Involvement of P2X4 and P2Y12 receptors in ATP-induced microglial chemotaxis

Microglial cell migration stimulated by ATP and C5a involve distinct molecular mechanisms: quantification of migration by a novel near-infrared method

Microglial cells, the macrophages of the brain, play an essential role in the propagation of neuroinflammation. Increased microglial cell migration in response to specific chemoattractants has been documented, but less is known about the differences between these stimuli and the signal transduction pathways that mediate their effects. Current methods to measure cell migration are often labor-intensive and rely on the manual counting of cell number, so more efficient and objective methods are needed. Here we present an improved and higher-throughput Boyden chamber technique that measures microglial cell migration by using DRAQ5, a nuclear dye that emits in the near-infrared. Out of a panel of chemoattractants tested, we found that ATP and C5a potently stimulate the migration of mouse primary microglial cells. The stimulatory effects of ATP and C5a displayed significant additivity, suggesting that each chemoattractant stimulated migration through independent molecular mechanisms. Accordingly, we found key differences in these responses: ATP stimulated a combination of both chemokinesis and chemotaxis, and this response was mediated by the ROCK signaling pathway; whereas C5a stimulated only chemotaxis and this response was mediated by the Rac1 signaling pathway. Finally, we found that functional PI3-kinase is only required for random basal microglial cell migration. Thus, our results show that distinct nonoverlapping signal transduction pathways control different modes of microglial cell migration and suggest that the targeting of these distinct molecular mechanisms should modulate different aspects of neuroinflammation propagation.

Endogenous signals initiating inflammation in the injured nervous system

Glial cells are known to respond to a variety of neural injuries and play an important role in tissue damage and repair in the injured nervous system. This glial response, which is initially characterized by the expression of proinflammatory cytokines and chemokines and the attraction of microglial cells toward sites of injury, literally occurs within seconds to minutes of the injury. This suggests that signals that are endogenous to the nervous system are responsible for initiating neuroinflammation. In this review, we summarize the most recent advances made in the identification of these endogenous signals and describe the receptors and signaling pathways by which these ligands stimulate the production of cytokines and chemokines. Among these endogenous damage signals are ligands for toll-like receptors, including several heat shock proteins and extracellular matrix components, as well as self-derived RNA and DNA and associated proteins. Growing evidence also suggests that nucleotides released upon injury and acting through P2 receptors, such as ATP and UTP or their analogues, could serve as endogenous signals for the rapid response of glial cells.

Sigma receptors suppress multiple aspects of microglial activation

During brain injury, microglia become activated and migrate to areas of degenerating neurons. These microglia release proinflammatory cytokines and reactive oxygen species causing additional neuronal death. Microglia express high levels of sigma receptors, however, the function of these receptors in microglia and how they may affect the activation of these cells remain poorly understood. Using primary rat microglial cultures, it was found that sigma receptor activation suppresses the ability of microglia to rearrange their actin cytoskeleton, migrate, and release cytokines in response to the activators adenosine triphosphate (ATP), monocyte chemoattractant protein 1 (MCP-1), and lipopolysaccharide (LPS). Next, the role of sigma receptors in the regulation of calcium signaling during microglial activation was explored. Calcium fluorometry experiments in vitro show that stimulation of sigma receptors suppressed both transient and sustained intracellular calcium elevations associated with the microglial response to these activators. Further experiments showed that sigma receptors suppress microglial activation by interfering with increases in intracellular calcium. In addition, sigma receptor activation also prevented membrane ruffling in a calcium-independent manner, indicating that sigma receptors regulate the function of microglia via multiple mechanisms.

Critical roles of NK and CD8+ T cells in central nervous system listeriosis

Listeria monocytogenes (LM) causes a life-threatening infectious disease affecting the brain of humans and domestic animals. Unfortunately, no adequate murine models for CNS listeriosis exist. Using intraparenchymal injection, we have established a new murine model for CNS listeriosis. Injection of a small volume of bacterial suspension limits the bacteria to the brain parenchyma with no leakage into the ventricular system. This new method enabled us to investigate the progression of and recovery from listerial brain infection, revealing roles for both innate and adaptive immune cells in CNS listeriosis. In the early phase of CNS listeriosis, NK cell-derived IFN-gamma is a critical cytokine in the limitation of bacterial growth by the host defense. During the later phase, CD8(+) but not CD4(+) T cells play a critical role and LM-specific CD8(+) T cells kill LM-infected microglia. Thus, innate and adaptive immune responses combine to successfully eliminate bacteria from the brain.

