Localization of the NBMPR-sensitive equilibrative nucleoside transporter, ENT1, in the rat dorsal root ganglion and lumbar spinal cord

Single-cell RNA sequencing reveals distinct transcriptional features of the purinergic signaling in mouse trigeminal ganglion

Trigeminal ganglion (TG) is the first station of sensory pathways in the orofacial region. The TG neurons communicate with satellite glial cells (SGCs), macrophages and other cells forming a functional unit that is responsible for processing of orofacial sensory information. Purinergic signaling, one of the most widespread autocrine and paracrine pathways, plays a crucial role in intercellular communication. The multidirectional action of purinergic signaling in different cell types contributes to the neuromodulation and orofacial sensation. To fully understand the purinergic signaling in these processes, it is essential to determine the shared and unique expression patterns of genes associated with purinergic signaling in different cell types. Here, we performed single-cell RNA sequencing of 22,969 cells isolated from normal mouse TGs. We identified 18 distinct cell populations, including 6 neuron subpopulations, 3 glial subpopulations, 7 immune cell subpopulations, fibroblasts, and endothelial cells. We also revealed the transcriptional features of genes associated with purinergic signaling, including purinergic receptors, extracellular adenosine triphosphate (eATP) release channels, eATP metabolism-associated enzymes, and eATP transporters in each cell type. Our results have important implications for understanding and predicting the cell type-specific roles of the purinergic signaling in orofacial signal processing in the trigeminal primary sensory system.

Pharmacological Tuning of Adenosine Signal Nuances Underlying Heart Failure With Preserved Ejection Fraction

Heart failure with preserved ejection fraction (HFpEF) roughly represents half of the cardiac failure events in developed countries. The proposed 'systemic microvascular paradigm' has been used to explain HFpHF presentation heterogeneity. The lack of effective treatments with few evidence-based therapeutic recommendations makes HFpEF one of the greatest unmet clinical necessities worldwide. The endogenous levels of the purine nucleoside, adenosine, increase significantly following cardiovascular events. Adenosine exerts cardioprotective, neuromodulatory, and immunosuppressive effects by activating plasma membrane-bound P1 receptors that are widely expressed in the cardiovascular system. Its proven benefits have been demonstrated in preclinical animal tests. Here, we provide a comprehensive and up-to-date critical review about the main therapeutic advantages of tuning adenosine signalling pathways in HFpEF, without discounting their side effects and how these can be seized.

Purinergic Signaling in Endometriosis-Associated Pain

Endometriosis is an estrogen-dependent gynecological disease, with an associated chronic inflammatory component, characterized by the presence of endometrial tissue outside the uterine cavity. Its predominant symptom is pain, a condition notably altering the quality of life of women with the disease. This review is intended to exhaustively gather current knowledge on purinergic signaling in endometriosis-associated pain. Altered extracellular ATP hydrolysis, due to changes in ectonucleotidase activity, has been reported in endometriosis; the resulting accumulation of ATP in the endometriotic microenvironment points to sustained activation of nucleotide receptors (P2 receptors) capable of generating a persistent pain message. P2X3 receptor, expressed in sensory neurons, mediates nociceptive, neuropathic, and inflammatory pain, and is enrolled in endometriosis-related pain. Pharmacological inhibition of P2X3 receptor is under evaluation as a pain relief treatment for women with endometriosis. The role of other ATP receptors is also discussed here, e.g., P2X4 and P2X7 receptors, which are involved in inflammatory cell-nerve and microglia-nerve crosstalk, and therefore in inflammatory and neuropathic pain. Adenosine receptors (P1 receptors), by contrast, mainly play antinociceptive and anti-inflammatory roles. Purinome-targeted drugs, including nucleotide receptors and metabolizing enzymes, are potential non-hormonal therapeutic tools for the pharmacological management of endometriosis-related pain.

Differential role of adenosine signaling cascade in acute and chronic pain

Adenosine is a signaling molecule induced under stress such as energy insufficiency and ischemic/hypoxic conditions. Adenosine controls multiple physiological and pathological cellular and tissue function by activation of four G protein-coupled receptors (GPCR). Functional role of adenosine signaling in acute pain has been widely studied. However, the role of adenosine signaling in chronic pain is poorly understood. At acute levels, adenosine can be beneficial to anti-pain whereas a sustained elevation of adenosine can be detrimental to promote chronic pain. In recent years, extensive progress has been made to define the role of adenosine signaling in chronic pain and to dissect molecular new insight underlying the development of chronic pain. In this review, we summarize the differential role of adenosine signaling cascade in acute and chronic pain with a major focus on recent studies revealing adenosine ADORA2B receptor activation in the pathology of chronic pain. We further provide a therapeutic outlook of how multiple adenosine signaling components can be useful to treat chronic pain.

