This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5′-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

In summary, P2Y(2) R appears to be involved in asthmatic airway inflammation by mediating ATP-triggered migration of mDCs and eosinophils, as well as reactive oxygen species production. Together our data suggest that targeting P2Y(2) R might be a therapeutic option for the treatment of asthma.

The angiotensin (Ang) type 1 receptor (AT1R) promotes functional and structural integrity of the arterial wall to contribute to vascular homeostasis, but this receptor also promotes hypertension. In our investigation of how Ang II signals are converted by the AT1R from physiological to pathological outputs, we found that the purinergic P2Y6 receptor (P2Y6R), an inflammation-inducible G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor (GPCR), promoted Ang II-induced hypertension in mice. In mice, deletion of P2Y6R attenuated Ang II-induced increase in blood pressure, vascular remodeling, oxidative stress, and endothelial dysfunction. AT1R and P2Y6R formed stable heterodimers, which enhanced G protein-dependent vascular hypertrophy but reduced β-arrestin-dependent AT1R internalization. Pharmacological disruption of AT1R-P2Y6R heterodimers by the P2Y6R antagonist MRS2578 suppressed Ang II-induced hypertension in mice. Furthermore, P2Y6R abundance increased with age in vascular smooth muscle cells. The increased abundance of P2Y6R converted AT1R-stimulated signaling in vascular smooth muscle cells from β-arrestin-dependent proliferation to G protein-dependent hypertrophy. These results suggest that increased formation of AT1R-P2Y6R heterodimers with age may increase the likelihood of hypertension induced by Ang II.

BelgiumAdenosine triphosphate has previously been shown to induce semi-mature human monocyte-derived dendritic cells (DC). These are characterized by the up-regulation of co-stimulatory molecules, the inhibition of IL-12 and the up-regulation of some genes involved in immune tolerance, such as thrombospondin-1 and indoleamine 2,3-dioxygenase. The actions of adenosine triphosphate are mediated by the P2Y 11 receptor; since there is no functional P2Y 11 gene in the murine genome, we investigated the action of adenine nucleotides on murine DC. Adenosine 5 0 -(3-thiotriphosphate) and adenosine inhibited the production of IL-12p70 by bone marrow-derived DC (BMDC). These inhibitions were relieved by 8-p-sulfophenyltheophylline, an adenosine receptor antagonist. The use of selective ligands and A 2B -/-BMDC indicated the involvement of the A 2B receptor. A microarray experiment, confirmed by quantitative PCR, showed that, in presence of LPS, 5 0 -(N-ethylcarboxamido) adenosine (NECA, the most potent A 2B receptor agonist) regulated the expression of several genes: arginase I and II, thrombospondin-1 and vascular endothelial growth factor were up-regulated whereas CCL2 and CCL12 were downregulated. We further showed that NECA, in combination with LPS, increased the arginase I enzymatic activity. In conclusion, the described actions of adenine nucleotides on BMDC are mediated by their degradation product, adenosine, acting on the A 2B receptor, and will possibly lead to an impairment of Th1 response or tolerance.

Objective Myogenic tone (MT) of resistance arteries ensures autoregulation of blood flow in organs and relies on the intrinsic property of smooth muscle to contract in response to stretch. Nucleotides released by mechanical strain on cells are responsible for pleiotropic vascular effects, including vasoconstriction. Here, we evaluated the contribution of extracellular nucleotides to MT. Approach and Results We measured MT and the associated pathway in mouse mesenteric resistance arteries using arteriography for small arteries and molecular biology. Of the P2 receptors in mouse mesenteric resistance arteries, mRNA expression of P2X1 and P2Y6 was dominant. P2Y6 fully sustained UDP/UTP-induced contraction (abrogated in P2ry6−/− arteries). Preventing nucleotide hydrolysis with the ectonucleotidase inhibitor ARL67156 enhanced pressure-induced MT by 20%, whereas P2Y6 receptor blockade blunted MT in mouse mesenteric resistance arteries and human subcutaneous arteries. Despite normal hemodynamic parameters, P2ry6−/− mice were protected against MT elevation in myocardial infarction–induced heart failure. Although both P2Y6 and P2Y2 receptors contributed to calcium mobilization, P2Y6 activation was mandatory for RhoA–GTP binding, myosin light chain, P42–P44, and c-Jun N-terminal kinase phosphorylation in arterial smooth muscle cells. In accordance with the opening of a nucleotide conduit in pressurized arteries, MT was altered by hemichannel pharmacological inhibitors and impaired in Cx43+/− and P2rx7−/− mesenteric resistance arteries. Conclusions Signaling through P2 nucleotide receptors contributes to MT. This mechanism encompasses the release of nucleotides coupled to specific autocrine/paracrine activation of the uracil nucleotide P2Y6 receptor and may contribute to impaired tissue perfusion in cardiovascular diseases.

