Adenosine receptors

Platelets isolation and ectonucleotidase assay: Revealing functional aspects of the communication between the vasculature and the immune system

Platelets are enucleated fragments of cells with a diversity of internal granules. They are responsible for functions related to hemostasis, coagulation, and inflammation. The activation of these processes depends on a cascade coordinated by cytokines, chemokines, and components of purinergic signaling, such as ATP, ADP, and adenosine. Platelets express distinct components of the purinergic system: P2X1, P2Y1, PY12, and P2Y14 receptors; and the ectonucleotidases NTPDase, NPP, and 5NTE (ecto-5'-nucleotidase). Except for P2Y14, which has not yet exhibited a known function, all other components relate to the biological processes mentioned before. Platelets are known to display specific responses to microorganisms, being capable of recognizing pathogen-associated molecular patterns (PAMPs), engulfing certain classes of viruses, and participating in NETosis. Platelet function dysregulation implicates various pathophysiological processes, including cardiovascular diseases (CVDs) and infections. In COVID-19 patients, platelets exhibit altered purinergic signaling and increased activation, contributing to inflammation. Excessive platelet activation can lead to complications from thrombosis, which can affect the circulation of vital organs. Therefore, controlling the activation is necessary to end the inflammatory process and restore homeostasis. Ectonucleotidases, capable of hydrolyzing ATP, ADP, and AMP, are of fundamental importance in activating platelets, promising pharmacological targets for clinical use as cardiovascular protective drugs. In this review, we revisit platelet biology, the purinergic receptors and ectonucleotidases on their surface, and their importance in platelet activity. Additionally, we describe methods for isolating platelets in humans and murine, as well as the main techniques for detecting the activity of ectonucleotidases in platelets. Considering the multitude of functions revealed by platelets and their potential use as potent bioreactors able to secrete and present molecules involved in the communication of the vasculature with the immune system, it is crucial to deeply understand platelet biology and purinergic signaling participation to contribute to the developing of therapeutic strategies in diseases of the cardiovascular, inflammatory, and immune systems.

Lessons from the physiological role of guanosine in neurodegeneration and cancer: Toward a multimodal mechanism of action?

Neurodegenerative diseases and brain tumours represent important health challenges due to their severe nature and debilitating consequences that require substantial medical care. Interestingly, these conditions share common physiological characteristics, namely increased glutamate, and adenosine transmission, which are often associated with cellular dysregulation and damage. Guanosine, an endogenous nucleoside, is safe and exerts neuroprotective effects in preclinical models of excitotoxicity, along with cytotoxic effects on tumour cells. However, the lack of well-defined mechanisms of action for guanosine hinders a comprehensive understanding of its physiological effects. In fact, the absence of specific receptors for guanosine impedes the development of structure-activity research programs to develop guanosine derivatives for therapeutic purposes. Alternatively, given its apparent interaction with the adenosinergic system, it is plausible that guanosine exerts its neuroprotective and anti-tumorigenic effects by modulating adenosine transmission through undisclosed mechanisms involving adenosine receptors, transporters, and purinergic metabolism. Here, several potential molecular mechanisms behind the protective actions of guanosine will be discussed. First, we explore its potential interaction with adenosine receptors (A1R and A2AR), including the A1R-A2AR heteromer. In addition, we consider the impact of guanosine on extracellular adenosine levels and the role of guanine-based purine-converting enzymes. Collectively, the diverse cellular functions of guanosine as neuroprotective and antiproliferative agent suggest a multimodal and complementary mechanism of action.

Renal ischemia and reperfusion impact the purinergic signaling in a vascular bed distant from the injured site

AIMS

Acute kidney injury (AKI) is a public health problem and represents a risk factor for cardiovascular diseases (CVD) and vascular damage. This study aimed to investigate the impact of AKI on purinergic components in mice aorta.

MAIN METHODS

The kidney ischemia was achieved by the occlusion of the left kidney pedicle for 60 min, followed by reperfusion for 8 (IR8) and 15 (IR15) days. Renal function was assessed through biochemical assays, while gene expression levels were evaluated by RT-qPCR.

KEY FINDINGS

Analyses of renal parameters showed renal remodeling through mass loss in the left kidney and hypertrophy of the right kidney in the IR15 group. Furthermore, after 15 days, local inflammation was evidenced in the aorta. Moreover, the aorta purinergic components were significantly impacted by the renal ischemia and reperfusion model, with increases in gene expression of the pro-inflammatory purinoceptors P2Y1, P2Y2, P2Y6, and P2X4, potentially contributing to the vessel inflammation. The expression of NTPDase2 and ecto-5'-nucleotidase were also significantly increased in the aorta of the same group. In addition, both ATP and AMP hydrolysis were significantly increased in the aorta from IR15 animals, driving the entire purinergic cascade to the production of the anti-inflammatory adenosine.

SIGNIFICANCE

In short, this is the first time that inflammation of the aorta due to AKI was shown to have an impact on purinergic signaling components, with emphasis on the adenosinergic pathway. This seems to be closely implicated in the establishment of vascular inflammation in this model of AKI and deserves to be further investigated.

The impact of purine nucleosides on neuroplasticity in the adult brain

Neuroplasticity refers to the nervous system's ability to adapt and reorganize its cell structures and neuronal networks in response to internal and external stimuli. In adults, this process involves neurogenesis, synaptogenesis, and synaptic and neurochemical plasticity. Several studies have reported the significant impact of the purinergic system on neuroplasticity modulation. And, there is considerable evidence supporting the role of purine nucleosides, such as adenosine, inosine, and guanosine, in this process. This review presents extensive research on how these nucleosides enhance the neuroplasticity of the adult central nervous system, particularly in response to damage. The mechanisms through which these nucleosides exert their effects involve complex interactions with various receptors and signaling pathways. Adenosine's influence on neurogenesis involves interactions with adenosine receptors, specifically A1R and A2AR. A1R activation appears to inhibit neuronal differentiation and promote astrogliogenesis, while A2AR activation supports neurogenesis, neuritogenesis, and synaptic plasticity. Inosine and guanosine positively impact cell proliferation, neurogenesis, and neuritogenesis. Inosine seems to modulate extracellular adenosine levels, and guanosine might act through interactions between purinergic and glutamatergic systems. Additionally, the review discusses the potential therapeutic implications of purinergic signaling in neurodegenerative and neuropsychiatric diseases, emphasizing the importance of these nucleosides in the neuroplasticity of brain function and recovery.

Neurobehavioral protective effects of Japanese sake yeast supplement against chronic stress-induced anxiety and depression-like symptoms in mice: Possible role of central adenosine receptors

RATIONALE

Using routine synthetic drugs in the treatment of psychiatric disorders may have some restrictions due to serious side effects and pharmacoresistance. Some natural agents may be promising alternatives in this case. The neuroprotective activity of the neuromodulator adenosine and its receptor, A1 receptor (A1R) in the central nervous system has been mentioned in different studies.

