Oligodendrocyte precursor cell maturation: role of adenosine receptors

Oligodendrocyte-formed myelin sheaths allow fast synaptic transmission in the brain and their degeneration leads to demyelinating diseases such as multiple sclerosis. Remyelination requires the differentiation of oligodendrocyte progenitor cells into mature oligodendrocytes but, in chronic neurodegenerative disorders, remyelination fails due to adverse environment. Therefore, a strategy to prompt oligodendrocyte progenitor cell differentiation towards myelinating oligodendrocytes is required. The neuromodulator adenosine, and its receptors(A1, A(2A), A(2B) and A3 receptors: A1R, A(2A)R, A(2B)R and A3R), are crucial mediators in remyelination processes. It is known that A1Rs facilitate oligodendrocyte progenitor cell maturation and migration whereas the A3Rs initiates apoptosis in oligodendrocyte progenitor cells. Our group of research contributed to the field by demonstrating that A(2A)R and A(2B)R inhibit oligodendrocyte progenitor cell maturation by reducing voltage-dependent K^+ currents necessary for cell differentiation. The present review summarizes the possible role of adenosine receptor ligands as potential therapeutic targets in demyelinating pathologies such as multiple sclerosis.

The seeming paradox of adenosine receptors as targets for the treatment of Alzheimer's disease: agonists or antagonists?

Alzheimer＇s disease （AD） is the most common neurodegenerative disorder, and its incidence is relatively high among elderly people, affecting about 1-2% of the population between 60-65 years old and rising dramatically （about 30%） in people aged 80 years or older （Selkoe, 2002）. Nowadays, considering the increasing mean lifespan of populations in developed countries, the disease is becoming more and more a health concern, and the search for an effective cure has turned into＂a real need＂.

Attenuation of gastric mucosal inflammation induced by aspirin through activation of A_(2A) adenosine receptor in rats

AIM： To determine whether a specific adenosine A2A receptor agonist （ATL-146e） can ameliorate aspirin-induced gastric mucosal lesions in rats, and reduce neutrophil accumulation and production of pro-inflammatory cytokines. METHODS： Gastric lesions were produced by oral gavage of aspirin （200 mg/kg） and HCI （0.15 mol/L, 8.0 mL/kg）. 4-{3-[6-Amino-9-（5-ethylcarbamoyl-3,4- dihydroxy-tetrahydro-furan-2-yl）-9H-purin-2-yl]-prop-2- ynyl}-cyclohexanecarboxylic acid methyl ester （ATL-146e, 2.5-5μg/kg, IP） was injected 30 min before the administration of aspirin. Tissue myeloperoxidase （MPO） concentration in gastric mucosa was measured as an index of neutrophil infiltration. Gastric mucosal concentrations of tumor necrosis factor-α （TNF-α） and interleukin-1β （IL-1β） were determined by ELISA. Also, we examined the effect of ATL-146e on tissue prostaglandin E2 （PGE2） production and gastric secretion. RESULTS： Intragastric administration of aspirin induced multiple hemorrhagic erosions in rat gastric mucosa. The total length of gastric erosions （ulcer index） in control rats was 29.8±7.75 mm and was reduced to 3.8±1.42 mm alter pretreatment with 5.0 g/kg ATL-146e （P〈 0.01）. The gastric contents of MPO and pro-inflammatory cytokines were all increased after the administration of aspirin and reduced to nearly normal levels by ATL-146e. Gastric mucosal PGE2 concentration was not affected by intraperitoneal injection of ATL-146e. CONCLUSION： The specific adenosine A2A receptor agonist, ATL-146e, has potent anti-ulcer effects presumably mediated by its anti-inflammatory properties.

