Microglial P2Y6 calcium signaling promotes phagocytosis and shapes neuroimmune responses in epileptogenesis

GABA-dependent microglial elimination of inhibitory synapses underlies neuronal hyperexcitability in epilepsy

Neuronal hyperexcitability is a common pathophysiological feature of many neurological diseases. Neuron-glia interactions underlie this process but the detailed mechanisms remain unclear. Here, we reveal a critical role of microglia-mediated selective elimination of inhibitory synapses in driving neuronal hyperexcitability. In epileptic mice of both sexes, hyperactive inhibitory neurons directly activate surveilling microglia via GABAergic signaling. In response, these activated microglia preferentially phagocytose inhibitory synapses, disrupting the balance between excitatory and inhibitory synaptic transmission and amplifying network excitability. This feedback mechanism depends on both GABA-GABAB receptor-mediated microglial activation and complement C3-C3aR-mediated microglial engulfment of inhibitory synapses, as pharmacological or genetic blockage of both pathways effectively prevents inhibitory synapse loss and ameliorates seizure symptoms in mice. Additionally, putative cell-cell interaction analyses of brain tissues from males and females with temporal lobe epilepsy reveal that inhibitory neurons induce microglial phagocytic states and inhibitory synapse loss. Our findings demonstrate that inhibitory neurons can directly instruct microglial states to control inhibitory synaptic transmission through a feedback mechanism, leading to the development of neuronal hyperexcitability in temporal lobe epilepsy.

Microglial MS4A4A Protects against Epileptic Seizures in Alzheimer's Disease

Alzheimer's disease (AD) is a predominant neurodegenerative disorder worldwide, with epileptic seizures being a common comorbidity that can exacerbate cognitive deterioration in affected individuals, thus highlighting the importance of early therapeutic intervention. It is determined that deletion of Ms4a4a, an AD-associated gene, exacerbates seizures in amyloid β (Aβ)-driven AD mouse model. MS4A4A is significantly upregulated in brain lesions in patients with epilepsy. Single-cell sequencing reveals that MS4A4A is highly expressed in microglia within these lesions, linked to enhanced phagocytic activity. Mechanistic investigation delineates that deletion of Ms4a4a impairs microglial phagocytosis, accompanied by diminished calcium influx and disruptions in mitochondrial metabolic fitness. The cytosolic fragment of Ms4a4a is anchored to the cytoskeletal components, supporting its critical role in mediating phagocytosis. Induction of Ms4a4a through central delivery of LNP-Il4 alleviates seizure conditions. Collectively, these findings identify Ms4a4a as a potential therapeutic target for managing seizures in AD treatment.

How microglia contribute to the induction and maintenance of neuropathic pain

Neuropathic pain is a debilitating condition caused by damage to the nervous system that results in changes along the pain pathway that lead to persistence of the pain sensation. Unremitting pain conditions are associated with maladaptive plasticity, disruption of neuronal activity that favours excitation over inhibition, and engagement of immune cells. The substantial progress made over the last two decades in the neuroimmune interaction research area points to a mechanistic role of spinal cord microglia, which are resident immune cells of the CNS. Microglia respond to and modulate neuronal activity during establishment and persistence of neuropathic pain states, and microglia-neuron pathways provide targets that can be exploited to attenuate abnormal neuronal activity and provide pain relief.

Modification of Neural Circuit Functions by Microglial P2Y6 Receptors in Health and Neurodegeneration

Neural circuits consisting of neurons and glial cells help to establish all functions of the CNS. Microglia, the resident immunocytes of the CNS, are endowed with UDP-sensitive P2Y6 receptors (P2Y6Rs) which regulate phagocytosis/pruning of excessive synapses during individual development and refine synapses in an activity-dependent manner during adulthood. In addition, this type of receptor plays a decisive role in primary (Alzheimer's disease, Parkinson's disease, neuropathic pain) and secondary (epilepsy, ischemic-, mechanical-, or irradiation-induced) neurodegeneration. A whole range of microglial cytokines controlled by P2Y6Rs, such as the interleukins IL-1β, IL-6, IL-8, and tumor necrosis factor-α (TNF-α), leads to neuroinflammation, resulting in neurodegeneration. Hence, small molecular antagonists of P2Y6Rs and genetic knockdown of this receptor provide feasible ways to alleviate inflammation-induced neurological disorders but might also interfere with the regulation of the synaptic circuitry. The present review aims at investigating this dual role of P2Y6Rs in microglia, both in shaping neural circuits by targeted phagocytosis and promoting neurodegenerative illnesses by fostering neuroinflammation through multiple transduction mechanisms.

