Mechanisms of release of nucleotides and integration of their action as P2X- and P2Y-receptor activating molecules

Enhancement of acid-sensing ion channel activity by metabotropic P2Y UTP receptors in primary sensory neurons

Peripheral purinergic signaling plays an important role in nociception. Increasing evidence suggests that metabotropic P2Y receptors are also involved, but little is known about the underlying mechanism. Herein, we report that selective P2Y receptor agonist uridine 5'-triphosphate (UTP) can exert an enhancing effect on the functional activity of acid-sensing ion channels (ASICs), key sensors for extracellular protons, in rat dorsal root ganglia (DRG) neurons. First, UTP dose-dependently increased the amplitude of ASIC currents. UTP also shifted the concentration-response curve for proton upwards, with a 56.6 ± 6.4% increase of the maximal current response to proton. Second, UTP potentiation of proton-gated currents can be mimicked by adenosine 5'-triphosphate (ATP), but not by P2Y1 receptor agonist ADP. Potentiation of UTP was blocked by P2Y receptor antagonist suramin and by inhibition of intracellular G protein, phospholipase C (PLC), protein kinase C (PKC), or protein interacting with C-kinase 1 (PICK1) signaling. Third, UTP altered acidosis-evoked membrane excitability of DRG neurons and caused a significant increase in the amplitude of the depolarization and the number of spikes induced by acid stimuli. Finally, UTP dose-dependently exacerbated nociceptive responses to injection of acetic acid in rats. These results suggest that UTP enhanced ASIC-mediated currents and nociceptive responses, which reveal a novel peripheral mechanism underlying UTP-sensitive P2Y2 receptor involvement in hyperalgesia by sensitizing ASICs in primary sensory neurons.

Astrocytic adenosine: from synapses to psychiatric disorders

Although it is considered to be the most complex organ in the body, the brain can be broadly classified into two major types of cells, neuronal cells and glial cells. Glia is a general term that encompasses multiple types of non-neuronal cells that function to maintain homeostasis, form myelin, and provide support and protection for neurons. Astrocytes, a major class of glial cell, have historically been viewed as passive support cells, but recently it has been discovered that astrocytes participate in signalling activities both with the vasculature and with neurons at the synapse. These cells have been shown to release D-serine, TNF-α, glutamate, atrial natriuretic peptide (ANP) and ATP among other signalling molecules. ATP and its metabolites are well established as important signalling molecules, and astrocytes represent a major source of ATP release in the nervous system. Novel molecular and genetic tools have recently shown that astrocytic release of ATP and other signalling molecules has a major impact on synaptic transmission. Via actions at the synapse, astrocytes have now been shown to regulate complex network signalling in the whole organism with impacts on respiration and the sleep-wake cycle. In addition, new roles for astrocytes are being uncovered in psychiatric disorders, and astrocyte signalling mechanisms represents an attractive target for novel therapeutic agents.

Emerging role of P2X7 receptors in CNS health and disease

Purinergic signalling in the brain is becoming an important focus in the study of CNS health and disease. Various purinergic receptors are found to be present in different brain cells in varying extent, which get activated upon binding of ATP or its analogues. Conventionally, ATP was considered only as a major metabolic fuel of the cell but its recognition as a neurotransmitter in early 1970s, brought meaningful insights in neuron glia crosstalk, participating in various physiological functions in the brain. P2X7R, a member of ligand gated purinergic receptor (P2X) family, is gaining attention in the field of neuroscience because of its emerging role in broad spectrum of ageing and age related neurological disorders. The aim of this review is to provide an overview about the structure and function of P2X7R highlighting its unique features which distinguish it from the other members of its family. This review critically analyzes the literature mentioning the details about the agonist and antagonist of the P2X7R. It also emphasizes the advancements in understanding the dual role of P2X7R in brain development and disorders inviting meaningful insights about its involvement in Alzheimer's disease, Huntington's disease, Multiple Sclerosis, Neuropathic pain, Spinal Cord Injury and NeuroAIDS. Exploring the roles of P2X7R in detail is critical to identify its therapeutic potential in the treatment of acute and chronic neurodegenerative diseases. Moreover, this review also helps to raise more interest in the neurobiology of the purinergic receptors and thus providing new avenues for future research.

Effects of Aerosolized Adenosine 5'-Triphosphate in Smokers and Patients With COPD

BACKGROUND

Extracellular adenosine 5'-triphosphate (ATP) stimulates vagal C and Aδ fibers in the lung, resulting in pronounced bronchoconstriction and cough mediated by P2X2/3 receptors located on vagal sensory nerve terminals. We investigated the effects of nebulized ATP on cough and symptoms in control subjects, healthy smokers, and patients with COPD and compared these responses to the effects of inhaled adenosine, the metabolite of ATP.

METHODS

We studied the effects of inhaled ATP and adenosine monophosphate (AMP) on airway caliber, perception of dyspnea assessed by the Borg score, cough sensitivity, and ATP in exhaled breath condensate in healthy nonsmokers (n = 10), healthy smokers (n = 14), and patients with COPD (n = 7).

RESULTS

In comparison with healthy subjects, ATP induced more dyspnea, cough, and throat irritation in smokers and patients with COPD, and the effects of ATP were more pronounced than those of AMP. The concentration of ATP in the exhaled breath condensate of patients with COPD was elevated compared with that of healthy subjects.

CONCLUSIONS

Smokers and patients with COPD manifest hypersensitivity to extracellular ATP, which may play a mechanistic role in COPD.

Rat, mouse, and primate models of chronic glaucoma show sustained elevation of extracellular ATP and altered purinergic signaling in the posterior eye

PURPOSE

The cellular mechanisms linking elevated IOP with glaucomatous damage remain unresolved. Mechanical strains and short-term increases in IOP can trigger ATP release from retinal neurons and astrocytes, but the response to chronic IOP elevation is unknown. As excess extracellular ATP can increase inflammation and damage neurons, we asked if sustained IOP elevation was associated with a sustained increase in extracellular ATP in the posterior eye.

METHODS

No ideal animal model of chronic glaucoma exists, so three different models were used. Tg-Myoc(Y437H) mice were examined at 40 weeks, while IOP was elevated in rats following injection of hypertonic saline into episcleral veins and in cynomolgus monkeys by laser photocoagulation of the trabecular meshwork. The ATP levels were measured using the luciferin-luciferase assay while levels of NTPDase1 were assessed using qPCR, immunoblots, and immunohistochemistry.

RESULTS

The ATP levels were elevated in the vitreal humor of rats, mice, and primates after a sustained period of IOP elevation. The ecto-ATPase NTPDase1 was elevated in optic nerve head astrocytes exposed to extracellular ATP for an extended period. NTPDase1 was also elevated in the retinal tissue of rats, mice, and primates, and in the optic nerve of rats, with chronic elevation in IOP.

CONCLUSIONS

A sustained elevation in extracellular ATP, and upregulation of NTPDase1, occurs in the posterior eye of rat, mouse, and primate models of chronic glaucoma. This suggests the elevation in extracellular ATP may be sustained in chronic glaucoma, and implies a role for altered purinergic signaling in the disease.

