Microglia release ATP by exocytosis

Role of microglia in stroke

Microglia play key roles in the post-ischemic inflammatory response and damaged tissue removal reacting rapidly to the disturbances caused by ischemia and working to restore the lost homeostasis. However, the modified environment, encompassing ionic imbalances, disruption of crucial neuron-microglia interactions, spreading depolarization, and generation of danger signals from necrotic neurons, induce morphological and phenotypic shifts in microglia. This leads them to adopt a proinflammatory profile and heighten their phagocytic activity. From day three post-ischemia, macrophages infiltrate the necrotic core while microglia amass at the periphery. Further, inflammation prompts a metabolic shift favoring glycolysis, the pentose-phosphate shunt, and lipid synthesis. These shifts, combined with phagocytic lipid intake, drive lipid droplet biogenesis, fuel anabolism, and enable microglia proliferation. Proliferating microglia release trophic factors contributing to protection and repair. However, some microglia accumulate lipids persistently and transform into dysfunctional and potentially harmful foam cells. Studies also showed microglia that either display impaired apoptotic cell clearance, or eliminate synapses, viable neurons, or endothelial cells. Yet, it will be essential to elucidate the viability of engulfed cells, the features of the local environment, the extent of tissue damage, and the temporal sequence. Ischemia provides a rich variety of region- and injury-dependent stimuli for microglia, evolving with time and generating distinct microglia phenotypes including those exhibiting proinflammatory or dysfunctional traits and others showing pro-repair features. Accurate profiling of microglia phenotypes, alongside with a more precise understanding of the associated post-ischemic tissue conditions, is a necessary step to serve as the potential foundation for focused interventions in human stroke.

Extracellular ATP inhibits excitatory synaptic input on parvalbumin positive interneurons and attenuates gamma oscillations via P2X4 receptors

BACKGROUND AND PURPOSE

P2X4 receptors (P2X4R) are ligand gated cation channels that are activated by extracellular ATP released by neurons and glia. The receptors are widely expressed in the brain and have fractional calcium currents comparable with NMDA receptors. Although P2X4Rs have been reported to modulate synaptic transmission and plasticity, their involvement in shaping neuronal network activity remains to be elucidated.

EXPERIMENTAL APPROACH

We investigated the effects of P2X receptors at network and synaptic level using local field potential electrophysiology, whole cell patch clamp recordings and calcium imaging in fast spiking parvalbumin positive interneurons (PVINs) in rat and mouse hippocampal slices. The stable ATP analogue ATPγS, selective antagonists and P2X4R knockout mice were used.

KEY RESULTS

The P2XR agonist ATPγS reversibly decreased the power of gamma oscillations. This inhibition could be antagonized by the selective P2X4R antagonist PSB-12062 and was not observed in P2X4-/- mice. The phasic excitatory inputs of CA3 PVINs were one of the main regulators of the gamma power. Associational fibre compound excitatory postsynaptic currents (cEPSCs) in CA3 PVINs were inhibited by P2X4R activation. This effect was reversible, dependent on intracellular calcium and dynamin-dependent internalization of AMPA receptors.

CONCLUSIONS AND IMPLICATIONS

The results indicate that P2X4Rs are an important source of dendritic calcium in CA3 PVINs, thereby regulating excitatory synaptic inputs onto the cells and presumably the state of gamma oscillations in the hippocampus. P2X4Rs represent an effective target to modulate hippocampal network activity in pathophysiological conditions such as Alzheimer's disease and schizophrenia.

Microglia regulate motor neuron plasticity via reciprocal fractalkine/adenosine signaling

Microglia are innate CNS immune cells that play key roles in supporting key CNS functions including brain plasticity. We now report a previously unknown role for microglia in regulating neuroplasticity within spinal phrenic motor neurons, the neurons driving diaphragm contractions and breathing. We demonstrate that microglia regulate phrenic long-term facilitation (pLTF), a form of respiratory memory lasting hours after repetitive exposures to brief periods of low oxygen (acute intermittent hypoxia; AIH) via neuronal/microglial fractalkine signaling. AIH-induced pLTF is regulated by the balance between competing intracellular signaling cascades initiated by serotonin vs adenosine, respectively. Although brainstem raphe neurons release the relevant serotonin, the cellular source of adenosine is unknown. We tested a model in which hypoxia initiates fractalkine signaling between phrenic motor neurons and nearby microglia that triggers extracellular adenosine accumulation. With moderate AIH, phrenic motor neuron adenosine 2A receptor activation undermines serotonin-dominant pLTF; in contrast, severe AIH drives pLTF by a unique, adenosine-dominant mechanism. Phrenic motor neuron fractalkine knockdown, cervical spinal fractalkine receptor inhibition on nearby microglia, and microglial depletion enhance serotonin-dominant pLTF with moderate AIH but suppress adenosine-dominant pLTF with severe AIH. Thus, microglia play novel functions in the healthy spinal cord, regulating hypoxia-induced neuroplasticity within the motor neurons responsible for breathing.

Extracellular ATP Is a Homeostatic Messenger That Mediates Cell-Cell Communication in Physiological Processes and Psychiatric Diseases

Neuronal activity is the basis of information encoding and processing in the brain. During neuronal activation, intracellular ATP (adenosine triphosphate) is generated to meet the high-energy demands. Simultaneously, ATP is secreted, increasing the extracellular ATP concentration and acting as a homeostatic messenger that mediates cell-cell communication to prevent aberrant hyperexcitability of the nervous system. In addition to the confined release and fast synaptic signaling of classic neurotransmitters within synaptic clefts, ATP can be released by all brain cells, diffuses widely, and targets different types of purinergic receptors on neurons and glial cells, making it possible to orchestrate brain neuronal activity and participate in various physiological processes, such as sleep and wakefulness, learning and memory, and feeding. Dysregulation of extracellular ATP leads to a destabilizing effect on the neural network, as found in the etiopathology of many psychiatric diseases, including depression, anxiety, schizophrenia, and autism spectrum disorder. In this review, we summarize advances in the understanding of the mechanisms by which extracellular ATP serves as an intercellular signaling molecule to regulate neural activity, with a focus on how it maintains the homeostasis of neural networks. In particular, we also focus on neural activity issues that result from dysregulation of extracellular ATP and propose that aberrant levels of extracellular ATP may play a role in the etiopathology of some psychiatric diseases, highlighting the potential therapeutic targets of ATP signaling in the treatment of these psychiatric diseases. Finally, we suggest potential avenues to further elucidate the role of extracellular ATP in intercellular communication and psychiatric diseases.

Lipopolysaccharide-mediated ATP signaling regulates interleukin-6 mRNA expression via the P2-purinoceptor in human dental pulp cells

Dental pulp cells play a crucial role in maintaining the balance of the pulp tissue. They actively respond to bacterial inflammation by producing proinflammatory cytokines, particularly interleukin-6 (IL-6). While many cell types release adenosine triphosphate (ATP) in response to various stimuli, the mechanisms and significance of ATP release in dental pulp cells under inflammatory conditions are not well understood. This study aimed to investigate ATP release and its relationship with IL-6 during the inflammatory response in immortalized human dental pulp stem cells (hDPSC-K4DT) following lipopolysaccharide (LPS) stimulation. We found that hDPSC-K4DT cells released ATP extracellularly when exposed to LPS concentrations above 10 μg/mL. ATP release was exclusively attenuated by N-ethylmaleimide, whereas other inhibitors, including clodronic acid (a vesicular nucleotide transporter inhibitor), probenecid (a selective pannexin-1 channel inhibitor), meclofenamic acid (a selective connexin 43 inhibitor), suramin (a nonspecific P2 receptor inhibitor), and KN-62 (a specific P2X7 antagonist), did not exhibit any effect. Additionally, LPS increased IL-6 mRNA expression, which was mitigated by the ATPase apyrase enzyme, N-ethylmaleimide, and suramin, but not by KN-62. Moreover, exogenous ATP induced IL-6 mRNA expression, whereas ATPase apyrase, N-ethylmaleimide, and suramin, but not KN-62, diminished ATP-induced IL-6 mRNA expression. Overall, our findings suggest that LPS-induced ATP release stimulates the IL-6 pathway through P2-purinoceptor, indicating that ATP may function as an anti-inflammatory signal, contributing to the maintenance of dental pulp homeostasis.