S100A9 protein activates microglia and stimulates phagocytosis, resulting in synaptic and neuronal loss

S100 calcium-binding protein A9 (S100A9, also known as calgranulin B) is expressed and secreted by myeloid cells under inflammatory conditions, and S100A9 can amplify inflammation. There is a large increase in S100A9 expression in the brains of patients with neurodegenerative diseases, such as Alzheimer's disease, and S100A9 has been suggested to contribute to neurodegeneration, but the mechanisms are unclear. Here we investigated the effects of extracellular recombinant S100A9 protein on microglia, neurons and synapses in primary rat brain neuronal-glial cell cultures. Incubation of cell cultures with 250-500 nM S100A9 caused neuronal loss without signs of apoptosis or necrosis, but accompanied by exposure of the "eat-me" signal - phosphatidylserine on neurons. S100A9 caused activation of microglial inflammation as evidenced by an increase in the microglial number, morphological changes, release of pro-inflammatory cytokines, and increased phagocytic activity. At lower concentrations, 10-100 nM S100A9 induced synaptic loss in the cultures. Depletion of microglia from the cultures prevented S100A9-induced neuronal and synaptic loss, indicating that neuronal and synaptic loss was mediated by microglia. These results suggest that extracellular S100A9 may contribute to neurodegeneration by activating microglial inflammation and phagocytosis, resulting in loss of synapses and neurons. This further suggests the possibility that neurodegeneration may be reduced by targeting S100A9 or microglia.

Elevated Astrocytic NFAT5 of the hippocampus increases epilepsy susceptibility in hypoxic-ischemic mice

OBJECTIVE

Hypoxic-ischemic brain damage (HIBD) is a leading cause of neonatal mortality, resulting in brain injury and persistent seizures that can last into the late neonatal period and beyond. Effective treatments and interventions for infants affected by hypoxia-ischemia remain lacking. Clinical investigations have indicated an elevation of nuclear factor of activated T cells 5 (NFAT5) in whole blood from umbilical cords of severely affected HIBD infants with epilepsy. Experimental research has demonstrated that NFAT5 has ambivalent effects on neuroprotection and neurologic damage. However, the mechanistic role of NFAT5 in HIBD remains unclear. This investigation aims to further clarify the role of NFAT5 in epilepsy following HIBD insult.

METHODS

We created a neonatal HIBD mouse model through left common carotid artery occlusion. By specifically knocking down astrocytic NFAT5 and its downstream molecule, Nedd4-2, using hippocampal delivery of adeno-associated virus 5-driven targeted shRNA, we investigated the role of astrocytic NFAT5 in epilepsy susceptibility in HIBD mice. This was assessed through electroencephalographic recordings, behavioral observations in vivo, and whole-cell recordings of hippocampal neuronal activity. In vitro, we evaluated the effects of astrocytic NFAT5 alteration on Kir4.1 expression and IKir4.1 in both brain slices from HIBD mice and cultured astrocytes treated with oxygen-glucose deprivation/reoxygenation.

RESULTS

Hypoxia-ischemia-induced upregulation of hippocampal NFAT5 occurs in astrocytes rather than in neurons. This upregulation leads to increased expression of the ubiquitin ligase Nedd4-2, resulting in excessive degradation of Kir4.1 in astrocytes. Consequently, astrocytic function in buffering extracellular K+ is impaired, causing depolarization of the resting potential and enhanced neuronal discharge. This disruption ultimately affects local neural network balance and increases susceptibility to epilepsy. In contrast, inhibiting or knocking down astrocytic NFAT5 almost completely reverses these effects.

SIGNIFICANCE

Our findings suggest that manipulating the NFAT5-Nedd4-2-Kir4.1 axis in astrocytes could provide a potential therapeutic strategy for the epileptic complications of HIBD.