Cardiomyogenesis of embryonic stem cells upon purinergic receptor activation by ADP and ATP

Purinergic signaling may be involved in embryonic development of the heart. In the present study, the effects of purinergic receptor stimulation on cardiomyogenesis of mouse embryonic stem (ES) cells were investigated. ADP or ATP increased the number of cardiac clusters and cardiac cells, as well as beating frequency. Cardiac-specific genes showed enhanced expression of α-MHC, MLC2v, α-actinin, connexin 45 (Cx45), and HCN4, on both gene and protein levels upon ADP/ATP treatment, indicating increased cardiomyogenesis and pacemaker cell differentiation. Real-time RT-PCR analysis of purinergic receptor expression demonstrated presence of P2X1, P2X4, P2X6, P2X7, P2Y1, P2Y2, P2Y4, and P2Y6 on differentiating ES cells. ATP and ADP as well as the P2X agonists β,γ-methylenadenosine 5'-triphosphate (β,γ-MetATP) and 8-bromoadenosine 5'-triphosphate (8-Br-ATP) but not UTP or UDP transiently increased the intracellular calcium concentration ([Ca(2+)](i)) as evaluated by the calcium indicator Fluo-4, whereas no changes in membrane potential were observed. [Ca(2+)](i) transients induced by ADP/ATP were abolished by the phospholipase C-β (PLC-β) inhibitor U-73122, suggesting involvement of metabotropic P2Y receptors. Furthermore, partial inhibition of [Ca(2+)](i) transients was achieved in presence of MRS2179, a selective P2Y1 receptor antagonist, whereas PPADS, a non-selective P2 receptor inhibitor, completely abolished the [Ca(2+)](i) response. Consequently, cardiomyocyte differentiation was decreased upon long term co-incubation of cells with ADP and P2 receptor antagonists. In summary, activation of purinoceptors and the subsequent [Ca(2+)](i) transients enhance the differentiation of ES cells toward cardiomyocytes. Purinergic receptor stimulation may be a promising strategy to drive the fate of pluripotent ES cells into a particular population of cardiomyocytes.

Advances in the pharmacology of lGICs auxiliary subunits

Ligand-gated ion channels (LGICs) are cell surface integral proteins that mediate the fast neurotransmission in the nervous system. LGICs require auxiliary subunits for their trafficking, assembly and pharmacological modulation. Auxiliary subunits do not form functional homomeric receptors, but are reported to assemble with the principal subunits in order to modulate their pharmacological profiles. For example, nACh receptors are built at least by co-assemble of α and β subunits, and the neuronal auxiliary subunits β3 and α5 and muscle type β, δ, γ, and ϵ determine the agonist affinity of these receptors. Serotonergic 5-HT3B, 5-HT3C, 5-HT3D and 5-HT3E are reported to assemble with the 5-HT3A subunit to modulate its pharmacological profile. Functional studies evaluating the role of γ2 and δ auxiliary subunits of GABAA receptors have made important advances in the understanding of the action of benzodiazepines, ethanol and neurosteroids. Glycine receptors are composed principally by α1-3 subunits and the auxiliary subunit β determines their synaptic location and their pharmacological response to propofol and ethanol. NMDA receptors appear to be functional as heterotetrameric channels. So far, the existence of NMDA auxiliary subunits is controversial. On the other hand, Kainate receptors are modulated by NETO 1 and 2. AMPA receptors are modulated by TARPs, Shisa 9, CKAMP44, CNIH2-3 auxiliary proteins reported that controls their trafficking, conductance and gating of channels. P2X receptors are able to associate with auxiliary Pannexin-1 protein to modulate P2X7 receptors. Considering the pharmacological relevance of different LGICs auxiliary subunits in the present work we will highlight the therapeutic potential of these modulator proteins.

Redox distress and genetic defects conspire in systemic autoinflammatory diseases

Inflammation is initiated by innate immune cell activation after contact with pathogens or tissue injury. An increasing number of observations have suggested that cellular stress, in the absence of infection or evident damage, can also induce inflammation. Thus, inflammation can be triggered by exogenous pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs)-so-called classic inflammation-or by endogenous stress resulting from tissue or cellular dysfunction. External triggers and cellular stress activate the same molecular pathways, possibly explaining why classic and stress-induced inflammation have similar clinical manifestations. In some systemic autoinflammatory diseases (SAIDs), inflammatory cells exhibit reduction-oxidation (redox) distress, having high levels of reactive oxygen species (ROS), which promote proinflammatory cytokine production and contribute to the subversion of mechanisms that self-limit inflammation. Thus, SAIDs can be viewed as a paradigm of stress-related inflammation, being characterized by recurrent flares or chronic inflammation (with no recognizable external triggers) and by a failure to downmodulate this inflammation. Here, we review SAID pathophysiology, focusing on the major cytokines and DAMPs, and on the key roles of redox distress. New therapeutic opportunities to tackle SAIDs by blocking stress-induced pathways and control the response to stress in patients are also discussed.

Extracellular ATP: a potential regulator of plant cell death

Adenosine 5'-triphosphate (ATP) has been regarded as an intracellular energy currency molecule for many years. In recent decades, it has been determined that ATP is released into the extracellular milieu by animal, plant and microbial cells. In animal cells, this extracellular ATP (eATP) functions as a signalling compound to mediate many cellular processes through its interaction with membrane-associated receptor proteins. It has also been reported that eATP is a signalling molecule required for the regulation of plant growth, development and responses to environmental stimuli. Recently, the first plant receptor for eATP was identified in Arabidopsis thaliana. Interestingly, some studies have shown that eATP is of particular importance in the control of plant cell death. In this review article, we summarize and discuss the theoretical and experimental advances that have been made with regard to the roles and mechanisms of eATP in plant cell death. We also make an attempt to address some speculative aspects to help develop and expand future research in this area.

Nucleotides Acting at P2Y Receptors: Connecting Structure and Function

Eight G protein-coupled P2Y receptor (P2YR) subtypes are important physiologic mediators. The human P2YRs are fully activated by ATP (P2Y2 and P2Y11), ADP (P2Y1, P2Y12, and P2Y13), UTP (P2Y2 and P2Y4), UDP (P2Y6 and P2Y14), and UDP glucose (P2Y14). Their structural elucidation is progressing rapidly. The X-ray structures of three ligand complexes of the Gi-coupled P2Y12R and two of the Gq-coupled P2Y1Rs were recently determined and will be especially useful in structure-based ligand design at two P2YR subfamilies. These high-resolution structures, which display unusual binding site features, complement mutagenesis studies for probing ligand recognition and activation. The structural requirements for nucleotide agonist recognition at P2YRs are relatively permissive with respect to the length of the phosphate moiety, but less so with respect to base recognition. Nucleotide-like antagonists and partial agonists are also known for P2Y1, P2Y2, P2Y4, and P2Y12Rs. Each P2YR subtype has the ability to be activated by structurally bifunctional agonists, such as dinucleotides, typically, dinucleoside triphosphates or tetraphosphates, and nucleoside polyphosphate sugars (e.g., UDP glucose) as well as the more conventional mononucleotide agonists. A range of dinucleoside polyphosphates, from triphosphates to higher homologs, occurs naturally. Earlier modeling predictions of the P2YRs were not very accurate, but recent findings have provided much detailed structural insight into this receptor family to aid in the rational design of new drugs.

Redox stress unbalances the inflammatory cytokine network: role in autoinflammatory patients and healthy subjects

The cell stress and redox responses are increasingly acknowledged as factors contributing to the generation and development of the inflammatory response. Several inflammation-inducing stressors have been identified, inside and outside of the cell. Furthermore, many hereditary diseases associate with inflammation and oxidative stress, suggesting a role for mutated proteins as stressors. The nucleotide-binding oligomerization domain, leucine-rich repeat-containing family, pyrin domain-containing 3 (NLRP3) inflammasome is an important node at the crossroad between redox response and inflammation. Remarkably, monocytes from patients with mutations in the NLRP3 gene undergo oxidative stress after stimulation with minute amounts of TLR agonists, resulting in unbalanced production of IL-1β and regulatory cytokines. Similar alterations in cytokine production are found in healthy monocytes upon TLR overstimulation. This mini-review summarizes recent progress in this field, discusses the molecular mechanisms underlying the loss of control of the cytokine network following oxidative stress, and proposes new therapeutic opportunities.