ATP metabolizing enzymes ENPP1, 2 and 3 are localized in sensory neurons of rat dorsal root ganglion

In dorsal root ganglion (DRG) neurons, ATP is an important neurotransmitter in nociceptive signaling through P2 receptors (P2Rs) such as P2X2/3R, and adenosine is also involved in anti-nociceptive signaling through adenosine A1R. Thus, the clearance system for adenine nucleotide/nucleoside plays a critical role in regulation of nociceptive signaling, but there is little information on it, especially ectoenzyme expression profiles in DRG. In this study, we examined expression and localization of ecto-nucleotide pyrophosphatase/phosphodiesterases (ENPPs), by which ATP is metabolized to AMP, in rat DRG. The mRNA expression levels of ENPP2 were greater than those of ENPP1 and ENPP3 in rat DRGs. On immunohistochemical analysis, ENPP1, 2 and 3 were found in soma of DRG neurons. Immunopositive rate of ENPP3 was greater than that of ENPP1 and ENPP2 in all DRG neurons. ENPP3, as compared with ENPP1 and ENPP2, was expressed mainly by isolectin B4-positive cells, and slightly by neurofilament 200-positive ones. In this way, the expression profile of ENPP1, 2 and 3 was different in DRGs, and they were mainly expressed in small/medium-sized DRG neurons. Moreover, ENPP1-, 2- and 3-immunoreactivities were colocalized with P2X2R, P2X3R and prostatic acid phosphatase (PAP), as an ectoenzyme for metabolism from AMP to adenosine. Additionally, PAP-immunoreactivity was colocalized with equilibrative nucleoside transporter (ENT) 1, as an adenosine uptake system. These results suggest that the clearance system consisted of ENPPs, PAP and ENT1 plays an important role in regulation of nociceptive signaling in sensory neurons.

ENT1 inhibition attenuates epileptic seizure severity via regulation of glutamatergic neurotransmission

Type 1 equilibrative nucleoside transporter (ENT1) promotes glutamate release by inhibition of adenosine signaling. However, whether ENT1 plays a role in epileptic seizure that involves elevated glutamatergic neurotransmission is unknown. Here, we report that both seizure rats and patients show increased expression of ENT1. Intrahippocampal injection of a specific inhibitor of ENT1, nitrobenzylthioinosine (NBTI), attenuates seizure severity and prolongs onset latency. In order to examine whether NBTI would be effective as antiepileptic after peripheral application, we injected NBTI intraperitoneally, and the results were similar to those obtained after intrahippocampal injection. NBTI administration leads to suppressed neuronal firing in seizure rats. In addition, increased mEPSC in seizure are inhibited by NBTI. Finally, NBTI results in deactivation of phosphorylated cAMP-response element-binding protein in the seizure rats. These results indicate that ENT1 plays an important role in the development of seizure. Inhibition of ENT1 might provide a novel therapeutic approach toward the control of epileptic seizure.

Hypoxia and P1 receptor activation regulate the high-affinity concentrative adenosine transporter CNT2 in differentiated neuronal PC12 cells

Under several adverse conditions, such as hypoxia or ischaemia, extracellular levels of adenosine are elevated because of increased energy demands and ATP metabolism. Because extracellular adenosine affects metabolism through G-protein-coupled receptors, its regulation is of high adaptive importance. CNT2 (concentrative nucleoside transporter 2) may play physiological roles beyond nucleoside salvage in brain as it does in other tissues. Even though nucleoside transport in brain has mostly been seen as being of equilibrative-type, in the present study, we prove that the rat phaeochromocytoma cell line PC12 shows a concentrative adenosine transport of CNT2-type when cells are differentiated to a neuronal phenotype by treatment with NGF (nerve growth factor). Differentiation of PC12 cells was also associated with the up-regulation of adenosine A1 receptors. Addition of adenosine receptor agonists to cell cultures increased CNT2-related activity by a mechanism consistent with A₁ and A2A receptor activation. The addition of adenosine to the culture medium also induced the phosphorylation of the intracellular regulatory kinase AMPK (AMP-activated protein kinase), with this effect being dependent upon adenosine transport. CNT2-related activity of differentiated PC12 cells was also dramatically down-regulated under hypoxic conditions. Interestingly, the analysis of nucleoside transporter expression after experimental focal ischaemia in rat brain showed that CNT2 expression was down-regulated in the infarcted tissue, with this effect somehow being restricted to other adenosine transporter proteins such as CNT3 and ENT1 (equilibrative nucleoside transporter 1). In summary, CNT2 is likely to modulate extracellular adenosine and cell energy balance in neuronal tissue.

