A role for P2X7in microglial proliferation

The human cathelicidin LL-37 modulates the activities of the P2X7 receptor in a structure-dependent manner

Extracellular ATP, released at sites of inflammation or tissue damage, activates the P2X(7) receptor, which in turn triggers a range of responses also including cell proliferation. In this study the ability of the human cathelicidin LL-37 to stimulate fibroblast growth was inhibited by commonly used P2X(7) blockers. We investigated the structural requirements of the growth-promoting activity of LL-37 and found that it did not depend on helix sense (the all-d analog was active) but did require a strong helix-forming propensity in aqueous solution (a scrambled analog and primate LL-37 orthologs devoid of this property were inactive). The involvement of P2X(7) was analyzed using P2X(7)-expressing HEK293 cells. LL-37 induced proliferation of these cells, triggered Ca(2+) influx, promoted ethidium bromide uptake, and synergized with benzoyl ATP to enhance the pore and channel functions of P2X(7). The activity of LL-37 had an absolute requirement for P2X(7) expression as it was blocked by the P2X(7) inhibitor KN-62, was absent in cells lacking P2X(7), and was restored by P2X(7) transfection. Of particular interest, LL-37 led to pore-forming activity in cells expressing a truncated P2X(7) receptor unable to generate the non-selective pore typical of the full-length receptor. Our results indicate that P2X(7) is involved in the proliferative cell response to LL-37 and that the structural/aggregational properties of LL-37 determine its capacity to modulate the activation state of P2X(7).

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.

P2X7 receptor activation amplifies lipopolysaccharide-induced vascular hyporeactivity via interleukin-1 beta release

Lipopolysaccharide (LPS) stimulates cytoplasmic accumulation of pro-interleukin (IL)-1beta. Activation of P2X(7) receptors stimulates conversion of pro-IL-1beta into mature IL-1beta, which is then secreted. Because both LPS (in vivo) and IL-1beta (in vitro) decrease vascular reactivity to contractile agents, we hypothesized the following: 1) P2X(7) receptor activation contributes to LPS-induced vascular hyporeactivity, and 2) IL-1beta mediates this change. Thoracic aortas were obtained from 12-week-old male C57BL/6 mice. The aortic rings were incubated for 24 h in Dulbecco's modified Eagle's medium, LPS, benzoylbenzoyl-ATP (BzATP; P2X(7) receptor agonist), LPS plus BzATP, oxidized ATP (oATP; P2X(7) receptor antagonist), or oATP plus LPS plus BzATP. After the treatment, the rings were either mounted in a myograph for evaluation of contractile activity or homogenized for IL-1beta and inducible nitric-oxide synthase (iNOS) protein measurement. In endothelium-intact aortic rings, phenylephrine (PE)-induced contractions were not altered by incubation with LPS or BzATP, but they significantly decreased in aortic rings incubated with LPS plus BzATP. Treatment with oATP or IL-1ra (IL-1beta receptor antagonist) reversed LPS plus BzATP-induced hyporeactivity to PE. In the presence of N(G)-nitro-l-arginine methyl ester or N-([3-(aminomethyl)phenyl]methyl)ethanimidamide (selective iNOS inhibitor), the vascular hyporeactivity induced by LPS plus BzATP on PE responses was not observed. BzATP augmented LPS-induced IL-1beta release and iNOS protein expression, and these effects were also inhibited by oATP. Moreover, incubation of endothelium-intact aortic rings with IL-1beta induced iNOS protein expression. Thus, activation of P2X(7) receptor amplifies LPS-induced hyporeactivity in mouse endothelium-intact aorta, which is associated with IL-1beta-mediated release of nitric oxide by iNOS.

The P2X7 receptor as a therapeutic target

BACKGROUND

The P2X7 receptor is present in a variety of cell types involved in pain, inflammatory processes and neurodegenerative conditions, thus it may be an appealing target for pharmacological intervention. The extensive use of high-throughput screening (HTS) followed by a hit-to-lead (HtL) program, has prompted a number of firms to identify highly selective and metabolically stable small-molecules possessing activity for both the rat and human P2X(7) receptor, which provide a novel therapeutic approach to the treatment of pain as well as neurodegenerative and inflammatory disorders.

OBJECTIVE

To describe the current status of and potential for development of P2X(7) receptor-antagonists.

METHODS

A literature review.

