P2X7 receptors regulate engulfing activity of non-stimulated resting astrocytes

P2X7 regulates ependymo-radial glial cell proliferation in adult Danio rerio following spinal cord injury

In contrast to mammals, zebrafish undergo successful neural regeneration following spinal cord injury. Spinal cord ependymo-radial glia (ERG) undergo injury-induced proliferation and neuronal differentiation to replace damaged cells and restore motor function. However, the molecular cues driving these processes remain elusive. Here, we demonstrate that the evolutionarily conserved P2X7 receptors are widely distributed on neurons and ERG within the zebrafish spinal cord. At the protein level, the P2X7 receptor expressed in zebrafish is a truncated splice variant of the full-length variant found in mammals. The protein expression of this 50 kDa isoform was significantly downregulated at 7 days post-injury (dpi) but returned to basal levels at 14 dpi when compared to naïve controls. Pharmacological activation of P2X7 following SCI resulted in a greater number of proliferating cells around the central canal by 7 dpi but did not affect neuronal differentiation at 14 dpi. Our findings suggest that unlike in mammals, P2X7 signaling may not play a maladaptive role following SCI in adult zebrafish and may also work to curb the proliferative response of ERG following injury.

Five-mer peptides prevent short-term spatial memory deficits in Aβ25-35-induced Alzheimer's model mouse by suppressing Aβ25-35 aggregation and resolving its aggregate form

BACKGROUND

The development of drugs for Alzheimer's disease (AD), which is related to the misfolding and aggregation of amyloid-β (Aβ), is high in demand due to the growing number of AD patients. In this study, we screened 22 kinds of 5-mer synthetic peptides derived from the Box A region of Tob1 protein to find a peptide effective against Aβ aggregation.

METHODS

A Thioflavin T (ThT) assay was performed to evaluate aggregation and screen aggregation inhibitors. Male ICR mice (6 weeks old) were administered saline, 9 nmol Aβ25-35, or a mixture of 9 nmol Aβ25-35 and 9 nmol GSGFK in the right lateral ventricle. Short-term spatial memory was assessed through Y-maze. Microglia cells (BV-)2 cells were plated on 24-well plates (4 × 10⁴ cells/well) and incubated for 48 h, and then, the cells were treated with 0.01, 0.05, 0.1, 0.2, or 0.5 mM GSGFK. After incubation for 24 h, bead uptake was evaluated using a laser confocal microscope and Cytation 5.

RESULTS

We found two kinds of peptides, GSGNR and GSGFK, that were not only suppressed by aggregation of Aβ25-35 but also resolved the aggregated Aβ25-35. Results obtained from the Y-maze test on an Aβ25-35-induced AD model mouse indicated that GSGFK prevents the deficits in short-term memory induced by Aβ25-35. The effect of GSGFK on phagocytosis in BV-2 cells proved that GSGFK activates the phagocytic ability of microglia.

CONCLUSIONS

In conclusion, 5-mer peptides prevent short-term memory deficit in Aβ25-35 induced AD model mouse by reducing the aggregated Aβ25-35. They may also upregulate the phagocytic ability of microglia, which makes 5-mer peptides suitable candidates as therapeutic drugs against AD.

Astroglial and Microglial Purinergic P2X7 Receptor as a Major Contributor to Neuroinflammation during the Course of Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune inflammatory disease of the central nervous system that leads to the progressive disability of patients. A characteristic feature of the disease is the presence of focal demyelinating lesions accompanied by an inflammatory reaction. Interactions between autoreactive immune cells and glia cells are considered as a central mechanism underlying the pathology of MS. A glia-mediated inflammatory reaction followed by overproduction of free radicals and generation of glutamate-induced excitotoxicity promotes oligodendrocyte injury, contributing to demyelination and subsequent neurodegeneration. Activation of purinergic signaling, in particular P2X7 receptor-mediated signaling, in astrocytes and microglia is an important causative factor in these pathological processes. This review discusses the role of astroglial and microglial cells, and in particular glial P2X7 receptors, in inducing MS-related neuroinflammatory events, highlighting the importance of P2X7R-mediated molecular pathways in MS pathology and identifying these receptors as a potential therapeutic target.