Olfactory receptors in neural regeneration in the central nervous system

Olfactory receptors are crucial for detecting odors and play a vital role in our sense of smell,influencing behaviors from food choices to emotional memories.These receptors also contribute to our perception of flavor and have potential applications in medical diagnostics and environmental monitoring.The ability of the olfactory system to regenerate its sensory neurons provides a unique model to study neural regeneration,a phenomenon largely absent in the central nervous system.Insights gained from how olfactory neurons continuously replace themselves and reestablish functional connections can provide strategies to promote similar regenerative processes in the central nervous system,where damage often results in permanent deficits.Understanding the molecular and cellular mechanisms underpinning olfactory neuron regeneration could pave the way for developing therapeutic approaches to treat spinal co rd injuries and neurodegenerative diseases like Alzheimer's disease.Olfa ctory receptors are found in almost any cell of eve ry orga n/tissue of the mammalian body.This ectopic expression provides insights into the chemical structures that can activate olfactory receptors.In addition to odors,olfactory receptors in ectopic expression may respond to endogenous compounds and molecules produced by mucosal colonizing microbiota.The analysis of the function of olfactory receptors in ectopic expression provides valuable information on the signaling pathway engaged upon receptor activation and the receptor's role in proliferation and cell differentiation mechanisms.This review explo res the ectopic expression of olfa ctory receptors and the role they may play in neural regeneration within the central nervous system,with particular attention to compounds that can activate these receptors to initiate regenerative processes.Evidence suggests that olfactory receptors could serve as potential therapeutic targets for enhancing neural repair and recovery following central nervous system injuries.

Prolonged intermittent theta burst stimulation restores the balance between A_(2A)R-and A_(1)R-mediated adenosine signaling in the 6-hydroxidopamine model of Parkinson's disease

An imbalance in adenosine-mediated signaling,particularly the increased A_(2A)R-mediated signaling,plays a role in the pathogenesis of Parkinson's disease.Existing therapeutic approaches fail to alter disease progression,demonstrating the need for novel approaches in PD.Repetitive transcranial magnetic stimulation is a non-invasive approach that has been shown to improve motor and non-motor symptoms of Parkinson's disease.However,the underlying mechanisms of the beneficial effects of repetitive transcranial magnetic stimulation remain unknown.The purpose of this study is to investigate the extent to which the beneficial effects of prolonged intermittent theta burst stimulation in the 6-hydroxydopamine model of experimental parkinsonism are based on modulation of adenosine-mediated signaling.Animals with unilateral 6-hydroxydopamine lesions underwent intermittent theta burst stimulation for 3 weeks and were tested for motor skills using the Rotarod test.Immunoblot,quantitative reverse transcription polymerase chain reaction,immunohistochemistry,and biochemical analysis of components of adenosine-mediated signaling were performed on the synaptosomal fraction of the lesioned caudate putamen.Prolonged intermittent theta burst stimulation improved motor symptoms in 6-hydroxydopamine-lesioned animals.A 6-hydroxydopamine lesion resulted in progressive loss of dopaminergic neurons in the caudate putamen.Treatment with intermittent theta burst stimulation began 7 days after the lesion,coinciding with the onset of motor symptoms.After treatment with prolonged intermittent theta burst stimulation,complete motor recovery was observed.This improvement was accompanied by downregulation of the e N/CD73-A_(2A)R pathway and a return to physiological levels of A_(1)R-adenosine deaminase 1 after 3 weeks of intermittent theta burst stimulation.Our results demonstrated that 6-hydroxydopamine-induced degeneration reduced the expression of A_(1)R and elevated the expression of A_(2A)R.Intermittent theta burst stimulation reversed these effects by restoring the abundances of A_(1)R and A_(2A)R to control levels.The shift in ARs expression likely restored the balance between dopamine-adenosine signaling,ultimately leading to the recovery of motor control.

Adenosine A_(2A)receptor blockade attenuates excitotoxicity in rat striatal medium spiny neurons during an ischemic-like insult

During brain ischemia,excitotoxicity and peri-infarct depolarization injuries occur and cause cerebral tissue damage.Indeed,anoxic depolarization,consisting of massive neuronal depolarization due to the loss of membrane ion gradients,occurs in vivo or in vitro during an energy failure.The neuromodulator adenosine is released in huge amounts during cerebral ischemia and exerts its effects by activating specific metabotropic receptors,namely:A_(1),A_(2A),A_(2B),and A_(3).The A_(2A)receptor subtype is highly expressed in striatal medium spiny neurons,which are particularly susceptible to ischemic damage.Evidence indicates that the A2Areceptors are upregulated in the rat striatum after stroke and the selective antagonist SCH58261 protects from exaggerated glutamate release within the first 4 hours from the insult and alleviates neurological impairment and histological injury in the following 24 hours.We recently added new knowledge to the mechanisms by which the adenosine A2Areceptor subtype participates in ischemia-induced neuronal death by performing patch-clamp recordings from medium spiny neurons in rat striatal brain slices exposed to oxygen and glucose deprivation.We demonstrated that the selective block of A2Areceptors by SCH58261 significantly reduced ionic imbalance and delayed the anoxic depolarization in medium spiny neurons during oxygen and glucose deprivation and that the mechanism involves voltage-gated K+channel modulation and a presynaptic inhibition of glutamate release by the A2Areceptor antagonist.The present review summarizes the latest findings in the literature about the possibility of developing selective ligands of A2Areceptors as advantageous therapeutic tools that may contribute to counteracting neurodegeneration after brain ischemia.