OBJECTIVE

We aimed to determine the anxiolytic, antidepressant and sedative effects of Japanese sake yeast as the first report.

METHOD

Mice were subjected to a one-week stress protocol and concomitantly treated orally with sake yeast at the dose levels of 100, 200 and 300 mg kg-1 once daily for a week. The anxiolytic, antidepressant, and sedative actions of sake yeast were evaluated with the related tests.

RESULTS

In all dose regiments, sake yeast significantly improved functions in the EPM and FST. 200 and 300 mg/kg of sake yeast significantly increased sleep duration and reduced sleep latency. Anxiolytic and antidepressant-like activities of sake yeast were maintained by the injection of ZM241385 (15 mg kg-1), a selective adenosine A2AR antagonist but completely counteracted by the injection of 8-cyclopentyltheophylline (10 mg kg-1), a selective adenosine A1R antagonist. 300 mg/kg of the yeast significantly increased the BDNF levels. Amygdala corticosterone levels did not show any significant changes at any dosage. Amygdala TNF-α, IL-6 and IL-1β levels also decreased significantly with all the sake regiments compared to the control group.

CONCLUSIONS

We conclude that oral sake yeast supplement exerts a neurobehavioral protective effect predominantly by activating central A1Rs.

Highly Sensitive Real-Time Monitoring of Adenosine Receptor Activities in Nonsmall Cell Lung Cancer Cells Using Carbon Nanotube Field-Effect Transistors

Adenosine metabolism through adenosine receptors plays a critical role in lung cancer biology. Although recent studies showed the potential of targeting adenosine receptors as drug targets for lung cancer treatment, conventional methods for investigating receptor activities often suffer from various drawbacks, including low sensitivity and slow analysis speed. In this study, adenosine receptor activities in nonsmall cell lung cancer (NSCLC) cells were monitored in real time with high sensitivity through a carbon nanotube field-effect transistor (CNT-FET). In this method, we hybridized a CNT-FET with NSCLC cells expressing A2A and A2B adenosine receptors to construct a hybrid platform. This platform could detect adenosine, an endogenous ligand of adenosine receptors, down to 1 fM in real time and sensitively discriminate adenosine among other nucleosides. Furthermore, we could also utilize the platform to detect adenosine in complicated environments, such as human serum. Notably, our hybrid platform allowed us to monitor pharmacological effects between adenosine and other drugs, including dipyridamole and theophylline, even in human serum samples. These results indicate that the NSCLC cell-hybridized CNT-FET can be a practical tool for biomedical applications, such as the evaluation and screening of drug-candidate substances.

Small molecule allosteric modulation of the adenosine A1 receptor

G protein-coupled receptors (GPCRs) represent the target for approximately a third of FDA-approved small molecule drugs. The adenosine A₁ receptor (A₁R), one of four adenosine GPCR subtypes, has important (patho)physiological roles in humans. A₁R has well-established roles in the regulation of the cardiovascular and nervous systems, where it has been identified as a potential therapeutic target for a number of conditions, including cardiac ischemia-reperfusion injury, cognition, epilepsy, and neuropathic pain. A₁R small molecule drugs, typically orthosteric ligands, have undergone clinical trials. To date, none have progressed into the clinic, predominantly due to dose-limiting unwanted effects. The development of A₁R allosteric modulators that target a topographically distinct binding site represent a promising approach to overcome current limitations. Pharmacological parameters of allosteric ligands, including affinity, efficacy and cooperativity, can be optimized to regulate A₁R activity with high subtype, spatial and temporal selectivity. This review aims to offer insights into the A₁R as a potential therapeutic target and highlight recent advances in the structural understanding of A₁R allosteric modulation.

Post-learning caffeine administration improves 'what-when' and 'what-where' components of episodic-like memory in rats

Episodic-like memory (ELM) consists in the capacity of nonhuman animals to remember 'where' and 'when' a specific episode occurred ('what'). Previous studies have showed that Wistar rats can form an ELM, but not after a 24 h retention delay. On the other hand, it has been demonstrated that caffeine can improve episodic memory consolidation in humans. Therefore, we verified whether acute post-sample caffeine administration could improve ELM consolidation in Wistar rats, as well if it could be related to neurochemical changes in the prefrontal cortex and hippocampus - regions related to episodic-like memory processing. 46 Male Wistar Rats, approximately 3 months-old, were divided into four groups as follows: untreated (n = 11), saline (n = 11), caffeine 10 mg ∕kg i.p (n = 12); caffeine 15 mg∕kgi.p (n = 12) and tested in WWWhen/ELM task. The animals treated with caffeine in different dosages (10 mg/kg and 15 mg/kg) discriminated temporally and spatially the objects, respectively. These groups also showed a dopamine renewal rate in the hippocampus, suggesting that there was an increase in the turnover compared with the groups with no caffeine administration. We can conclude that caffeine leads to an improvement in the consolidation of the temporal ('what-when') and spatial ('what-where') aspects of episodic-like memory.

Adenosine A1 and A2A receptors are involved on guanosine protective effects against oxidative burst and mitochondrial dysfunction induced by 6-OHDA in striatal slices

6-Hydroxydopamine (6-OHDA) is the most used toxin in experimental Parkinson's disease (PD) models. 6-OHDA shows high affinity for the dopamine transporter and once inside the neuron, it accumulates and undergoes non-enzymatic auto-oxidation, promoting reactive oxygen species (ROS) formation and selective damage of catecholaminergic neurons. In this way, our group has established a 6-OHDA in vitro protocol with rat striatal slices as a rapid and effective model for screening of new drugs with protective effects against PD. We have shown that co-incubation with guanosine (GUO, 100 μM) prevented the 6-OHDA-induced damage in striatal slices. As the exact GUO mechanism of action remains unknown, the aim of this study was to investigate if adenosine A1 (A1R) and/or A2A receptors (A2AR) are involved on GUO protective effects on striatal slices. Pre-incubation with DPCPX, an A1R antagonist prevented guanosine effects on 6-OHDA-induced ROS formation and mitochondrial membrane potential depolarization, while CCPA, an A1R agonist, did not alter GUO effects. Regarding A2AR, the antagonist SCH58261 had similar protective effect as GUO in ROS formation and mitochondrial membrane potential. Additionally, SCH58261 did not affect GUO protective effects. The A2AR agonist CGS21680, although, completely blocked GUO effects. Finally, the A1R antagonist DPCPX, and the A2AR agonist CGS21680 also abolished the preventive guanosine effect on 6-OHDA-induced ATP levels decrease. These results reinforce previous evidence for a putative interaction of GUO with A1R-A2AR heteromer as its molecular target and clearly indicate a dependence on adenosine receptors modulation to GUO protective effect.