Zinc-aggravated M1 microglia regulate astrocytic engulfment via P2×7 receptors

BACKGROUND

Glial cells such as astrocytes and microglia play an important role in the central nervous system via communication between these glial cells. Activated microglia can exhibit either the inflammatory M1 phenotype or the anti-inflammatory M2 phenotype, which influences astrocytic neuroprotective functions, including engulfment of cell debris. Recently, extracellular zinc has been shown to promote the inflammatory M1 phenotype in microglia through intracellular zinc accumulation and reactive oxygen species (ROS) generation.

PURPOSE

Here, we investigated whether the zinc-enhanced inflammatory M1 phenotype of microglia affects the astrocytic engulfing activity.

METHODS

Engulfing activity was assessed in astrocytes treated with microglial-conditioned medium (MCM) from lipopolysaccharide (LPS)-activated or from ZnCl2-pretreated LPS-activated M1 microglia. The effect of zinc on microglia phenotype was also validated using the zinc chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) and the ROS scavenger Trolox.

RESULTS

Although treatment of astrocytes with LPS showed no significant effect on the engulfing activity, MCM from LPS-induced M1 microglia increased the beads uptake by astrocytes. This increased uptake activity was suppressed when MCM from LPS-induced M1 microglia pretreated with ZnCl2 was applied to astrocytes, which was further abolished by the intracellular zinc chelator TPEN and the ROS scavenger Trolox. In addition, expression of P2×7 receptors (P2×7R) was increased in astrocytes treated with MCM derived from M1 microglia but not in the M1 microglia pretreated with ZnCl2.

CONCLUSION

These findings suggest that zinc pre-treatment abolishes the ability of LPS-induced M1 microglia to increase the engulfing activity in astrocytes via alteration of astrocytic P2×7R.

P2X7 receptor signaling during adult hippocampal neurogenesis

Neurogenesis is a persistent and essential feature of the adult mammalian hippocampus. Granular neurons generated from resident pools of stem or progenitor cells provide a mechanism for the formation and consolidation of new memories. Regulation of hippocampal neurogenesis is complex and multifaceted, and numerous signaling pathways converge to modulate cell proliferation, apoptosis, and clearance of cellular debris, as well as synaptic integration of newborn immature neurons. The expression of functional P2X7 receptors in the central nervous system has attracted much interest and the regulatory role of this purinergic receptor during adult neurogenesis has only recently begun to be explored. P2X7 receptors are exceptionally versatile: in their canonical role they act as adenosine triphosphate-gated calcium channels and facilitate calcium-signaling cascades exerting control over the cell via calcium-encoded sensory proteins and transcription factor activation. P2X7 also mediates transmembrane pore formation to regulate cytokine release and facilitate extracellular communication, and when persistently stimulated by high extracellular adenosine triphosphate levels large P2X7 pores form, which induce apoptotic cell death through cytosolic ion dysregulation. Lastly, as a scavenger receptor P2X7 directly facilitates phagocytosis of the cellular debris that arises during neurogenesis, as well as during some disease states. Understanding how P2X7 receptors regulate the physiology of stem and progenitor cells in the adult hippocampus is an important step towards developing useful therapeutic models for regenerative medicine. This review considers the relevant aspects of adult hippocampal neurogenesis and explores how P2X7 receptor activity may influence the molecular physiology of the hippocampus, and neural stem and progenitor cells.

P2X7 as a scavenger receptor for innate phagocytosis in the brain

The P2X7 receptor has been widely studied for its ATP-induced pro-inflammatory effect, but in the absence of a ligand, P2X7 has a second function as a scavenger receptor, which is active in the development of the human brain. The scavenger activity of P2X7 is only evident in the absence of serum but is fully active in cerebrospinal fluid. P2X7 on the cell surface is present as a membrane complex, and an attachment to non-muscle myosin of the cytoskeleton is required for particle engulfment. Selective antagonists of P2X7 pro-inflammatory function have little effect on phagocytosis, but inheritance of a variant haplotype spanning the P2RX7 and P2RX4 genes has been associated with loss of P2X7-mediated phagocytosis. Recent studies in mice suggest that the innate phagocytosis mediated by P2X7 receptors declines with ageing. Thus, defective P2X7-mediated phagocytosis may contribute to age-related neuro-degenerative diseases including Alzheimer's disease, age-related macular degeneration and primary progressive multiple sclerosis.