IRE1α/XBP1-mediated branch of the unfolded protein response regulates osteoclastogenesis

The unfolded protein response (UPR) is a cellular adaptive mechanism that is activated in response to the accumulation of unfolded proteins in the endoplasmic reticulum. The inositol-requiring protein-1α/X-box-binding protein-mediated (IRE1α/XBP1-mediated) branch of the UPR is highly conserved and has also been shown to regulate various cell-fate decisions. Herein, we have demonstrated a crucial role for the IREα/XBP1-mediated arm of the UPR in osteoclast differentiation. Using murine models, we found that the conditional abrogation of IRE1α in bone marrow cells increases bone mass as the result of defective osteoclastic bone resorption. In osteoclast precursors, IRE1α was transiently activated during osteoclastogenesis, and suppression of the IRE1α/XBP1 pathway in these cells substantially inhibited the formation of multinucleated osteoclasts in vitro. We determined that XBP1 directly binds the promoter and induces transcription of the gene encoding the master regulator of osteoclastogenesis nuclear factor of activated T cells cytoplasmic 1 (NFATc1). Moreover, activation of IRE1α was partially dependent on Ca2+ oscillation mediated by inositol 1,4,5-trisphosphate receptors 2 and 3 (ITPR2 and ITPR3) in the endoplasmic reticulum, as pharmacological inhibition or deletion of these receptors markedly decreased Xbp1 mRNA processing. The present study thus reveals an intracellular pathway that integrates the UPR and osteoclast differentiation through activation of the IRE1α/XBP1 pathway.

Hypoxia is an effective stimulus for vesicular release of ATP from human umbilical vein endothelial cells

Introduction

Hypoxia induces dilatation of the umbilical vein by releasing autocoids from endothelium; prostaglandins (PGs), adenosine and nitric oxide (NO) have been implicated. ATP is vasoactive, thus we tested whether hypoxia releases ATP from primary Human Umbilical Vein Endothelial Cells (HUVEC).

Methods

HUVEC were grown on inserts under no-flow conditions. ATP was assayed by luciferin–luciferase and visualised by quinacrine labeling. Intracellular Ca²⁺ ([Ca²⁺]_i) was imaged with Fura-2.

Results

ATP release occurred constitutively and was increased by hypoxia (PO₂: 150–8 mmHg), ∼10-fold more from apical, than basolateral surface. Constitutive ATP release was decreased, while hypoxia-induced release was abolished by brefeldin or monensin A, inhibitors of vesicular transport, and LY294002 or Y27632, inhibitors of phosphoinositide 3-kinases (PI₃K) and Rho-associated protein kinase (ROCK). ATP release was unaffected by NO donor, but increased by calcium ionophore, by >60-fold from apical, but <25% from basolateral surface. Hypoxia induced a small increase in [Ca²⁺]_i compared with ATP (10 μM); hypoxia inhibited the ATP response. Quinacrine-ATP fluorescent loci in the perinuclear space, were diminished by hypoxia and monensin, whereas brefeldin A increased fluorescence intensity, consistent with inhibition of anterograde transport.

Discussion.

Hypoxia within the physiological range releases ATP from HUVEC, particularly from apical/adluminal surfaces by exocytosis, via an increase in [Ca²⁺]_i, PI₃K and ROCK, independently of NO. We propose that hypoxia releases ATP at concentrations sufficient to induce umbilical vein dilation via PGs and NO and improve fetal blood flow, but curbs amplification of ATP release by autocrine actions of ATP, so limiting its pro-inflammatory effects.

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Hypoxia releases ATP from Human umbilical vein endothelial cells (HUVEC).

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This ATP release is preferentially from apical surfaces.

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Polarised ATP release is also triggered by Ca²⁺ ionophore.

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Hypoxia-induced ATP release occurs from vesicles, as visualised by quinacrine.

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It is attenuated by inhibitors of vesicular trafficking, PI₃K and ROCK.

5'-Ectonucleotidase/CD73 expression on lymph-circulating lymphocytes and lymphatic endothelial cells offers new paths to explore barrier function

5'-Nucleotidase/CD73 is a key enzyme in the regulation of purinergic signaling, hydrolyzing extracellular AMP to produce adenosine, which is critical in the blood vascular system and in immunosuppression. CD73 is expressed by both blood endothelial cells and lymphatic endothelial cells. Although the role of CD73 on blood endothelial cells in controlling vascular permeability and leukocyte trafficking has been studied, the role of lymphatic CD73 has thus far remained unknown. In this issue of European Journal of Immunology, Yegutkin et al. [Eur. J. Immunol. 2015. 45: 562-573] compare CD73 activity in the endothelia of lymphatics and blood vessels and investigate the CD73(+) lymphocyte subpopulations possibly involved in immunoregulation. This Commentary will discuss how the authors' work sheds light on the differential use of CD73 by these two cell populations to control endothelial permeability and sprouting.

Role of the P2Y13 receptor in the differentiation of bone marrow stromal cells into osteoblasts and adipocytes

Accumulating evidence indicates that extracellular nucleotides, signaling through purinergic receptors, play a significant role in bone remodeling. Mesenchymal stem cells (MSCs) express functional P2Y receptors whose expression level is regulated during osteoblast or adipocyte differentiation. P2Y13 -deficient mice were previously shown to exhibit a decreased bone turnover associated with a reduction in the number of both osteoblasts and osteoclasts on the bone surfaces. We therefore examined whether P2Y13 R activation was involved in the osteogenic differentiation of MSC. Our study demonstrated that ADP stimulation of P2Y13 R(+/+) (but not P2Y13 R(-/-) ) adherent bone marrow stromal cells (BMSCs) increased significantly the formation of alkaline phosphatase-colony-forming units (CFU-ALP) as well as the expression of osteoblastic markers (osterix, alkaline phosphatase, and collagen I) involved in the maturation of preosteoblasts into osteoblasts. The number of CFU-ALP obtained from P2Y13 R(-/-) BMSC and the level of osteoblastic gene expression after osteogenic stimulation were strongly reduced compared to those obtained in wild-type cell cultures. In contrast, when P2Y13 R(-/-) BMSCs were incubated in an adipogenic medium, the number of adipocytes generated and the level of adipogenic gene expression (PPARγ2 and Adipsin) were higher than those obtained in P2Y13 R(+/+) MSC. Interestingly, we observed a significant increase of the number of bone marrow adipocytes in tibia of P2Y13 R(-/-) mice. In conclusion, our findings indicate that the P2Y13 R plays an important role in the balance of osteoblast and adipocyte terminal differentiation of bone marrow progenitors. Therefore, the P2Y13 receptor can be considered as a new pharmacological target for the treatment of bone diseases like osteoporosis. STEM Cells 2013;31:2747-2758.

ATP signaling in skeletal muscle: from fiber plasticity to regulation of metabolism

Tetanic electrical stimulation releases adenosine triphosphate (ATP) from muscle fibers through pannexin-1 channels in a frequency-dependent manner; extracellular ATP activates signals that ultimately regulate gene expression and is able to increase glucose transport through activation of P2Y receptors, phosphatidylinositol 3-kinase, Akt, and AS160. We hypothesize that this mechanism is an important link between exercise and the regulation of muscle fiber plasticity and metabolism.

Role of transient receptor potential and pannexin channels in cigarette smoke-triggered ATP release in the lung

BACKGROUND

COPD is an inflammatory disease usually associated with cigarette smoking (CS) with an increasing global prevalence and no effective medication. Extracellular ATP is increased in the COPD affected lung and may play a key role in driving CS-induced airway inflammation, but the mechanism involved in ATP release has eluded researchers. Recently, the transient receptor potential (TRP) and pannexin-1 channels have been suggested to play a role in other experimental paradigms. Thus, the aim of this work is to investigate if these channels are involved in CS-induced ATP release in the lung.

METHODS

Primary human cells were exposed to CS and extracellular ATP levels measured. Mice were exposed to mainstream CS and airway inflammation assessed. TRPV1/4 mRNA expression was assessed in human lung parenchyma.

RESULTS

CS exposure caused a dose-related increase in ATP from primary airway bronchial epithelial cells. This was attenuated by blockers of TRPV1, TRPV4 and pannexin-1 channels. Parallel data was obtained using murine acute CS-driven model systems. Finally, TRPV1/4 mRNA expression was increased in lung tissue samples from patients with COPD.