P2X7 Receptor-Induced Human Mast Cell Degranulation Is Enhanced by Interleukin 33

MCs are tissue-resident immune cells that strategically reside in barrier organs and respond effectively to a wide range of stimuli, such as IL-33, a mediator released upon epithelial damage. Adenosine triphosphate (ATP) accumulates at sites of tissue injury and is known to modulate MC activities. This study investigated how an inflammatory tissue environment rich in IL-33 modulates the ATP-mediated activation of MCs. Human primary MCs primed with IL-33 displayed a strongly increased response to ATP but not ADP. This resulted in increased degranulation, IL-8 release, and pERK1/2 signalling. Such effects are unique to IL-33 stimulation and not shared by the epithelial alarmin, TSLP. MC exposure to IL-33 also increased membrane expression of purinergic and ATP-binding P2X receptors. The use of selective P2X receptor inhibitors identified P2X7 receptor as the key mediator of the enhanced ATP-induced ERK1/2 signalling and degranulation in IL-33-primed MCs. Whilst the inhibition of P2X1 and P2X4 receptors had no effect on MC degranulation, inhibiting these receptors together with P2X7 resulted in further decreased MC-mediated degranulation. These data therefore point toward the potential mechanisms by which IL-33 contributes to the modulation of ATP-mediated activation in human MCs.

Extracellular ATP/adenosine dynamics in the brain and its role in health and disease

Extracellular ATP and adenosine are neuromodulators that regulate numerous neuronal functions in the brain. Neuronal activity and brain insults such as ischemic and traumatic injury upregulate these neuromodulators, which exert their effects by activating purinergic receptors. In addition, extracellular ATP/adenosine signaling plays a pivotal role in the pathogenesis of neurological diseases. Virtually every cell type in the brain contributes to the elevation of ATP/adenosine, and various mechanisms underlying this increase have been proposed. Extracellular adenosine is thought to be mainly produced via the degradation of extracellular ATP. However, adenosine is also released from neurons and glia in the brain. Therefore, the regulation of extracellular ATP/adenosine in physiological and pathophysiological conditions is likely far more complex than previously thought. To elucidate the complex mechanisms that regulate extracellular ATP/adenosine levels, accurate methods of assessing their spatiotemporal dynamics are needed. Several novel techniques for acquiring spatiotemporal information on extracellular ATP/adenosine, including fluorescent sensors, have been developed and have started to reveal the mechanisms underlying the release, uptake and degradation of ATP/adenosine. Here, we review methods for analyzing extracellular ATP/adenosine dynamics as well as the current state of knowledge on the spatiotemporal dynamics of ATP/adenosine in the brain. We focus on the mechanisms used by neurons and glia to cooperatively produce the activity-dependent increase in ATP/adenosine and its physiological and pathophysiological significance in the brain.

Lactiplantibacillus plantarum fermentation enhanced the protective effect of kiwifruit on intestinal injury in rats: Based on mitochondrial morphology and function

Fermented foods have protective effects on body health. In our previously study, we found Lactiplantibacillus plantarum fermentation enhanced antioxidant activity of kiwifruit in vitro digestion. Then, in this work we explored the protective effect of fermented kiwi on intestinal injury induced by acute lipopolysaccharide (LPS) stress. Compared to non-fermented kiwi pulp (KP), Lactiplantibacillus plantarum fermented kiwi pulp (FKP-LP) contained more peptides, hormones and vitamins contents, lesser nucleic acid and carbohydrate contents. FKP-LP could relieve the intestinal injury by improving morphological of tight junction and upregulating tight junction proteins mRNA expression. Fermented kiwi maintained the mitochondrial morphology, mitochondrial respiratory function, and mitochondrial homeostasis, and relieved the LPS induced injury by regulating the contents of energy substances, and the respiratory chain complex enzyme activity through the pathway of AMPK and its downstream factors including PGC-1α, NRF1, NRF2, TFAM, and ULK2.

Microglial voltage-dependent anion channel 1 signaling modulates sleep deprivation-induced transition to chronic postsurgical pain

STUDY OBJECTIVES

This study verified that sleep deprivation before and after skin/muscle incision and retraction (SMIR) surgery increased the risk of chronic pain and investigated the underlying roles of microglial voltage-dependent anion channel 1 (VDAC1) signaling.

METHODS

Adult mice received 6 hours of total sleep deprivation from 1 day prior to SMIR until the third day after surgery. Mechanical and heat-evoked pain was assessed before and within 21 days after surgery. Microglial activation and changes in VDAC1 expression and oligomerization were measured. Minocycline was injected to observe the effects of inhibiting microglial activation on pain maintenance. The VDAC1 inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and oligomerization inhibitor VBIT-4 were used to determine the roles of VDAC1 signaling on microglial adenosine 5' triphosphate (ATP) release, inflammation (IL-1β and CCL2), and chronicity of pain.

RESULTS

Sleep deprivation significantly increased the pain duration after SMIR surgery, activated microglia, and enhanced VDAC1 signaling in the spinal cord. Minocycline inhibited microglial activation and alleviated sleep deprivation-induced pain maintenance. Lipopolysaccharide (LPS)-induced microglial activation was accompanied by increased VDAC1 expression and oligomerization, and more VDAC1 was observed on the cell membrane surface compared with control. DIDS and VBIT-4 rescued LPS-induced microglial ATP release and IL-1β and CCL2 expression. DIDS and VBIT-4 reversed sleep loss-induced microglial activation and pain chronicity in mice, similar to the effects of minocycline. No synergistic effects were found for minocycline plus VBIT-4 or DIDS.

CONCLUSIONS

Perioperative sleep deprivation activated spinal microglia and increases the risk of chronic postsurgical pain in mice. VDAC1 signaling regulates microglial activation-related ATP release, inflammation, and chronicity of pain.

Age-dependent changes on fractalkine forms and their contribution to neurodegenerative diseases

The chemokine fractalkine (FKN, CX3CL1), a member of the CX3C subfamily, contributes to neuron-glia interaction and the regulation of microglial cell activation. Fractalkine is expressed by neurons as a membrane-bound protein (mCX3CL1) that can be cleaved by extracellular proteases generating several sCX3CL1 forms. sCX3CL1, containing the chemokine domain, and mCX3CL1 have high affinity by their unique receptor (CX3CR1) which, physiologically, is only found in microglia, a resident immune cell of the CNS. The activation of CX3CR1contributes to survival and maturation of the neural network during development, glutamatergic synaptic transmission, synaptic plasticity, cognition, neuropathic pain, and inflammatory regulation in the adult brain. Indeed, the various CX3CL1 forms appear in some cases to serve an anti-inflammatory role of microglia, whereas in others, they have a pro-inflammatory role, aggravating neurological disorders. In the last decade, evidence points to the fact that sCX3CL1 and mCX3CL1 exhibit selective and differential effects on their targets. Thus, the balance in their level and activity will impact on neuron-microglia interaction. This review is focused on the description of factors determining the emergence of distinct fractalkine forms, their age-dependent changes, and how they contribute to neuroinflammation and neurodegenerative diseases. Changes in the balance among various fractalkine forms may be one of the mechanisms on which converge aging, chronic CNS inflammation, and neurodegeneration.