In mammalian species, including humans, the hippocampal dentate gyrus (DG) is a primary region of adult neurogenesis. Aberrant adult hippocampal neurogenesis is associated with neurological pathologies. Understanding the cellular mechanisms controlling adult hippocampal neurogenesis is expected to open new therapeutic strategies for mental disorders. Microglia is intimately associated with neural progenitor cells in the hippocampal DG and has been implicated, under varying experimental conditions, in the control of the proliferation, differentiation and survival of neural precursor cells. But the underlying mechanisms remain poorly defined. Using fluorescent in situ hybridization we show that microglia in brain express the ADP-activated P2Y13 receptor under basal conditions and that P2ry13 mRNA is absent from neurons, astrocytes, and neural progenitor cells. Disrupting P2ry13 decreases structural complexity of microglia in the hippocampal subgranular zone (SGZ). But it increases progenitor cell proliferation and new neuron formation. Our data suggest that P2Y13 receptor-activated microglia constitutively attenuate hippocampal neurogenesis. This identifies a signaling pathway whereby microglia, via a nucleotide-mediated mechanism, contribute to the homeostatic control of adult hippocampal neurogenesis. Selective P2Y13R antagonists could boost neurogenesis in pathological conditions associated with impaired hippocampal neurogenesis.

ATP release and subsequent activation of purinergic receptors has been suggested to be one of the key transduction pathways activated by mechanical stimulation of bone. The P2Y 13 receptor, recently found to be expressed by osteoblasts, has been suggested to provide a negative feedback pathway for ATP release in different cell types. Therefore, we hypothesized that the P2Y 13 receptor may contribute to the mediation of osteogenic responses to mechanical stimulation by regulating ATP metabolism by osteoblasts. To test this hypothesis, wild-type (WT) and P2Y 13 receptor knockout (P2Y 13 R À/À ) mice were subject to non-invasive axial mechanical loading of the left tibiae to induce an osteogenic response. Micro-computed tomography analysis showed mechanical loading induced an osteogenic response in both strains of mice in terms of increased total bone volume and cortical bone volume, with the P2Y 13 R À/À mice having a significantly greater response. The extent of the increased osteogenic response was defined by dynamic histomorphometry data showing dramatically increased bone formation and mineral apposition rates in P2Y 13 R À/À mice compared with controls. In vitro, primary P2Y 13 R À/À osteoblasts had an accumulation of mechanically induced extracellular ATP and reduced levels of hydrolysis. In addition, P2Y 13 R À/À osteoblasts also had a reduction in their maximal alkaline phosphatase (ALP) activity, one of the main ecto-enzymes expressed by osteoblasts, which hydrolyzes extracellular ATP. In conclusion, deletion of the P2Y 13 receptor leads to an enhanced osteogenic response to mechanical loading in vivo, possibly because of the reduced extracellular ATP degradation by ALP. The augmented osteogenic response to mechanical stimulation, combined with suppressed bone remodeling activities and protection from OVXinduced bone loss after P2Y 13 receptor depletion as previously described, suggests a potential role for P2Y 13 receptor antagonist-based therapy, possibly in combination with mechanical loading, for the treatment of osteoporosis.

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