P2X7 Receptors Regulate Phagocytosis and Proliferation in Adult Hippocampal and SVZ Neural Progenitor Cells: Implications for Inflammation in Neurogenesis

Identifying the signaling mechanisms that regulate adult neurogenesis is essential to understanding how the brain may respond to neuro‐inflammatory events. P2X7 receptors can regulate pro‐inflammatory responses, and in addition to their role as cation channels they can trigger cell death and mediate phagocytosis. How P2X7 receptors may regulate adult neurogenesis is currently unclear. Here, neural progenitor cells (NPCs) derived from adult murine hippocampal subgranular (SGZ) and cerebral subventricular (SVZ) zones were utilized to characterize the roles of P2X7 in adult neurogenesis, and assess the effects of high extracellular ATP, characteristic of inflammation, on NPCs. Immunocytochemistry found NPCs in vivo and in vitro expressed P2X7, and the activity of P2X7 in culture was demonstrated using calcium influx and pore formation assays. Live cell and confocal microscopy, in conjunction with flow cytometry, revealed P2X7⁺ NPCs were able to phagocytose fluorescent beads, and this was inhibited by ATP, indicative of P2X7 involvement. Furthermore, P2X7 receptors were activated with ATP or BzATP, and 5‐ethynyl‐2′‐deoxyuridine (EdU) used to observe a dose‐dependent decrease in NPC proliferation. A role for P2X7 in decreased NPC proliferation was confirmed using chemical inhibition and NPCs from P2X7^−/− mice. Together, these data present three distinct roles for P2X7 during adult neurogenesis, depending on extracellular ATP concentrations: (a) P2X7 receptors can form transmembrane pores leading to cell death, (b) P2X7 receptors can regulate rates of proliferation, likely via calcium signaling, and (c) P2X7 can function as scavenger receptors in the absence of ATP, allowing NPCs to phagocytose apoptotic NPCs during neurogenesis. stem cells2018;36:1764–1777

The scavenger activity of the human P2X7 receptor differs from P2X7 pore function by insensitivity to antagonists, genetic variation and sodium concentration: Relevance to inflammatory brain diseases

Activation of P2X7 receptors is widely recognised to initiate proinflammatory responses. However P2X7 also has a dual function as a scavenger receptor which is active in the absence of ATP and plasma proteins and may be important in central nervous system (CNS) diseases. Here, we investigated both P2X7 pore formation and its phagocytic function in fresh human monocytes (as a model of microglia) by measuring ATP-induced ethidium dye uptake and fluorescent bead uptake respectively. This was studied in monocytes expressing various polymorphic variants as well as in the presence of different P2X7 antagonists and ionic media. P2X7-mediated phagocytosis was found to account for about half of Latrunculin (or Cytochalasin D)-sensitive bead engulfment by fresh human monocytes. Monocytes harbouring P2X7 Ala348Thr or Glu496Ala polymorphic variants showed increase or loss of ethidium uptake respectively, but these changes in pore formation did not always correspond to the changes in phagocytosis of YG beads. Unlike pore function, P2X7-mediated phagocytosis was not affected by three potent selective P2X7 antagonists and remained identical in Na⁺ and K⁺ media. Taken together, our results show that P2X7 is a scavenger receptor with important function in the CNS but its phagocytic function has features distinct from its pore function. Both P2X7 pore formation and P2X7-mediated phagocytosis should be considered in the design of new P2X7 antagonists for the treatment of CNS diseases.

Oxidative stress-induced increase of intracellular zinc in astrocytes decreases their functional expression of P2X7 receptors and engulfing activity

Exposure of astrocytes to oxidative stress induces an increase of intracellular labile zinc and a decrease of functional expression of P2X7 receptorviaits translocation from the plasma membrane to the cytosol by altering the expression profile of P2X7 receptor and its splice variants, leading to a decrease of their engulfing activity.