Magnitude and Mechanism of Phrenic Long-term Facilitation Shift Between Daily Rest Versus Active Phase

Plasticity is a fundamental property of the neural system controlling breathing. One key example of respiratory motor plasticity is phrenic long-term facilitation (pLTF), a persistent increase in phrenic nerve activity elicited by acute intermittent hypoxia (AIH). pLTF can arise from distinct cell signaling cascades initiated by serotonin versus adenosine receptor activation, respectively, and interact via powerful cross-talk inhibition. Here, we demonstrate that the daily rest/active phase and the duration of hypoxic episodes within an AIH protocol have profound impact on the magnitude and mechanism of pLTF due to shifts in serotonin/adenosine balance. Using the historical "standard" AIH protocol (3, 5-min moderate hypoxic episodes), we demonstrate that pLTF magnitude is unaffected by exposure in the midactive versus midrest phase, yet the mechanism driving pLTF shifts from serotonin-dominant (midrest) to adenosine-dominant (midactive). This mechanistic "flip" results from combined influences of hypoxia-evoked adenosine release and daily fluctuations in basal spinal adenosine. Since AIH evokes less adenosine with shorter (15, 1-min) hypoxic episodes, midrest pLTF is amplified due to diminished adenosine constraint on serotonin-driven plasticity; in contrast, elevated background adenosine during the midactive phase suppresses serotonin-dominant pLTF. These findings demonstrate the importance of the serotonin/adenosine balance in regulating the amplitude and mechanism of AIH-induced pLTF. Since AIH is emerging as a promising therapeutic modality to restore respiratory and nonrespiratory movements in people with spinal cord injury or ALS, knowledge of how time-of-day and hypoxic episode duration impact the serotonin/adenosine balance and the magnitude and mechanism of pLTF has profound biological, experimental, and translational implications.

Coordinated Transcriptional Waves Define the Inflammatory Response of Primary Microglial Culture

The primary role of microglia is to maintain homeostasis by effectively responding to various disturbances. Activation of transcriptional programs determines the microglia's response to external stimuli. In this study, we stimulated murine neonatal microglial cells with benzoyl ATP (bzATP) and lipopolysaccharide (LPS), and monitored their ability to release pro-inflammatory cytokines. When cells are exposed to bzATP, a purinergic receptor agonist, a short-lived wave of transcriptional changes, occurs. However, only combining bzATP and LPS led to a sustainable and robust response. The transcriptional profile is dominated by induced cytokines (e.g., IL-1α and IL-1β), chemokines, and their membrane receptors. Several abundant long noncoding RNAs (lncRNAs) are induced by bzATP/LPS, including Ptgs2os2, Bc1, and Morrbid, that function in inflammation and cytokine production. Analyzing the observed changes through TNF (Tumor necrosis factor) and NF-κB (nuclear factor kappa light chain enhancer of activated B cells) pathways confirmed that neonatal glial cells exhibit a distinctive expression program in which inflammatory-related genes are upregulated by orders of magnitude. The observed capacity of the microglial culture to activate a robust inflammatory response is useful for studying neurons under stress, brain injury, and aging. We propose the use of a primary neonatal microglia culture as a responsive in vitro model for testing drugs that may interact with inflammatory signaling and the lncRNA regulatory network.

Emerging non-proinflammatory roles of microglia in healthy and diseased brains

Microglia, the resident myeloid cells of the central nervous system, are the first line of defense against foreign pathogens, thereby confining the extent of brain injury. However, the role of microglia is not limited to macrophage-like functions. In addition to proinflammatory response mediation, microglia are involved in neurodevelopmental remodeling and homeostatic maintenance in the absence of disease. An increasing number of studies have also elucidated microglia-mediated regulation of tumor growth and neural repair in diseased brains. Here, we review the non-proinflammatory roles of microglia, with the aim of promoting a deeper understanding of the functions of microglia in healthy and diseased brains and contributing to the development of novel therapeutics that target microglia in neurological disorders.

Mild inflammation impairs acute intermittent hypoxia-induced phrenic long-term facilitation by a spinal adenosine-dependent mechanism

Inflammation undermines neuroplasticity, including serotonin-dependent phrenic long-term facilitation (pLTF) following moderate acute intermittent hypoxia (mAIH: 3, 5-min episodes, arterial Po2: 40-50 mmHg; 5-min intervals). Mild inflammation elicited by a low dose of the TLR-4 receptor agonist, lipopolysaccharide (LPS; 100 µg/kg, ip), abolishes mAIH-induced pLTF by unknown mechanisms. In the central nervous system, neuroinflammation primes glia, triggering ATP release and extracellular adenosine accumulation. As spinal adenosine 2 A (A2A) receptor activation impairs mAIH-induced pLTF, we hypothesized that spinal adenosine accumulation and A2A receptor activation are necessary in the mechanism whereby LPS impairs pLTF. We report that 24 h after LPS injection in adult male Sprague Dawley rats: 1) adenosine levels increase in ventral spinal segments containing the phrenic motor nucleus (C3-C5; P = 0.010; n = 7/group) and 2) cervical spinal A2A receptor inhibition (MSX-3, 10 µM, 12 µL intrathecal) rescues mAIH-induced pLTF. In LPS vehicle-treated rats (saline, ip), MSX-3 enhanced pLTF versus controls (LPS: 110 ± 16% baseline; controls: 53 ± 6%; P = 0.002; n = 6/group). In LPS-treated rats, pLTF was abolished as expected (4 ± 6% baseline; n = 6), but intrathecal MSX-3 restored pLTF to levels equivalent to MSX-3-treated control rats (120 ± 14% baseline; P < 0.001; n = 6; vs. LPS controls with MSX-3: P = 0.539). Thus, inflammation abolishes mAIH-induced pLTF by a mechanism that requires increased spinal adenosine levels and A2A receptor activation. As repetitive mAIH is emerging as a treatment to improve breathing and nonrespiratory movements in people with spinal cord injury or ALS, A2A inhibition may offset undermining effects of neuroinflammation associated with these neuromuscular disorders.NEW & NOTEWORTHY Mild inflammation undermines motor plasticity elicited by mAIH. In a model of mAIH-induced respiratory motor plasticity (phrenic long-term facilitation; pLTF), we report that inflammation induced by low-dose lipopolysaccharide undermines mAIH-induced pLTF by a mechanism requiring increased cervical spinal adenosine and adenosine 2 A receptor activation. This finding advances the understanding of mechanisms impairing neuroplasticity, potentially undermining the ability to compensate for the onset of lung/neural injury or to harness mAIH as a therapeutic modality.

Gap Junctions and Connexins in Microglia-Related Oxidative Stress and Neuroinflammation: Perspectives for Drug Discovery

Microglia represent the immune system of the brain. Their role is central in two phenomena, neuroinflammation and oxidative stress, which are at the roots of different pathologies related to the central nervous system (CNS). In order to maintain the homeostasis of the brain and re-establish the equilibrium after a threatening imbalance, microglia communicate with each other and other cells within the CNS by receiving specific signals through membrane-bound receptors and then releasing neurotrophic factors into either the extracellular milieu or directly into the cytoplasm of nearby cells, such as astrocytes and neurons. These last two mechanisms rely on the activity of protein structures that enable the formation of channels in the membrane, namely, connexins and pannexins, that group and form gap junctions, hemichannels, and pannexons. These channels allow the release of gliotransmitters, such as adenosine triphosphate (ATP) and glutamate, together with calcium ion (Ca2+), that seem to play a pivotal role in inter-cellular communication. The aim of the present review is focused on the physiology of channel protein complexes and their contribution to neuroinflammatory and oxidative stress-related phenomena, which play a central role in neurodegenerative disorders. We will then discuss how pharmacological modulation of these channels can impact neuroinflammatory phenomena and hypothesize that currently available nutraceuticals, such as carnosine and N-acetylcysteine, can modulate the activity of connexins and pannexins in microglial cells and reduce oxidative stress in neurodegenerative disorders.

Salt-loading promotes extracellular ATP release mediated by glial cells in the hypothalamic paraventricular nucleus of rats

Previously, we have shown that purinergic signalling is involved in the control of hyperosmotic-induced sympathoexcitation at the level of the PVN, via activation of P2X receptors. However, the source(s) of ATP that drives osmotically-induced increases in sympathetic outflow remained undetermined. Here, we tested the two competing hypotheses that either (1) higher extracellular ATP in PVN during salt loading (SL) is a result of a failure of ectonucleotidases to metabolize ATP; and/or (2) SL can stimulate PVN astrocytes to release ATP. Rats were salt loaded with a 2 % NaCl solution replacing drinking water up to 4 days, an experimental model known to cause a gradual increase in blood pressure and plasma osmolarity. Immunohistochemical assessment of glial-fibrillary acidic protein (GFAP) revealed increased glial cell reactivity in the PVN of rats after 4 days of high salt exposure. ATP and adenosine release measurements via biosensors in hypothalamic slices showed that baseline ATP release was increased 17-fold in the PVN while adenosine remained unchanged. Disruption of Ca²⁺-dependent vesicular release mechanisms in PVN astrocytes by virally-driven expression of a dominant-negative SNARE protein decreased the release of ATP. The activity of ectonucleotidases quantified in vitro by production of adenosine from ATP was increased in SL group. Our results showed that SL stimulates the release of ATP in the PVN, at least in part, from glial cells by a vesicle-mediated route and likely contributes to the neural control of circulation during osmotic challenges.