Regulation of epithelial sodium channel ^-subunit expression by adenosine receptor A2a in alveolar epithelial ceils

Background The amiloride-sensitive epithelial sodium channel a-subunit （a-ENaC） is an important factor for alveolar fluid clearance during acute lung injury. The relationship between adenosine receptor A2a （A2aAR） expressed in alveolar epithelial cells and aα-ENaC is poorly understood. We targeted the A2aAR in this study to investigate its role in the expression of αa-ENaC and in acute lung injury.Methods A549 cells were incubated with different concentrations of A2aAR agonist CGS-21680 and with 100 μmol/L CGS-21680 for various times. Rats were treated with lipopolysaccharide （LPS） after CGS-21680 was injected. Animals were sacrificed and tissue was harvested for evaluation of lung injury by analysis of the lung wet-to-dry weight ratio, lung permeability and myeloperoxidase activity. RT-PCR and Western blotting were used to determine the mRNA and protein expression levels of α-ENaC in A549 cells and alveolar type II epithelial cells.Results Both mRNA and protein levels of α-ENaC were markedly higher from 4 hours to 24 hours after exposure to 100μmol/L CGS-21680. There were significant changes from 0.1 umol/L to 100 μmol/L CGS-21680, with a positive correlation between increased concentrations of CGS-21680 and expression of α-ENaC. Treatment with CGS-21680during LPS induced lung injury protected the lung and promoted α-ENaC expression in the alveolar epithelial cells.Conclusion Activation of A2aAR has a protective effect during the lung injury, which may be beneficial to the prognosis of acute lung injury

Hypoxia induces T-cell apoptosis by inhibiting chemokine C receptor 7 expression:the role of adenosine receptor A2

Hypoxia is a major characteristic of the tumor microenvironment,and its effects on immune cells are proposed to be important factors for the process of tumor immune escape.It has been reported that hypoxia affects the function of dendritic cells and the antitumor function of T cells.Here we discuss the effects of hypoxia on T-cell survival.Our results showed that hypoxia induced apoptosis of T cells.Adenosine and adenosine receptors(AR)are important to the hypoxia-related signaling pathway.Using AR agonists and antagonists,we demonstrated that hypoxia-induced apoptosis of T cells was mediated by A^(2a)and A^(2b)receptors.Furthermore,we are the first,to our knowledge,to report that hypoxia significantly inhibited the expression of chemokine C receptor 7(CCR7)of T cells via the A^(2)R signal pathway,perhaps representing a mechanism of hypoxia-induced apoptosis of T cells.Collectively,our research demonstrated that hypoxia induces T-cell apoptosis by the A^(2)R signaling pathway partly by suppressing CCR7.Blocking the A^(2)R signaling pathway and/or activation of CCR7 can increase the anti-apoptosis function of T cells and may become a new strategy to improve antitumor potential.

Expression of adenosine receptors in human retinal pigment epithelium cells in vitro

Background Adenosine receptors （ADORs） have been reported to play a role in experimental myopia. This study aimed to determine the distribution of ADORs in human retinal pigment epithelium （RPE） cells cultured in vitro.Methods Human RPE cells （cell line D407） were cultured in vitro. ADOR mRNA in RPE was detected by reverse transcription polymerase chain reaction. ADOR protein expression in RPE was confirmed by Western blotting analysis of cell lysates. Confocal fluorescence microscopy was used to study the subcellular distribution of ADORs.Results All four subtypes of ADORs mRNA and protein were expressed in human RPE. This was confirmed by Western blotting analysis. The ADOR subtypes were differently distributed within the cells. ADORA1 was expressed in nucleus, perinucleus and cytoplasm of RPE. ADORA2A was concentrated mainly in one side of the perinucleus and cytoplasm of RPE. ADORA2B was strongly expressed in the nucleus, perinucleus and the cytoplasm, and ADORA3 was expressed weakly in the cytoplasm of RPE.Conclusions ADORs are expressed in human RPE. The different distribution at the subcellular level suggests different functions of ADOR subtypes.