The P2X7 Receptor

The P2X7 receptor is a trimeric ion channel gated by extracellular adenosine 5'-triphosphate. The receptor is present on an increasing number of different cells types including stem, blood, glial, neural, ocular, bone, dental, exocrine, endothelial, muscle, renal and skin cells. The P2X7 receptor induces various downstream events in a cell-specific manner, including inflammatory molecule release, cell proliferation and death, metabolic events, and phagocytosis. As such this receptor plays important roles in heath and disease. Increasing knowledge about the P2X7 receptor has been gained from studies of, but not limited to, protein chemistry including cloning, site-directed mutagenesis, crystal structures and atomic modeling, as well as from studies of primary tissues and transgenic mice. This chapter focuses on the P2X7 receptor itself. This includes the P2RX7 gene and its products including splice and polymorphic variants. This chapter also reviews modulators of P2X7 receptor activation and inhibition, as well as the transcriptional regulation of the P2RX7 gene via its promoter and enhancer regions, and by microRNA and long-coding RNA. Furthermore, this chapter discusses the post-translational modification of the P2X7 receptor by N-linked glycosylation, adenosine 5'-diphosphate ribosylation and palmitoylation. Finally, this chapter reviews interaction partners of the P2X7 receptor, and its cellular localisation and trafficking within cells.

P2X7 from j774 murine macrophages acts as a scavenger receptor for bacteria but not yeast

We studied the effects of extracellular ATP and Ca²⁺ on uptake of bacteria (Staphylococcus aureus or Escherichia coli) and live yeast (Candida glabrata) by J774 macrophages to determine the role of endogenous P2X7 receptors in phagocytosis. Our findings show that phagocytosis of bio-particles coated with S. aureus or E. coli was blocked by ATP and the P2X7 receptor agonist BzATP, while yeast phagocytosis was not. A438079, an antagonist of P2X7 receptors, partially reverted the effects of ATP on bacterial phagocytosis. To determine if P2X7-mediated Ca²⁺ entry into macrophages was blocking the engulfment of bacteria, we measured phagocytic activity in the absence or presence of 2 mM extracellular Ca²⁺ with or without ATP. Ca²⁺, in the absence of ATP, was required for engulfment of E. coli and C. glabrata but not S. aureus. Adding ATP inhibited phagocytosis of S. aureus and E. coli regardless of Ca²⁺, suggesting that Ca²⁺ entry was not important for inhibiting phagocytosis. On the other hand, phagocytosis of normal or hyper-adherent C. glabrata mutants had an absolute requirement for extracellular Ca²⁺ due to yeast adhesion to macrophages mediated by Ca²⁺-dependent adhesion proteins. We conclude that unstimulated P2X7 from J774 cells act as scavenger receptor for the uptake of S. aureus and E. coli but not of yeast; Ca²⁺ entry via P2X7 receptors play no role in phagocytosis of S. aureus and E. coli; while the effect of Ca²⁺ on C. glabrata phagocytosis was mediated by the adhesins Epa1, Epa6 and Epa7.

Innate phagocytosis by peripheral blood monocytes is altered in Alzheimer's disease

Sporadic Alzheimer's disease (AD) is characterised by the deposition and accumulation of specific protein aggregates. Failure of clearance could underlie this process, and recent genetic association studies point towards involvement of the phagocytosis and autophagy pathways. We developed a real-time tri-color flow cytometry method to quantitate the phagocytic function of human peripheral blood monocyte subsets including non-classic CD14(dim)CD16(+), intermediate CD14(+)CD16(+) and classic CD14(+)CD16(-) monocytes. Using this method, we have measured the phagocytic ability of fresh monocytes in a study of preclinical, prodromal and clinical AD, matched with cognitively normal healthy control subjects. Basal levels of phagocytosis in all three subsets of monocytes were similar between healthy controls and AD patients, while a significant increase of basal phagocytosis was found in subjects with high Aβ-amyloid burden as assessed by PET scans. Pre-treating cells with Copaxone (CPX, to stimulate phagocytosis) or ATP (an inhibitor of P2X7-mediated phagocytosis) showed a differential response depending on clinical or Aβ-burden status, indicating a relative functional deficit. Overall the results are consistent with a perturbation of basal and stimulated innate phagocytosis in sporadic AD.