Astrocyte Immune Functions and Glaucoma

Astrocytes, a non-neuronal glial cell type in the nervous system, are essential for regulating physiological functions of the central nervous system. In various injuries and diseases of the central nervous system, astrocytes often change their phenotypes into neurotoxic ones that participate in pro-inflammatory responses (hereafter referred to as "immune functions"). Such astrocytic immune functions are not only limited to brain diseases but are also found in ocular neurodegenerative diseases such as glaucoma, a retinal neurodegenerative disease that is the leading cause of blindness worldwide. The eye has two astrocyte-lineage cells: astrocytes and Müller cells. They maintain the physiological environment of the retina and optic nerve, thereby controlling visual function. Dysfunction of astrocyte-lineage cells may be involved in the onset and progression of glaucoma. These cells become reactive in glaucoma patients, and animal studies have suggested that their immune responses may be linked to glaucoma-related events: tissue remodeling, neuronal death, and infiltration of peripheral immune cells. In this review, we discuss the role of the immune functions of astrocyte-lineage cells in the pathogenesis of glaucoma.

Ocular P2 receptors and glaucoma

Adenosine triphosphate (ATP), an energy source currency in cells, is released or leaked to the extracellular space under both physiological and pathological conditions. Extracellular ATP functions as an intercellular signaling molecule through activation of purinergic P2 receptors. Ocular tissue and cells release ATP in response to physiological stimuli such as intraocular pressure (IOP), and P2 receptor activation regulates IOP elevation or reduction. Dysregulated purinergic signaling may cause abnormally elevated IOP, which is one of the major risk factors for glaucoma. Glaucoma, a leading cause of blindness worldwide, is characterized by progressive degeneration of optic nerves and retinal ganglion cells (RGCs), which are essential retinal neurons that transduce visual information to the brain. An elevation in IOP may stress RGCs and increase the risk for glaucoma pathogenesis. In the aqueous humor of human patients with glaucoma, the ATP level is significantly elevated. Such excess amount of ATP may directly cause RGC death via a specific subtype of P2 receptors. Dysregulated purinergic signaling may also trigger inflammation, oxidative stress, and excitotoxicity via activating non-neuronal cell types such as glial cells. In this review, we discussed the physiological roles of extracellular nucleotides in the ocular tissue and their potential role in the pathogenesis of glaucoma. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

P2Y1 Receptor as a Catalyst of Brain Neurodegeneration

Different brain disorders display distinctive etiologies and pathogenic mechanisms. However, they also share pathogenic events. One event systematically occurring in different brain disorders, both acute and chronic, is the increase of the extracellular ATP levels. Accordingly, several P2 (ATP/ADP) and P1 (adenosine) receptors, as well as the ectoenzymes involved in the extracellular catabolism of ATP, have been associated to different brain pathologies, either with a neuroprotective or neurodegenerative action. The P2Y1 receptor (P2Y1R) is one of the purinergic receptors associated to different brain diseases. It has a widespread regional, cellular, and subcellular distribution in the brain, it is capable of modulating synaptic function and neuronal activity, and it is particularly important in the control of astrocytic activity and in astrocyte–neuron communication. In diverse brain pathologies, there is growing evidence of a noxious gain-of-function of P2Y1R favoring neurodegeneration by promoting astrocyte hyperactivity, entraining Ca2+-waves, and inducing the release of glutamate by directly or indirectly recruiting microglia and/or by increasing the susceptibility of neurons to damage. Here, we review the current evidence on the involvement of P2Y1R in different acute and chronic neurodegenerative brain disorders and the underlying mechanisms.

The role of snare proteins in cortical development

Neural communication in the adult nervous system is mediated primarily through chemical synapses, where action potentials elicit Ca²⁺ signals, which trigger vesicular fusion and neurotransmitter release in the presynaptic compartment. At early stages of development, the brain is shaped by communication via trophic factors and other extracellular signaling, and by contact-mediated cell-cell interactions including chemical synapses. The patterns of early neuronal impulses and spontaneous and regulated neurotransmitter release guide the precise topography of axonal projections and contribute to determining cell survival. The study of the role of specific proteins of the synaptic vesicle release machinery in the establishment, plasticity, and maintenance of neuronal connections during development has only recently become possible, with the advent of mouse models where various members of the N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex have been genetically manipulated. We provide an overview of these models, focusing on the role of regulated vesicular release and/or cellular excitability in synaptic assembly, development and maintenance of cortical circuits, cell survival, circuit level excitation-inhibition balance, myelination, refinement, and plasticity of key axonal projections from the cerebral cortex. These models are important for understanding various developmental and psychiatric conditions, and neurodegenerative diseases.

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

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

Excess intracellular ATP causes neuropathic pain following spinal cord injury

Intractable neuropathic pain following spinal cord injury (NP-SCI) reduces a patient's quality of life. Excessive release of ATP into the extracellular space evokes neuroinflammation via purinergic receptor. Neuroinflammation plays an important role in the initiation and maintenance of NP. However, little is known about whether or not extracellular ATP cause NP-SCI. We found in the present study that excess of intracellular ATP at the lesion site evokes at-level NP-SCI. No significant differences in the body weight, locomotor function, or motor behaviors were found in groups that were negative and positive for at-level allodynia. The intracellular ATP level at the lesion site was significantly higher in the allodynia-positive mice than in the allodynia-negative mice. A metabolome analysis revealed that there were no significant differences in the ATP production or degradation between allodynia-negative and allodynia-positive mice. Dorsal horn neurons in allodynia mice were found to be inactivated in the resting state, suggesting that decreased ATP consumption due to neural inactivity leads to a build-up of intracellular ATP. In contrast to the findings in the resting state, mechanical stimulation increased the neural activity of dorsal horn and extracellular ATP release at lesion site. The forced production of intracellular ATP at the lesion site in non-allodynia mice induced allodynia. The inhibition of P2X4 receptors in allodynia mice reduced allodynia. These results suggest that an excess buildup of intracellular ATP in the resting state causes at-level NP-SCI as a result of the extracellular release of ATP with mechanical stimulation.

Diversity of function and mechanism in a family of organic anion transporters

Originally identified as transporters for inorganic phosphate, solute carrier 17 (SLC17) family proteins subserve diverse physiological roles. The vesicular glutamate transporters (VGLUTs) package the principal excitatory neurotransmitter glutamate into synaptic vesicles (SVs). In contrast, the closely related sialic acid transporter sialin mediates the flux of sialic acid in the opposite direction, from lysosomes to the cytoplasm. The two proteins couple in different ways to the H⁺ electrochemical gradient driving force, and high-resolution structures of the Escherichia coli homolog d-galactonate transporter (DgoT) and more recently rat VGLUT2 now begin to suggest the mechanisms involved as well as the basis for substrate specificity.

ATP indirectly stimulates hippocampal CA1 and CA3 pyramidal neurons via the activation of neighboring P2X7 receptor-bearing astrocytes and NG2 glial cells, respectively

There is ongoing dispute on the question whether CNS neurons possess ATP-sensitive P2X7 receptors (Rs) or whether only non-neuronal cells bear this receptor-type and indirectly signal to the neighboring neurons. We genetically deleted P2X7Rs specifically in astrocytes, oligodendrocytes and microglia, and then recorded current responses in neurons to the prototypic agonist of this receptor, dibenzoyl-ATP (Bz-ATP). These experiments were made in brain slice preparations taken from the indicated variants of the P2X7R KO animals. In hippocampal CA3, but not CA1 pyramidal neurons, the deletion of oligodendrocytic (NG2 glial) P2X7Rs abolished the Bz-ATP-induced current responses. In contrast to the Bz-ATP-induced currents in CA3 pyramidal neurons, current amplitudes evoked by the ionotropic glutamate/GABA_AR agonists AMPA/muscimol were not inhibited at all. Whereas in the CA3 area NG2 glia appeared to mediate the P2X7R-mediated stimulation of pyramidal neurons, in the CA1 area, astrocytic P2X7Rs had a somewhat similar effect. This was shown by recording the frequencies and amplitudes of spontaneous excitatory currents (sPSCs) in brain slice preparations. Bz-ATP increased the sPSC frequency in CA1, but not CA3 pyramidal neurons without altering the amplitude, indicating a P2X7R-mediated increase of the neuronal input. Micro-injection of the selective astrocytic toxin L-α-aminoadipate into both hippocampi, or the in vitro application of the GABA_AR antagonistic gabazine, completely blocked the frequency increases of sPSCs. Hence, CA1 and CA3 pyramidal neurons of the mouse did not possess P2X7Rs, but were indirectly modulated by astrocytic and oligodendrocytic P2X7Rs, respectively.