A nonolfactory shark adenosine receptor activates CFTR with unique pharmacology and structural features

Adenosine receptors (ADORs) are G protein-coupled purinoceptors that have several functions including regulation of chloride secretion via cystic fibrosis transmembrane conductance regulator (CFTR) in human airway and kidney. We cloned an ADOR from Squalus acanthias (shark) that likely regulates CFTR in the rectal gland. Phylogenic and expression analyses indicate that elasmobranch ADORs are nonolfactory and appear to represent extant predecessors of mammalian ADORs. We therefore designate the shark ADOR as the A₀ receptor. We coexpressed A₀ with CFTR in Xenopus laevis oocytes and characterized the coupling of A₀ to the chloride channel. Two-electrode voltage clamping was performed, and current-voltage (I-V) responses were recorded to monitor CFTR status. Only in A₀- and CFTR-coinjected oocytes did adenosine analogs produce a significant concentration-dependent activation of CFTR consistent with its electrophysiological signature. A pharmacological profile for A₀ was obtained for ADOR agonists and antagonists that differed markedly from all mammalian ADOR subtypes [agonists: R-phenyl-isopropyl adenosine (R-PIA) > S-phenyl-isopropyl adenosine (S-PIA) > CGS21680 > N⁶-cyclopentyladenosine (CPA) > 2-chloroadenosine (2ClAdo) > CV1808 = N⁶-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine (DPMA) > N-ethyl-carboxyl adenosine (NECA); and antagonists: 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) > PD115199 > 1,3-dimethyl-8-phenylxanthine (8PT) > CGS15943]. Structures of human ADORs permitted a high-confidence homology model of the shark A₀ core that revealed unique structural features of ancestral receptors. We conclude that 1) A₀ is a novel and unique adenosine receptor ancestor by functional and structural criteria; 2) A₀ likely activates CFTR in vivo, and this receptor activates CFTR in oocytes, indicating an evolutionary coupling between ADORs and chloride secretion; and 3) A₀ appears to be a nonolfactory evolutionary ancestor of all four mammalian ADOR subtypes.

Allosteric Interactions between Adenosine A2A and Dopamine D2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A_2A and dopamine D₂ receptors (R). Stimulation of A_2AR inhibits and blockade of A_2AR enhances D₂R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D₂R via its interaction with A_2AR. Reciprocal A_2AR/D₂R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A_2AR and D₂R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor–receptor interactions within A_2AR/D₂R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson’s disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A_2AR/D₂R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

G protein-coupled receptors expressed and studied in yeast. The adenosine receptor as a prime example

G protein-coupled receptors (GPCRs) are the largest class of membrane proteins with around 800 members in the human genome/proteome. Extracellular signals such as hormones and neurotransmitters regulate various biological processes via GPCRs, with GPCRs being the bodily target of 30-40% of current drugs on the market. Complete identification and understanding of GPCR functionality will provide opportunities for novel drug discovery. Yeast expresses three different endogenous GPCRs regulating pheromone and sugar sensing, with the pheromone pathway offering perspectives for the characterization of heterologous GPCR signaling. Moreover, yeast offers a ''null" background for studies on mammalian GPCRs, including GPCR activation and signaling, ligand identification, and characterization of disease-related mutations. This review focuses on modifications of the yeast pheromone signaling pathway for functional GPCR studies, and on opportunities and usage of the yeast system as a platform for human GPCR studies. Finally, this review discusses in some further detail studies of adenosine receptors heterologously expressed in yeast, and what Geoff Burnstock thought of this approach.

Involvement of adenosine A1 and A2A receptors on guanosine-mediated anti-tremor effects in reserpinized mice

Parkinson's disease (PD) signs and symptoms regularly include tremor. Interestingly, the nucleoside guanosine (GUO) has already proven to be effective in reducing reserpine-induced tremulous jaw movements (TJMs) in rodent models, thus becoming a promising antiparkinsonian drug. Here, we aimed at revealing the mechanism behind GUO antiparkinsonian efficacy by assessing the role of adenosine A₁ and A_2A receptors (A₁R and A_2AR) on GUO-mediated anti-tremor effects in the reserpinized mouse model of PD. Reserpinized mice showed elevated reactive oxygen species (ROS) production and cellular membrane damage in striatal slices assessed ex vivo and GUO treatment reversed ROS production. Interestingly, while the simultaneous administration of sub-effective doses of GUO (5 mg/kg) and SCH58261 (0.01 mg/kg), an A_2AR antagonist, precluded reserpine-induced TJMs, these were ineffective on reverting ROS production in ex vivo experiments. Importantly, GUO was able to reduce TJM and ROS production in reserpinized mouse lacking the A_2AR, thus suggesting an A_2AR-independent mechanism of GUO-mediated effects. Conversely, the administration of DPCPX (0.75 mg/kg), an A₁R antagonist, completely abolished both GUO-mediated anti-tremor effects and blockade of ROS production. Overall, these results indicated that GUO anti-tremor and antioxidant effects in reserpinized mice were A₁R dependent but A_2AR independent, thus suggesting a differential participation of adenosine receptors in GUO-mediated effects.

Adenosine A2A and A3 Receptors Are Able to Interact with Each Other. A Further Piece in the Puzzle of Adenosine Receptor-Mediated Signaling

The aim of this paper was to check the possible interaction of two of the four purinergic P1 receptors, the A_2A and the A₃. Discovery of the A_2A-A₃ receptor complex was achieved by means of immunocytochemistry and of bioluminescence resonance energy transfer. The functional properties and heteromer print identification were addressed by combining binding and signaling assays. The physiological role of the novel heteromer is to provide a differential signaling depending on the pre-coupling to signal transduction components and/or on the concentration of the endogenous agonist. The main feature was that the heteromeric context led to a marked decrease of the signaling originating at A₃ receptors. Interestingly from a therapeutic point of view, A_2A receptor antagonists overrode the blockade, thus allowing A₃ receptor-mediated signaling. The A_2A-A₃ receptor heteromer print was detected in primary cortical neurons. These and previous results suggest that all four adenosine receptors may interact with each other. Therefore, each adenosine receptor could form heteromers with distinct properties, expanding the signaling outputs derived from the binding of adenosine to its cognate receptors.