Morphine enhances microglial migration through modulation of P2X4 receptor signaling

Opioids, although fundamental to the treatment of pain, are limited in efficacy by side effects including tolerance and hyperalgesia. Using an in vitro culture system, we report that morphine increased microglial migration via a novel interaction between mu-opioid and P2X(4) receptors, which is dependent upon PI3K/Akt pathway activation. Morphine at 100 nm enhanced migration of primary microglial cells toward adenosine diphosphate by 257, 247, 301, 394, and 345% following 2, 6, 12, 24, and 48 h of stimulation, respectively. This opioid-dependent migration effect was inhibited by naloxone and confirmed to be mu-opioid receptor-dependent through the use of selective agonists and antagonists. PPADS [pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid)], a P2X(1-3,5-7) antagonist, had no effect on microglial migration; however, TNP-ATP [2',3'-O-(2,4,6-trinitrophenyl)-ATP], a P2X(1-7) antagonist, inhibited morphine-induced migration, suggesting a P2X(4) receptor-mediated effect. The PI3K inhibitors wortmannin and LY294002 decreased morphine-induced microglial migration. Iba1 protein, a microglial marker, and P2X(4) receptor expression were significantly increased after 6, 12, 24, and 48 h of morphine stimulation. Together, these results provide evidence for two phases of morphine effects on microglia. The initial phase takes place in minutes, involves PI3K/Akt pathway activation and leads to acutely enhanced migration. The longer-term phase occurs on the order of hours and involves increased expression of Iba1 and P2X(4) receptor protein, which imparts a promigratory phenotype and is correlated with even greater migration. These data provide the first necessary step in supporting microglial migration as an attractive target for the prevention or attenuation of morphine-induced side effects including tolerance and hyperalgesia.

Trophic functions of nucleotides in the central nervous system

In addition to short-term effects, one of the fundamental roles of extracellular nucleotides in the central nervous system involves long-term trophic effects. Physiological outcomes include neurogenesis, neuronal differentiation, glial proliferation, migration, growth arrest and apoptosis. Nucleotides exert these functions via P2-receptor-mediated mechanisms that can also interact with polypeptide-growth-factor-mediated or integrin-mediated signaling pathways. In addition, pathogenic roles for extracellular nucleotides in response to central nervous system injury including trauma and ischemia have been observed after the release of nucleotides by damaged and dying cells and in the development of neuropathic and inflammatory pain. Here, we illuminate the contribution of extracellular nucleotides to the development, growth, cellular plasticity and death of neural cells and the mechanisms regulating these trophic effects.

Mechanisms underlying fibronectin-induced up-regulation of P2X4R expression in microglia: distinct roles of PI3K-Akt and MEK-ERK signalling pathways

Abstract

Microglia

are resident immune cells in the central nervous system that become activated and produce pro-inflammatory and neurotrophic factors upon activation of various cell-surface receptors. The P2X₄ receptor (P2X₄R) is a sub-type of the purinergic ion-channel receptors expressed in microglia. P2X₄R expression is up-regulated under inflammatory or neurodegenerative conditions, and this up-regulation is implicated in disease pathology. However, the molecular mechanism underlying up-regulation of P2X₄R in microglia remains unknown. In the present study, we investigated the intracellular signal transduction pathway that promotes P2X₄R expression in microglia in response to fibronectin, an extracellular matrix protein that has previously been shown to stimulate P2X₄R expression. We found that in fibronectin-stimulated microglia, activation of phosphatidylinositol 3-kinase (PI3K)–Akt and mitogen-activated protein kinase kinase (MAPK kinase, MEK)–extracellular signal-regulated kinase (ERK) signalling cascades occurred divergently downstream of Src-family kinases (SFKs). Pharmacological interference of PI3K–Akt signalling inhibited fibronectin-induced P2X₄R gene expression. Activation of PI3K–Akt signalling resulted in a decrease in the protein level of the transcription factor p53 via mouse double minute 2 (MDM2), an effect that was prevented by MG-132, an inhibitor of the proteasome. In microglia pre-treated with MG-132, fibronectin failed to up-regulate P2X₄R expression. Conversely, an inhibitor of p53 caused increased expression of P2X₄R, implying a negative regulatory role of p53. On the other hand, inhibiting MEK–ERK signalling activated by fibronectin suppressed an increase in P2X₄R protein but interestingly did not affect the level of P2X₄R mRNA. We also found that fibronectin stimulation resulted in the activation of the translational factor eIF4E via MAPK-interacting protein kinase-1 (MNK1) in an MEK–ERK signalling-dependent manner, and an MNK1 inhibitor attenuated the increase in P2X₄R protein. Together, these results suggest that the PI3K–Akt and MEK–ERK signalling cascades have distinct roles in the up-regulation of P2X₄R expression in microglia at transcriptional and post-transcriptional levels, respectively.