ATPase Inhibitory Factor 1 Is Critical for Regulating Sevoflurane-Induced Microglial Inflammatory Responses and Caspase-3 Activation

Postoperative delirium (POD) is one of the most important complications after surgery with general anesthesia, for which the neurotoxicity of general anesthetics is a high-risk factor. However, the mechanism remains largely unknown, which also hinders the effective treatment of POD. Here, we confirmed that a clinical concentration of the general anesthetic sevoflurane increased the expression of inflammatory factors and activated the caspase-3 by upregulating ATPase inhibitory factor 1 (ATPIF1) expression in microglia. Upregulation of ATPIF1 decreased the synthesis of ATP which is an important signaling molecule secreted by microglia. Extracellular supplementation with ATP attenuated the microglial inflammatory response and caspase-3 activation caused by sevoflurane or overexpression of ATPIF1. Additionally, the microglial inflammatory response further upregulated ATPIF1 expression, resulting in a positive feedback loop. Animal experiments further indicated that intraperitoneal injection of ATP significantly alleviated sevoflurane anesthesia-induced POD-related anxiety behavior and memory damage in mice. This study reveals that ATPIF1, an important protein regulating ATP synthesis, mediates sevoflurane-induced neurotoxicity in microglia. ATP supplementation may be a potential clinical treatment to alleviate sevoflurane-induced POD.

Respiratory neuroplasticity: Mechanisms and translational implications of phrenic motor plasticity

Widespread appreciation that neuroplasticity is an essential feature of the neural system controlling breathing has emerged only in recent years. In this chapter, we focus on respiratory motor plasticity, with emphasis on the phrenic motor system. First, we define related but distinct concepts: neuromodulation and neuroplasticity. We then focus on mechanisms underlying two well-studied models of phrenic motor plasticity: (1) phrenic long-term facilitation following brief exposure to acute intermittent hypoxia; and (2) phrenic motor facilitation after prolonged or recurrent bouts of diminished respiratory neural activity. Advances in our understanding of these novel and important forms of plasticity have been rapid and have already inspired translation in multiple respects: (1) development of novel therapeutic strategies to preserve/restore breathing function in humans with severe neurological disorders, such as spinal cord injury and amyotrophic lateral sclerosis; and (2) the discovery that similar plasticity also occurs in nonrespiratory motor systems. Indeed, the realization that similar plasticity occurs in respiratory and nonrespiratory motor neurons inspired clinical trials to restore leg/walking and hand/arm function in people living with chronic, incomplete spinal cord injury. Similar application may be possible to other clinical disorders that compromise respiratory and non-respiratory movements.

The Purinergic System as a Target for the Development of Treatments for Bipolar Disorder

The neurobiological and neurochemical mechanisms underlying the pathophysiology of bipolar disorder are complex and not yet fully understood. From circadian disruption to neuroinflammation, many pathways and signaling molecules are important contributors to bipolar disorder development, some specific to a disease subtype or a cycling episode. Pharmacological agents for bipolar disorder have shown only partial efficacy, including mood stabilizers and antipsychotics. The purinergic hypothesis for bipolar disorder emerges in this scenario as a promising target for further research and drug development, given its role in neurotransmission and neuroinflammation that results in behavioral and mood regulation. Here, we review the basic concepts of purinergic signaling in the central nervous system and its contribution to bipolar disorder pathophysiology. Allopurinol and novel P2X7 receptor antagonists are promising candidates for treating bipolar disorder. We further explore currently available pharmacotherapies and the emerging new purinergic targets for drug development in bipolar disorder.

Expression of Ectonucleoside Triphosphate Diphosphohydrolase 2 (NTPDase2) Is Negatively Regulated Under Neuroinflammatory Conditions In Vivo and In Vitro

Ectonucleoside triphosphate diphosphohydrolase 2 (NTPDase2) hydrolyzes extracellular ATP to ADP, which is the ligand for P2Y1,12,13 receptors. The present study describes the distribution of NTPDase2 in adult rat brains in physiological conditions, and in hippocampal neurodegeneration induced by trimethyltin (TMT). The study also describes the regulation of NTPDase2 by inflammatory mediators in primary astrocytes and oligodendroglial cell line OLN93. In physiological conditions, NTPDase2 protein was most abundant in the hippocampus, where it was found in fibrous astrocytes and synaptic endings in the synaptic-rich hippocampal layers. In TMT-induced neurodegeneration, NTPDase2-mRNA acutely decreased at 2-dpi and then gradually recovered to the control level at 7-dpi and 21-dpi. As determined by immunohistochemistry and double immunofluorescence, the decrease was most pronounced in the dentate gyrus (DG), where NTPDase2 withdrew from the synaptic boutons in the polymorphic layer of DG, whereas the recovery of the expression was most profound in the subgranular layer. Concerning the regulation of NTPDase2 gene expression, proinflammatory cytokines IL-6, IL-1β, TNFα, and IFNγ negatively regulated the expression of NTPDase2 in OLN93 cells, while did not altering the expression in primary astrocytes. Different cell-intrinsic stressors, such as depletion of intracellular energy store, oxidative stress, endoplasmic reticulum stress, and activation of protein kinase C, also massively disturbed the expression of the NTPDase2 gene. Together, our results suggest that the expression and the activity of NTPDase2 transiently cease in neurodegeneration and brain injury, most likely as a part of the acute adaptive response designed to promote cell defense, survival, and recovery.

The Repertoire of Small-Molecule PET Probes for Neuroinflammation Imaging: Challenges and Opportunities beyond TSPO

Neuroinflammation is an adaptive response of the central nervous system to diverse potentially injurious stimuli, which is closely associated with neurodegeneration and typically characterized by activation of microglia and astrocytes. As a noninvasive and translational molecular imaging tool, positron emission tomography (PET) could provide a better understanding of neuroinflammation and its role in neurodegenerative diseases. Ligands to translator protein (TSPO), a putative marker of neuroinflammation, have been the most commonly studied in this context, but they suffer from serious limitations. Herein we present a repertoire of different structural chemotypes and novel PET ligand design for classical and emerging neuroinflammatory targets beyond TSPO. We believe that this Perspective will support multidisciplinary collaborations in academic and industrial institutions working on neuroinflammation and facilitate the progress of neuroinflammation PET probe development for clinical use.

Inhibitors of ectonucleotidases have paradoxical effects on synaptic transmission in the mouse cortex

Extracellular adenosine plays prominent roles in the brain in both physiological and pathological conditions. Adenosine can be generated following the degradation of extracellular nucleotides by various types of ectonucleotidases. Several ectonucleotidases are present in the brain parenchyma: ecto-nucleotide triphosphate diphosphohydrolases 1 and 3 (NTPDase 1 and 3), ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP 1), ecto-5'-nucleotidase (eN), and tissue non-specific alkaline phosphatase (TNAP, whose function in the brain has received little attention). Here we examined, in a living brain preparation, the role of these ectonucleotidases in generating extracellular adenosine. We recorded local field potentials evoked by electrical stimulation of the lateral olfactory tract in the mouse piriform cortex in vitro. Variations in adenosine level were evaluated by measuring changes in presynaptic inhibition generated by adenosine A1 receptors (A1Rs) activation. A1R-mediated presynaptic inhibition was present endogenously and was enhanced by bath-applied AMP and ATP. We hypothesized that inhibiting ectonucleotidases would reduce extracellular adenosine concentration, which would result in a weakening of presynaptic inhibition. However, inhibiting TNAP had no effect in controlling endogenous adenosine action and no effect on presynaptic inhibition induced by bath-applied AMP. Furthermore, contrary to our expectation, inhibiting TNAP reinforced, rather than reduced, presynaptic inhibition induced by bath-applied ATP. Similarly, inhibition of NTPDase 1 and 3, NPP1 and eN induced stronger, rather than weaker, presynaptic inhibition, both in endogenous condition and with bath-applied ATP and AMP. Consequently, attempts to suppress the functions of extracellular adenosine by blocking its extracellular synthesis in living brain tissue could have functional impacts opposite to those anticipated.