Identification of new potent A1 adenosine receptor antagonists using a multistage virtual screening approach

The use of antagonists for each adenosine receptor (AR) subtype as potent clinical candidates is of growing interest due to their involvement in the treatment of various diseases. The recent resolution of several A₁ and A_2A ARs X-ray structures provides opportunities for structure-based drug design. In this study, we describe the discovery of novel A₁AR antagonists by applying a multistage virtual screening approach, which is based on random forest (RF), e-pharmacophore modeling and docking methods. A multistage virtual screening approach was applied to screen the ChemDiv library (1,492,362 compounds). Among the final hits, 22 compounds were selected for further radioligand binding assay analysis against human A₁AR, and 18 compounds (81.82% success) exhibited nanomolar or low micromolar binding potency (K_i). Then, we selected six compounds (pK_i > 6) to further evaluate their antagonist profile in a cAMP functional assay, and we found that they had low micromolar antagonistic activity (pIC₅₀ = 5.51-6.38) for the A₁AR. Particularly, four of six compounds (pK_i > 6) showed very good affinity (pK_i = 6.11-7.13) and selectively (>100-fold) for A₁AR over A_2AAR. Moreover, the novelty analysis suggested that four of six compounds (pK_i > 6) were dissimilar to existing A₁AR antagonists and hence represented novel A₁AR antagonists. Further molecular docking and molecular dynamics (MD) studies showed that the three selective compounds 15, 20 and 22 were stabilized (RMS_lig value ≤ 2 Å) inside the binding pocket of A₁AR with similar orientations to the docking pose in 100-ns MD simulations, whereas they escaped from the binding area of A_2AAR with larger values of RMS_lig (RMS_lig ≥ 2 Å). We hope that these findings provide new insights into the discovery of drugs targeting A₁AR and facilitate research on new drugs and treatments for A₁AR-related human pathologies.

Methoxy substituted 2-benzylidene-1-indanone derivatives as A1 and/or A2A AR antagonists for the potential treatment of neurological conditions

A prior study reported on hydroxy substituted 2-benzylidene-1-indanone derivatives as A1 and/or A2A antagonists for the potential treatment of neurological conditions. A lead compound (1a) was identified with both A1 and A2A affinity in the micromolar range. The current study explored the structurally related methoxy substituted 2-benzylidene-1-indanone derivatives with various substitutions on ring A and B of the benzylidene indanone scaffold in order to enhance A1 and A2A affinity. This led to compounds with both A1 and A2A affinity in the nanomolar range, namely 2c (A1 K i (rat) = 41 nM; A2A K i (rat) = 97 nM) with C4-OCH3 substitution on ring A together with meta (3') hydroxy substitution on ring B and 2e (A1 K i (rat) = 42 nM; A2A K i (rat) = 78 nM) with C4-OCH3 substitution on ring A together with meta (3') and para (4') dihydroxy substitution on ring B. Additionally, 2c is an A1 antagonist. Consequently, the methoxy substituted 2-benzylidene-1-indanone scaffold is highly promising for the design of novel A1 and A2A antagonists.

Coffee, caffeine, chlorogenic acid, and the purinergic system

Coffee is a drink prepared from roasted coffee beans and is lauded for its aroma and flavour. It is the third most popular beverage in the world. This beverage is known by its stimulant effect associated with the presence of methylxanthines. Caffeine, a purine-like molecule (1,3,7 trymetylxantine), is the most important bioactive compound in coffee, among others such as chlorogenic acid (CGA), diterpenes, and trigonelline. CGA is a phenolic acid with biological properties as antioxidant, anti-inflammatory, neuroprotector, hypolipidemic, and hypoglicemic. Purinergic system plays a key role inneuromodulation and homeostasis. Extracellular ATP, other nucleotides and adenosine are signalling molecules that act through their specific receptors, namely purinoceptors, P1 for nucleosides and P2 for nucleotides. They regulate many pathological processes, since adenosine, for instance, can limit the damage caused by ATP in the excitotoxicity from the neuronal cells. The primary purpose of this review is to discuss the effects of coffee, caffeine, and CGA on the purinergic system. This review focuses on the relationship/interplay between coffee, caffeine, CGA, and adenosine, and their effects on ectonucleotidases activities as well as on the modulation of P1 and P2 receptors from central nervous system and also in peripheral tissue.

Synaptic transmission and excitability during hypoxia with inflammation and reoxygenation in hippocampal CA1 neurons

Although a number of experimental and clinical studies have shown that hypoxia typically accompanies acute inflammatory responses, the combinatorial effect of the two insults on basic neural function has not been thoroughly investigated. Previous studies have predominantly suggested that hypoxia reduces network activity; however, several studies suggest the opposite effect. Of note, inflammation is known to increase neural activity. In the current study, we examined the effects of limited oxygen in combination with an inflammatory stimulus, as well as the effects of reoxygenation, on synaptic transmission and excitability. We observed a significant reduction of both synaptic transmission and excitability when hypoxia and inflammation occurred in combination, whereas reoxygenation caused hyperexcitability of neurons. Further, we found that the observed reduction in synaptic transmission was due to compromised presynaptic release efficiency based on an adenosine-receptor-dependent increase in synaptic facilitation. Excitability changes in both directions were attributable to dynamic regulation of the hyperpolarization-activated cation current (I_h) and to changes in the input resistance and the voltage difference between resting membrane potential and action potential threshold. We found that zatebradine, an I_h current inhibitor, reduced the fluctuation in excitability, suggesting that it may have potential as a drug to ameliorate reperfusion brain injury.

Functional roles of three cues that provide nonsynaptic modes of communication in the brain: electromagnetic field, oxygen, and carbon dioxide

The integrative actions of the brain depend on the exchange of information among its computational elements. Hence, this phenomenon plays the key role in driving the complex dynamics of the central nervous system, in which true computations interact with noncomputational dynamical processes to generate brain representations of the body and of the body in the external world, and hence the finalistic behavior of the organism. In this context, it should be pointed out that, besides the intercellular interactions mediated by classical electrochemical signals, other types of interactions, namely, "cues" and "coercions," also appear to be exploited by the system to achieve its function. The present review focuses mainly on cues present in the environment and on those produced by cells of the body, which "pervade" the brain and contribute to its dynamics. These cues can also be metabolic substrates, and, in most cases, they are of fundamental importance to brain function and the survival of the entire organism. Three of these highly pervasive cues will be analyzed in greater detail, namely, oxygen, carbon dioxide, and electromagnetic fields (EMF). Special emphasis will be placed on EMF, since several authors have suggested that these highly pervasive energy fluctuations may play an important role in the global integrative actions of the brain; hence, EMF signaling may transcend classical connectionist models of brain function. Thus the new concept of "broadcasted neuroconnectomics" has been introduced, which transcends the current connectomics view of the brain.

5-Substituted 2-benzylidene-1-tetralone analogues as A1 and/or A2A antagonists for the potential treatment of neurological conditions

Adenosine A₁ and A_2A receptors are attracting great interest as drug targets for their role in cognitive and motor deficits, respectively. Antagonism of both these adenosine receptors may offer therapeutic benefits in complex neurological diseases, such as Alzheimer's and Parkinson's disease. The aim of this study was to explore the affinity and selectivity of 2-benzylidene-1-tetralone derivatives as adenosine A₁ and A_2A receptor antagonists. Several 5-hydroxy substituted 2-benzylidene-1-tetralone analogues with substituents on ring B were synthesized and assessed as antagonists of the adenosine A₁ and A_2A receptors via radioligand binding assays. The results indicated that hydroxy substitution in the meta and para position of phenyl ring B, displayed the highest selectivity and affinity for the adenosine A₁ receptor with K_i values in the low micromolar range. Replacement of ring B with a 2-amino-pyrimidine moiety led to compound 12 with an increase of affinity and selectivity for the adenosine A_2A receptor. These substitution patterns led to enhanced adenosine A₁ and A_2A receptor binding affinity. The para-substituted 5-hydroxy analogue 3 behaved as an adenosine A₁ receptor antagonists in a GTP shift assay performed with rat whole brain membranes expressing adenosine A₁ receptors. In conclusion, compounds 3 and 12, showed the best adenosine A₁ and A_2A receptor affinity respectively, and therefore represent novel adenosine receptor antagonists that may have potential with further structural modifications as drug candidates for neurological disorders.

