Activation of UTP-sensitive P2Y2 receptor induces the expression of cholinergic genes in cultured cortical neurons: a signaling cascade triggered by Ca2+ mobilization and extracellular regulated kinase phosphorylation

Quantitative Super-Resolution Imaging for the Analysis of GPCR Oligomerization

G-protein coupled receptors (GPCRs) are known to form homo- and hetero- oligomers which are considered critical to modulate their function. However, studying the existence and functional implication of these complexes is not straightforward as controversial results are obtained depending on the method of analysis employed. Here, we use a quantitative single molecule super-resolution imaging technique named qPAINT to quantify complex formation within an example GPCR. qPAINT, based upon DNA-PAINT, takes advantage of the binding kinetics between fluorescently labelled DNA imager strands to complementary DNA docking strands coupled to protein targeting antibodies to quantify the protein copy number in nanoscale dimensions. We demonstrate qPAINT analysis via a novel pipeline to study the oligomerization of the purinergic receptor Y2 (P2Y₂), a rhodopsin-like GPCR, highly expressed in the pancreatic cancer cell line AsPC-1, under control, agonistic and antagonistic conditions. Results reveal that whilst the density of P2Y₂ receptors remained unchanged, antagonistic conditions displayed reduced percentage of oligomers, and smaller numbers of receptors in complexes. Yet, the oligomeric state of the receptors was not affected by agonist treatment, in line with previous reports. Understanding P2Y₂ oligomerization under agonistic and antagonistic conditions will contribute to unravelling P2Y₂ mechanistic action and therapeutic targeting.

Astrocyte-mediated purinergic signaling is upregulated in a mouse model of Fragile X syndrome

Fragile X syndrome (FXS) is the leading monogenic cause of intellectual disability and autism spectrum disorders. With increasing investigation into the molecular mechanisms underlying FXS, there is growing evidence that perturbations in glial signaling are widely associated with neurological pathology. Purinergic signaling, which utilizes nucleoside triphosphates as signaling molecules, provides one of the most ubiquitous signaling systems for glial-neuronal and glial-glial crosstalk. Here, we sought to identify whether purinergic signaling is dysregulated within the FXS mouse cortex, and whether this dysregulation contributes to aberrant intercellular communication. In primary astrocyte cultures derived from the Fmr1 knockout (KO) mouse model of FXS, we found that application of exogenous ATP and UTP evoked elevated intracellular calcium responses compared to wildtype levels. Accordingly, purinergic P2Y₂ and P2Y₆ receptor expression was increased in Fmr1 KO astrocytes both in vitro and in acutely dissociated tissue, while P2Y antagonism via suramin prevented intracellular calcium elevations, suggesting a role for these receptors in aberrant FXS astrocyte activation. To investigate the impact of elevated purinergic signaling on astrocyte-mediated synaptogenesis, we quantified synaptogenic protein TSP-1, known to be regulated by P2Y activation. TSP-1 secretion and expression were both heightened in Fmr1 KO vs wildtype astrocytes following UTP application, while naïve TSP-1 cortical expression was also transiently elevated in vivo, indicating increased potential for excitatory TSP-1-mediated synaptogenesis in the FXS cortex. Together, our results demonstrate novel and significant purinergic signaling elevations in Fmr1 KO astrocytes, which may serve as a potential therapeutic target to mitigate the signaling aberrations observed in FXS.

Regulation of CD39 expression in ATP-P2Y2R-mediated alcoholic liver steatosis and inflammation

Inflammation plays a central role in the progression of alcoholic liver disease. ATP-P2Y2R signaling and CD39 play an important role in various diseases, but little is known about their role in alcoholic liver steatosis and inflammation. As a transmembrane hydrolase, CD39 hydrolyzes ATP, while the mutual regulation of CD39 and ATP-P2Y2R in alcoholic steatohepatitis is poorly understood. Here, we found that the expression of ATP, P2Y2R, and CD39 is increased significantly both in the liver of alcohol-fed mice and alcohol-induced RAW264.7 cell lines. In this study, C57BL/6 mice were intrapretationally injected with P2Y2R inhibitor suramin from day 4 until day 10 during the induction of a chronic/binge drinking model. Pharmacological blockade of P2Y2R largely prevents liver damage, lipid accumulation, and inflammation, with concomitant down-expression of CD39 in liver. We found that the inhibition of P2Y2R in vitro reduces inflammation via down-expression of interleukin 6 (IL-6), interleukin-1β (IL-1β), and tumor necrosis factor-alpha (TNF-α), and the expression of CD39 was reduced, whereas the activation of P2Y2R showed an opposite effect. Silencing of CD39 promoted the expression of ATP and P2Y2R. These results indicate that CD39 attenuates alcohol-induced steatohepatitis by scavenging extracellular ATP to indirectly regulate the expression of P2Y2R. Interestingly, P2Y2R paradoxically boosts CD39 activity. Thus, blockade of the extracellular ATP-P2Y2R signalling represents a potential therapeutic approach against alcoholic liver disease, and CD39 is a potential therapeutic target.