Chemogenetic activation of microglial Gi signaling decreases microglial surveillance and impairs neuronal synchronization

Microglia actively survey the brain and dynamically interact with neurons to maintain brain homeostasis. Microglial Gi protein-coupled receptors (Gi-GPCRs) play a critical role in microglia-neuron communications. However, the impact of temporally activating microglial Gi signaling on microglial dynamics and neuronal activity in the homeostatic brain remains largely unknown. In this study, we used Gi-based designer receptors exclusively activated by designer drugs (Gi-DREADD) to selectively and temporally modulate microglial Gi signaling pathway. By integrating this chemogenetic approach with in vivo two-photon imaging, we observed that exogenous activation of microglial Gi signaling transiently inhibited microglial process dynamics, reduced neuronal activity, and impaired neuronal synchronization. These altered neuronal functions were associated with a decrease in interactions between microglia and neuron somata. Together, this study demonstrates that acute, exogenous activation of microglial Gi signaling regulates neuronal circuit function, offering a potential pharmacological target for the neuromodulation through microglia.

Extracellular ATP regulates phagocytic activity, mitochondrial respiration, and cytokine secretion of human astrocytic cells

The two main glial cell types of the central nervous system (CNS), astrocytes and microglia, are responsible for neuroimmune homeostasis. Recent evidence indicates astrocytes can participate in removal of pathological structures by becoming phagocytic under conditions of neurodegenerative disease when microglia, the professional phagocytes, are impaired. We hypothesized that adenosine triphosphate (ATP), which acts as damage-associated molecular pattern (DAMP), when released at high concentrations into extracellular space, upregulates phagocytic activity of human astrocytes. This study is the first to measure changes in phagocytic activity and mitochondrial respiration of human astrocytic cells in response to extracellular ATP. We demonstrate that ATP-induced phagocytic activity of U118 MG astrocytic cells is accompanied by upregulated mitochondrial oxidative phosphorylation, which likely supports this energy-dependent process. Application of a selective antagonist A438079 provides evidence identifying astrocytic purinergic P2X7 receptor (P2X7R) as the potential regulator of their phagocytic function. We also report a rapid ATP-induced increase in intracellular calcium ([Ca2+]i), which could serve as regulator of both the phagocytic activity and mitochondrial metabolism, but this hypothesis will need to be tested in future studies. Since ATP upregulates interleukin (IL)-8 secretion by astrocytes but has no effect on their cytotoxicity towards neuronal cells, we conclude that extracellular ATP affects only specific functions of astrocytes. The selectivity of P2X7R-dependent regulation of astrocyte functions by extracellular ATP could allow targeting this receptor-ligand interaction to upregulate their phagocytic function. This could have beneficial outcomes in neurodegenerative disorders, such as Alzheimer's disease, that are characterized by reactive astrocytes and defective phagocytic processes.

Microglial TREM2 promotes phagocytic clearance of damaged neurons after status epilepticus

In the central nervous system, triggering receptor expressed on myeloid cells 2 (TREM2) is exclusively expressed by microglia and is critical for microglial proliferation, migration, and phagocytosis. Microglial TREM2 plays an important role in neurodegenerative diseases, such as Alzheimer's disease and amyotrophic lateral sclerosis. However, little is known about how TREM2 affects microglial function within epileptogenesis. To investigate this, we utilized male TREM2 knockout (KO) mice within the intra-amygdala kainic acid seizure model. Electroencephalographic analysis, immunocytochemistry, and RNA sequencing revealed that TREM2 deficiency significantly promoted seizure-induced pathology. We found that TREM2 KO increased both the severity of acute status epilepticus and the number of spontaneous recurrent seizures characteristic of chronic focal epilepsy. Phagocytic clearance of damaged neurons by microglia was also impaired by TREM2 KO and reduced phagocytic activity correlated with increased spontaneous seizures. Analysis of human tissue from patients who underwent surgical resection for drug resistant temporal lobe epilepsy also showed a negative correlation between expression of the microglial phagocytic marker CD68 and focal to bilateral tonic-clonic generalized seizure history. These results indicate that microglial TREM2 and phagocytic activity are important to epileptogenic pathology.