Purinergic signalling in inflammation of the central nervous system

Inflammation is the most fundamental body reaction to noxious stimuli. No vascularized tissue, organ or apparatus is free from this response. Several mediators of inflammation, originating from outside (exogenous) or inside (endogenous) the body, are known. Among the endogenous factors, extracellular nucleotides and nucleosides are attracting interest for their ubiquity and striking ability to modulate diverse immune responses. Until recently, it was doubted that the central nervous system (CNS), reportedly an 'immunoprivileged organ', could be the site of immune reactions. Nowadays, it is acknowledged that inflammation and immunity have a key role in a vast range of CNS diseases. Likewise, it is clear that purinergic signalling profoundly affects neuroinflammation. Here, we provide a brief update of the state of the art in this expanding field.

P2Y(6)-Evoked Microglial Phagocytosis

While it was reported that microglia is engaged in the clearance of dead cells or dangerous debris, the mechanism of phagocytosis is still unclear. Recently, we found that purinergic system has a very important role for the chemotaxis and phagocytosis of microglia. When neighboring cells are injured, the cells release or leak ATP into extracellular space and microglia rapidly move toward or extend a process to the nucleotides as chemotaxis through P2Y(12) receptors of microglia. In the meanwhile, microglia expressing metabotropic P2Y(6) receptors show phagocytosis by the stimulation of uridine 5'-diphosphate (UDP), an agonist of P2Y(6). UDP/UTP is leaked when hippocampal neurons are damaged by kainic acid (KA) in vivo and in vitro. Systemic administration of KA in rats results in neuronal cell death in the hippocampal CA1 and CA3 regions, where mRNA for P2Y(6) receptors increases activated microglia. Thus, the P2Y(6) receptor is upregulated when neurons are damaged, and would function as a sensor for phagocytosis by sensing diffusible UDP signals.

Acute lung inflammation and ventilator-induced lung injury caused by ATP via the P2Y receptors: an experimental study

BACKGROUND

Extracellular adenosine 5'-triphosphate (ATP) is an endogenous signaling molecule involved in multiple biological phenomena, including inflammation. The effects of extracellular ATP in the lung have not been fully clarified. This study examined 1) the biological roles of extracellular ATP in the pathogenesis of lung inflammation and 2) the possibility of involvement of extracellular ATP in mechanical ventilation-induced lung injury.

METHODS

The effects of intratracheal ATP on lung permeability, edema or lung inflammation were assessed by measurements of the lung wet-to-dry weight ratio and lung permeability index, immunohistochemistry and expression of key cytokines by real-time polymerase chain reaction. The ATP concentration in broncho-alveolar lavage (BAL) fluid from mice mechanically ventilated was measured by luciferin-luciferase assay. The suppressive effects of a P2 receptor antagonist on ventilator-induced lung inflammation were also examined.

RESULTS

ATP induced inflammatory reactions in the lung mainly via the ATP-P2Y receptor system. These reactions were alleviated by the co-administration of a specific P2 receptor antagonist. Mechanical ventilation with a large tidal volume caused lung inflammation and increased the ATP concentration in BAL fluid. P2 receptor antagonism partially mitigated the inflammatory effects of large tidal volume ventilation.

CONCLUSION

Our observations suggest that the ATP-P2Y receptor system is partially involved in the pathogenesis of ventilator-induced lung injury.