Dystrophic mdx mouse myoblasts exhibit elevated ATP/UTP-evoked metabotropic purinergic responses and alterations in calcium signalling

Pathophysiology of Duchenne Muscular Dystrophy (DMD) is still elusive. Although progressive wasting of muscle fibres is a cause of muscle deterioration, there is a growing body of evidence that the triggering effects of DMD mutation are present at the earlier stage of muscle development and affect myogenic cells. Among these abnormalities, elevated activity of P2X7 receptors and increased store-operated calcium entry myoblasts have been identified in mdx mouse. Here, the metabotropic extracellular ATP/UTP-evoked response has been investigated. Sensitivity to antagonist, effect of gene silencing and cellular localization studies linked these elevated purinergic responses to the increased expression of P2Y2 but not P2Y4 receptors. These alterations have physiological implications as shown by reduced motility of mdx myoblasts upon treatment with P2Y2 agonist. However, the ultimate increase in intracellular calcium in dystrophic cells reflected complex alterations of calcium homeostasis identified in the RNA seq data and with significant modulation confirmed at the protein level, including a decrease of Gq11 subunit α, plasma membrane calcium ATP-ase, inositol-2,4,5-trisphosphate-receptor proteins and elevation of phospholipase Cβ, sarco-endoplamatic reticulum calcium ATP-ase and sodium‑calcium exchanger. In conclusion, whereas specificity of dystrophic myoblast excitation by extracellular nucleotides is determined by particular receptor overexpression, the intensity of such altered response depends on relative activities of downstream calcium regulators that are also affected by Dmd mutations. Furthermore, these phenotypic effects of DMD emerge as early as in undifferentiated muscle. Therefore, the pathogenesis of DMD and the relevance of current therapeutic approaches may need re-evaluation.

Sepsis decreases the activity of acetylcholinesterase by reducing its expression at the neuromuscular junction

Our previous study demonstrated that sepsis may decrease the activity of acetylcholinesterase (AChE) at the neuromuscular junction (NMJ) of the diaphragm at 24 h, and thus improve the antagonistic action of neostigmine on rocuronium. The present study aimed to determine the effects of sepsis on AChE activity over 2 weeks, which is a more clinically relevant time period. Furthermore, the present study aimed to elucidate the association between AChE activity and its expression at the NMJ during sepsis. Male adult Sprague‑Dawley rats were randomly divided into the sham or sepsis groups. Sepsis was induced by cecal ligation and puncture. On days 1, 3, 7 and 14 after surgery, AChE activity at the NMJ of the diaphragm was detected using a modified Karnovsky and Roots method. Furthermore, AChE expression levels at the NMJ, and in the whole muscle fibers of the diaphragm, were detected by immunohistofluorescence staining and western blot analysis, respectively. AChE activity was significantly decreased in the sepsis group, with its lowest level detected on day 7; however, its activity had partially recovered on day 14 (P<0.01). AChE activity was positively correlated (r=0.975, P=0.025) with its expression at the NMJ, which showed a similar trend over 2 weeks of sepsis. The protein expression levels of AChE in the whole muscle fibers of the diaphragm were significantly decreased on days 1, 3 and 7 in the sepsis group (P<0.01), with the lowest level observed on day 3. In conclusion, sepsis decreased AChE activity by reducing its expression at the NMJ over 14 days; the reduced expression of AChE at the NMJ might be as a result of its reduced muscular production.

Regulation of proteasome activity by P2Y2 receptor underlies the neuroprotective effects of extracellular nucleotides

The Ubiquitin-Proteasome System (UPS) is essential for the regulation of the cellular proteostasis. Indeed, it has been postulated that an UPS dysregulation is the common mechanism that underlies several neurological disorders. Considering that extracellular nucleotides, through their selective P2Y₂ receptor (P2Y₂R), play a neuroprotective role in various neurological disorders that course with an UPS impairment, we wonder if this neuroprotective capacity resulted from their ability to modulate the UPS. Using a cellular model expressing two different UPS reporters, we found that the stimulation of P2Y₂R by its selective agonist Up₄U induced a significant reduction of UPS reporter levels. This reduction was due to an increase in two of the three peptidase proteasome activities, chymotrypsin and postglutamyl, caused by an increased expression of proteasome constitutive catalytic subunits β1 and β5. The intracellular signaling pathway involved required the activation of IP₃/MEK1/2/ERK but was independent of PKC or PKA. Interestingly, the P2Y₂R activation was able to revert both UPS-reporter accumulation and the cell death induced by a prolonged inhibition of UPS. Finally, we also observed that intracerebroventricular administration of Up₄U induced a significant increase both of chymotrypsin and postglutamyl activities as well as an increased expression of proteasome subunits β1 and β5 in the hippocampus of wild-type mice, but not in P2Y₂R KO mice. All these results strongly suggest that the capacity to modulate the UPS activity via P2Y₂R is the molecular mechanism which is how the nucleotides play a neuroprotective role in neurological disorders.