Expression level of P2X7 receptor is a determinant of ATP-induced death of mouse cultured neurons

Activation of P2X7 receptor (P2X7R), a purinergic receptor, expressed by neurons is well-known to induce their death, but whether or not their sensitivity to ATP depends on its expression levels remains unclear. Here, we examined the effect of the expression level of P2X7Rs on cell viability using pure neuron cultures, co-cultures with astrocytes derived from SJL- and ddY-strain mice, and mouse P2X7R-expressing HEK293T cell systems. Treatment of pure neuron cultures with 5mM ATP for 2h, followed by 3-h incubation in fresh medium, resulted in death of both types of neurons, and their death was prevented by administration of P2X7R-specific antagonists. In both SJL- and ddY-neurons, ATP-induced neuronal death was inhibited by a mitochondrial permeability transition pore inhibitor cyclosporine A, mitochondrial dysfunction being involved in their death. The ATP-induced neuronal death was greater for SJL-neurons than for ddY-ones, this being correlated with the expression level of P2X7R in them, and the same results were obtained for the HEK293T cell systems. Co-culture of neurons with astrocytes increased the ATP-induced neuronal death compared to the case of pure neuron cultures. Overall, we reveal that neuronal vulnerability to ATP depends on the expression level of P2X7R, and co-existence of astrocytes exacerbates ATP-induced neuronal death.

P2X7 receptors mediate innate phagocytosis by human neural precursor cells and neuroblasts

During early human neurogenesis there is overproduction of neuroblasts and neurons accompanied by widespread programmed cell death (PCD). While it is understood that CD68(+) microglia and astrocytes mediate phagocytosis during target-dependent PCD, little is known of the cell identity or the scavenger molecules used to remove apoptotic corpses during the earliest stages of human neurogenesis. Using a combination of multiple-marker immunohistochemical staining, functional blocking antibodies and antagonists, we showed that human neural precursor cells (hNPCs) and neuroblasts express functional P2X7 receptors. Furthermore, using live-cell imaging, flow cytometry, phagocytic assays, and siRNA knockdown, we showed that in a serum-free environment, doublecortin(+) (DCX) neuroblasts and hNPCs can clear apoptotic cells by innate phagocytosis mediated via P2X7. We found that both P2X7(high) DCX(low) hNPCs and P2X7(high) DCX(high) neuroblasts, derived from primary cultures of human fetal telencephalon, phagocytosed targets including latex beads, apoptotic ReNcells, and apoptotic hNPC/neuroblasts. Pretreatment of neuroblasts and hNPCs with 1 mM adenosine triphosphate (ATP), 100 µM OxATP (P2X7 antagonist), or siRNA knockdown of P2X7 inhibited phagocytosis of these targets. Our results show that P2X7 functions as a scavenger receptor under serum-free conditions resembling those in early neurogenesis. This is the first demonstration that hNPCs and neuroblasts may participate in clearance of apoptotic corpses during pre target-dependent neurogenesis and mediate phagocytosis using P2X7 as a scavenger receptor.

Ultrastructural investigation of microcalcification and the role of oxygen-glucose deprivation in cultured rat hippocampal slices

Intracellular calcium accumulation is associated with cell death in several neuropathological disorders including brain ischemia, but the exact mechanisms of calcification need to be clarified. We used organotypic hippocampal slice culture - cultures subjected to oxygen-glucose deprivation (OGD) mimicking the in vivo situation to investigate the events underlying ectopic calcification. Alizarin red staining indicating calcium deposition was observed in the cornu ammonis (CA)1 and dentate gyrus regions in control hippocampal slices despite no specific labeling for cell death markers. Electron microscopy using the osmium/potassium dichromate method revealed scattered degenerated cells throughout the normally appearing CA1 region. They contained electron-dense precipitates within mitochondria, and electron probe microanalysis confirmed that they were calcifying mitochondria. Selective calcium deposition was noted within, but not beyond, mitochondria in these mineralized cells. They showed ultrastructural features of non-necrotic, non-apoptotic cell death and retained their compact ultrastructure, even after the majority of mitochondria were calcified. Unexpectedly, no intracellular calcification was noted in necrotic CA1 pyramidal cells after OGD, and there was no progression of calcification in OGD-lesioned slices. In addition, mineralized cells in both control and OGD-lesioned slices were closely associated with or completely engulfed by astrocytes but not microglia. These astrocytes were laden with heterogeneous cytoplasmic inclusions that appeared to be related with their phagocytic activity. These data demonstrate that microcalcification specifically associated with mitochondria might lead to a novel type of cell death and suggest that astrocytes may be involved in the phagocytosis of these mineralized cells and possibly in the regulation of ectopic calcification.

