Microglial P2X7 receptor expression is accompanied by neuronal damage in the cerebral cortex of the APPswe/PS1dE9 mouse model of Alzheimer's disease

The possibility that P2X7 receptor (P2X7R) expression in microglia would mediate neuronal damage via reactive oxygen species (ROS) production was examined in the APPswe/PS1dE9 mouse model of Alzheimer's disease (AD). P2X7R was predominantly expressed in CD11b-immunopositive microglia from 3 months of age before Abeta plaque formation. In addition, gp91phox, a catalytic subunit of NADPH oxidase, and ethidium fluorescence were detected in P2X7R-positive microglial cells of animals at 6 months of age, indicating that P2X7R-positive microglia could produce ROS. Postsynaptic density 95-positive dendrites showed significant damage in regions positive for P2X7R in the cerebral cortex of 6 month-old mice. Taken together, up-regulation of P2X7R activation and ROS production in microglia are parallel with Abeta increase and correlate with synaptotoxicity in AD.

Activation of nicotinic acetylcholine receptor prevents the production of reactive oxygen species in fibrillar beta amyloid peptide (1-42)-stimulated microglia

Recent studies have reported that the "cholinergic anti-inflammatory pathway" regulates peripheral inflammatory responses via alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs) and that acetylcholine and nicotine regulate the expression of proinflammatory mediators such as TNF-alpha and prostaglandin E2 in microglial cultures. In a previous study we showed that ATP released by beta-amyloid-stimulated microglia induced reactive oxygen species (ROS) production, in a process involving the P2X7 receptor (P2X7R), in an autocrine fashion. These observations led us to investigate whether stimulation by nicotine could regulate fibrillar beta amyloid peptide (1-42) (fA beta(1-42))-induced ROS production by modulating ATP efflux-mediated Ca2+ influx through P2X7R. Nicotine inhibited ROS generation in fA beta(1-42)-stimulated microglial cells, and this inhibition was blocked by mecamylamine, a non-selective nAChR antagonist, and a-bungarotoxin, a selective alpha7 nAChR antagonist. Nicotine inhibited NADPH oxidase activation and completely blocked Ca2+ influx in fA beta(1-42)-stimulated microglia. Moreover, ATP release from fA beta(1-42)-stimulated microglia was significantly suppressed by nicotine treatment. In contrast, nicotine did not inhibit 2',3'-O-(4-benzoyl)-benzoyl ATP (BzATP)-induced Ca2+ influx, but inhibited ROS generation in BzATP-stimulated microglia, indicating an inhibitory effect of nicotine on a signaling process downstream of P2X7R. Taken together, these results suggest that the inhibitory effect of nicotine on ROS production in fA beta(1-42)-stimulated microglia is mediated by indirect blockage of ATP release and by directly altering the signaling process downstream from P2X7R.

Cross talk between P2 purinergic receptors modulates extracellular ATP-mediated interleukin-10 production in rat microglial cells

Previously we demonstrated that ATP released from LPS-activated microglia induced IL-10 expression in a process involving P2 receptors, in an autocrine fashion. Therefore, in the present study we sought to determine which subtype of P2 receptor was responsible for the modulation of IL-10 expression in ATP-stimulated microglia. We found that the patterns of IL-10 production were dose-dependent (1, 10, 100, 1,000 micrometer) and bell-shaped. The concentrations of ATP, ATP-gammaS, ADP, and ADP-beta S that showed maximal IL-10 release were 100, 10, 100, and 100 micrometer respectively. The rank order of agonist potency for IL-10 production was 2'-3'-O-(4-benzoyl)-benzoyl ATP (BzATP) = dATP > 2-methylthio-ADP (2-meSADP). On the other hand, 2-methylthio-ATP (2-meSATP), alpha,beta-methylene ATP (alpha,beta-meATP), UTP, and UDP did not induce the release of IL-10 from microglia. Further, we obtained evidence of crosstalk between P2 receptors, in a situation where intracellular Ca2+ release and/or cAMP-activated PKA were the main contributors to extracellular ATP-(or ADP)-mediated IL-10 expression, and IL-10 production was down- regulated by either MRS2179 (a P2Y1 antagonist) or 5'-AMPS (a P2Y11 antagonist), indicating that both the P2Y1 and P2Y11 receptors are major receptors involved in IL-10 expression. In addition, we found that inhibition of IL-10 production by high concentrations of ATP-gammaS (100 micrometer) was restored by TNP-ATP (an antagonist of the P2X1, P2X3, and P2X4 receptors), and that IL-10 production by 2-meSADP was restored by 2meSAMP (a P2Y12 receptor antagonist) or pertusis toxin (PTX; a Gi protein inhibitor), indicating that the P2X1, P2X3, P2X4 receptor group, or the P2Y12 receptor, negatively modulate the P2Y11 receptor or the P2Y1 receptor, respectively.

Interleukin-10 endogenously expressed in microglia prevents lipopolysaccharide-induced neurodegeneration in the rat cerebral cortex in vivo

A degree of brain inflammation is required for repair of damaged tissue, but excessive inflammation causes neuronal cell death. Here, we observe that IL-10 is expressed in LPS-injected rat cerebral cortex, contributing to neuronal survival. Cells immunopositive for IL-10 were detected as early as 8 h post-injection and persisted for up to 3 d, in parallel with the expression of IL-1beta, TNF-alpha, and iNOS. Double immunofluorescence staining showed that IL-10 expression was localized mainly in activated microglia. Next, we examined the neuroprotective effects of IL-10 using IL-10 neutralizing antibody (IL-10NA). Blockade of IL-10 action caused a significant loss of neurons both 3 d and 7 d after LPS injection. Further, the induction of mRNA species encoding IL-1beta, TNF-alpha, and iNOS, reactive oxygen species (ROS) production, and NADPH oxidase activation, increased after co-injection of LPS and IL-10NA, compared to the levels seen after injection of LPS alone. Taken together, these results clearly suggest that LPS-induced endogenous expression of IL-10 in microglia contributes to neuronal survival by inhibiting brain inflammation.

ATP released from beta-amyloid-stimulated microglia induces reactive oxygen species production in an autocrine fashion

Present study demonstrated that fibrillar beta-amyloid peptide (fAbeta(1-42)) induced ATP release, which in turn activated NADPH oxidase via the P2X(7) receptor (P2X(7)R). Reactive oxygen species (ROS) production in fAbeta(1-42)-treated microglia appeared to require Ca2+ influx from extracellular sources, because ROS generation was abolished to control levels in the absence of extracellular Ca2+. Considering previous observation of superoxide generation by Ca2+ influx through P2X(7)R in microglia, we hypothesized that ROS production in fAbeta-stimulated microglia might be mediated by ATP released from the microglia. We therefore examined whether fAbeta(1-42)-induced Ca2+ influx was mediated through P2X(7)R activation. In serial experiments, we found that microglial pretreatment with the P2X(7)R antagonists Pyridoxal-phosphate-6-azophenyl-2',4'- disulfonate (100 micrometer) or oxidized ATP (100 micrometer) inhibited fAbeta-induced Ca2+ influx and reduced ROS generation to basal levels. Furthermore, ATP efflux from fAbeta(1-42)-stimulated microglia was observed, and apyrase treatment decreased the generation of ROS. These findings provide conclusive evidence that fAbeta-stimulated ROS generation in microglial cells is regulated by ATP released from the microglia in an autocrine manner.