CONCLUSIONS

Extracellular ATP is increased in the COPD affected lung and may play a key role in driving disease pathophysiology. These experiments uncover a novel mechanism which may be responsible for CS-induced ATP release. These findings highlight novel targets that could lead to the development of medicine to treat this devastating disease.

Extracellular UDP and P2Y6 function as a danger signal to protect mice from vesicular stomatitis virus infection through an increase in IFN-β production

Extracellular nucleotides that constitute a "danger signal" play an important role in the regulation of immune responses. However, the function and mechanism of extracellular UDP and P2Y6 in antiviral immunity remain unknown. In this study, we demonstrated the in vitro and in vivo protection of UDP/P2Y6 signaling in vesicular stomatitis virus (VSV) infection. First, we demonstrated that VSV-infected cells secrete UDP from the cytoplasm as a danger signal to arouse surrounding cells. Meanwhile, expression of the UDP-specific receptor P2Y6 also was enhanced by VSV. Consequently, UDP protects RAW 264.7 cells, murine embryonic fibroblasts, bone marrow-derived macrophages, and L929 cells from VSV and GFP lentivirus infection. This protection can be blocked by the P2Y6 selective antagonist MRS2578 or IFN-α/β receptor-blocking Ab. VSV-induced cell death and virus replication were both enhanced significantly by knocking down and knocking out P2Y6 in different cells. Mechanistically, UDP facilitates IFN-β secretion through the p38/JNK- and ATF-2/c-Jun-signaling pathways, which are crucial in promoting antiviral immunity. Interestingly, UDP was released through a caspase-cleaved pannexin-1 channel in VSV-induced apoptotic cells and protected cells from infection through P2Y6 receptor in an autocrine or paracrine manner. Furthermore, UDP also protected mice from VSV infection through P2Y6 receptors in an acute neurotropic infection mouse model. Taken together, these results demonstrate the important role of extracellular UDP and P2Y6 as a danger signal in antiviral immune responses and suggest a potential therapeutic role for UDP/P2Y6 in preventing and controlling viral diseases.

Toll-like receptor-triggered calcium mobilization protects mice against bacterial infection through extracellular ATP release

Extracellular ATP (eATP), released as a "danger signal" by injured or stressed cells, plays an important role in the regulation of immune responses, but the relationship between ATP release and innate immune responses is still uncertain. In this study, we demonstrated that ATP was released through Toll-like receptor (TLR)-associated signaling in both Escherichia coli-infected mice and lipopolysaccharide (LPS)- or Pam3CSK4-treated macrophages. This ATP release could be blocked completely only by N-ethylmaleimide (NEM), not by carbenoxolone (CBX), flufenamic acid (FFA), or probenecid, suggesting the key role of exocytosis in this process. Furthermore, LPS-induced ATP release could also be reduced dramatically through suppressing calcium mobilization by use of U73122, caffeine, and thapsigargin (TG). In addition, the secretion of interleukin-1β (IL-1β) and CCL-2 was enhanced significantly by ATP, in a time- and dose-dependent manner. Meanwhile, macrophage-mediated phagocytosis of bacteria was also promoted significantly by ATP stimulation. Furthermore, extracellular ATP reduced the number of invading bacteria and protected mice from peritonitis by activating purinergic receptors. Mechanistically, phosphorylation of AKT and ERK was overtly increased by ATP in antibacterial immune responses. Accordingly, if we blocked the P2X- and P2Y-associated signaling pathway by using suramin and pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid), tetrasodium salt (PPADS), the ATP-enhanced immune response was restrained significantly. Taken together, our findings reveal an internal relationship between danger signals and TLR signaling in innate immune responses, which suggests a potential therapeutic significance of calcium mobilization-mediated ATP release in infectious diseases.

The two-hit hypothesis for neuroinflammation: role of exogenous ATP in modulating inflammation in the brain

Brain inflammation is a common occurrence following responses to varied insults such as bacterial infections, stroke, traumatic brain injury and neurodegenerative disorders. A common mediator for these varied inflammatory responses is prostaglandin E₂ (PGE₂), produced by the enzymatic activity of cyclooxygenases (COX) 1 and 2. Previous attempts to reduce neuronal inflammation through COX inhibition, by use of nonsteroidal anti-inflammatory drugs (NSAIDs), have met with limited success. We are proposing the two-hit model for neuronal injury—an initial localized inflammation mediated by PGE₂ (first hit) and the simultaneous release of adenosine triphosphate (ATP) by injured cells (second hit), which significantly enhances the inflammatory response through increased synthesis of PGE₂. Several evidences on the role of exogenous ATP in inflammation have been reported, including contrary instances where extracellular ATP reduces inflammatory events. In this review, we will examine the current literature on the role of P2 receptors, to which ATP binds, in modulating inflammatory reactions during neurodegeneration. Targeting the P2 receptors, therefore, provides a therapeutic alternative to reduce inflammation in the brain. P2 receptor-based anti-inflammatory drugs (PBAIDs) will retain the activities of essential COX enzymes, yet will significantly reduce neuroinflammation by decreasing the enhanced production of PGE₂ by extracellular ATP.

Chemotherapeutic drugs induce ATP release via caspase-gated pannexin-1 channels and a caspase/pannexin-1-independent mechanism

Anti-tumor immune responses have been linked to the regulated release of ATP from apoptotic cancer cells to engage P2 purinergic receptor signaling cascades in nearby leukocytes. We used the Jurkat T cell acute lymphocytic leukemia model to characterize the role of pannexin-1 (Panx1) channels in the release of nucleotides during chemotherapeutic drug-induced apoptosis. Diverse pro-apoptotic drugs, including topoisomerase II inhibitors, kinase inhibitors, and proteosome inhibitors, induced functional activation of Panx1 channels via caspase-3-mediated cleavage of the Panx1 autoinhibitory C-terminal domain. The caspase-activated Panx1 channels mediated efflux of ATP, but also ADP and AMP, with the latter two comprising >90% of the released adenine nucleotide pool as cells transitioned from the early to late stages of apoptosis. Chemotherapeutic drugs also activated an alternative caspase- and Panx1-independent pathway for ATP release from Jurkat cells in the presence of benzyloxycarbonyl-VAD, a pan-caspase inhibitor. Comparison of Panx1 levels indicated much higher expression in leukemic T lymphocytes than in normal, untransformed T lymphoblasts. This suggests that signaling roles for Panx1 may be amplified in leukemic leukocytes. Together, these results identify chemotherapy-activated pannexin-1 channels and ATP release as possible mediators of paracrine interaction between dying tumor cells and the effector leukocytes that mediate immunogenic anti-tumor responses.

Responses of macrophages to the danger signals released from necrotic cells

The immune system maintains homeostasis by recognizing and responding to cell death caused by various stresses. The immune response is considered to be elicited by 'danger signals' released from necrotic cells. However, the identity of the danger signals remains elusive. In this study, we focused on the expression of chemokines by macrophages stimulated with necrotic cells. In mouse bone-marrow-derived macrophages, the chemokine monocyte chemoattractant protein (MCP)-3 was induced at both the mRNA and protein levels in response to heat-killed murine cells. The induction of MCP-3 was also observed in MyD88-deficient macrophages, indicating that Toll-like receptors and the IL-1 receptor are not involved in this response. Consistent with this observation, the activation of NF-κB was not detected in RAW264.7 macrophages stimulated with necrotic cells. Treatments with proteinase K, DNaseI or RNaseA did not affect the ' STIMULATING ACTIVITY': of necrotic cells. In contrast, treatment with apyrase, which removes phosphates from nucleoside tri- and di-phosphates, abolished the inducing activity. Purified UDP at 30 µM concentration elicited similar induction of MCP-3 in RAW264.7 macrophages. Small interfering RNA-mediated knock-down of the UDP receptor P2Y6 in RAW264.7 cells significantly reduced the induction of MCP-3 in response to necrotic cells, but not its induction by lipopolysaccharide. Furthermore, ectopic expression of the P2Y6 receptor in HEK293 cells conferred responsiveness to necrotic cells. These results suggest that UDP released by necrotic cells plays a critical role as an endogenous danger signal and that P2Y6 is required for the induction of MCP-3 in response to necrotic cells.