Evaluation of 2-benzylidene-1-tetralone derivatives as antagonists of A1 and A2A adenosine receptors

Antagonists of the adenosine receptors (A₁ and A_2A ) are thought to be beneficial in neurological disorders, such as Alzheimer's and Parkinson's disease. The aim of this study was to explore 2-benzylidene-1-tetralone derivatives as antagonists of A₁ and/or A_2A adenosine receptors. In general, the test compounds were found to be selective for the A₁ adenosine receptor, with only three test compounds possessing affinity for both the A₁ and A_2A adenosine receptor. The 2-benzylidene-1-tetralones bearing a hydroxyl substituent at either position C5, C6 or C7 of ring A displayed favourable adenosine A₁ receptor binding, while C5 hydroxy substitution led to favourable A_2A adenosine receptor affinity. Interestingly, para-hydroxy substitution on ring B in combination with ring A bearing a hydroxy at position C6 or C7 provided the 2-benzylidene-1-tetralones with both A₁ and A_2A adenosine receptor affinity. Compounds 4 and 8 displayed the highest A₁ and A_2A adenosine receptor affinity with values below 7 μm. Both these compounds behaved as A₁ adenosine receptor antagonists in the performed GTP shift assays. In conclusion, the 2-benzylidene-1-tetralone derivatives can be considered as lead compounds to design a new class of dual acting adenosine A₁ /A_2A receptor antagonists that may have potential in treating both dementia and locomotor deficits in Parkinson's disease.

Preclinical Evaluation and Quantification of 18F-Fluoroethyl and 18F-Fluoropropyl Analogs of SCH442416 as Radioligands for PET Imaging of the Adenosine A2A Receptor in Rat Brain

The cerebral adenosine A_2A receptor is an attractive therapeutic target for neuropsychiatric disorders. ¹⁸F-fluoroethyl and ¹⁸F-fluoropropyl analogs of ¹⁸F-labeled pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH442416) (¹⁸F-FESCH and ¹⁸F-FPSCH, respectively) were developed as A_2A receptor-specific PET ligands. Our aim was to determine an appropriate compartmental model for tracer kinetics, evaluate a reference tissue approach, and select the most suitable PET ligand. Methods: A 90-min dynamic PET scan with arterial blood sampling and metabolite analysis was acquired for 22 healthy male Wistar rats starting at the time of ¹⁸F-FESCH (n = 12) and ¹⁸F-FPSCH (n = 10) injection. For each tracer, half the animals were vehicle-treated whereas the other half were pretreated with the A_2A receptor-selective antagonist KW-6002, inducing full blocking. Regional tissue total volume of distribution (V_T) was estimated by 1- and 2-tissue-compartment modeling (1TCM and 2TCM, respectively) and Logan graphical analysis. Midbrain, cerebellum, and hippocampus were evaluated as the reference region by comparing baseline V_T with V_T under full blocking conditions and comparing striatal nondisplaceable binding potential (BP_ND) using a simplified reference tissue model (SRTM) with distribution volume ratio minus 1 (DVR - 1) for 60- and 90-min scans. Results: On the basis of the Akaike information criterion, 1TCM and 2TCM were the most appropriate models for ¹⁸F-FPSCH (baseline striatal V_T, 3.7 ± 1.1) and ¹⁸F-FESCH (baseline striatal V_T, 5.0 ± 2.0), respectively. Baseline striatal V_T did not significantly differ between tracers. After pretreatment, striatal V_T was reduced significantly, with no significant decrease in hippocampus, midbrain, or cerebellum V_T Baseline striatal SRTM BP_ND did not differ significantly from DVR - 1 except for ¹⁸F-FPSCH when using a 60-min scan and midbrain as the reference region, whereas Bland-Altman analysis found a smaller bias for ¹⁸F-FESCH and a 60-min scan. After pretreatment, striatal SRTM BP_ND did not significantly differ from zero except for ¹⁸F-FPSCH when using hippocampus as the reference region. Striatal SRTM BP_ND using midbrain or cerebellum as the reference region was significantly lower for ¹⁸F-FPSCH (range, 1.41-2.62) than for ¹⁸F-FESCH (range, 1.64-3.36). Conclusion: Dynamic PET imaging under baseline and blocking conditions determined ¹⁸F-FESCH to be the most suitable PET ligand for quantifying A_2A receptor expression in the rat brain. Accurate quantification is achieved by a 60-min dynamic PET scan and the use of either cerebellum or midbrain as the reference region.

Extracellular Loop 2 of the Adenosine A1 Receptor Has a Key Role in Orthosteric Ligand Affinity and Agonist Efficacy

The adenosine A₁ G protein-coupled receptor (A₁AR) is an important therapeutic target implicated in a wide range of cardiovascular and neuronal disorders. Although it is well established that the A₁AR orthosteric site is located within the receptor's transmembrane (TM) bundle, prior studies have implicated extracellular loop 2 (ECL2) as having a significant role in contributing to orthosteric ligand affinity and signaling for various G protein-coupled receptors (GPCRs). We thus performed extensive alanine scanning mutagenesis of A₁AR-ECL2 to explore the role of this domain on A₁AR orthosteric ligand pharmacology. Using quantitative analytical approaches and molecular modeling, we identified ECL2 residues that interact either directly or indirectly with orthosteric agonists and antagonists. Discrete mutations proximal to a conserved ECL2-TM3 disulfide bond selectively affected orthosteric ligand affinity, whereas a cluster of five residues near the TM4-ECL2 juncture influenced orthosteric agonist efficacy. A combination of ligand docking, molecular dynamics simulations, and mutagenesis results suggested that the orthosteric agonist 5'-N-ethylcarboxamidoadenosine binds transiently to an extracellular vestibule formed by ECL2 and the top of TM5 and TM7, prior to entry into the canonical TM bundle orthosteric site. Collectively, this study highlights a key role for ECL2 in A₁AR orthosteric ligand binding and receptor activation.