Who's Down With UDP?-Not Epilepsy; Purinergic Microglial Ca2+ Signaling Mediates Epileptogenesis

Purinergic mechanisms mediate microglial Ca 2 + signaling in epileptogenesis Umpierre, AD, Li, B, Katayoun, A, Simon, WL, Zhao, S, Xie, M, Thyen, G, Hur, B, Zheng, J, Liang, Y, Bosco, DB, Maynes, MA, Wu, Z, Yu, X, Sung, J, Johnson, AJ, Li, Y, Wu,J-L. 2024. Microglial P2Y6 calcium signaling promotes phagocytosis and shapes neuroimmune responses in epileptogenesis. Neuron 112:1959-1977. Microglial calcium signaling is rare in a baseline state but strongly engaged during early epilepsy development. The mechanism(s) governing microglial calcium signaling are not known. By developing an in vivo uridine diphosphate (UDP) fluorescent sensor, GRABUDP1.0, we discovered that UDP release is a conserved response to seizures and excitotoxicity across brain regions. UDP can signal through the microglial-enriched P2Y6 receptor to increase calcium activity during epileptogenesis. P2Y6 calcium activity is associated with lysosome biogenesis and enhanced production of NF-kB-related cytokines. In the hippocampus, knockout of the P2Y6 receptor prevents microglia from fully engulfing neurons. Attenuating microglial calcium signaling through calcium extruder (“CalEx”) expression recapitulates multiple features of P2Y6 knockout, including reduced lysosome biogenesis and phagocytic interactions. Ultimately, P2Y6 knockout mice retain more CA3 neurons and better cognitive task performance during epileptogenesis. Our results demonstrate that P2Y6 signaling impacts multiple aspects of myeloid cell immune function during epileptogenesis.

Calcium dynamics of skin-resident macrophages during homeostasis and tissue injury

Immune cells depend on rapid changes in intracellular calcium activity to modulate cell function. Skin contains diverse immune cell types and is critically dependent on calcium signaling for homeostasis and repair, yet the dynamics and functions of calcium in skin immune cells remain poorly understood. Here, we characterize calcium activity in Langerhans cells, skin-resident macrophages responsible for surveillance and clearance of cellular debris after tissue damage. Langerhans cells reside in the epidermis and extend dynamic dendrites in close proximity to adjacent keratinocytes and somatosensory peripheral axons. We find that homeostatic Langerhans cells exhibit spontaneous and transient changes in calcium activity, with calcium flux occurring primarily in the cell body and rarely in the dendrites. Triggering somatosensory axon degeneration increases the frequency of calcium activity in Langerhans cell dendrites. By contrast, we show that Langerhans cells exhibit a sustained increase in intracellular calcium following engulfment of damaged keratinocytes. Altering intracellular calcium activity leads to a decrease in engulfment efficiency of keratinocyte debris. Our findings demonstrate that Langerhans cells exhibit context-specific changes in calcium activity and highlight the utility of skin as an accessible model for imaging calcium dynamics in tissue-resident macrophages.

Macrophage P2Y6R activation aggravates psoriatic inflammation through IL-27-mediated Th1 responses

Purinergic signaling plays a causal role in the modulation of immune inflammatory response in the course of psoriasis, but its regulatory mechanism remains unclear. As a member of purinoceptors, P2Y6R mainly distributed in macrophages was significantly up-expressed in skin lesions from patients with psoriasis in the present study. Here, the severity of psoriasis was alleviated in imiquimod-treated mice with macrophages conditional knockout of P2Y6R, while the cell-chat algorithm showed there was a correlation between macrophage P2Y6R and Th1 cells mediated by IL-27. Mechanistically, P2Y6R enhanced PLC β /p-PKC/MAPK activation to induce IL-27 release dependently, which subsequently regulated the differentiation of Th1 cells, leading to erythematous and scaly plaques of psoriasis. Interestingly, we developed a novel P2Y6R inhibitor FS-6, which bonds with the ARG266 side chain of P2Y6R, exhibited remarkable anti-psoriasis effects targeting P2Y6R. Our study provides insights into the role of P2Y6R in the pathogenesis of psoriasis and suggests its potential as a target for the development of therapeutic interventions. A novel P2Y6R inhibitor FS-6 could be developed as an anti-psoriasis drug candidate for the clinic.