Regulation of extracellular ATP in human erythrocytes infected with Plasmodium falciparum

In human erythrocytes (h-RBCs) various stimuli induce increases in [cAMP] that trigger ATP release. The resulting pattern of extracellular ATP accumulation (ATPe kinetics) depends on both ATP release and ATPe degradation by ectoATPase activity. In this study we evaluated ATPe kinetics from primary cultures of h-RBCs infected with P. falciparum at various stages of infection (ring, trophozoite and schizont stages). A "3V" mixture containing isoproterenol (β-adrenergic agonist), forskolin (adenylate kinase activator) and papaverine (phosphodiesterase inhibitor) was used to induce cAMP-dependent ATP release. ATPe kinetics of r-RBCs (ring-infected RBCs), t-RBCs (trophozoite-infected RBCs) and s-RBCs (schizont-infected RBCs) showed [ATPe] to peak acutely to a maximum value followed by a slower time dependent decrease. In all intraerythrocytic stages, values of ΔATP1 (difference between [ATPe] measured 1 min post-stimulus and basal [ATPe]) increased nonlinearly with parasitemia (from 2 to 12.5%). Under 3V exposure, t-RBCs at parasitemia 94% (t94-RBCs) showed 3.8-fold higher ΔATP1 values than in h-RBCs, indicative of upregulated ATP release. Pre-exposure to either 100 µM carbenoxolone, 100 nM mefloquine or 100 µM NPPB reduced ΔATP1 to 83-87% for h-RBCs and 63-74% for t94-RBCs. EctoATPase activity, assayed at both low nM concentrations (300-900 nM) and 500 µM exogenous ATPe concentrations increased approx. 400-fold in t94-RBCs, as compared to h-RBCs, while intracellular ATP concentrations of t94-RBCs were 65% that of h-RBCs. In t94-RBCs, production of nitric oxide (NO) was approx. 7-fold higher than in h-RBCs, and was partially inhibited by L-NAME pre-treatment. In media with L-NAME, ΔATP1 values were 2.7-times higher in h-RBCs and 4.2-times higher in t94-RBCs, than without L-NAME. Results suggest that P. falciparum infection of h-RBCs strongly activates ATP release via Pannexin 1 in these cells. Several processes partially counteracted ATPe accumulation: an upregulated ATPe degradation, an enhanced NO production, and a decreased intracellular ATP concentration.

Adenosine 5'-triphosphate (ATP) inhibits schwann cell demyelination during Wallerian degeneration

Adenosine 5'-triphosphate (ATP) is implicated in intercellular communication as a neurotransmitter in the peripheral nervous system. In addition, ATP is known as lysosomal exocytosis activator. In this study, we investigated the role of extracellular ATP on demyelination during Wallerian degeneration (WD) using ex vivo and in vivo nerve degeneration models. We found that extracellular ATP inhibited myelin fragmentation and axonal degradation during WD. Furthermore, metformin and chlorpromazine, lysosomal exocytosis antagonists blocked the effect of ATP on the inhibition of demyelination. Thus, these findings indicate that ATP-induced-lysosomal exocytosis may be involved in demyelination during WD.

Purinergic signaling via P2X7 receptor mediates IL-1β production in Kupffer cells exposed to silica nanoparticle

There is extensive evidence that nanoparticles (NPs) cause adverse effects in multiple organs, including liver, though the mechanisms involved remain to be fully established. Kupffer cells are macrophages resident in the liver, and play important roles in liver inflammation induced by various toxic agents, including lipopolysaccharide (LPS). Interleukin-1 (IL-1) family members IL-1α,β are released from LPS-primed macrophages exposed to NPs, including silica NPs (SNPs), via activation of nucleotide-binding oligomerization domain-like receptor family pyrin domain-containing 3 inflammasomes. Here, we investigated the mechanism of production of IL-1β via activation of inflammasomes in mouse Kupffer cell line KUP5, focusing on the role of purinergic signaling via P2X7 receptor. IL-1β production by LPS-primed KUP5 cells exposed to SNPs was increased dose-dependently, and was greatest in response to SNPs with a diameter of 30 nm (SNP30), as compared with 70-nm and 300-nm SNPs (SNP70 and SNP300). ATP release was also highest in cells exposed to SNP30. Treatment of LPS-primed KUP5 cells with ATP also induced a high level of IL-1β production, similar to that induced by SNP30. IL-1β production was significantly inhibited by apyrase (an ecto-nucleotidase) and A438079 (a P2X7 antagonist/ATP-release inhibitor). Production of reactive oxygen species (ROS) was confirmed in cells exposed to SNP30. In conclusion, ATP released from P2X7 receptor in response to stimulation of KUP5 cells with SNP30 induces ROS production via cell-membrane NADPH oxidase. The ROS causes activation of inflammasomes, leading to caspase-1-dependent processing of IL-1β.

Inhibition of G protein-coupled P2Y2 receptor induced analgesia in a rat model of trigeminal neuropathic pain

BACKGROUNDS

ATP and P2X receptors play important roles in the modulation of trigeminal neuropathic pain, while the role of G protein-coupled P2Y₂ receptors and the underlying mechanisms are less clear. The threshold and frequency of action potentials, fast inactivating transient K+ channels (IA) are important regulators of membrane excitability in sensory neurons because of its vital role in the control of the spike onset. In this study, pain behavior tests, QT-RT-PCR, immunohistochemical staining, and patch-clamp recording, were used to investigate the role of P2Y₂ receptors in pain behaviour.

RESULTS

In control rats: 1) UTP, an agonist of P2Y₂/P2Y₄ receptors, caused a significant decrease in the mean threshold intensities for evoking action potentials and a striking increase in the mean number of spikes evoked by TG neurons. 2) UTP significantly inhibited IA and the expression of Kv1.4, Kv3.4 and Kv4.2 subunits in TG neurons, which could be reversed by the P2 receptor antagonist suramin and the ERK antagonist U0126. In ION-CCI (chronic constriction injury of infraorbital nerve) rats: 1) mRNA levels of Kv1.4, Kv3.4 and Kv4.2 subunits were significantly decreased, while the protein level of phosphorylated ERK was significantly increased. 2) When blocking P2Y₂ receptors by suramin or injection of P2Y2R antisense oligodeoxynucleotides both led to a time- and dose-dependent reverse of allodynia in ION-CCI rats. 3) Injection of P2Y₂ receptor antisense oligodeoxynucleotides induced a pronounced decrease in phosphorylated ERK expression and a significant increase in Kv1.4, Kv3.4 and Kv4.2 subunit expression in trigeminal ganglia.

CONCLUSIONS

Our data suggest that inhibition of P2Y₂ receptors leads to down-regulation of ERK-mediated phosphorylation and increase of the expression of I(A)-related Kv channels in trigeminal ganglion neurons, which might contribute to the clinical treatment of trigeminal neuropathic pain.

Extracellular ATP drives systemic inflammation, tissue damage and mortality

Systemic inflammatory response syndromes (SIRS) may be caused by both infectious and sterile insults, such as trauma, ischemia-reperfusion or burns. They are characterized by early excessive inflammatory cytokine production and the endogenous release of several toxic and damaging molecules. These are necessary to fight and resolve the cause of SIRS, but often end up progressively damaging cells and tissues, leading to life-threatening multiple organ dysfunction syndrome (MODS). As inflammasome-dependent cytokines such as interleukin-1β are critically involved in the development of MODS and death in SIRS, and ATP is an essential activator of inflammasomes in vitro, we decided to analyze the ability of ATP removal to prevent excessive tissue damage and mortality in a murine LPS-induced inflammation model. Our results indeed indicate an important pro-inflammatory role for extracellular ATP. However, the effect of ATP is not restricted to inflammasome activation at all. Removing extracellular ATP with systemic apyrase treatment not only prevented IL-1β accumulation but also the production of inflammasome-independent cytokines such as TNF and IL-10. In addition, ATP removal also prevented systemic evidence of cellular disintegration, mitochondrial damage, apoptosis, intestinal barrier disruption and even mortality. Although blocking ATP receptors with the broad-spectrum P2 purinergic receptor antagonist suramin imitated certain beneficial effects of apyrase treatment, it could not prevent morbidity or mortality at all. We conclude that removal of systemic extracellular ATP could be a valuable strategy to dampen systemic inflammatory damage and toxicity in SIRS.