Unlocking antitumor immunity with adenosine receptorblockers

Tumors survive by creating a tumor microenvironment(TME)that suppresses antitumor immunity.The TME suppresses the immune system by limiting antigen presentation,inhibiting lymphocyte and natural killer(NK)cell activation,and facilitating T cell exhaustion.Checkpoint inhibitors like anti-PD-1 and anti-CTLA4 are immunostimulatory antibodies,and their blockade extends the survival of some but not all cancer patients.Extracellular adenosine triphosphate(ATP)is abundant in inflamed tumors,and its metabolite,adenosine(ADO),is a driver of immunosuppression mediated by adenosine A2A receptors(A2AR)and adenosine A2B receptors(A2BR)found on tumor-associated lymphoid and myeloid cells.This review will focus on adenosine as a key checkpoint inhibitor-like immunosuppressive player in the TME and how reducing adenosine production or blocking A2AR and A2BR enhances antitumor immunity.

Comparative pharmacophore modeling of human adenosine receptor A1 and A3 antagonists

Adenosine receptors are promising therapeutic targets in drug discovery. In this study, three-dimensional pharmacophore mod- els of human adenosine receptor A1 and A3 antagonists were developed based on 26 and 23 diverse compounds, respectively. The best A1 pharmacophore model （A1-Hopyl） consists of four features： one hydrogen bond donor, one hydrophobic point and two ring aromatics, while the best A3 pharmacophore model （A3_Hopyl） also has four features： one hydrogen bond ac- ceptor, one hydrophobic point and two ring aromatics. The correlation coefficients were 0.840 for A1 test set with 146 diverse compounds and 0.827 for A3 test set with 238 diverse compounds. In the simulated virtual screening experiments, high en- richment factors of 6.51 and 6.90 were obtained for A1_Hopyl and A3_Hopyl models, respectively. Moreover, two models also showed high subtype-selectivity in the simulated virtual screening experiments. These results could be helpful for the dis- covery of novel potent and selective A1 and A3 antagonists.

Motor Effects of 1,3-Disubstituted 8-Styrylxanthines as A_(1) and A_(2) Adenosine-Receptor Antagonists in Rats

A series of 1,3-substituted 8-styrylxanthines (11a-d) was synthesized, under chemo- and regioselective conditions, in a good overall yield. The compounds showed affinity towards both A1 and A2A-adenosine receptors by radioligand binding by means of in vitro assays. The (E)-3-ethyl-1-propyl-8-styrylxanthine (11a) showed the greatest affinity towards the A2A receptor, whereas (E)-3-pentyl-1-propyl-8-styrylxanthine (11d) showed the greatest affinity for the A1 receptor. When the 8-styrylxanthines 11a (A15Et) and 11c (A15Bu) were administrated in rats, which were previously injured with 6-hydroxydopamine at the substantia nigra pars compacta (SNc), the turning behavior decreased 50%. Based on these results we propose to A15Et as a potential compound to treat some symptoms of Parkinson’s disease.

Pretreatment with adenosine and adenosine A1 receptor agonist protects against intestinal ischemia-reperfusion injury in rat

AIM： To examine the effects of adenosine and A1 receptor activation on reperfusion-induced small intestinal injury. METHODS： Rats were randomized into groups with sham operation, ischemia and reperfusion, and systemic treatments with either adenosine or 2-chloro-N^6-cyclopentyladenosine, A1 receptor agonist or 8-cyclopentyl- 1,3-clipropylxanthine, A1 receptor antagonist, plus adenosine before ischemia. Following reperfusion, contractions of ileum segments in response to KCl, carbachol and substance P were recorded. Tissue myeloperoxidase,malondialdehyde, and reduced glutathione levels were measured. RESULTS： Ischemia significantly decreased both contraction and reduced glutathione level which were ameliorated by adenosine and agonist administration. Treatment also decreased neutrophil infiltration and membrane lipid peroxidation. Beneficial effects of adenosine were abolished by pretreatment with A1 receptor antagonist. CONCLUSION： The data suggest that adenosine and A1 receptor stimulation attenuate ischemic intestinal injury via decreasing oxidative stress, lowering neutrophil infiltration, and increasing reduced glutathione content.