RESULTS/CONCLUSIONS

We describe the recent discoveries of novel P2X(7) receptor-selective antagonists, along with their biological activity and therapeutic potential.

Lipopolysaccharide is a frequent and significant contaminant in microglia-activating factors

Lipopolysaccharide (LPS/endotoxin) is a potent immunologic stimulant. Many commercial-grade reagents used in research are not screened for LPS contamination. LPS induces a wide spectrum of proinflammatory responses in microglia, the immune cells of the brain. Recent studies have demonstrated that a broad range of endogenous factors including plasma-derived proteins and bioactive phospholipids can also activate microglia. However, few of these studies have reported either the LPS levels found in the preparations used or the effect of LPS inhibitors such as polymyxin B (PMX) on factor-induced responses. Here, we used the Limulus amoebocyte lysate assay to screen a broad range of commercial- and pharmaceutical-grade proteins, peptides, lipids, and inhibitors commonly used in microglia research for contamination with LPS. We then characterized the ability of PMX to alter a representative set of factor-induced microglial activation parameters including surface antigen expression, metabolic activity/proliferation, and NO/cytokine/chemokine release in both the N9 microglial cell line and primary microglia. Significant levels of LPS contamination were detected in a number of commercial-grade plasma/serum- and nonplasma/serum-derived proteins, phospholipids, and synthetic peptide preparations, but not in pharmaceutical-grade recombinant proteins or pharmacological inhibitors. PMX had a significant inhibitory effect on the microglia-activating potential of a number of commercial-, but not pharmaceutical-grade, protein preparations. Novel PMX-resistant responses to alpha(2)-macroglobulin and albumin were incidentally observed. Our results indicate that LPS is a frequent and significant contaminant in commercial-grade preparations of previously reported microglia-activating factors. Careful attention to LPS levels and appropriate controls are necessary for future studies in the neuroinflammation field.

Immunosuppression after traumatic or ischemic CNS damage: it is neuroprotective and illuminates the role of microglial cells

Acute traumatic and ischemic events in the central nervous system (CNS) invariably result in activation of microglial cells as local representatives of the immune system. It is still under debate whether activated microglia promote neuronal survival, or whether they exacerbate the original extent of neuronal damage. Protagonists of the view that microglial cells cause secondary damage have proposed that inhibition of microglial activation by immunosuppression is beneficial after acute CNS damage. It is the aim of this review to analyse the effects of immunosuppressants on isolated microglial cells and neurons, and to scrutinize the effects of immunosuppression in different in vivo models of acute CNS trauma or ischemia. It is found that the immunosuppressants cytosine-arabinoside, different steroids, cyclosporin A, FK506, rapamycin, mycophenolate mofetil, and minocycline all have direct inhibitory effects on microglial cells. These effects are mainly exerted by inhibiting microglial proliferation or microglial secretion of neurotoxic substances such as proinflammatory cytokines and nitric oxide. Furthermore, immunosuppression after acute CNS trauma or ischemia results in improved structure preservation and, mostly, in enhanced function. However, all investigated immunosuppressants also have direct effects on neurons, and some immunosuppressants affect other glial cells such as astrocytes. In summary, it is safe to conclude that immunosuppression after acute CNS trauma or ischemia is neuroprotective. Furthermore, circumferential evidence indicates that microglial activation after traumatic or ischemic CNS damage is not beneficial to adjacent neurons in the immediate aftermath of such acute lesions. Further experiments with more specific agents or genetic approaches that specifically inhibit microglial cells are needed in order to fully answer the question of whether microglial activation is "good or bad".

ERK1/2 and p38 MAP kinases control prion protein fragment 90-231-induced astrocyte proliferation and microglia activation