P2Y2 receptor activation inhibits the expression of the sodium-chloride cotransporter NCC in distal convoluted tubule cells

Luminal nucleotide stimulation is known to reduce Na(+) transport in the distal nephron. Previous studies suggest that this mechanism may involve the thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC), which plays an essential role in NaCl reabsorption in the cells lining the distal convoluted tubule (DCT). Here we show that stimulation of mouse DCT (mDCT) cells with ATP or UTP promoted Ca(2+) transients and decreased the expression of NCC at both mRNA and protein levels. Specific siRNA-mediated silencing of P2Y2 receptors almost completely abolished ATP/UTP-induced Ca(2+) transients and significantly reduced ATP/UTP-induced decrease of NCC expression. To test whether local variations in the intracellular Ca(2+) concentration ([Ca(2+)]i) may control NCC transcription, we overexpressed the Ca(2+)-binding protein parvalbumin selectively in the cytosol or in the nucleus of mDCT cells. The decrease in NCC mRNA upon nucleotide stimulation was abolished in cells overexpressing cytosolic PV but not in cells overexpressing either a nuclear-targeted PV or a mutated PV unable to bind Ca(2+). Using a firefly luciferase reporter gene strategy, we observed that the activity of NCC promoter region from -1 to -2,200 bp was not regulated by changes in [Ca(2+)]i. In contrast, high cytosolic calcium level induced instability of NCC mRNA. We conclude that in mDCT cells: (1) P2Y2 receptor is essential for the intracellular Ca(2+) signaling induced by ATP/UTP stimulation; (2) P2Y2-mediated increase of cytoplasmic Ca(2+) concentration down-regulates the expression of NCC; (3) the decrease of NCC expression occurs, at least in part, via destabilization of its mRNA.

Cellular mechanisms of tissue fibrosis. 6. Purinergic signaling and response in fibroblasts and tissue fibrosis

Tissue fibrosis occurs as a result of the dysregulation of extracellular matrix (ECM) synthesis. Tissue fibroblasts, resident cells responsible for the synthesis and turnover of ECM, are regulated via numerous hormonal and mechanical signals. The release of intracellular nucleotides and their resultant autocrine/paracrine signaling have been shown to play key roles in the homeostatic maintenance of tissue remodeling and in fibrotic response post-injury. Extracellular nucleotides signal through P2 nucleotide and P1 adenosine receptors to activate signaling networks that regulate the proliferation and activity of fibroblasts, which, in turn, influence tissue structure and pathologic remodeling. An important component in the signaling and functional responses of fibroblasts to extracellular ATP and adenosine is the expression and activity of ectonucleotideases that attenuate nucleotide-mediated signaling, and thereby integrate P2 receptor- and subsequent adenosine receptor-initiated responses. Results of studies of the mechanisms of cellular nucleotide release and the effects of this autocrine/paracrine signaling axis on fibroblast-to-myofibroblast conversion and the fibrotic phenotype have advanced understanding of tissue remodeling and fibrosis. This review summarizes recent findings related to purinergic signaling in the regulation of fibroblasts and the development of tissue fibrosis in the heart, lungs, liver, and kidney.

Desensitization properties of P2X3 receptors shaping pain signaling

ATP-gated P2X3 receptors are mostly expressed by nociceptive sensory neurons and participate in transduction of pain signals. P2X3 receptors show a combination of fast desensitization onset and slow recovery. Moreover, even low nanomolar agonist concentrations unable to evoke a response, can induce desensitization via a phenomenon called “high affinity desensitization.” We have also observed that recovery from desensitization is agonist-specific and can range from seconds to minutes. The recovery process displays unusually high temperature dependence. Likewise, recycling of P2X3 receptors in peri-membrane regions shows unexpectedly large temperature sensitivity. By applying kinetic modeling, we have previously shown that desensitization characteristics of P2X3 receptor are best explained with a cyclic model of receptor operation involving three agonist molecules binding a single receptor and that desensitization is primarily developing from the open receptor state. Mutagenesis experiments suggested that desensitization depends on a certain conformation of the ATP binding pocket and on the structure of the transmembrane domains forming the ion pore. Further molecular determinants of desensitization have been identified by mutating the intracellular N- and C-termini of P2X3 receptor. Unlike other P2X receptors, the P2X3 subtype is facilitated by extracellular calcium that acts via specific sites in the ectodomain neighboring the ATP binding pocket. Thus, substitution of serine275 in this region (called “left flipper”) converts the natural facilitation induced by extracellular calcium to receptor inhibition. Given their strategic location in nociceptive neurons and unique desensitization properties, P2X3 receptors represent an attractive target for development of new analgesic drugs via promotion of desensitization aimed at suppressing chronic pain.

P2X receptors as targets for the treatment of status epilepticus

Prolonged seizures are amongst the most common neurological emergencies. Status epilepticus is a state of continuous seizures that is life-threatening and prompt termination of status epilepticus is critical to protect the brain from permanent damage. Frontline treatment comprises parenteral administration of anticonvulsants such as lorazepam that facilitate γ-amino butyric acid (GABA) transmission. Because status epilepticus can become refractory to anticonvulsants in a significant proportion of patients, drugs which act on different neurotransmitter systems may represent potential adjunctive treatments. P2X receptors are a class of ligand-gated ion channel activated by ATP that contributes to neuro- and glio-transmission. P2X receptors are expressed by both neurons and glia in various brain regions, including the hippocampus. Electrophysiology, pharmacology and genetic studies suggest certain P2X receptors are activated during pathologic brain activity. Expression of several members of the family including P2X₂, P2X₄, and P2X₇ receptors has been reported to be altered in the hippocampus following status epilepticus. Recent studies have shown that ligands of the P2X₇ receptor can have potent effects on seizure severity during status epilepticus and mice lacking this receptor display altered seizures in response to chemoconvulsants. Antagonists of the P2X₇ receptor also modulate neuronal death, microglial responses and neuroinflammatory signaling. Recent work also found altered neuronal injury and inflammation after status epilepticus in mice lacking the P2X₄ receptor. In summary, members of the P2X receptor family may serve important roles in the pathophysiology of status epilepticus and represent novel targets for seizure control and neuroprotection.