Status epilepticus induces a particular microglial activation state characterized by enhanced purinergic signaling

Microglia cells are the resident macrophages of the CNS, and their activation plays a critical role in inflammatory reactions associated with many brain disorders, including ischemia, Alzheimer's and Parkinson's diseases, and epilepsy. However, the changes of microglia functional properties in epilepsy have rarely been studied. Here, we used a model of status epilepticus (SE) induced by intraperitoneal kainate injections to characterize the properties of microglial cells in hippocampal slices from CX3CR1(eGFP/+) mice. SE induced within 3 h an increased expression of inflammatory mediators in the hippocampus, followed by a modification of microglia morphology, a microglia proliferation, and a significant neurodegeneration in CA1. Changes in electrophysiological intrinsic membrane properties of hippocampal microglia were detected at 24-48 h after SE with, in particular, the appearance of new voltage-activated potassium currents. Consistent with the observation of an upregulation of purinergic receptor mRNAs in the hippocampus, we also provide pharmacological evidence that microglia membrane currents mediated by the activation of P2 receptors, including P2X(7), P2Y(6), and P2Y(12), were increased 48 h after SE. As a functional consequence of this modification of purinergic signaling, motility of microglia processes toward a source of P2Y(12) receptor agonist was twice as fast in the epileptic hippocampus. This study is the first functional description of microglia activation in an in vivo model of inflammation and provides evidence for the existence of a particular microglial activation state after a status epilepticus.

Purinergic signalling and disorders of the central nervous system

Purines have key roles in neurotransmission and neuromodulation, with their effects being mediated by the purine and pyrimidine receptor subfamilies, P1, P2X and P2Y. Recently, purinergic mechanisms and specific receptor subtypes have been shown to be involved in various pathological conditions including brain trauma and ischaemia, neurodegenerative diseases involving neuroimmune and neuroinflammatory reactions, as well as in neuropsychiatric diseases, including depression and schizophrenia. This article reviews the role of purinergic signalling in CNS disorders, highlighting specific purinergic receptor subtypes, most notably A(2A), P2X(4) and P2X(7), that might be therapeutically targeted for the treatment of these conditions.

Lysophosphatidic acid-induced membrane ruffling and brain-derived neurotrophic factor gene expression are mediated by ATP release in primary microglia

We examined the effects of lysophosphatidic acid (LPA) on microglia, which may play an important role in the development and maintenance of neuropathic pain. LPA caused membrane ruffling as detected by scanning electron microscopy, and increased the expression of brain-derived neurotrophic factor (BDNF) in a primary culture of rat microglia, which express LPA(3), but not LPA(1) or LPA(2) receptors. These actions were inhibited by a Galpha(q/11)-antisense oligodeoxynucleotide (AS-ODN), U73122, an inhibitor of phospholipase C (PLC), and apyrase, which specifically degrades ATP and ADP. When ATP release was measured using a luciferin-luciferase bioluminescence assay, LPA was shown to increase it in an LPA(3) and PLC inhibitor-reversible manner. However, LPA-induced ATP release was also blocked by the Galpha(q/11) AS-ODN, but not by pertussis toxin. These results suggest that LPA induces the release of ATP from rat primary cultured microglia via the LPA(3) receptor, Galpha(q/11) and PLC, and that the released ATP or ectopically converted ADP may in turn cause membrane ruffling via P2Y(12) receptors and Galpha(i/o) activation, and BDNF expression via activation of P2X(4) receptors.

Akt activation is involved in P2Y12 receptor-mediated chemotaxis of microglia

Microglia play a variety of significant roles in the central nervous system (CNS), and in one of those roles they undergo morphological change in response to neural injury and migrate to the injured region. We previously reported that ATP/ADP promotes microglial chemotaxis via the Gi/o-coupled P2Y12 receptor; however, the intracellular signaling underlying P2Y12-receptor-mediated microglial chemotaxis is not fully understood. In this study, we examined the role of phospholipase C (PLC) and calcium signaling in ADP-induced microglial chemotaxis. A PLC inhibitor, U73122, significantly suppressed the chemotaxis and completely blocked the ADP-evoked intracellular calcium response, and a calcium chelator, BAPTA-AM, inhibited the chemotaxis. These results indicate that ADP-induced microglial chemotaxis is regulated by a PLC-mediated calcium pathway. ADP stimulation induced Akt phosphorylation in microglia, and the phosphorylation was inhibited by a P2Y12 receptor antagonist, AR-C69931MX. The Akt phosphorylation was blocked by U73122 and BAPTA-AM as well as by a phosphatidylinositol 3-kinase (PI3K) inhibitor, wortmannin, and inhibition of the Akt activation resulted in failure of chemotaxis. These results indicate that Akt activation is dependent on the PI3K pathway and a PLC-mediated increase in intracellular calcium and suggest that Akt activation is involved in ADP-induced microglial chemotaxis.