Quinacrine is not a vital fluorescent probe for vesicular ATP storage

Quinacrine, a fluorescent amphipathic amine, has been used as a vital fluorescent probe to visualize vesicular storage of ATP in the field of purinergic signaling. However, the mechanism(s) by which quinacrine represents vesicular ATP storage remains to be clarified. The present study investigated the validity of the use of quinacrine as a vial fluorescent probe for ATP-storing organelles. Vesicular nucleotide transporter (VNUT), an essential component for vesicular storage and ATP release, is present in very low density lipoprotein (VLDL)-containing secretory vesicles in hepatocytes. VNUT gene knockout (Vnut^-/-) or clodronate treatment, a VNUT inhibitor, disappeared vesicular ATP release (Tatsushima et al., Biochim Biophys Acta Molecular Basis of Disease 2021, e166013). Upon incubation of mice's primary hepatocytes, quinacrine accumulates in a granular pattern into the cytoplasm, sensitive to 0.1-μM bafilomycin A₁, a vacuolar ATPase (V-ATPase) inhibitor. Neither Vnut^-/- nor treatment of clodronate affected quinacrine granular accumulation. In vitro, quinacrine is accumulated into liposomes upon imposing inside acidic transmembranous pH gradient (∆pH) irrespective of the presence or absence of ATP. Neither ATP binding on VNUT nor VNUT-mediated uptake of ATP was affected by quinacrine. Consistently, VNUT-mediated uptake of quinacrine was negligible or under the detection limit. From these results, it is concluded that vesicular quinacrine accumulation is not due to a consequence of its interaction with ATP but due to ∆pH-driven concentration across the membranes as an amphipathic amine. Thus, quinacrine is not a vital fluorescent probe for vesicular ATP storage.

Extracellular Vesicles from Human Teeth Stem Cells Trigger ATP Release and Promote Migration of Human Microglia through P2X4 Receptor/MFG-E8-Dependent Mechanisms

Extracellular vesicles (EVs) effectively suppress neuroinflammation and induce neuroprotective effects in different disease models. However, the mechanisms by which EVs regulate the neuroinflammatory response of microglia remains largely unexplored. Here, we addressed this issue by testing the action of EVs derived from human exfoliated deciduous teeth stem cells (SHEDs) on immortalized human microglial cells. We found that EVs induced a rapid increase in intracellular Ca²⁺ and promoted significant ATP release in microglial cells after 20 min of treatment. Boyden chamber assays revealed that EVs promoted microglial migration by 20%. Pharmacological inhibition of different subtypes of purinergic receptors demonstrated that EVs activated microglial migration preferentially through the P2X4 receptor (P2X4R) pathway. Proximity ligation and co-immunoprecipitation assays revealed that EVs promote association between milk fat globule-epidermal growth factor-factor VIII (MFG-E8) and P2X4R proteins. Furthermore, pharmacological inhibition of αVβ3/αVβ5 integrin suppressed EV-induced cell migration and formation of lipid rafts in microglia. These results demonstrate that EVs promote microglial motility through P2X4R/MFG-E8-dependent mechanisms. Our findings provide novel insights into the molecular mechanisms through which EVs target human microglia that may be exploited for the development of new therapeutic strategies targeting disease-associated neuroinflammation.

Exocytosis in Astrocytes

Until recently, astrocytes were thought to be a part of a simple "brain glue" providing only a supporting role for neurons. However, the discoveries of the last two decades have proven astrocytes to be dynamic partners participating in brain metabolism and actively influencing communication between neurons. The means of astrocyte-neuron communication are diverse, although regulated exocytosis has received the most attention but also caused the most debate. Similar to most of eukaryotic cells, astrocytes have a complex range of vesicular organelles which can undergo exocytosis as well as intricate molecular mechanisms that regulate this process. In this review, we focus on the components needed for regulated exocytosis to occur and summarise the knowledge about experimental evidence showing its presence in astrocytes.

Inhibition of phosphatidylinositol kinase-III alpha induces or facilitates lysosome exocytosis from microglia

Besides degradation, lysosomes can also carry molecules for secretion out of the cell, such as ATP and cytokines, during unconventional secretion. Phosphatidylinositols and their metabolizing enzymes play important roles in the sorting and trafficking of lysosomal materials through the trans-Golgi network. The present study reveals a new function of phosphatidylinositol kinase-III alpha in the 'kiss-and-run' fusion of lysosomes at the plasma membrane to release ATP from microglia.

Role of Purinergic Signalling in Endothelial Dysfunction and Thrombo-Inflammation in Ischaemic Stroke and Cerebral Small Vessel Disease

The cerebral endothelium is an active interface between blood and the central nervous system. In addition to being a physical barrier between the blood and the brain, the endothelium also actively regulates metabolic homeostasis, vascular tone and permeability, coagulation, and movement of immune cells. Being part of the blood–brain barrier, endothelial cells of the brain have specialized morphology, physiology, and phenotypes due to their unique microenvironment. Known cardiovascular risk factors facilitate cerebral endothelial dysfunction, leading to impaired vasodilation, an aggravated inflammatory response, as well as increased oxidative stress and vascular proliferation. This culminates in the thrombo-inflammatory response, an underlying cause of ischemic stroke and cerebral small vessel disease (CSVD). These events are further exacerbated when blood flow is returned to the brain after a period of ischemia, a phenomenon termed ischemia-reperfusion injury. Purinergic signaling is an endogenous molecular pathway in which the enzymes CD39 and CD73 catabolize extracellular adenosine triphosphate (eATP) to adenosine. After ischemia and CSVD, eATP is released from dying neurons as a damage molecule, triggering thrombosis and inflammation. In contrast, adenosine is anti-thrombotic, protects against oxidative stress, and suppresses the immune response. Evidently, therapies that promote adenosine generation or boost CD39 activity at the site of endothelial injury have promising benefits in the context of atherothrombotic stroke and can be extended to current CSVD known pathomechanisms. Here, we have reviewed the rationale and benefits of CD39 and CD39 therapies to treat endothelial dysfunction in the brain.

Purinergic Receptors of the Central Nervous System: Biology, PET Ligands, and Their Applications

Purinergic receptors play important roles in central nervous system (CNS). These receptors are involved in cellular neuroinflammatory responses that regulate functions of neurons, microglial and astrocytes. Based on their endogenous ligands, purinergic receptors are classified into P1 or adenosine, P2X and P2Y receptors. During brain injury or under pathological conditions, rapid diffusion of extracellular adenosine triphosphate (ATP) or uridine triphosphate (UTP) from the damaged cells, promote microglial activation that result in the changes in expression of several of these receptors in the brain. Imaging of the purinergic receptors with selective Positron Emission Tomography (PET) radioligands has advanced our understanding of the functional roles of some of these receptors in healthy and diseased brains. In this review, we have accumulated a list of currently available PET radioligands of the purinergic receptors that are used to elucidate the receptor functions and participations in CNS disorders. We have also reviewed receptors lacking radiotracer, laying the foundation for future discoveries of novel PET radioligands to reveal these receptors roles in CNS disorders.