ATP released via pannexin-1 hemichannels mediates bladder overactivity triggered by urothelial P2Y6 receptors

In contrast to the well-known signaling role of urothelial ATP to control bladder function, the hypothesis that uracil nucleotides (UTP and/or UDP) also exert autocrine/paracrine actions only recently gained experimental support. Urothelial cells express UDP-sensitive P2Y6 receptors, yet their role in the control of bladder activity has been mostly neglected. This study was designed to investigate the ability of PSB0474, a stable UDP analogue which exhibits selectivity for P2Y6 receptors, to modulate urodynamic responses in the anaesthetized rat in vivo. Instillation of PSB0474 into the bladder increased the voiding frequency (VF) without affecting the amplitude (A) and the duration (Δt) of bladder contractions. PSB0474-induced bladder overactivity was prevented by the selective P2Y6 antagonist, MRS2578. The increase in the VF produced by PSB0474 was also blocked by inhibitors of pannexin-1 hemichannels, (10)Panx or carbenoxolone, when these drugs were applied inside the bladder lumen but not when they were administered intravenously. Reduction of hemichannels pore permeability with H1152 also prevented PSB0474-induced bladder overactivity, but the exocytosis inhibitor, Exo-1, was inactive. PSB0474 increased by 3-fold the urinary ATP content. Implication of hemichannels permeability on PSB0474-induced ATP release was demonstrated by real-time fluorescence video-microscopy measuring the uptake of propidium iodide by intact urothelial cells in the absence and in the presence of MRS2578 or carbenoxolone. Confocal microscopy studies confirmed the co-localization of pannexin-1 and P2Y6 receptors in the rat urothelium. Data indicate that activation of P2Y6 receptors causes bladder overactivity in the anaesthetized rat indirectly by releasing ATP from the urothelium via pannexin-1 hemichannels.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Regulation of cellular gas exchange, oxygen sensing, and metabolic control

Cells must continuously monitor and couple their metabolic requirements for ATP utilization with their ability to take up O2 for mitochondrial respiration. When O2 uptake and delivery move out of homeostasis, cells have elaborate and diverse sensing and response systems to compensate. In this review, we explore the biophysics of O2 and gas diffusion in the cell, how intracellular O2 is regulated, how intracellular O2 levels are sensed and how sensing systems impact mitochondrial respiration and shifts in metabolic pathways. Particular attention is paid to how O2 affects the redox state of the cell, as well as the NO, H2S, and CO concentrations. We also explore how these agents can affect various aspects of gas exchange and activate acute signaling pathways that promote survival. Two kinds of challenges to gas exchange are also discussed in detail: when insufficient O2 is available for respiration (hypoxia) and when metabolic requirements test the limits of gas exchange (exercising skeletal muscle). This review also focuses on responses to acute hypoxia in the context of the original "unifying theory of hypoxia tolerance" as expressed by Hochachka and colleagues. It includes discourse on the regulation of mitochondrial electron transport, metabolic suppression, shifts in metabolic pathways, and recruitment of cell survival pathways preventing collapse of membrane potential and nuclear apoptosis. Regarding exercise, the issues discussed relate to the O2 sensitivity of metabolic rate, O2 kinetics in exercise, and influences of available O2 on glycolysis and lactate production.

Current status and proposed roles for nitric oxide as a key mediator of the effects of extracellular nucleotides on plant growth

Recent data indicate that nucleotides are released into the extracellular matrix during plant cell growth, and that these extracellular nucleotides induce signaling changes that can, in a dose-dependent manner, increase or decrease the cell growth. After activation of a presumed receptor, the earliest signaling change induced by extracellular nucleotides is an increase in the concentration of cytosolic Ca(2+), but rapidly following this change is an increase in the cellular level of nitric oxide (NO). In Arabidopsis, mutants deficient in nitrate reductase activity (nia1nia2) have drastically reduced nitric oxide production and cannot transduce the effects of applied nucleotides into growth changes. Both increased levels of extracellular nucleotides and increased NO production inhibit auxin transport and inhibit growth, and these effects are potentially due to disruption of the localization and/or function of auxin transport facilitators. However, because NO- and auxin-induced signaling pathways can intersect at multiple points, there may be diverse ways by which the induction of NO by extracellular ATP could modulate auxin signaling and thus influence growth. This review will discuss these optional mechanisms and suggest possible regulatory routes based on current experimental data and predictive computational analyses.

Bradykinin-induced Ca2+ signaling in human subcutaneous fibroblasts involves ATP release via hemichannels leading to P2Y12 receptors activation

Background

Chronic musculoskeletal pain involves connective tissue remodeling triggered by inflammatory mediators, such as bradykinin. Fibroblast cells signaling involve changes in intracellular Ca²⁺ ([Ca²⁺]_i). ATP has been related to connective tissue mechanotransduction, remodeling and chronic inflammatory pain, via P2 purinoceptors activation. Here, we investigated the involvement of ATP in bradykinin-induced Ca²⁺ signals in human subcutaneous fibroblasts.

Results

Bradykinin, via B₂ receptors, caused an abrupt rise in [Ca²⁺]_i to a peak that declined to a plateau, which concentration remained constant until washout. The plateau phase was absent in Ca²⁺-free medium; [Ca²⁺]_i signal was substantially reduced after depleting intracellular Ca²⁺ stores with thapsigargin. Extracellular ATP inactivation with apyrase decreased the [Ca²⁺]_i plateau. Human subcutaneous fibroblasts respond to bradykinin by releasing ATP via connexin and pannexin hemichannels, since blockade of connexins, with 2-octanol or carbenoxolone, and pannexin-1, with ¹⁰Panx, attenuated bradykinin-induced [Ca²⁺]_i plateau, whereas inhibitors of vesicular exocytosis, such as brefeldin A and bafilomycin A1, were inactive. The kinetics of extracellular ATP catabolism favors ADP accumulation in human fibroblast cultures. Inhibition of ectonucleotidase activity and, thus, ADP formation from released ATP with POM-1 or by Mg²⁺ removal from media reduced bradykinin-induced [Ca²⁺]_i plateau. Selective blockade of the ADP-sensitive P2Y₁₂ receptor with AR-C66096 attenuated bradykinin [Ca²⁺]_i plateau, whereas the P2Y₁ and P2Y₁₃ receptor antagonists, respectively MRS 2179 and MRS 2211, were inactive. Human fibroblasts exhibited immunoreactivity against connexin-43, pannexin-1 and P2Y₁₂ receptor.

Conclusions

Bradykinin induces ATP release from human subcutaneous fibroblasts via connexin and pannexin-1-containing hemichannels leading to [Ca²⁺]_i mobilization through the cooperation of B₂ and P2Y₁₂ receptors.

Cellular and molecular mechanisms of dental nociception

Due, in part, to the unique structure of the tooth, dental pain is initiated via distinct mechanisms. Here we review recent advances in our understanding of inflammatory tooth pain and discuss 3 hypotheses proposed to explain dentinal hypersensitivity: The first hypothesis, supported by functional expression of temperature-sensitive transient receptor potential channels, emphasizes the direct transduction of noxious temperatures by dental primary afferent neurons. The second hypothesis, known as hydrodynamic theory, attributes dental pain to fluid movement within dentinal tubules, and we discuss several candidate cellular mechanical transducers for the detection of fluid movement. The third hypothesis focuses on the potential sensory function of odontoblasts in the detection of thermal or mechanical stimuli, and we discuss the accumulating evidence that supports their excitability. We also briefly update on a novel strategy for local nociceptive anesthesia via nociceptive transducer molecules in dental primary afferents with the potential to specifically silence pain fibers during dental treatment. Further understanding of the molecular mechanisms of dental pain would greatly enhance the development of therapeutics that target dental pain.

The mechanisms of energy crisis in human astrocytes after subarachnoid hemorrhage

BACKGROUND

Calcium (Ca2+) is a cofactor of multiple cellular processes. The mechanisms that lead to elevated cytosolic Ca2+ concentration are unclear.

OBJECTIVE

To illuminate how bloody cerebrospinal fluid (bCSF) from patients with intraventricular hemorrhage causes cell death of cultured human astrocytes.

METHODS

Cultured astrocytes were incubated with bCSF. In control experiments, native CSF was used. Cytosolic Ca2+ concentration was measured by fura-2 fluorescence. Apoptosis and necrosis were evaluated by staining with Hoechst-3342 and propidium iodide.

RESULTS

Incubation of astrocytes with bCSF provoked a steep Ca2+ concentration peak that was followed by a slow Ca2+ rise during the observation period of 50 minutes. Necrosis, but not apoptosis, was induced. Blockade of ATP-sensitive P2 receptors with suramin inhibited the bCSF-induced initial Ca2+ peak and necrosis. Blockade of P1 receptors with 8-phenyltheophylline or of N-methyl-D-aspartate receptors with D(-)-2-amino-5-phosphopentanoic acid had no significant effect. Preincubation with xestospongin D, a blocker of inositol 1,4,5-trisphosphate receptors, prevented the initial Ca2+ rise and reduced the rate of necrosis. Preemptying of the endoplasmic reticulum with thapsigargin protected astrocytes from the bCSF-induced Ca2+ peak. Inhibition of mitochondrial permeability transition pores opening with cyclosporin A reduced the rate of astrocytic necrosis significantly, although it did not influence the initial Ca peak.