A rare P2X7 variant Arg307Gln with absent pore formation function protects against neuroinflammation in multiple sclerosis

Multiple sclerosis (MS) is a chronic relapsing-remitting inflammatory disease of the central nervous system characterized by oligodendrocyte damage, demyelination and neuronal death. Genetic association studies have shown a 2-fold or greater prevalence of the HLA-DRB1*1501 allele in the MS population compared with normal Caucasians. In discovery cohorts of Australasian patients with MS (total 2941 patients and 3008 controls), we examined the associations of 12 functional polymorphisms of P2X7, a microglial/macrophage receptor with proinflammatory effects when activated by extracellular adenosine triphosphate (ATP). In discovery cohorts, rs28360457, coding for Arg307Gln was associated with MS and combined analysis showed a 2-fold lower minor allele frequency compared with controls (1.11% for MS and 2.15% for controls, P = 0.0000071). Replication analysis of four independent European MS case-control cohorts (total 2140 cases and 2634 controls) confirmed this association [odds ratio (OR) = 0.69, P = 0.026]. A meta-analysis of all Australasian and European cohorts indicated that Arg307Gln confers a 1.8-fold protective effect on MS risk (OR = 0.57, P = 0.0000024). Fresh human monocytes heterozygous for Arg307Gln have >85% loss of 'pore' function of the P2X7 receptor measured by ATP-induced ethidium uptake. Analysis shows Arg307Gln always occurred with 270His suggesting a single 307Gln-270His haplotype that confers dominant negative effects on P2X7 function and protection against MS. Modeling based on the homologous zP2X4 receptor showed Arg307 is located in a region rich in basic residues located only 12 Å from the ligand binding site. Our data show the protective effect against MS of a rare genetic variant of P2RX7 with heterozygotes showing near absent proinflammatory 'pore' function.

P2X7R activation drives distinct IL-1 responses in dendritic cells compared to macrophages

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P2X₇R-driven IL-1 responses are markedly enhanced in BMDC compared to BMM.

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P2X₇R antagonist A-740003 inhibitedIL-1 release in BMDC and BMM.

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P2X₇R-induced pore formation was greater in BMDC than in BMM.

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LPS-primed BMDC resisted ATP-induced cell death in comparison to BMM.

The P2X₇R is a functionally distinct member of the P2X family of non-selective cation channels associated with rapid activation of the inflammasome complex and signalling interleukin (IL)-1β release in macrophages. The main focus of this investigation was to compare P2X₇R-driven IL-1 production by primary murine bone marrow derived dendritic cells (BMDC) and macrophages (BMM). P2X₇R expression in murine BMDC and BMM at both transcriptional (P2X₇A variant) and protein levels was demonstrated. Priming with lipopolysaccharide (LPS) and receptor activation with adenosine triphosphate (ATP) resulted in markedly enhanced IL-1 (α and β) secretion in BMDC compared with BMM. In both cell types IL-1 production was profoundly inhibited with a P2X₇R-specific inhibitor (A-740003) demonstrating that this release is predominantly a P2X₇R-dependent process. These data also suggest that P2X₇R and caspase-1 activation drive IL-1α release from BMDC. Both cell types expressed constitutively the gain-of-function P2X₇K as well as the full P2X₇A variant at equivalent levels. LPS priming reduced significantly levels of P2X₇A but not P2X₇K transcripts in both BMDC and BMM. P2X₇R-induced pore formation, assessed by YO-PRO-1 dye uptake, was greater in BMDC, and these cells were protected from cell death. These data demonstrate that DC and macrophages display distinct patterns of cytokine regulation, particularly with respect to IL-1, as a consequence of cell-type specific differences in the physicochemical properties of the P2X₇R. Understanding the cell-specific regulation of these cytokines is essential for manipulating such responses in health and disease.

