacological inhibition of microglial displacement of GABAergic presynapses or selective ablation of microglia in CD11b^DTR mice promotes the generation of complex FSs. Blocking or deleting the P2Y₁₂ receptor (P2Y₁₂R) reduces microglial displacement of GABAergic presynapses and shortens the latency of complex FSs. Together, microglial displacement of GABAergic presynapses, regulated by P2Y₁₂R, reduces neuronal excitability to mitigate the generation of complex FSs. Microglial displacement is a protective event during the pathological process of complex FSs.