Can the immune system be harnessed to repair the CNS?

Experimental and clinical data have demonstrated that activating the immune system in the CNS can be destructive. However, other studies have shown that enhancing an immune response can be therapeutic, and several clinical trials have been initiated with the aim of boosting immune responses in the CNS of individuals with spinal cord injury, multiple sclerosis and Alzheimer's disease. Here, we evaluate the controversies in the field and discuss the remaining scientific challenges that are associated with enhancing immune function in the CNS to treat neurological diseases.

P2Y12 receptors in spinal microglia are required for neuropathic pain after peripheral nerve injury

Extracellular nucleotides have been implicated as signaling molecules used by microglia to sense adverse physiological conditions, such as neuronal damage. They act through purinoceptors, especially the G-protein-coupled P2Y receptor P2Y(12)R. Emerging evidence has indicated that activated spinal microglia responding to nerve injury are key cellular intermediaries in the resulting highly debilitating chronic pain state, namely neuropathic pain. However, the role of microglial P2Y(12)Rs in neuropathic pain remains unknown. Here, we show that the level of P2Y(12)R mRNA expression was markedly increased in the spinal cord ipsilateral to the nerve injury and that this expression was highly restricted to ionized binding calcium adapter molecule 1-positive microglia. An increase in the immunofluorescence of P2Y(12)R protein in the ipsilateral spinal cord was also observed after nerve injury, and P2Y(12)R-positive cells were double labeled with the microglial marker OX-42. Blocking spinal P2Y(12)R by the intrathecal administration of its antagonist AR-C69931MX prevented the development of tactile allodynia (pain hypersensitivity to innocuous stimuli), a hallmark of neuropathic pain syndrome. Furthermore, mice lacking P2ry(12) (P2ry(12)(-/-)) displayed impaired tactile allodynia after nerve injury without any change in basal mechanical sensitivity. Moreover, a single intrathecal administration of AR-C69931MX or oral administration of clopidogrel (a P2Y(12)R blocker clinically in use) to nerve-injured rats produced a striking alleviation of existing tactile allodynia. Together, our findings indicate that activation of P2Y(12)Rs in spinal microglia may be a critical event in the pathogenesis of neuropathic pain and suggest that blocking microglial P2Y(12)R might be a viable therapeutic strategy for treating neuropathic pain.

Prostaglandin E2 reduces extracellular ATP-induced migration in cultured rat microglia

Treatment with 100 microM adenosine triphosphate (ATP) for 120 min augmented migration of cultured rat microglia by about 4-fold. This augmentation was effectively reduced by 0.1-10 microM prostaglandin E(2) (PGE(2)). PGE(2)-mediated reduction was reversed by the EP2 antagonist AH6809 at 10 microM. The EP2 agonist butaprost also reduced ATP-induced migration at 10 microM, whereas the EP1 agonist 17-phenyl trinor PGE(2), the EP3 agonist sulprostone, and the EP4 agonist PGE(1) alcohol all had no effect at 10 microM. In addition, ATP-induced migration was reduced by the adenylate cyclase activator forskolin at 100 microM, whereas the adenylate cyclase inhibitor SQ22536 reversed the effect of PGE(2) on ATP-induced migration at 100 microM. Over the same experimental duration, PGE(2), butaprost, and forskolin had little effect on cell viability. These findings indicate that ATP-induced microglial migration is reduced by PGE(2) through EP2 and adenylate cyclase.

P2Y12 receptor upregulation in activated microglia is a gateway of p38 signaling and neuropathic pain

Microglia in the spinal cord may play an important role in the development and maintenance of neuropathic pain. A metabotropic ATP receptor, P2Y(12), has been shown to be expressed in spinal microglia constitutively and be involved in chemotaxis. Activation of p38 mitogen-activated protein kinase (MAPK) occurs in spinal microglia after nerve injury and may be related to the production of cytokines and other mediators, resulting in neuropathic pain. However, it remains unknown whether any type of P2Y receptor in microglia is involved in the activation of p38 MAPK and the pain behaviors after nerve injury. Using the partial sciatic nerve ligation (PSNL) model in the rat, we found that P2Y(12) mRNA and protein increased in the spinal cord and peaked at 3 d after PSNL. Double labeling studies revealed that cells expressing increased P2Y(12) mRNA and protein after nerve injury were exclusively microglia. Both pharmacological blockades by intrathecal administration of P2Y(12) antagonist and antisense knockdown of P2Y(12) expression suppressed the development of pain behaviors and the phosphorylation of p38 MAPK in spinal microglia after PSNL. The intrathecal infusion of the P2Y(12) agonist 2-(methythio) adenosine 5'-diphosphate trisodium salt into naive rats mimicked the nerve injury-induced activation of p38 in microglia and elevated pain behaviors. These data suggest a new mechanism of neuropathic pain, in which the increased P2Y(12) works as a gateway of the following events in microglia after nerve injury. Activation of this receptor by released ATP or the hydrolyzed products activate p38 MAPK pathway and may play a crucial role in the generation of neuropathic pain.