Group B streptococcal infection and activation of human astrocytes

Background

Streptococcus agalactiae (Group B Streptococcus, GBS) is the leading cause of life-threatening meningitis in human newborns in industrialized countries. Meningitis results from neonatal infection that occurs when GBS leaves the bloodstream (bacteremia), crosses the blood-brain barrier (BBB), and enters the central nervous system (CNS), where the bacteria contact the meninges. Although GBS is known to invade the BBB, subsequent interaction with astrocytes that physically associate with brain endothelium has not been well studied.

Methodology/Principal Findings

We hypothesize that human astrocytes play a unique role in GBS infection and contribute to the development of meningitis. To address this, we used a well- characterized human fetal astrocyte cell line, SVG-A, and examined GBS infection in vitro. We observed that all GBS strains of representative clinically dominant serotypes (Ia, Ib, III, and V) were able to adhere to and invade astrocytes. Cellular invasion was dependent on host actin cytoskeleton rearrangements, and was specific to GBS as Streptococcus gordonii failed to enter astrocytes. Analysis of isogenic mutant GBS strains deficient in various cell surface organelles showed that anchored LTA, serine-rich repeat protein (Srr1) and fibronectin binding (SfbA) proteins all contribute to host cell internalization. Wild-type GBS also displayed an ability to persist and survive within an intracellular compartment for at least 12 h following invasion. Moreover, GBS infection resulted in increased astrocyte transcription of interleukin (IL)-1β, IL-6 and VEGF.

Conclusions/Significance

This study has further characterized the interaction of GBS with human astrocytes, and has identified the importance of specific virulence factors in these interactions. Understanding the role of astrocytes during GBS infection will provide important information regarding BBB disruption and the development of neonatal meningitis.

C/EBPβ regulates P2X7 receptor expression in response to glucose challenge in intestinal epithelial cells

Activation of the ATP-dependent P2X7 receptor modulates glucose transport in intestinal epithelial cells through the downregulation of glucose transporter GLUT2. In the present study, we show that an increase in glucose concentration stimulates P2X7 receptor transcription via modulation of CCAAT/enhancer binding proteins (C/EBPs) α and β expression. The described human P2X7 receptor promoter region (GenBank Y12851) was cloned upstream of a luciferase reporter gene in pGL4.10 plasmid and used to determine whether C/EBPs, namely C/EBPα and C/EBPβ, are able to stimulate the transcription of P2X7 receptor. Results show that C/EBPβ was the main regulator of P2X7 receptor expression in response to a glucose challenge. Chromatin immunoprecipitation (ChIP) assays further revealed that C/EBPβ occupied the –213 to +6 nt P2X7 promoter region. Surprisingly, C/EBPα was also able to bind this region as revealed by ChIP assays, but without inducing receptor transcription. In fact, C/EBPα and the C/EBPβ-LIP isoform blocked the C/EBPβ-dependent regulation of P2X7 receptor transcription. These findings suggest that glucose is not only the major source of energy for cell function but may also act as a signaling molecule to stimulate the expression of regulatory proteins.

Chronic stress induces prolonged suppression of the P2X7 receptor within multiple regions of the hippocampus: a cumulative threshold spectra analysis

A number of studies have identified that mutations in the P2X7 receptor occur with a significantly higher incidence in individuals with major depression. Consistent with these findings, a number of preclinical studies have identified that mice in which the P2X7 receptor has been deleted exhibit a higher level of resilience-like behaviour to acutely aversive situations. At present, however, no studies have examined changes in P2X7 receptor expression in otherwise healthy animals exposed to persistently stressful situations. This is significant as several lines of evidence have demonstrated that it is exposure to persistently aversive, rather than acutely aversive, situations that is associated with the emergence of mood disturbance. Accordingly, the objective of the current study was to examine whether chronic exposure to restraint stress was associated with alterations in the expression of P2X7 within the hippocampal formation. The study involved three principal groups: acute stress (1 session), chronic stress (21 sessions, 1 per day) and a chronic stress with recovery group (21 sessions, 1 per day followed by 7days of no stress) and appropriate control groups. The results of the analysis indicate that all forms of stress, regardless of the duration, provoked a reduction in P2X7 receptor expression. Comparative analysis on normalised data indicated that the magnitude of the P2X7 reduction was significantly greater in the chronic stress relative to the acute stress group. We additionally found that there was a gradual rebound in P2X7 expression, in two of nine regions examined, in animals that were allowed to recover for 7days following the final stress session. Collectively, these findings provide the first evidence that exposure to chronic restraint stress produces a pronounced and relatively persistent suppression of the P2X7 receptor within the hippocampus.