Electroacupuncture-induced neuroprotection against focal cerebral ischemia in the rat is mediated by adenosine A1 receptors

The activation of adenosine A1 receptors is important for protecting against ischemic brain injury and pretreatment with electroacupuncture has been shown to mitigate ischemic brain insult. The aim of this study was to test whether the adenosine A1 receptor mediates electroacupuncture pretreatment-induced neuroprotection against ischemic brain injury. We first performed 30 minutes of electroacupuncture pretreatment at the Baihui acupoint（GV20）, delivered with a current of 1 mA, a frequency of 2/15 Hz, and a depth of 1 mm. High-performance liquid chromatography found that adenosine triphosphate and adenosine levels peaked in the cerebral cortex at 15 minutes and 120 minutes after electroacupuncture pretreatment, respectively. We further examined the effect of 15 or 120 minutes electroacupuncture treatment on ischemic brain injury in a rat middle cerebral artery-occlusion model. We found that at 24 hours reperfusion,120 minutes after electroacupuncture pretreatment, but not for 15 minutes, significantly reduced behavioral deficits and infarct volumes. Last, we demonstrated that the protective effect gained by 120 minutes after electroacupuncture treatment before ischemic injury was abolished by pretreatment with the A1-receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine（1 mg/kg, intraperitoneally）. Our results suggest that pretreatment with electroacupuncture at the Baihui acupoint elicits protection against transient cerebral ischemia via action at adenosine A1 receptors.

Upregulated adenosine 2A receptor accelerates post-infectious irritable bowel syndrome by promoting CD4+T cells’T helper 17 polarization

BACKGROUND Post-infectious irritable bowel syndrome(PI-IBS)is generally regarded as a functional disease.Several recent studies have reported the involvement of lowgrade inflammation and immunological dysfunction in PI-IBS.T helper 17(Th17)polarization occurs in IBS.Adenosine and its receptors participate in intestinal inflammation and immune regulation.AIM To investigate the role of Th17 polarization of CD4+T cells regulated by adenosine 2A receptor(A2AR)in PI-IBS.METHODS A PI-IBS model was established by infecting mice with Trichinella spiralis.The intestinal A2AR and CD4+T lymphocytes were detected by immunohistochemistry,and the inflammatory cytokines were detected by enzyme-linked immunoassay.CD4+T lymphocytes present in the animal’s spleen were separated and cultured with or without A2AR agonist and antagonist.Western blotting and real-time quantitative polymerase chain reaction were performed to determine the effect of A2AR on the cells and intestinal tissue.Cytokine production was determined.The protein and mRNA levels of A2AR associated signaling pathway molecules were also evaluated.Furthermore,A2AR agonist and antagonist were injected into the mouse model and the clinical features were observed.RESULTS The PI-IBS mouse model showed increased expression of ATP and A2AR(P<0.05),and inhibition of A2AR improved the clinical features in PI-IBS,including the abdominal withdrawal reflex and colon transportation test(P<0.05).The number of intestinal CD4+T cells and interleukin-17(IL-17)protein levels increased during PI-IBS,which was reversed by administration of the A2AR antagonist(P<0.05).CD4+T cells expressed A2AR and produced IL-17 in vitro,which was regulated by the A2AR agonist and antagonist.The A2AR antagonist increased the production of IL-17 by CD4+T cells via the Janus kinase-signal transducer and activator of transcriptionreceptor-related orphan receptorγsignaling pathway.CONCLUSION The results of the present study suggested that the upregulation of A2AR increases PI-IBS by promoting the Th17 polarization of CD4+T cells.