The Effects of Hypoxia and Inflammation on Synaptic Signaling in the CNS

Normal brain function is highly dependent on oxygen and nutrient supply and when the demand for oxygen exceeds its supply, hypoxia is induced. Acute episodes of hypoxia may cause a depression in synaptic activity in many brain regions, whilst prolonged exposure to hypoxia leads to neuronal cell loss and death. Acute inadequate oxygen supply may cause anaerobic metabolism and increased respiration in an attempt to increase oxygen intake whilst chronic hypoxia may give rise to angiogenesis and erythropoiesis in order to promote oxygen delivery to peripheral tissues. The effects of hypoxia on neuronal tissue are exacerbated by the release of many inflammatory agents from glia and neuronal cells. Cytokines, such as TNF-α, and IL-1β are known to be released during the early stages of hypoxia, causing either local or systemic inflammation, which can result in cell death. Another growing body of evidence suggests that inflammation can result in neuroprotection, such as preconditioning to cerebral ischemia, causing ischemic tolerance. In the following review we discuss the effects of acute and chronic hypoxia and the release of pro-inflammatory cytokines on synaptic transmission and plasticity in the central nervous system. Specifically we discuss the effects of the pro-inflammatory agent TNF-α during a hypoxic event.

Involvement of purinergic system in the release of cytokines by macrophages exposed to glioma-conditioned medium

Macrophages are involved in cancer progression. M1 macrophages have an antitumor effect, whereas M2 phenotype are associated with tumor growth. The progression of gliomas involves the participation of an inflammatory microenvironment. Adenosine triphosphate (ATP) can act as pro-inflammatory signal, whereas adenosine has opposite properties. The biological effects of extracellular nucleotides/nucleosides mediated by purinergic receptors are controlled by ectonucleotidases. In the present work, we evaluated whether glioma-conditioned medium (GL-CM) modulates macrophage differentiation and the participation of ATP and adenosine in the release of pro-and anti-inflammatory cytokines by these cells. The results show that macrophages exposed to GL-CM were modulated to an M2-like phenotype. HPLC analysis of GL-CM demonstrated the presence of significant amounts of ATP and its metabolites. Macrophages exposed to GL-CM presented decreased ATP and AMP hydrolysis and increased IL-10 and MCP-1 secretion, effects that were diminished by P1 or P2 antagonists. GL-CM did not alter the release of IL-6 by macrophages, although treatment with ATP promoted an increase in the release of IL-6, which was prevented by a P2X7 antagonist. In summary, we found that A2A and P2X7 activation is necessary for IL-10, MCP-1, and IL-6 release by macrophages exposed to GL-CM, which, in turn, modulates the macrophages to M2-phenotype. The present study establishes a relationship between M2-like polarization, cytokine release and purinergic receptor activation in macrophages exposed to GL-CM. Therefore, the data presented herein contributes to advancing in the field of cancer-related inflammation and point specific purinergic receptors as targets for modulation of the phenotype of glioma-associated macrophages.

Association of ADORA1 rs2228079 and ADORA2A rs5751876 Polymorphisms with Gilles de la Tourette Syndrome in the Polish Population

BACKGROUND

Gilles de la Tourette syndrome (GTS) is a neurodevelopmental disorder characterized by motor and vocal tics. Hyperactivity of dopaminergic transmission is considered a prime abnormality in the pathophysiology of tics. There are reciprocal antagonistic interactions between adenosine and dopamine transmission. The aim of the study was to analyze the association of two polymorphisms, rs2228079 in ADORA1 and rs5751876 in ADORA2A, with the risk of GTS and co-morbid disorders.

MATERIAL AND METHODS

A total of 162 Polish GTS patients and 270 healthy persons were enrolled in the study. Two polymorphisms were selected on the basis of knowledge of SNPs frequencies in ADORA1 and ADORA2A. Chi-square test was used for allelic and genotypic association studies. Association of genotypes with age of tic onset was analyzed with Mann-Whitney test. Multivariate logistic regression was used to find independent predictors of GTS risk.

RESULTS

We found that the risk of GTS was associated with rs2228079 and rs5751876 polymorphisms. The GG+GT genotypes of rs2228079 in ADORA1 were underrepresented in GTS patients (p = 0.011), whereas T allele of rs5751876 in ADORA2A was overrepresented (p = 0.017). The GG genotype of rs2228079 was associated with earlier age of tic onset (p = 0.046). We found also that the minor allele G of rs2228079 was more frequent in GTS patients with depression as compared to the patients without depression (p = 0.015). Also the genotype GG was significantly more frequent in patients with obsessive compulsive disorder/behavior (OCD/OCB, p = 0.021) and depression (p = 0.032), as compared to the patients without these co-morbidities. The minor allele T frequency of rs5751876 was lower in GTS patients with co-morbid attention deficit hyperactivity disorder (p = 0.022), and TT+TC genotypes were less frequent in the non-OCD anxiety disorder group (p = 0.045).

CONCLUSION

ADORA1 and ADORA2A variants are associated with the risk of GTS, co-morbid disorders, and may affect the age of tic onset.

Photomodulation of G protein-coupled adenosine receptors by a novel light-switchable ligand

The adenosinergic system operates through G protein-coupled adenosine receptors, which have become promising therapeutic targets for a wide range of pathological conditions. However, the ubiquity of adenosine receptors and the eventual lack of selectivity of adenosine-based drugs have frequently diminished their therapeutic potential. Accordingly, here we aimed to develop a new generation of light-switchable adenosine receptor ligands that change their intrinsic activity upon irradiation, thus allowing the spatiotemporal control of receptor functioning (i.e., receptor activation/inactivation dependent on location and timing). Therefore, we synthesized an orthosteric, photoisomerizable, and nonselective adenosine receptor agonist, nucleoside derivative MRS5543 containing an aryl diazo linkage on the N⁶ substituent, which in the dark (relaxed isomer) behaved as a full adenosine A₃ receptor (A₃R) and partial adenosine A_2A receptor (A_2AR) agonist. Conversely, upon photoisomerization with blue light (460 nm), it remained a full A₃R agonist but became an A_2AR antagonist. Interestingly, molecular modeling suggested that structural differences encountered within the third extracellular loop of each receptor could modulate the intrinsic, receptor subtype-dependent, activity. Overall, the development of adenosine receptor ligands with photoswitchable activity expands the pharmacological toolbox in support of research and possibly opens new pharmacotherapeutic opportunities.

[Stimulation of adenosine receptors on myeloid cells enhance leukocyte migration at the site of burn injury]

Adenosine, endogenous purine nucleoside, is an ATP metabolite that also acts as an extracellular signaling molecule. The concentration of extracellular adenosine rises during hypoxia and cell damage leading to numerous pleiotropic effects. Although a high concentration of adenosine was found at burn injury, the effect has not been well elucidated. We have studied human peripheral blood myeloid cell, due to their expression of specific adenosine receptors and capacity to migrate to the site of burn injury. We have shown that myeloid cells after 72 hours of stimulation of adenosine receptors develop altered expression of surface antigens: preserved monocyte's marker CD14 with already expressed dendritic cell markers (CD209, CD1a). Whereas untreated cells have already lost monocyte marker in 72 hours, and express CD1a more abundantly. Adenosine modified myeloid cells express also higher levels of mRNA of proinflammatory cytokines and chemoattractants (IL-6, IL-8, IL-1 b). Using mouse model of the burn injury we have shown, that adenosine modified bone marrow derived myeloid cells injected in the site of the injury promote migration of granulocytes, monocytes, macrophages, and fibroblasts on the 7th day after burn. Thus, stimulation of adenosine receptors alters differentiation and function of myeloid cells. In the site of burn injury adenosine modified myeloid cells augment cell migration due to paracrine factors.