Sodium-dependent regulation of renal amiloride-sensitive currents by apical P2 receptors

The epithelial sodium channel (ENaC) plays a major role in the regulation of sodium balance and BP by controlling Na(+) reabsorption along the renal distal tubule and collecting duct (CD). ENaC activity is affected by extracellular nucleotides acting on P2 receptors (P2R); however, there remain uncertainties over the P2R subtype(s) involved, the molecular mechanism(s) responsible, and their physiologic role. This study investigated the relationship between apical P2R and ENaC activity by assessing the effects of P2R agonists on amiloride-sensitive current in the rat CD. Using whole-cell patch clamp of principal cells of split-open CD from Na(+)-restricted rats, in combination with immunohistochemistry and real-time PCR, we found that activation of metabotropic P2R (most likely the P2Y(2) and/or (4) subtype), via phospholipase C, inhibited ENaC activity. In addition, activation of ionotropic P2R (most likely the P2X(4) and/or (4/6) subtype), via phosphatidylinositol-3 kinase, either inhibited or potentiated ENaC activity, depending on the extracellular Na(+) concentration; therefore, it is proposed that P2X(4) and/or (4/6) receptors might function as apical Na(+) sensors responsible for local regulation of ENaC activity in the CD and could thereby help to regulate Na(+) balance and systemic BP.

The role of nucleotides in the neuron--glia communication responsible for the brain functions

Accumulating findings indicate that nucleotides play an important role in cell-to-cell communication through P2 purinoceptors, even though ATP is recognized primarily to be a source of free energy and nucleotides are key molecules in cells. P2 purinoceptors are divided into two families, ionotropic receptors (P2X) and metabotropic receptors (P2Y). P2X receptors (7 types; P2X(1)-P2X(7)) contain intrinsic pores that open by binding with ATP. P2Y (8 types; P2Y(1, 2, 4, 6, 11, 12, 13,) and (14)) are activated by nucleotides and couple to intracellular second-messenger systems through heteromeric G-proteins. Nucleotides are released or leaked from non-excitable cells as well as neurons in physiological and pathophysiological conditions. One of the most exciting cells in non-excitable cells is the glia cells, which are classified into astrocytes, oligodendrocytes, and microglia. Astrocytes express many types of P2 purinoceptors and release the 'gliotransmitter' ATP to communicate with neurons, microglia and the vascular walls of capillaries. Microglia also express many types of P2 purinoceptors and are known as resident macrophages in the CNS. ATP and other nucleotides work as 'warning molecules' especially through activating microglia in pathophysiological conditions. Microglia play a key role in neuropathic pain and show phagocytosis through nucleotide-evoked activation of P2X(4) and P2Y(6) receptors, respectively. Such strong molecular, cellular and system-level evidence for extracellular nucleotide signaling places nucleotides in the central stage of cell communications in glia/CNS.

Neuronal 'On' and 'Off' signals control microglia

Recent findings indicate that neurons are not merely passive targets of microglia but rather control microglial activity. The variety of different signals that neurons use to control microglia can be divided into two categories: 'Off' signals constitutively keep microglia in their resting state and antagonize proinflammatory activity. 'On' signals are inducible and include purines, chemokines, glutamate. They instruct microglia activation under pathological conditions towards a beneficial or detrimental phenotype. Various neuronal signaling molecules thus actively control microglia function, thereby contribute to the inflammatory milieu of the central nervous system. Thus, neurons should be envisaged as key immune modulators in the brain.