Astrogliosis and microglial activation are a common feature during prion diseases, causing the release of chemoattractant and proinflammatory factors as well as reactive free radicals, involved in neuronal degeneration. The recombinant protease-resistant domain of the prion protein (PrP90-231) displays in vitro neurotoxic properties when refolded in a beta-sheet-rich conformer. Here, we report that PrP90-231 induces the secretion of several cytokines, chemokines, and nitric oxide (NO) release, in both type I astrocytes and microglial cells. PrP90-231 elicited in both cell types the activation of ERK1/2 MAP kinase that displays, in astrocytes, a rapid kinetics and a proliferative response. Conversely, in microglia, PrP90-231-dependent MAP kinase activation was delayed and long lasting, inducing functional activation and growth arrest. In microglial cells, NO release, dependent on the expression of the inducible NO synthase (iNOS), and the secretion of the chemokine CCL5 were Ca(2+) dependent and under the control of the MAP kinases ERK1/2 and p38: ERK1/2 inhibition, using PD98059, reduced iNOS expression, while p38 blockade by PD169316 inhibited CCL5 release. In summary, we demonstrate that glial cells are activated by extracellular misfolded PrP90-231 resulting in a proliferative/secretive response of astrocytes and functional activation of microglia, both dependent on MAP kinase activation. In particular, in microglia, PrP90-231 activated a complex signalling cascade involved in the regulation of NO and chemokine release. These data argue in favor of a causal role for misfolded prion protein in sustaining glial activation and, possibly, glia-mediated neuronal death.

Synaptic terminals from mice midbrain exhibit functional P2X7 receptor

P2X(7) receptor has been recently localized in mice cerebellar granule neuron fibers. Here, the expression of this subunit has been detected in wild type mice midbrain, by quantitative real time-polymerase chain reaction, immunocytochemistry and Western blot assays. The functionality of this P2X(7) subunit has been confirmed using microfluorimetric experiments in isolated synaptic terminals from mice midbrain. 2'-3'-O-(4-benzoylbenzoyl)-ATP (BzATP) was 30-fold more potent than ATP and EC(50) values were 20 microM and 630 microM respectively. Brilliant Blue G (BBG) and 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62) produced an inhibition in the responses induced by BzATP, with IC(50) values of 0.027 nM and 2.23 nM, respectively. In addition, P2X(7) inhibitors as ZnSO(4), BBG and suramin abolished partially or totally the responses induced by the physiological agonist ATP. According to immunochemical and PCR assays the presence of a "P2X(7)-like" protein in synaptosomes from validated P2X(7) knockout (KO) model have been detected. In KO animals, BzATP was sixfold more potent than ATP and the EC(50) values were 87 microM and 590 microM respectively. BBG and KN-62 also produced an inhibition in the responses induced by BzATP, with IC(50) value of 0.61 nM and 118 nM respectively, both of them higher than in wild type mice. Moreover, the calcium mobilization ability of native P2X(7) receptors was higher in control compared with KO mice. These biochemical and pharmacological experiments are consistent with the presence of a functional P2X(7) receptor in wild type mice midbrain, and the existence of a less efficient "P2X(7)-like" receptor in the KO model.

P2X(7) receptor stimulation upregulates Egr-1 biosynthesis involving a cytosolic Ca(2+) rise, transactivation of the EGF receptor and phosphorylation of ERK and Elk-1

The P2X(7) receptor is an ATP-gated ionotropic receptor that is permeable for small cations including Ca(2+) ions. Using 293 cells expressing P2X(7) receptors, we show that the P2X(7) receptor-specific ligand 2',3'-O-(4-benzoyl-benzoyl)-ATP (BzATP) induces a signaling cascade leading to the biosynthesis of biologically active Egr-1, a zinc finger transcription factor. BzATP-triggered Egr-1 biosynthesis was attenuated by the mitogen-activated protein kinase kinase inhibitor PD98059, by BAPTA-AM, the acetoxymethylester of the cytosolic Ca(2+) chelator BAPTA, and by an epidermal growth factor (EGF) receptor-specific tyrosine kinase inhibitor (AG1478). These results indicate that phosphorylation and activation of extracellular signal-regulated protein kinase ERK, elevated levels of intracellular Ca(2+) and the transactivation of the EGF receptor are essential for BzATP-induced upregulation of Egr-1. The requirement of Ca(2+) within the signaling cascade was upstream of Raf kinase activation. Lentiviral-mediated expression of MAP kinase phosphatase-1 (MKP-1), a dual-specific phosphatase that dephosphorylates and inactivates ERK in the nucleus, inhibited Egr-1 biosynthesis following BzATP stimulation, indicating that MKP-1 functions as a nuclear shut-off device. Furthermore, the ternary complex factor Elk-1 was phosphorylated and the transcriptional activation potential of Elk-1 was enhanced following P2X(7) receptor stimulation. Expression of a dominant-negative mutant of Elk-1 impaired BzATP-induced upregulation of Egr-1 biosynthesis. Thus, Elk-1 connects the intracellular signaling cascade elicited by activation of P2X(7) receptors with the transcription of the Egr-1 gene.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.