Modulation of Ca2+-currents by sequential and simultaneous activation of adenosine A1 and A 2A receptors in striatal projection neurons

D(1)- and D(2)-types of dopamine receptors are located separately in direct and indirect pathway striatal projection neurons (dSPNs and iSPNs). In comparison, adenosine A(1)-type receptors are located in both neuron classes, and adenosine A(2A)-type receptors show a preferential expression in iSPNs. Due to their importance for neuronal excitability, Ca(2+)-currents have been used as final effectors to see the function of signaling cascades associated with different G protein-coupled receptors. For example, among many other actions, D(1)-type receptors increase, while D(2)-type receptors decrease neuronal excitability by either enhancing or reducing, respectively, CaV1 Ca(2+)-currents. These actions occur separately in dSPNs and iSPNs. In the case of purinergic signaling, the actions of A(1)- and A(2A)-receptors have not been compared observing their actions on Ca(2+)-channels of SPNs as final effectors. Our hypotheses are that modulation of Ca(2+)-currents by A(1)-receptors occurs in both dSPNs and iSPNs. In contrast, iSPNs would exhibit modulation by both A(1)- and A2A-receptors. We demonstrate that A(1)-type receptors reduced Ca(2+)-currents in all SPNs tested. However, A(2A)-type receptors enhanced Ca(2+)-currents only in half tested neurons. Intriguingly, to observe the actions of A(2A)-type receptors, occupation of A(1)-type receptors had to occur first. However, A(1)-receptors decreased Ca(V)2 Ca(2+)-currents, while A(2A)-type receptors enhanced current through Ca(V)1 channels. Because these channels have opposing actions on cell discharge, these differences explain in part why iSPNs may be more excitable than dSPNs. It is demonstrated that intrinsic voltage-gated currents expressed in SPNs are effectors of purinergic signaling that therefore play a role in excitability.

New approaches to thyroid hormones and purinergic signaling

It is known that thyroid hormones influence a wide variety of events at the molecular, cellular, and functional levels. Thyroid hormones (TH) play pivotal roles in growth, cell proliferation, differentiation, apoptosis, development, and metabolic homeostasis via thyroid hormone receptors (TRs) by controlling the expression of TR target genes. Most of these effects result in pathological and physiological events and are already well described in the literature. Even so, many recent studies have been devoted to bringing new information on problems in controlling the synthesis and release of these hormones and to elucidating mechanisms of the action of these hormones unconventionally. The purinergic system was recently linked to thyroid diseases, including enzymes, receptors, and enzyme products related to neurotransmitter release, nociception, behavior, and other vascular systems. Thus, throughout this text we intend to relate the relationship between the TH in physiological and pathological situations with the purinergic signaling.

Resveratrol mediates anti-atherogenic effects on cholesterol flux in human macrophages and endothelium via PPARγ and adenosine

Resveratrol is a bioactive molecule used in dietary supplements and herbal medicines and consumed worldwide. Known cardioprotective and anti-inflammatory properties of resveratrol have spurred investigation of the mechanisms involved. The present study explored potential atheroprotective actions of resveratrol on cholesterol metabolism in cells of the arterial wall, including human macrophages and arterial endothelium. Using QRT-PCR and Western blotting techniques, we measured expression of the proteins involved in reverse cholesterol transport (ABCA1, ABCG1 and SR-B1) and the scavenger receptors responsible for uptake of modified cholesterol (CD36, SR-A1 and LOX-1). We analyzed the effect of resveratrol on apoA-1-and HDL-mediated cholesterol efflux in human THP-1 macrophages. The effect of resveratrol on oxLDL internalization and foam cell formation were evaluated using confocal and light microscopy. Our data indicate that resveratrol regulates expression of major proteins involved in cholesterol transport, promotes apoA-1 and HDL-mediated efflux, downregulates oxLDL uptake and diminishes foam cell formation. Mechanistically, resveratrol effects were dependent upon PPAR-γ and adenosine 2A receptor pathways. For the first time we demonstrate that resveratrol regulates expression of the cholesterol metabolizing enzyme cytochrome P450 27-hydroxylase, providing efficient cholesterol elimination via formation of oxysterols. This study establishes that resveratrol attenuates lipid accumulation in cultured human macrophages via effects on cholesterol transport. Further in vivo studies are needed to determine whether resveratrol may be an additional resource available to reduce lipid deposition and atherosclerosis in humans.

G protein-coupled receptor oligomerization and brain integration: focus on adenosinergic transmission

The control of glutamatergic corticostriatal transmission is essential for the induction and expression of plasticity mechanisms in the striatum, a phenomenon thickly regulated by G protein-coupled receptors (GPCRs). Interestingly, in addition to dopamine receptors, adenosine and metabotropic glutamate receptors also play a key role in striatal functioning. The existence of a supramolecular organization (i.e. oligomer) containing dopamine, adenosine and metabotropic glutamate receptors in the striatal neurons is now being widely accepted by the scientific community. Indeed, these oligomers may enhance the diversity and performance by which extracellular striatal signals are transferred to the G-proteins in the process of receptor transduction, and also may allow unpredictable receptor-receptor allosteric regulations. Overall, here we want to review how formations of adenosine, dopamine and metabotropic glutamate receptors-containing oligomers impinge into striatal functioning in both normal and pathological conditions. This article is part of a Special Issue entitled: Brain Integration.

Purine nucleosides: endogenous neuroprotectants in hypoxic brain

Even a short blockade of oxygen flow in brain may lead to the inhibition of oxidative phosphorylation and depletion of cellular ATP, which results in profound deficiencies in cellular function. Following ischemia, dying, injured, and hypoxic cells release soluble purine-nucleotide and -nucleoside pools. Growing evidence suggests that purine nucleosides might act as trophic factors in the CNS and PNS. In addition to equilibrative nucleoside transporters (ENTs) regulating purine nucleoside concentrations intra- and extracellularly, specific extracellular receptor subtypes for these compounds are expressed on neurons, glia, and endothelial cells, mediating stunningly diverse effects. Such effects range from induction of cell differentiation, apoptosis, mitogenesis, and morphogenetic changes, to stimulation of synthesis and/or release of cytokines and neurotrophic factors under both physiological and pathological conditions. Multiple signaling pathways regulate the critical balance between cell death and survival in hypoxia-ischemia. A convergent pathway for the regulation of multiple modalities involved in O₂ sensing is the mitogen activated protein kinase (p42/44 MAPK) or (ERK1/2 extracellular signal-regulated kinases) pathway terminating in a variety of transcription factors, for example, hypoxia-inducible factor 1α. In this review, the coherence of purine nucleoside-related pathways and MAPK activation in the endogenous neuroprotective regulation of the nervous system's development and neuroplasticity under hypoxic stress will be discussed.