Involvement of P2X and P2Y receptors in microglial activation in vivo

Microglial cells are the primary immune effector cells in the brain. Extracellular ATP, e.g., released after brain injury, may initiate microglial activation via stimulation of purinergic receptors. In the rat nucleus accumbens (NAc), the involvement of P2X and P2Y receptors in the generation of microglial reaction in vivo was investigated. A stab wound in the NAc increased immunoreactivity (IR) for P2X_1,2,4,7 and P2Y_1,2,4,6,12 receptors on microglial cells when visualized with confocal laser scanning microscopy. A prominent immunolabeling of P2X₇ receptors with antibodies directed against the ecto- or endodomain was found on Griffonia simplicifolia isolectin-B4-positive cells. Additionally, the P2X₇ receptor was colocalized with active caspase 3 but not with the anti-apoptotic marker pAkt. Four days after local application of the agonists α,βmeATP, ADPβS, 2MeSATP, and BzATP, an increase in OX 42- and G. simplicifolia isolectin-IR was observed around the stab wound, quantified both densitometrically and by counting the number of ramified and activated microglial cells, whereas UTPγS appeared to be ineffective. The P2 receptor antagonists PPADS and BBG decreased the injury-induced increase of these IRs when given alone and in addition inhibited the agonist effects. Further, the intra-accumbally applied P2X₇ receptor agonist BzATP induced an increase in the number of caspase-3-positive cells. These results indicate that ATP, acting via different P2X and P2Y receptors, is a signaling molecule in microglial cell activation after injury in vivo. The up-regulation of P2X₇-IR after injury suggests that this receptor is involved in apoptotic rather than proliferative effects.

Neurotransmitter receptors on microglia

Microglia are the intrinsic immune cells of the brain and express chemokine and cytokine receptors that interact with the peripheral immune cells. Recent studies have indicated that microglia also respond to the brain's classical signalling substances, the neurotransmitters. Here, we review the evidence for the expression of neurotransmitter receptors on microglia and the consequences of this receptor activation for microglial behaviour. It is evident that neurotransmitters instruct microglia to perform distinct types of responses, such as triggering an inflammatory cascade or acquiring a neuroprotective phenotype. Understanding how microglia respond to different neurotransmitters will thus have important implications for controlling the reactivity of these cells in acute injury, as well as for treating chronic neurodegenerative diseases.

Modification of neuropathic pain sensation through microglial ATP receptors

Neuropathic pain that typically develops when peripheral nerves are damaged through surgery, bone compression in cancer, diabetes, or infection is a major factor causing impaired quality of life in millions of people worldwide. Recently, there has been a rapidly growing body of evidence indicating that spinal glia play a critical role in the pathogenesis of neuropathic pain. Accumulating findings also indicate that nucleotides play an important role in neuron-glia communication through P2 purinoceptors. Damaged neurons release or leak nucleotides including ATP and UTP to stimulate microglia through P2 purinoceptors expressing on microglia. It was shown in an animal model of neuropathic pain that microglial P2X₄ and P2X₇ receptors are crucial in pain signaling after peripheral nerve lesion. In this review, we describe the modification of neuropathic pain sensation through microglial P2X₄ and P2X₇, with the possibility of P2Y₆ and P2Y₁₂ involvement.

Optimized isolation enables ex vivo analysis of microglia from various central nervous system regions

Ex vivo analysis is an accurate and convenient way to study in vivo microglia phenotype and function. However, current microglia isolation protocols for ex vivo analysis show many differences in isolation steps (perfusion, removal of meninges and blood vessels, mechanical dissociation, enzymatic dissociation, density separation, immunomagnetic separation, and fluorescence-activated cell sorting), often without addressing their effects on microglia purity, number, phenotype, and function. Therefore, the aim of this study was to provide an optimized isolation protocol with emphasis on microglia purity and number to enable ex vivo analysis of adult mouse microglia. The application of this protocol for ex vivo phenotype and functional analysis is corroborated by results from flow cytometry, gene expression analysis, chemotaxis, and phagocytosis assays. In addition, this study shows the possibility to analyze microglia isolated from various central nervous system regions such as optic nerve, striatum, hippocampus, spinal cord, cerebellum, and cerebral cortex. Furthermore, this is the first study presenting DRAQ5 as a superior alternative to propidium iodide for the discrimination between living and dead cells. DRAQ5 staining facilitated the identification of microglia upon flow cytometry without the need of additional fluorescent markers. Along with a favorable emission spectrum, DRAQ5 proved a valuable tool for flow cytometry of microglia. The presented optimized microglia isolation protocol for ex vivo analysis offers the opportunity to obtain more insight into both general and region-specific microglia behavior.