ATP and adenosine-Two players in the control of seizures and epilepsy development

Despite continuous advances in understanding the underlying pathogenesis of hyperexcitable networks and lowered seizure thresholds, the treatment of epilepsy remains a clinical challenge. Over one third of patients remain resistant to current pharmacological interventions. Moreover, even when effective in suppressing seizures, current medications are merely symptomatic without significantly altering the course of the disease. Much effort is therefore invested in identifying new treatments with novel mechanisms of action, effective in drug-refractory epilepsy patients, and with the potential to modify disease progression. Compelling evidence has demonstrated that the purines, ATP and adenosine, are key mediators of the epileptogenic process. Extracellular ATP concentrations increase dramatically under pathological conditions, where it functions as a ligand at a host of purinergic receptors. ATP, however, also forms a substrate pool for the production of adenosine, via the action of an array of extracellular ATP degrading enzymes. ATP and adenosine have assumed largely opposite roles in coupling neuronal excitability to energy homeostasis in the brain. This review integrates and critically discusses novel findings regarding how ATP and adenosine control seizures and the development of epilepsy. This includes purine receptor P1 and P2-dependent mechanisms, release and reuptake mechanisms, extracellular and intracellular purine metabolism, and emerging receptor-independent effects of purines. Finally, possible purine-based therapeutic strategies for seizure suppression and disease modification are discussed.

ATP and cancer immunosurveillance

While intracellular adenosine triphosphate (ATP) occupies a key position in the bioenergetic metabolism of all the cellular compartments that form the tumor microenvironment (TME), extracellular ATP operates as a potent signal transducer. The net effects of purinergic signaling on the biology of the TME depend not only on the specific receptors and cell types involved, but also on the activation status of cis- and trans-regulatory circuitries. As an additional layer of complexity, extracellular ATP is rapidly catabolized by ectonucleotidases, culminating in the accumulation of metabolites that mediate distinct biological effects. Here, we discuss the molecular and cellular mechanisms through which ATP and its degradation products influence cancer immunosurveillance, with a focus on therapeutically targetable circuitries.

Reactive astrocyte-driven epileptogenesis is induced by microglia initially activated following status epilepticus

Extensive activation of glial cells during a latent period has been well documented in various animal models of epilepsy. However, it remains unclear whether activated glial cells contribute to epileptogenesis, i.e., the chronically persistent process leading to epilepsy. Particularly, it is not clear whether interglial communication between different types of glial cells contributes to epileptogenesis, because past literature has mainly focused on one type of glial cell. Here, we show that temporally distinct activation profiles of microglia and astrocytes collaboratively contributed to epileptogenesis in a drug-induced status epilepticus model. We found that reactive microglia appeared first, followed by reactive astrocytes and increased susceptibility to seizures. Reactive astrocytes exhibited larger Ca2+ signals mediated by IP3R2, whereas deletion of this type of Ca2+ signaling reduced seizure susceptibility after status epilepticus. Immediate, but not late, pharmacological inhibition of microglial activation prevented subsequent reactive astrocytes, aberrant astrocyte Ca2+ signaling, and the enhanced seizure susceptibility. These findings indicate that the sequential activation of glial cells constituted a cause of epileptogenesis after status epilepticus. Thus, our findings suggest that the therapeutic target to prevent epilepsy after status epilepticus should be shifted from microglia (early phase) to astrocytes (late phase).

Caffeine and Its Neuroprotective Role in Ischemic Events: A Mechanism Dependent on Adenosine Receptors

Ischemia is characterized by a transient, insufficient, or permanent interruption of blood flow to a tissue, which leads to an inadequate glucose and oxygen supply. The nervous tissue is highly active, and it closely depends on glucose and oxygen to satisfy its metabolic demand. Therefore, ischemic conditions promote cell death and lead to a secondary wave of cell damage that progressively spreads to the neighborhood areas, called penumbra. Brain ischemia is one of the main causes of deaths and summed with retinal ischemia comprises one of the principal reasons of disability. Although several studies have been performed to investigate the mechanisms of damage to find protective/preventive interventions, an effective treatment does not exist yet. Adenosine is a well-described neuromodulator in the central nervous system (CNS), and acts through four subtypes of G-protein-coupled receptors. Adenosine receptors, especially A₁ and A_2A receptors, are the main targets of caffeine in daily consumption doses. Accordingly, caffeine has been greatly studied in the context of CNS pathologies. In fact, adenosine system, as well as caffeine, is involved in neuroprotection effects in different pathological situations. Therefore, the present review focuses on the role of adenosine/caffeine in CNS, brain and retina, ischemic events.

Loss of P2Y12 Has Behavioral Effects in the Adult Mouse

While microglia have been established as critical mediators of synaptic plasticity, the molecular signals underlying this process are still being uncovered. Increasing evidence suggests that microglia utilize these signals in a temporally and regionally heterogeneous manner. Subsequently, it is necessary to understand the conditions under which different molecular signals are employed by microglia to mediate the physiological process of synaptic remodeling in development and adulthood. While the microglial purinergic receptor P2Y12 is required for ocular dominance plasticity, an adolescent form of experience-dependent plasticity, it remains unknown whether P2Y12 functions in other forms of plasticity at different developmental time points or in different brain regions. Using a combination of ex vivo characterization and behavioral testing, we examined how the loss of P2Y12 affects developmental processes and behavioral performance in adulthood in mice. We found P2Y12 was not required for an early form of plasticity in the developing visual thalamus and did not affect microglial migration into barrels in the developing somatosensory cortex. In adult mice, however, the loss of P2Y12 resulted in alterations in recognition and social memory, as well as anxiety-like behaviors, suggesting that while P2Y12 is not a universal regulator of synaptic plasticity, the loss of P2Y12 is sufficient to cause functional defects.

Ectonucleotidases in Acute and Chronic Inflammation

Ectonucleotidases are extracellular enzymes with a pivotal role in inflammation that hydrolyse extracellular purine and pyrimidine nucleotides, e.g., ATP, UTP, ADP, UDP, AMP and NAD⁺. Ectonucleotidases, expressed by virtually all cell types, immune cells included, either as plasma membrane-associated or secreted enzymes, are classified into four main families: 1) nucleoside triphosphate diphosphohydrolases (NTPDases), 2) nicotinamide adenine dinucleotide glycohydrolase (NAD glycohydrolase/ADP-ribosyl cyclase/cyclic ADP-ribose hydrolase 1), 3) ecto-5′-nucleotidase (NT5E), and 4) ecto-nucleotide pyrophosphatase/phosphodiesterases (NPPs). Concentration of ATP, UTP and NAD⁺ can be increased in the extracellular space thanks to un-regulated, e.g., cell damage or cell death, or regulated processes. Regulated processes include secretory exocytosis, connexin or pannexin hemichannels, ATP binding cassette (ABC) transporters, calcium homeostasis modulator (CALMH) channels, the ATP-gated P2X7 receptor, maxi-anion channels (MACs) and volume regulated ion channels (VRACs). Hydrolysis of extracellular purine nucleotides generates adenosine, an important immunosuppressant. Extracellular nucleotides and nucleosides initiate or dampen inflammation via P2 and P1 receptors, respectively. All these agents, depending on their level of expression or activation and on the agonist concentration, are potent modulators of inflammation and key promoters of host defences, immune cells activation, pathogen clearance, tissue repair and regeneration. Thus, their knowledge is of great importance for a full understanding of the pathophysiology of acute and chronic inflammatory diseases. A selection of these pathologies will be briefly discussed here.

Extracellular ATP: A Feasible Target for Cancer Therapy

Adenosine triphosphate (ATP) is one of the main biochemical components of the tumor microenvironment (TME), where it can promote tumor progression or tumor suppression depending on its concentration and on the specific ecto-nucleotidases and receptors expressed by immune and cancer cells. ATP can be released from cells via both specific and nonspecific pathways. A non-regulated release occurs from dying and damaged cells, whereas active release involves exocytotic granules, plasma membrane-derived microvesicles, specific ATP-binding cassette (ABC) transporters and membrane channels (connexin hemichannels, pannexin 1 (PANX1), calcium homeostasis modulator 1 (CALHM1), volume-regulated anion channels (VRACs) and maxi-anion channels (MACs)). Extracellular ATP acts at P2 purinergic receptors, among which P2X7R is a key mediator of the final ATP-dependent biological effects. Over the years, P2 receptor- or ecto-nucleotidase-targeting for cancer therapy has been proposed and actively investigated, while comparatively fewer studies have explored the suitability of TME ATP as a target. In this review, we briefly summarize the available evidence suggesting that TME ATP has a central role in determining tumor fate and is, therefore, a suitable target for cancer therapy.