Mechanism of nitrogen metabolism-related parameters and enzyme activities in the pathophysiology of autism

BACKGROUND

There is evidence that impaired metabolism play an important role in the etiology of many neuropsychiatric disorders. Although this has not been investigated to date, several recent studies proposed that nitrogen metabolism-related parameters may have a pathophysiological role in autism.

METHODS

The study enrolled 20 Saudi boys with autism aged 4 to 12 years and 20 healthy controls matched for age and gender. Levels of creatine, urea, ammonia, gamma-aminobutyric acid (GABA), glutamate:glutamine (Glu:Gln) ratio, and enzymatic activities of glutamate dehydrogenase, 5'-nucleotidase, and adenosine deaminase (ADA) were determined in plasma samples from both groups.

RESULTS

We found a significant elevation of creatine, 5'-nucleotidase, GABA, and glutamic acid and a significant decrease in the enzymatic activity of ADA and glutamine level in patients with autism compared with healthy controls. The most significant variation between the two groups was found in the Glu:Gln ratio.

CONCLUSION

A raised Glu:Gln ratio together with positive correlations in creatine, GABA, and 5'-nucleotidase levels could contribute to the pathophysiology of autism, and might be useful diagnostic markers. The mechanism through which these parameters might be related to autism is discussed in detail.

CD73 protects kidney from ischemia-reperfusion injury through reduction of free radicals

Renal ischemia-reperfusion injury (IRI) may cause severe systemic diseases. Extracellular adenosine is anti-inflammatory especially during hypoxemia. As ecto-5'-nucleotidase (CD73) is the rate-limiting enzyme for extracellular adenosine generation, it may protect renal IRI through adenosine production. In the current studies, we investigated the effects of CD73 in genetically modified mice. We found that renal IRI caused more serious histological injury, vascular permeability, and lipid peroxidation in CD73(-/-) than that in CD73(+/+) mice. In addition, AMP and free radical concentrations were much higher in CD73(-/-) than that in CD73(+/+) mice. Our data support the fact that CD73 may protect the kidney from IRI through adenosine production and a reduction of free radicals.

Regulation of G protein-coupled receptor function by Na+/H+ exchange regulatory factors

Many G protein-coupled receptors (GPCR) exert patterns of cell-specific signaling and function. Mounting evidence now supports the view that cytoplasmic adapter proteins contribute critically to this behavior. Adapter proteins recognize highly conserved motifs such as those for Src homology 3 (SH3), phosphotyrosine-binding (PTB), and postsynaptic density 95/discs-large/zona occludens (PDZ) docking sequences in candidate GPCRs. Here we review the behavior of the Na+/H+ exchange regulatory factor (NHERF) family of PDZ adapter proteins on GPCR signalling, trafficking, and function. Structural determinants of NHERF proteins that allow them to recognize targeted GPCRs are considered. NHERF1 and NHERF2 are capable also of modifying the assembled complex of accessory proteins such as β-arrestins, which have been implicated in regulating GPCR signaling. In addition, NHERF1 and NHERF2 modulate GPCR signaling by altering the G protein to which the receptor binds or affect other regulatory proteins that affect GTPase activity, protein kinase A, phospholipase C, or modify downstream signaling events. Small molecules targeting the site of NHERF1-GPCR interaction are being developed and may become important and selective drug candidates.

Adenosine receptor containing oligomers: their role in the control of dopamine and glutamate neurotransmission in the brain

While the G protein-coupled receptor (GPCR) oligomerization has been questioned during the last fifteen years, the existence of a multi-receptor complex involving direct receptor-receptor interactions, called receptor oligomers, begins to be widely accepted. Eventually, it has been postulated that oligomers constitute a distinct functional form of the GPCRs with essential receptorial features. Also, it has been proven, under certain circumstances, that the GPCR oligomerization phenomenon is crucial for the receptor biosynthesis, maturation, trafficking, plasma membrane diffusion, and pharmacology and signalling. Adenosine receptors are GPCRs that mediate the physiological functions of adenosine and indeed these receptors do also oligomerize. Accordingly, adenosine receptor oligomers may improve the molecular mechanism by which extracellular adenosine signals are transferred to the G proteins in the process of receptor transduction. Importantly, these adenosine receptor-containing oligomers may allow not only the control of the adenosinergic function but also the fine-tuning modulation of other neurotransmitter systems (i.e. dopaminergic and glutamatergic transmission). Overall, we underscore here recent significant developments based on adenosine receptor oligomerization that are essential for acquiring a better understanding of neurotransmission in the central nervous system under normal and pathological conditions.

Optimization of arylindenopyrimidines as potent adenosine A(2A)/A(1) antagonists

Two reactive metabolites were identified in vivo for the dual A(2A)/A(1) receptor antagonist 1. Two strategies were implemented to successfully mitigate the metabolic liabilities associated with 1. Optimization of the arylindenopyrimidines led to a number of amide, ether, and amino analogs having comparable in vitro and in vivo activity.

Adenosine receptors interacting proteins (ARIPs): Behind the biology of adenosine signaling

Adenosine is a well known neuromodulator in the central nervous system. As a consequence, adenosine can be beneficial in certain disorders and adenosine receptors will be potential targets for therapy in a variety of diseases. Adenosine receptors are G protein-coupled receptors, and are also expressed in a large variety of cells and tissues. Using these receptors as a paradigm of G protein-coupled receptors, the present review focus on how protein-protein interactions might contribute to neurotransmitter/neuromodulator regulation, based on the fact that accessory proteins impinge on the receptor/G protein interaction and therefore modulate receptor functioning. Besides affecting receptor signaling, these accessory components also play a key role in receptor trafficking, internalization and desensitization, as it will be reviewed here. In conclusion, the finding of an increasing number of adenosine receptors interacting proteins, and specially the molecular and functional integration of these accessory proteins into receptorsomes, will open new perspectives in the understanding of particular disorders where these receptors have been proved to be involved.

Regulation of leukocyte migration across endothelial barriers by ECTO-5'-nucleotidase-generated adenosine

CD73-deficient mice are valuable for evaluating the ability of CD73-generated adenosine to modulate adenosine receptor-mediated responses. Here we report the role of CD73 in regulating lymphocyte migration across two distinct barriers. In the first case, CD73-generated adenosine restricts the migration of lymphocytes across high endothelial venules (HEV) into draining lymph nodes after an inflammatory stimulus, apparently by triggering A(2B) receptors on HEV. Secondly, CD73 promotes the migration of pathogenic T cells into the central nervous system during experimental autoimmune encephalomyelitis. Experiments are in progress to determine whether this effect is also adenosine receptor-mediated and to identify the relevant adenosine receptor.