Restricted collision coupling of the A2A receptor revisited: evidence for physical separation of two signaling cascades

Low Density Lipoprotein Cholesterol Decreases the Expression of Adenosine A2A Receptor and Lipid Rafts-Protein Flotillin-1: Insights on Cardiovascular Risk of Hypercholesterolemia

High blood levels of low-density lipoprotein (LDL)-cholesterol (LDL-C) are associated with atherosclerosis, mainly by promoting foam cell accumulation in vessels. As cholesterol is an essential component of cell plasma membranes and a regulator of several signaling pathways, LDL-C excess may have wider cardiovascular toxicity. We examined, in untreated hypercholesterolemia (HC) patients, selected regardless of the cause of LDL-C accumulation, and in healthy participants (HP), the expression of the adenosine A2A receptor (A2AR), an anti-inflammatory and vasodilatory protein with cholesterol-dependent modulation, and Flotillin-1, protein marker of cholesterol-enriched plasma membrane domains. Blood cardiovascular risk and inflammatory biomarkers were measured. A2AR and Flotillin-1 expression in peripheral blood mononuclear cells (PBMC) was lower in patients compared to HP and negatively correlated to LDL-C blood levels. No other differences were observed between the two groups apart from transferrin and ferritin concentrations. A2AR and Flotillin-1 proteins levels were positively correlated in the whole study population. Incubation of HP PBMCs with LDL-C caused a similar reduction in A2AR and Flotillin-1 expression. We suggest that LDL-C affects A2AR expression by impacting cholesterol-enriched membrane microdomains. Our results provide new insights into the molecular mechanisms underlying cholesterol toxicity, and may have important clinical implication for assessment and treatment of cardiovascular risk in HC.

Cathepsin D interacts with adenosine A2A receptors in mouse macrophages to modulate cell surface localization and inflammatory signaling

Adenosine A_2A receptor (A_2AR)–dependent signaling in macrophages plays a key role in the regulation of inflammation. However, the processes regulating A_2AR targeting to the cell surface and degradation in macrophages are incompletely understood. For example, the C-terminal domain of the A_2AR and proteins interacting with it are known to regulate receptor recycling, although it is unclear what role potential A_2AR-interacting partners have in macrophages. Here, we aimed to identify A_2AR-interacting partners in macrophages that may effect receptor trafficking and activity. To this end, we performed a yeast two-hybrid screen using the C-terminal tail of A_2AR as the “bait” and a macrophage expression library as the “prey.” We found that the lysosomal protease cathepsin D (CtsD) was a robust hit. The A_2AR–CtsD interaction was validated in vitro and in cellular models, including RAW 264.7 and mouse peritoneal macrophage (IPMΦ) cells. We also demonstrated that the A_2AR is a substrate of CtsD and that the blockade of CtsD activity increases the density and cell surface targeting of A_2AR in macrophages. Conversely, we demonstrate that A_2AR activation prompts the maturation and enzymatic activity of CtsD in macrophages. In summary, we conclude that CtsD is a novel A_2AR-interacting partner and thus describe molecular and functional interplay that may be crucial for adenosine-mediated macrophage regulation in inflammatory processes.

Repurposing Dipyridamole in Niemann Pick Type C Disease: A Proof of Concept Study

Niemann Pick type C disease (NPC) is a rare disorder characterized by lysosomal lipid accumulation that damages peripheral organs and the central nervous system. Currently, only miglustat is authorized for NPC treatment in Europe, and thus the identification of new therapies is necessary. The hypothesis addressed in this study is that increasing adenosine levels may represent a new therapeutic approach for NPC. In fact, a reduced level of adenosine has been shown in the brain of animal models of NPC; moreover, the compound T1-11, which is able to weakly stimulate A_2A receptor and to increase adenosine levels by blocking the equilibrative nucleoside transporter ENT1, significantly ameliorated the pathological phenotype and extended the survival in a mouse model of the disease. To test our hypothesis, fibroblasts from NPC1 patients were treated with dipyridamole, a clinically-approved drug with inhibitory activity towards ENT1. Dipyridamole significantly reduced cholesterol accumulation in fibroblasts and rescued mitochondrial deficits; the mechanism elicited by dipyridamole relies on activation of the adenosine A_2AR subtype subsequent to the increased levels of extracellular adenosine due to the inhibition of ENT1. In conclusion, our results provide the proof of concept that targeting adenosine tone could be beneficial in NPC.