P2X7 receptor: an emerging target in central nervous system diseases

The ATP-sensitive homomeric P2X7 receptor (P2X7R) has received particular attention as a potential drug target because of its widespread involvement in inflammatory diseases as a key regulatory element of the inflammasome complex. However, it has only recently become evident that P2X7Rs also play a pivotal role in central nervous system (CNS) pathology. There is an explosion of data indicating that genetic deletion and pharmacological blockade of P2X7Rs alter responsiveness in animal models of neurological disorders, such as stroke, neurotrauma, epilepsy, neuropathic pain, multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), Alzheimer's disease, Parkinson's disease, and Huntington's disease. Moreover, recent studies suggest that P2X7Rs regulate the pathophysiology of psychiatric disorders, including mood disorders, implicating P2X7Rs as drug targets in a variety of CNS pathology.

Regulation of activity of P2X7 receptor by its splice variants in cultured mouse astrocytes

Of purinergic receptors, P2X7 receptor (P2X7R, defined as a full-length receptor) has unique characteristics, and its activation leads to ion channel activity and pore formation, causing cell death. Previously, we demonstrated that P2X7R expressed by nonstimulated astrocyte cultures obtained from SJL-strain mice exhibits constitutive activation, implying its role in maintenance of cellular homeostasis. To obtain novel insights into its physiological roles, we examined whether constitutive activation of P2X7R is regulated by expression of its splice variants in such resting astrocytes, and whether their distinct expression profiles in different mouse strains affect activation levels of astrocytic P2X7Rs. In SJL- and ddY-mouse astrocytes, spontaneous YO-PRO-1 uptake, an indicator of pore activity of P2X7R, was detected, but the uptake by the formers was significantly greater than that by the latter. Between the two mouse strains, there was a difference in their sensitivity of YO-PRO-1 uptake to antagonists, but not in the expression levels and sequences of P2X7R and pannexin-1. Regarding expression of splice variants of P2X7R, expression of P2X7R variant-3 (P2X7R-v3) and -4 (P2X7R-v4), but not variant-2 and -k, was lower in SJL-mouse astrocytes than in ddY-mouse ones. On transfection of P2X7R-v3 and -v4 into SJL-mouse astrocytes, the pore activity was attenuated as in the case of the HEK293T cell-expression system. These findings demonstrate that basal activity of P2X7R expressed by resting astrocytes is negatively regulated by P2X7R-v3 and -v4, and that their distinct expression profiles result in the different activation levels of astrocytic P2X7Rs in different mouse strains.

Heteromeric assembly of P2X subunits

Transcripts and/or proteins of P2X receptor (P2XR) subunits have been found in virtually all mammalian tissues. Generally more than one of the seven known P2X subunits have been identified in a given cell type. Six of the seven cloned P2X subunits can efficiently form functional homotrimeric ion channels in recombinant expression systems. This is in contrast to other ligand-gated ion channel families, such as the Cys-loop or glutamate receptors, where homomeric assemblies seem to represent the exception rather than the rule. P2XR mediated responses recorded from native tissues rarely match exactly the biophysical and pharmacological properties of heterologously expressed homomeric P2XRs. Heterotrimerization of P2X subunits is likely to account for this observed diversity. While the existence of heterotrimeric P2X2/3Rs and their role in physiological processes is well established, the composition of most other P2XR heteromers and/or the interplay between distinct trimeric receptor complexes in native tissues is not clear. After a description of P2XR assembly and the structure of the intersubunit ATP-binding site, this review summarizes the distribution of P2XR subunits in selected mammalian cell types and the biochemically and/or functionally characterized heteromeric P2XRs that have been observed upon heterologous co-expression of P2XR subunits. We further provide examples where the postulated heteromeric P2XRs have been suggested to occur in native tissues and an overview of the currently available pharmacological tools that have been used to discriminate between homo- and heteromeric P2XRs.