Overview of the role of purinergic signaling and insights into its role in cancer therapy

Innovation of cancer therapy has received a dramatic acceleration over the last fifteen years thanks to the introduction of the novel immune checkpoint inhibitors (ICI). On the other hand, the conspicuous scientific knowledge accumulated in purinergic signaling since the early seventies is finally being transferred to the clinic. Several Phase I/II clinical trials are currently underway to investigate the effect of drugs interfering with purinergic signaling as stand-alone or combination therapy in cancer. This is supporting the novel concept of "purinergic immune checkpoint" (PIC) in cancer therapy. In the present review we will address a) the basic pharmacology and cell biology of the purinergic system; b) principles of its pathophysiology in human diseases; c) implications for cell death, cell proliferation and cancer; d) novel molecular tools to investigate nucleotide homeostasis in the extracellular environment; e) recent developments in the pharmacology of P1, P2 receptors and related ecto-enzymes; f) P1 and P2 ligands as novel diagnostic tools; g) current issues in PIC-based anti-cancer therapy. This review will provide an appraisal of the current status of purinergic signaling in cancer and will help identify future avenues of development.

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.

The microglial P2Y6 receptor as a therapeutic target for neurodegenerative diseases

Neurodegenerative diseases are associated with chronic neuroinflammation in the brain, which can result in microglial phagocytosis of live synapses and neurons that may contribute to cognitive deficits and neuronal loss. The microglial P2Y6 receptor (P2Y6R) is a G-protein coupled receptor, which stimulates microglial phagocytosis when activated by extracellular uridine diphosphate, released by stressed neurons. Knockout or inhibition of P2Y6R can prevent neuronal loss in mouse models of Alzheimer's disease (AD), Parkinson's disease, epilepsy, neuroinflammation and aging, and prevent cognitive deficits in models of AD, epilepsy and aging. This review summarises the known roles of P2Y6R in the physiology and pathology of the brain, and its potential as a therapeutic target to prevent neurodegeneration and other brain pathologies.

Neuroimmune modulation in liver pathophysiology

The liver, the largest organ in the human body, plays a multifaceted role in digestion, coagulation, synthesis, metabolism, detoxification, and immune defense. Changes in liver function often coincide with disruptions in both the central and peripheral nervous systems. The intricate interplay between the nervous and immune systems is vital for maintaining tissue balance and combating diseases. Signaling molecules and pathways, including cytokines, inflammatory mediators, neuropeptides, neurotransmitters, chemoreceptors, and neural pathways, facilitate this complex communication. They establish feedback loops among diverse immune cell populations and the central, peripheral, sympathetic, parasympathetic, and enteric nervous systems within the liver. In this concise review, we provide an overview of the structural and compositional aspects of the hepatic neural and immune systems. We further explore the molecular mechanisms and pathways that govern neuroimmune communication, highlighting their significance in liver pathology. Finally, we summarize the current clinical implications of therapeutic approaches targeting neuroimmune interactions and present prospects for future research in this area.

Rod-shaped microglia interact with neuronal dendrites to regulate cortical excitability in TDP-43 related neurodegeneration

Motor cortical hyperexcitability is well-documented in the presymptomatic stage of amyotrophic lateral sclerosis (ALS). However, the mechanisms underlying this early dysregulation are not fully understood. Microglia, as the principal immune cells of the central nervous system, have emerged as important players in sensing and regulating neuronal activity. Here we investigated the role of microglia in the motor cortical circuits in a mouse model of TDP-43 neurodegeneration (rNLS8). Utilizing multichannel probe recording and longitudinal in vivo calcium imaging in awake mice, we observed neuronal hyperactivity at the initial stage of disease progression. Spatial and single-cell RNA sequencing revealed that microglia are the primary responders to motor cortical hyperactivity. We further identified a unique subpopulation of microglia, rod-shaped microglia, which are characterized by a distinct morphology and transcriptional profile. Notably, rod-shaped microglia predominantly interact with neuronal dendrites and excitatory synaptic inputs to attenuate motor cortical hyperactivity. The elimination of rod-shaped microglia through TREM2 deficiency increased neuronal hyperactivity, exacerbated motor deficits, and further decreased survival rates of rNLS8 mice. Together, our results suggest that rod-shaped microglia play a neuroprotective role by attenuating cortical hyperexcitability in the mouse model of TDP-43 related neurodegeneration.