Expression of equilibrative nucleoside transporter type 1 protein in elderly patients with schizophrenia

Alterations in glutamatergic neurotransmission are thought to be involved in several psychiatric disorders, including schizophrenia. Equilibrative nucleoside transporter type 1 (ENT1) regulates glutamate levels by regulating excitatory amino acid transporter expression and activity in the brain. In this study, we investigated whether ENT1 is abnormally expressed in the brain of elderly patients with schizophrenia. We measured protein expression of ENT1 in the superior temporal gyrus (STG) and anterior cingulate cortex (ACC) in patients with schizophrenia (STG, n=22; ACC, n=34) and a comparison group (STG, n=24; ACC, n=29). We found decreased ENT1 expression in the STG in patients with schizophrenia, supporting the hypothesis of altered glutamate transport in this illness.

Antihyperalgesic activity of nucleoside transport inhibitors in models of inflammatory pain in guinea pigs

Background and methods

The role of the endogenous purine nucleoside, adenosine, in nociception is well established. Inhibition of the equilibrative nucleoside transporter (ENT1) prevents adenosine uptake into cells, and could therefore enhance the antinociceptive properties of adenosine. The effects of ENT1 inhibition were studied in two animal models of inflammatory pain. Analgesic activity was assessed in a complete Freund’s adjuvant (CFA)-induced and carrageenan-induced mechanical and thermal hyperalgesia model in the guinea pig.

Results

Draflazine, dipyridamole, dilazep, lidoflazine, soluflazine, and KF24345 showed efficacy in the CFA thermal hyperalgesia model. Draflazine, the most potent compound in this test, was further characterized in the CFA model of mechanical hyperalgesia and the carrageenan inflammation model of thermal and mechanical hyperalgesia, where it completely reversed the hypersensitivity. The antihyperalgesic effects of draflazine (10 mg/kg, administered subcutaneously) were attenuated by the A1 receptor antagonist, cyclopentyltheophylline (5–40 mg/kg, administered intraperitoneally), by the nonselective adenosine antagonist, caffeine (10–40 mg/kg intraperitoneally), and by the A2 antagonist, DMPX (10 mg/kg administered intraperitoneally).

Conclusion

ENT1 inhibition is an effective way of reversing mechanical and thermal inflammatory hyperalgesia in the guinea pig, and these effects are mediated by enhancement of endogenous adenosine levels. Both A1 and A2 adenosine receptor subtypes are likely to be involved.

Roles of purines in synaptic modulation evoked by hypercapnia in isolated spinal cord of neonatal rat in vitro

BACKGROUND AND PURPOSE

The purine compounds, adenosine 5'-triphosphate (ATP) and adenosine, are known to accumulate in the extracellular space and to elicit various cellular responses during hypoxia/ischemia, whereas the roles of purines during hypercapnia are poorly understood. In this study, we examined the effects of various drugs affecting purine turnover on the responses to hypercapnia in the spinal cord.

EXPERIMENTAL APPROACH

Electrically evoked reflex potentials were measured in an in vitro preparation of the isolated spinal cord of the neonatal rat by extracellular recording. Extracellular adenosine concentrations were assayed by high performance liquid chromatography (HPLC) methods.

KEY RESULTS

Hypercapnia (20% CO2) depressed the reflex potentials, which were partially reversed by an adenosine A1 receptor antagonist, 8-cyclopentyl theophylline, but not by a P2 receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid. Exogenous adenosine and ATP also depressed the reflex potentials via adenosine A1 receptors. The hypercapnia-evoked depression was not reversed by inhibitors of gap junction hemichannels, anion channels, P2X7 receptors or equilibrative nucleoside transporters, all of which might be involved in purine efflux pathways. The adenosine accumulation evoked by hypercapnia was not inhibited by tetrodotoxin, ethylene glycol-bis(beta-amino ethyl ether) tetraacetic acid (EGTA) or an ecto-ATPase inhibitor, ARL 67156. Homocysteine thiolactone, used to trap intracellular adenosine, significantly reduced extracellular adenosine accumulation during hypercapnia.

CONCLUSIONS AND IMPLICATIONS

These results suggest that hypercapnia released adenosine itself from intracellular sources, using pathways different from the conventional exocytotic mechanism, and that this adenosine depressed spinal synaptic transmission via adenosine A1 receptors.

Nucleoside transporters: from scavengers to novel therapeutic targets

Hydrophilic purine and pyrimidine nucleosides rely on specialized carrier proteins for their membrane translocation. The recent identification of two gene families encoding equilibrative and concentrative nucleoside transporters in mammals and other organisms has provided the essential breakthrough to a more complete understanding of the biological significance of nucleoside transport. Although nucleoside salvage is a primary function of these proteins, recent data indicate functions beyond metabolic recycling. In brain and spinal cord, for example, nucleoside transporters have the potential to regulate synaptic levels of neuroactive purines such as adenosine and, thereby, indirectly modulate physiological processes through G-protein-coupled purine P1 receptors. As described in this review, recent research indicates novel putative functions for CNS nucleoside transporters in sleep, arousal, drug and alcohol addiction, nociception and analgesia. The therapeutic use of nucleoside analogue drugs and nucleoside transporter inhibitors in viral, neoplastic, cardiovascular and infectious disease is also described.