Neonatal Seizures and Purinergic Signalling

Neonatal seizures are one of the most common comorbidities of neonatal encephalopathy, with seizures aggravating acute injury and clinical outcomes. Current treatment can control early life seizures; however, a high level of pharmacoresistance remains among infants, with increasing evidence suggesting current anti-seizure medication potentiating brain damage. This emphasises the need to develop safer therapeutic strategies with a different mechanism of action. The purinergic system, characterised by the use of adenosine triphosphate and its metabolites as signalling molecules, consists of the membrane-bound P1 and P2 purinoreceptors and proteins to modulate extracellular purine nucleotides and nucleoside levels. Targeting this system is proving successful at treating many disorders and diseases of the central nervous system, including epilepsy. Mounting evidence demonstrates that drugs targeting the purinergic system provide both convulsive and anticonvulsive effects. With components of the purinergic signalling system being widely expressed during brain development, emerging evidence suggests that purinergic signalling contributes to neonatal seizures. In this review, we first provide an overview on neonatal seizure pathology and purinergic signalling during brain development. We then describe in detail recent evidence demonstrating a role for purinergic signalling during neonatal seizures and discuss possible purine-based avenues for seizure suppression in neonates.

Purinergic signaling orchestrating neuron-glia communication

This review discusses the evidence supporting a role for ATP signaling (operated by P₂X and P₂Y receptors) and adenosine signaling (mainly operated by A₁ and A_2A receptors) in the crosstalk between neurons, astrocytes, microglia and oligodendrocytes. An initial emphasis will be given to the cooperation between adenosine receptors to sharpen information salience encoding across synapses. The interplay between ATP and adenosine signaling in the communication between astrocytes and neurons will then be presented in context of the integrative properties of the astrocytic syncytium, allowing to implement heterosynaptic depression processes in neuronal networks. The process of microglia 'activation' and its control by astrocytes and neurons will then be analyzed under the perspective of an interplay between different P₂ receptors and adenosine A_2A receptors. In spite of these indications of a prominent role of purinergic signaling in the bidirectional communication between neurons and glia, its therapeutical exploitation still awaits obtaining an integrated view of the spatio-temporal action of ATP signaling and adenosine signaling, clearly distinguishing the involvement of both purinergic signaling systems in the regulation of physiological processes and in the control of pathogenic-like responses upon brain dysfunction or damage.

Adenosine Triphosphate Accumulated Following Cerebral Ischemia Induces Neutrophil Extracellular Trap Formation

In ischemic stroke, neutrophils infiltrate damaged brain tissue immediately following the ischemic insult and aggravate inflammation via various mechanisms which include neutrophil extracellular traps (NETs) formation. In the present study, we showed that adenosine triphosphate (ATP), a DAMP molecule, accumulates in the brain and induces NETosis in brain parenchyma and in circulating neutrophils (PMNs) isolated from a murine model of stroke induced by middle cerebral artery occlusion (MCAO). Expression of peptidylarginine deiminase-4 (PAD4), which induces citrullination of histones H3 (CitH3) and initiates NETosis, was significantly enhanced in brain parenchyma and blood PMNs following MCAO. ATP or BzATP (a prototypic P2X7R agonist) significantly enhanced the inductions of PAD4 and CitH3 in a P2X7R-dependent manner and intracellular Ca²⁺ influx, PKCα activation, and NADPH oxidase-dependent reactive oxygen species (ROS) production play critical roles in this ATP-P2X7R-mediated NETosis. In our MCAO animal model, NETosis was markedly suppressed by treatment with apyrase, an enzyme hydrolyzing ATP, but enhanced by co-treatment of BzATP, confirming ATP-P2X7R-mediated NETosis. Since ATP not only induced NETosis but was also extruded after NETosis, our results indicate that ATP accumulated in the ischemic brain induces NETosis, mediating a cross-talk linking NETosis with neuronal damage that might aggravate inflammation and brain damage.

Platelet-rich plasma in umbilical cord blood reduces neuropathic pain in spinal cord injury by altering the expression of ATP receptors

BACKGROUND

Neuropathic pain following injury or dysfunction of the peripheral or CNS is one of the most important medical challenges to treat. Humane platelet-rich plasma (HPRP), which is a rich source of growth factors, may be able to treat and reduce pain caused by spinal cord injury (SCI). In this study, the effect of HPRP on neuropathic pain caused by SCI was investigated.

METHODS

Sixty adult male Wistar rats were randomly divided into 6 groups: control, sham, SCI, vehicle (SCI+platelet-poor plasma), SCI+ PRP2day (injection 48 hrs after SCI) and SCI+PRP14day (injection 14 days after SCI). SCI was induced at the T12-T13 level. Behavioral tests were conducted weekly after injury for six weeks. Allodynia and hyperalgesia were assessed using acetone drops, plantar test and von Frey filament. Cavity size and the number of fibroblasts were determined by H&E stain, and the expression of mTOR, p-mTOR, P₂×₃R and P₂Y₄R were determined using the western blot technique. Data were analyzed using PRISM & SPSS software.

RESULTS

PRP injection showed a higher pain threshold in mechanical allodynia (p<0.0001), cold allodynia (p<0.0001) and thermal hyperalgesia (p<0.0001) than those in the spinal. Animals treated with PRP also reduced cavity size, fibroblast number, p-mTOR/mTOR ratio, and P₂×₃R expression, and increased P₂Y₄R expression. The difference between the two groups was not statistically significant.

CONCLUSIONS

The results showed that PRP reduced SCI-induced allodynia and hyperalgesia by regulating ATP signaling. Using HPRP can open a new window in the treatment of pain caused by damage to the nervous system.

Implication of Neuronal Versus Microglial P2X4 Receptors in Central Nervous System Disorders

The P2X4 receptor (P2X4) is an ATP-gated cation channel that is highly permeable to Ca²⁺ and widely expressed in neuronal and glial cell types throughout the central nervous system (CNS). A growing body of evidence indicates that P2X4 plays key roles in numerous central disorders. P2X4 trafficking is highly regulated and consequently in normal situations, P2X4 is present on the plasma membrane at low density and found mostly within intracellular endosomal/lysosomal compartments. An increase in the de novo expression and/or surface density of P2X4 has been observed in microglia and/or neurons during pathological states. This review aims to summarize knowledge on P2X4 functions in CNS disorders and provide some insights into the relative contributions of neuronal and glial P2X4 in pathological contexts. However, determination of the cell-specific functions of P2X4 along with its intracellular and cell surface roles remain to be elucidated before its potential as a therapeutic target in multiple disorders can be defined.

Extracellular ATP as an Inter-Kingdom Signaling Molecule: Release Mechanisms by Bacteria and Its Implication on the Host

The purine adenosine 5′-triphosphate (ATP) is not only a universal intracellular energy carrier but plays also an important role as extracellular signaling molecule. Purinergic signaling is involved in many physiological and pathological processes like coagulation, inflammation, or sepsis in mammals. ATP is well-known as a messenger for intercellular communications in multicellular organisms, but phylogenetically much older unicellular organisms like yeast or bacteria use ATP as an extracellular signaling molecule as well. However, the mechanisms of ATP secretion by bacteria and its extracellular implications still have to be elucidated. This review will provide an overview of the current knowledge about bacterial extracellular ATP (eATP) under homeostatic conditions and during growth. Possible secretion mechanisms of ATP by bacteria will be discussed and implications of bacterial ATP are shown, with a focus on bacteria–host interactions.

Physiopathological roles of vesicular nucleotide transporter (VNUT), an essential component for vesicular ATP release

Vesicular nucleotide transporter (VNUT) is the last identified member of the SLC17 organic anion transporter family, which plays a central role in vesicular storage in ATP-secreting cells. The discovery of VNUT demonstrated that, despite having been neglected for a long time, vesicular ATP release represents a major pathway for purinergic chemical transmission, which had been mainly attributed to ATP permeation channels. This article summarizes recent advances in our understanding of the mechanism of VNUT and its physiopathological roles as well as the development of inhibitors. Regulating the activity and/or the expression of VNUT represents a new and promising therapeutic strategy for the treatment of multiple diseases.