CONCLUSION

bCSF elicits a steep, transient Ca rise when administered to human astrocytes by activation of ATP-sensitive P2 receptors and subsequent inositol 1,4,5-trisphosphate-dependent Ca release from endoplasmic reticulum. This massive Ca overload leads to subsequent mitochondrial permeability transition pores opening and necrosis of the cells.

Loss of ectonucleotidases from the coronary vascular bed after ischemia-reperfusion in isolated rat heart

Background

Ectonucleotidase plays an important role in the regulation of cardiac function by controlling extracellular levels of adenine nucleotides and adenosine. To determine the influence of ischemia-reperfusion injury on ectonucleotidase activity in coronary vascular bed, we compared the metabolic profile of adenine nucleotides during the coronary circulation in pre- and post-ischemic heart.

Methods

Langendorff-perfused rat hearts were used to assess the intracoronary metabolism of adenine nucleotides. The effects of ischemia on the adenine nucleotide metabolism were examined after 30 min of ischemia and 30 min of reperfusion. Adenine nucleotide metabolites were measured by high performance liquid chromatography.

Results

ATP, ADP and AMP were rapidly metabolized to adenosine and inosine during the coronary circulation. After ischemia, ectonucleotidase activity of the coronary vascular bed was significantly decreased. In addition, the perfusate from the ischemic heart contained a considerable amount of enzymes degrading ATP, AMP and adenosine. Immunoblot analysis revealed that the perfusate from the ischemic heart dominantly contained ectonucleoside triphosphate diphosphohydrolase 1, and, to a lesser extent, ecto-5’-nucleotidase. The leakage of nucleotide metabolizing enzymes from the coronary vascular bed by ischemia-reperfusion was more remarkable in aged rats, in which post-ischemic cardiac dysfunction was more serious.

Conclusion

Ectonucleotidases were liberated from the coronary vascular bed by ischemia-reperfusion, resulting in an overall decrease in ectonucleotidase activity in the post-ischemic coronary vascular bed. These results suggest that decreased ectonucleotidase activity by ischemia may exacerbate subsequent reperfusion injury, and that levels of circulating ectonucleotidase may reflect the severity of ischemic vascular injury.

Mechanical stimulation-induced calcium wave propagation in cell monolayers: the example of bovine corneal endothelial cells

Intercellular communication is essential for the coordination of physiological processes between cells in a variety of organs and tissues, including the brain, liver, retina, cochlea and vasculature. In experimental settings, intercellular Ca(2+)-waves can be elicited by applying a mechanical stimulus to a single cell. This leads to the release of the intracellular signaling molecules IP3 and Ca(2+) that initiate the propagation of the Ca(2+)-wave concentrically from the mechanically stimulated cell to the neighboring cells. The main molecular pathways that control intercellular Ca(2+)-wave propagation are provided by gap junction channels through the direct transfer of IP3 and by hemichannels through the release of ATP. Identification and characterization of the properties and regulation of different connexin and pannexin isoforms as gap junction channels and hemichannels are allowed by the quantification of the spread of the intercellular Ca(2+)-wave, siRNA, and the use of inhibitors of gap junction channels and hemichannels. Here, we describe a method to measure intercellular Ca(2+)-wave in monolayers of primary corneal endothelial cells loaded with Fluo4-AM in response to a controlled and localized mechanical stimulus provoked by an acute, short-lasting deformation of the cell as a result of touching the cell membrane with a micromanipulator-controlled glass micropipette with a tip diameter of less than 1 μm. We also describe the isolation of primary bovine corneal endothelial cells and its use as model system to assess Cx43-hemichannel activity as the driven force for intercellular Ca(2+)-waves through the release of ATP. Finally, we discuss the use, advantages, limitations and alternatives of this method in the context of gap junction channel and hemichannel research.

ATP release and Ca2+ signalling by human bronchial epithelial cells following Alternaria aeroallergen exposure

  Exposure of human bronchial epithelial (HBE) cells from normal and asthmatic subjects to extracts from Alternaria alternata evoked a rapid and sustained release of ATP with greater efficacy observed in epithelial cells from asthmatic patients. Previously, Alternaria allergens were shown to produce a sustained increase in intracellular Ca2+ concentration ([Ca2+]i) that was dependent on the coordinated activation of specific purinergic receptor (P2Y2 and P2X7) subtypes. In the present study, pretreatment with a cell-permeable Ca2+-chelating compound (BAPTA-AM) significantly inhibited ATP release, indicating dependency on [Ca2+]i. Alternaria-evoked ATP release exhibited a greater peak response and a slightly lower EC50 value in cells obtained from asthmatic donors compared to normal control cells. Furthermore, the maximum increase in [Ca2+]i resulting from Alternaria treatment was greater in cells from asthmatic patients compared to normal subjects. The vesicle transport inhibitor brefeldin A and BAPTA-AM significantly blocked Alternaria-stimulated incorporation of fluorescent lipid (FM1-43)-labelled vesicles into the plasma membrane and ATP release. In addition, inhibiting uptake of ATP into exocytotic vesicles with bafilomycin also reduced ATP release comparable to the effects of brefeldin A and BAPTA-AM. These results indicate that an important mechanism for Alternaria-induced ATP release is Ca2+ dependent and involves exocytosis of ATP. Serine and cysteine protease inhibitors also reduced Alternaria-induced ATP release; however, the sustained increase in [Ca2+]i typically observed following Alternaria exposure appeared to be independent of protease-activated receptor (PAR2) stimulation.

Inflammation and immune response in COPD: where do we stand?

Increasing evidence indicates that chronic inflammatory and immune responses play key roles in the development and progression of COPD. Recent data provide evidence for a role in the NLRP3 inflammasome in the airway inflammation observed in COPD. Cigarette smoke activates innate immune cells by triggering pattern recognition receptors (PRRs) to release “danger signal”. These signals act as ligands to Toll-like receptors (TLRs), triggering the production of cytokines and inducing innate inflammation. In smokers who develop COPD there appears to be a specific pattern of inflammation in the airways and parenchyma as a result of both innate and adaptive immune responses, with the predominance of CD8+ and CD4+ cells, and in the more severe disease, with the presence of lymphoid follicles containing B lymphocytes and T cells. Furthermore, viral and bacterial infections interfere with the chronic inflammation seen in stable COPD and exacerbations via pathogen-associated molecular patterns (PAMPs). Finally, autoimmunity is another novel aspect that may play a critical role in the pathogenesis of COPD. This review is un update of the currently discussed roles of inflammatory and immune responses in the pathogenesis of COPD.

Mediator mechanisms involved in TRPV1, TRPA1 and P2X receptor-mediated sensory transduction of pulmonary ROS by vagal lung C-fibers in rats

We investigated the mediator mechanisms involved in the sensory transduction of pulmonary reactive oxygen species (ROS) by vagal lung C-fibers in anesthetized rats. Airway challenge of aerosolized H₂O₂ (0.4%) stimulated these afferent fibers. The H₂O₂-induced responses were reduced by a cyclooxygenase inhibitor or ATP scavengers and also attenuated by an antagonist of TRPV1, TRPA1 or P2X receptors. The suppressive effect of the cyclooxygenase inhibitor was not affected by a combined treatment with the TRPV1 or TRPA1 antagonist, but was amplified by a combined treatment with the P2X antagonists. The suppressive effect of ATP scavengers was not affected by a combined treatment with the P2X antagonist, but was amplified by a combined treatment with the TRPV1 or TRPA1 antagonist. Thus, the actions of cyclooxygenase metabolites are mediated through the functioning of the TRPV1 and TRPA1 receptors, whereas the action of ATP is mediated through the functioning of P2X receptors.