Adenosine 2A receptor contributes to the facilitation of postinfectious irritable bowel syndrome by γδ T cells via the PKA/CREB/NF-κB signaling pathway

BACKGROUND Immunological dysfunction-induced low-grade inflammation is regarded as one of the predominant pathogenetic mechanisms in post-infectious irritable bowel syndrome(PI-IBS).γδT cells play a crucial role in innate and adaptive immunity.Adenosine receptors expressed on the surface ofγδT cells participate in intestinal inflammation and immunity regulation.AIM To investigate the role ofγδT cell regulated by adenosine 2A receptor(A2AR)in PI-IBS.METHODS The PI-IBS mouse model has been established with Trichinella spiralis(T.spiralis)infection.The intestinal A2AR and A2AR inγδT cells were detected by immunohistochemistry,and the inflammatory cytokines were measured by western blot.The role of A2AR on the isolatedγδT cells,including proliferation,apoptosis,and cytokine production,were evaluated in vitro.Their A2AR expression was measured by western blot and reverse transcription polymerase chain reaction(RT-PCR).The animals were administered with A2AR agonist,or A2AR antagonist.Besides,γδT cells were also injected back into the animals,and the parameters described above were examined,as well as the clinical features.Furthermore,the A2AR-associated signaling pathway molecules were assessed by western blot and RT-PCR.RESULTS PI-IBS mice exhibited elevated ATP content and A2AR expression(P<0.05),and suppression of A2AR enhanced PI-IBS clinical characteristics,indicated by the abdominal withdrawal reflex and colon transportation test.PI-IBS was associated with an increase in intestinal T cells,and cytokine levels of interleukin-1(IL-1),IL-6,IL-17A,and interferon-α(IFN-α).Also,γδT cells expressed A2AR in vitro and generated IL-1,IL-6,IL-17A,and IFN-α,which can be controlled by A2AR agonist and antagonist.Mechanistic studies demonstrated that the A2AR antagonist improved the function ofγδT cells through the PKA/CREB/NF-κB signaling pathway.CONCLUSION Our results revealed that A2AR contributes to the facilitation of PI-IBS by regulating the function ofγδT cells via the PKA/CREB/NF-κB signaling pathway.

Brain-derived neurotrophic factor signaling in the neuromuscular junction during developmental axonal competition and synapse elimination

During the development of the nervous system,there is an overproduction of neurons and synapses.Hebbian competition between neighboring nerve endings and synapses performing different activity levels leads to their elimination or strengthening.We have extensively studied the involvement of the brain-derived neurotrophic factor-Tropomyosin-related kinase B receptor neurotrophic retrograde pathway,at the neuromuscular junction,in the axonal development and synapse elimination process versus the synapse consolidation.The purpose of this review is to describe the neurotrophic influence on developmental synapse elimination,in relation to other molecular pathways that we and others have found to regulate this process.In particular,we summarize our published results based on transmitter release analysis and axonal counts to show the different involvement of the presynaptic acetylcholine muscarinic autoreceptors,coupled to downstream serine-threonine protein kinases A and C(PKA and PKC)and voltage-gated calcium channels,at different nerve endings in developmental competition.The dynamic changes that occur simultaneously in several nerve terminals and synapses converge across a postsynaptic site,influence each other,and require careful studies to individualize the mechanisms of specific endings.We describe an activity-dependent balance(related to the extent of transmitter release)between the presynaptic muscarinic subtypes and the neurotrophin-mediated TrkB/p75NTR pathways that can influence the timing and fate of the competitive interactions between the different axon terminals.The downstream displacement of the PKA/PKC activity ratio to lower values,both in competing nerve terminals and at postsynaptic sites,plays a relevant role in controlling the elimination of supernumerary synapses.Finally,calcium entry through L-and P/Q-subtypes of voltage-gated calcium channels(both channels are present,together with the N-type channel in developing nerve terminals)contributes to reduce transmitter release and promote withdrawal of the most unfavorable nerve terminals during elimination(the weakest in acetylcholine release and those that have already become silent).The main findings contribute to a better understanding of punishment-rewarding interactions between nerve endings during development.Identifying the molecular targets and signaling pathways that allow synapse consolidation or withdrawal of synapses in different situations is important for potential therapies in neurodegenerative diseases.