New perspectives for multi-level regulations of neuronal acetylcholinesterase by dioxins

Acetylcholinesterase (AChE; EC 3.1.1.7) is a vital functional enzyme in cholinergic neurotransmission which can rapidly hydrolyze neurotransmitter, acetylcholine, in the central and peripheral nervous systems. Emerging evidence showed that in addition to classical environmental AChE inhibitors, e.g. organophosphate and carbamate pesticides, dioxins are a new type of xenobiotic causing impairment of AChE. Dioxin can transcriptionally or post-transcriptionally suppress AChE expression in human neuroblastoma cells or mouse immune cells via the aryl hydrocarbon receptor (AhR) pathway, respectively. Dioxins can affect gene expression through other mechanisms, such as cross-talk with other signaling cascades and epigenetic modulations. Therefore, in this review, by summarizing the known mechanisms of AChE regulation and dioxin-induced gene alteration, potential signaling cascades and epigenetic mechanisms are proposed for dioxin-mediated AChE regulation. Mitogen activated protein (MAP) kinase, 3', 5'-cyclic adenosine monophosphate (cAMP) and calcium-related singaling pathways, as well as potential epigenetic mechanisms, such as DNA methylation, and post-transcriptional regulation via microRNAs, including hsa-miR-132, hsa-miR-212 and hsa-miR-25-3p are discussed here. These proposed mechanisms may be invaluable not only to promote comprehensive understanding of the action mechanisms for dioxin, but to illustrate the molecular basis of dioxin-induced health impacts.

Caveolin-1 Regulates the P2Y2 Receptor Signaling in Human 1321N1 Astrocytoma Cells

Damage to the CNS can cause a differential spatio-temporal release of multiple factors, such as nucleotides, ATP and UTP. The latter interact with neuronal and glial nucleotide receptors. The P2Y2 nucleotide receptor (P2Y2R) has gained prominence as a modulator of gliotic responses after CNS injury. Still, the molecular mechanisms underlying these responses in glia are not fully understood. Membrane-raft microdomains, such as caveolae, and their constituent caveolins, modulate receptor signaling in astrocytes; yet, their role in P2Y2R signaling has not been adequately explored. Hence, this study evaluated the role of caveolin-1 (Cav-1) in modulating P2Y2R subcellular distribution and signaling in human 1321N1 astrocytoma cells. Recombinant hP2Y2R expressed in 1321N1 cells and Cav-1 were found to co-fractionate in light-density membrane-raft fractions, co-localize via confocal microscopy, and co-immunoprecipitate. Raft localization was dependent on ATP stimulation and Cav-1 expression. This hP2Y2R/Cav-1 distribution and interaction was confirmed with various cell model systems differing in the expression of both P2Y2R and Cav-1, and shRNA knockdown of Cav-1 expression. Furthermore, shRNA knockdown of Cav-1 expression decreased nucleotide-induced increases in the intracellular Ca(2+) concentration in 1321N1 and C6 glioma cells without altering TRAP-6 and carbachol Ca(2+) responses. In addition, Cav-1 shRNA knockdown also decreased AKT phosphorylation and altered the kinetics of ERK1/2 activation in 1321N1 cells. Our findings strongly suggest that P2Y2R interaction with Cav-1 in membrane-raft caveolae of 1321N1 cells modulates receptor coupling to its downstream signaling machinery. Thus, P2Y2R/Cav-1 interactions represent a novel target for controlling P2Y2R function after CNS injury.

Reduced Expression of P2Y2 Receptor and Acetylcholinesterase at Neuromuscular Junction of P2Y1 Receptor Knock-out Mice

ATP is co-stored and co-released with acetylcholine (ACh) at the pre-synaptic vesicles in vertebrate neuromuscular junction (nmj). Several lines of studies demonstrated that binding of ATP to its corresponding P2Y1 and P2Y2 receptors in the muscle regulated post-synaptic gene expressions. To further support the notion that P2Y receptors are playing indispensable role in formation of post-synaptic specifications at the nmj, the knock-out mice of P2Y1 receptor (P2Y1R (-/-)) were employed here for analyses. In P2Y1R (-/-) mice, the expression of P2Y2 receptor in muscle was reduced by over 50 %, as compared to P2Y1R (+/+) mice. In parallel, the expression of acetylcholinesterase (AChE) in muscle was markedly decreased. In the analysis of the expression of anchoring subunits of AChE in P2Y1R (-/-) mice, the proline-rich membrane anchor (PRiMA) subunit was reduced by 60 %; while the collagen tail (ColQ) subunit was reduced by 50 %. AChE molecular forms in the muscle were not changed, except the amount of enzyme was reduced. Immuno-staining of P2Y1R (-/-) mice nmj, both AChE and AChR were still co-localized at the nmj, and the staining was diminished. Taken together our data demonstrated that P2Y1 receptor regulated the nmj gene expression.

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.