Adenosine amine congener ameliorates cisplatin-induced hearing loss

AIM: To investigate a novel pharmacological intervention to mitigate cisplatin ototoxicity using a selective adenosine A1 receptor agonist adenosine amine congener(ADAC).METHODS: Male Wistar rats(8-10 wk) were exposed to a two-cycle cisplatin treatment similar to clinical course of cancer chemotherapy. Each cycle comprised 4 d of intraperitoneal cisplatin injections(1 mg/kg twice daily) separated by 10 d of rest. ADAC(100 μg/kg) or drug vehicle solution(control) was administered intraperitoneally for 5 d at 24 h intervals during the second cisplatin cycle(Regime 1), or upon completion of the cisplatin treatment(Regime 2). Hearing thresholds were measured using auditory brainstem responses(ABR) before cisplatin administration(baseline) and 7 d after the end of cisplatin treatment. Histological analysis of cochlear tissues included hair cell counting and qualitative assessment of apoptosis using terminal deoxynucleotidyl transferase mediated d UTP nick end labelling(TUNEL) staining.RESULTS: ABR threshold shifts in cisplatin-treated Wistar rats ranged from 5-29 d B across the frequency range used in the study(4-24 k Hz). Higher frequencies(16-24 k Hz) were mostly affected by cisplatin ototoxicity(mean threshold shift 25-29 d B). ADAC treatment during the second cisplatin cycle reduced cisplatininduced threshold shifts by 12-16 d B(P < 0.01) at higher frequencies compared to control vehicle-treated rats. However, the treatment was ineffective if ADAC administration was delayed until after the completion of the cisplatin regime. Functional recovery was supported by increased survival of hair cells in the cochlea. Qualitative analysis using TUNEL staining demonstrated reduced apoptosis of the outer hair cells and marginal cells in the stria vascularis in animals treated with ADAC during the second cisplatin cycle.CONCLUSION: A1 adenosine receptor agonist ADAC mitigates cisplatin-induced cochlear injury and hearing loss, however its potential interference with antineoplastic effects of cisplatin needs to be established.

Structural insight into the dual-antagonistic mechanism of AB928 on adenosine A_(2)receptors

The adenosine subfamily G protein-coupled receptors A_(2A)R and A_(2B)R have been identified as promising cancer immunotherapy candidates.One of the A_(2A)R/A_(2B)R dual antagonists,AB928,has progressed to a phaseⅡclinical trial to treat rectal cancer.However,the precise mechanism underlying its dual-antagonistic properties remains elusive.Herein,we report crystal structures of the A_(2A)R complexed with AB928 and a selective A_(2A)R antagonist 2-118.The structures revealed a common binding mode on A_(2A)R,wherein the ligands established extensive interactions with residues from the orthosteric and secondary pockets.In contrast,the cAMP assay and A_(2A)R and A_(2B)R molecular dynamics simulations indicated that the ligands adopted distinct binding modes on A_(2B)R.Detailed analysis of their chemical structures suggested that AB928 readily adapted to the A_(2B)R pocket,while 2-118 did not due to intrinsic differences.This disparity potentially accounted for the difference in inhibitory efficacy between A_(2B)R and A_(2A)R.This study serves as a valuable structural template for the future development of selective or dual inhibitors targeting A_(2A)R/A_(2B)R for cancer therapy.

Targeting the adenosine A2A receptor for neuroprotection and cognitive improvement in traumatic brain injury and Parkinson's disease

Adenosine exerts its dual functions of homeostasis and neuromodulation in the brain by acting at mainly 2 G-protein coupled receptors,called A1 and A2A receptors.The adenosine A2A receptor(A2AR)antagonists have been clinically pursued for the last 2 decades,leading to final approval of the istradefylline,an A2AR antagonist,for the treatment of OFF-Parkinson's disease(PD)patients.The approval paves the way to develop novel therapeutic methods for A2AR antagonists to address 2 major unmet medical needs in PD and traumatic brain injury(TBI),namely neuroprotection or improving cognition.In this review,we first consider the evidence for aberrantly increased adenosine signaling in PD and TBI and the sufficiency of the increased A2AR signaling to trigger neurotoxicity and cognitive impairment.We further discuss the increasing preclinical data on the reversal of cognitive deficits in PD and TBI by A2AR antagonists through control of degenerative proteins and synaptotoxicity,and on protection against TBI and PD pathologies by A2AR antagonists through control of neuroinflammation.Moreover,we provide the supporting evidence from multiple human prospective epidemiological studies which revealed an inverse relation between the consumption of caffeine and the risk of developing PD and cognitive decline in aging population and Alzheimer's disease patients.Collectively,the convergence of clinical,epidemiological and experimental evidence supports the validity of A2AR as a new therapeutic target and facilitates the design of A2AR antagonists in clinical trials for disease-modifying and cognitive benefit in PD and TBI patients.