Allosteric modulation of the adenosine A2A receptor by cholesterol

Cholesterol is a major component of the cell membrane and commonly regulates membrane protein function. Here, we investigate how cholesterol modulates the conformational equilibria and signaling of the adenosine A2A receptor (A2AR) in reconstituted phospholipid nanodiscs. This model system conveniently excludes possible effects arising from cholesterol-induced phase separation or receptor oligomerization and focuses on the question of allostery. GTP hydrolysis assays show that cholesterol weakly enhances the basal signaling of A2AR while decreasing the agonist EC50. Fluorine nuclear magnetic resonance (19F NMR) spectroscopy shows that this enhancement arises from an increase in the receptor's active state population and a G-protein-bound precoupled state. 19F NMR of fluorinated cholesterol analogs reveals transient interactions with A2AR, indicating a lack of high-affinity binding or direct allosteric modulation. The combined results suggest that the observed allosteric effects are largely indirect and originate from cholesterol-mediated changes in membrane properties, as shown by membrane fluidity measurements and high-pressure NMR.

Calcium modulates calmodulin/α-actinin 1 interaction with and agonist-dependent internalization of the adenosine A2A receptor

Adenosine receptors are G protein-coupled receptors that sense extracellular adenosine to transmit intracellular signals. One of the four adenosine receptor subtypes, the adenosine A_2A receptor (A_2AR), has an exceptionally long intracellular C terminus (A_2AR-ct) that mediates interactions with a large array of proteins, including calmodulin and α-actinin. Here, we aimed to ascertain the α-actinin 1/calmodulin interplay whilst binding to A_2AR and the role of Ca²⁺ in this process. First, we studied the A_2AR-α-actinin 1 interaction by means of native polyacrylamide gel electrophoresis, isothermal titration calorimetry, and surface plasmon resonance, using purified recombinant proteins. α-Actinin 1 binds the A_2AR-ct through its distal calmodulin-like domain in a Ca²⁺-independent manner with a dissociation constant of 5-12μM, thus showing an ~100 times lower affinity compared to the A_2AR-calmodulin/Ca²⁺ complex. Importantly, calmodulin displaced α-actinin 1 from the A_2AR-ct in a Ca²⁺-dependent fashion, disrupting the A_2AR-α-actinin 1 complex. Finally, we assessed the impact of Ca²⁺ on A_2AR internalization in living cells, a function operated by the A_2AR-α-actinin 1 complex. Interestingly, while Ca²⁺ influx did not affect constitutive A_2AR endocytosis, it abolished agonist-dependent internalization. In addition, we demonstrated that the A_2AR/α-actinin interaction plays a pivotal role in receptor internalization and function. Overall, our results suggest that the interplay of A_2AR with calmodulin and α-actinin 1 is fine-tuned by Ca²⁺, a fact that might power agonist-mediated receptor internalization and function.

Structure-Activity Relationships of the Sustained Effects of Adenosine A2A Receptor Agonists Driven by Slow Dissociation Kinetics

The duration of action of adenosine A2A receptor (A2A) agonists is critical for their clinical efficacy, and we sought to better understand how this can be optimized. The in vitro temporal response profiles of a panel of A2A agonists were studied using cAMP assays in recombinantly (CHO) and endogenously (SH-SY5Y) expressing cells. Some agonists (e.g., 3cd; UK-432,097) but not others (e.g., 3ac; CGS-21680) demonstrated sustained wash-resistant agonism, where residual receptor activation continued after washout. The ability of an antagonist to reverse pre-established agonist responses was used as a surrogate read-out for agonist dissociation kinetics, and together with radioligand binding studies suggested a role for slow off-rate in driving sustained effects. One compound, 3ch, showed particularly marked sustained effects, with a reversal t1/2 > 6 hours and close to maximal effects that remained for at least 5 hours after washing. Based on the structure-activity relationship of these compounds, we suggest that lipophilic N6 and bulky C2 substituents can promote stable and long-lived binding events leading to sustained agonist responses, although a high compound logD is not necessary. This provides new insight into the binding interactions of these ligands and we anticipate that this information could facilitate the rational design of novel long-acting A2A agonists with improved clinical efficacy.

How does adenosine control neuronal dysfunction and neurodegeneration?

The adenosine modulation system mostly operates through inhibitory A₁ (A₁ R) and facilitatory A_2A receptors (A_2A R) in the brain. The activity-dependent release of adenosine acts as a brake of excitatory transmission through A₁ R, which are enriched in glutamatergic terminals. Adenosine sharpens salience of information encoding in neuronal circuits: high-frequency stimulation triggers ATP release in the 'activated' synapse, which is locally converted by ecto-nucleotidases into adenosine to selectively activate A_2A R; A_2A R switch off A₁ R and CB1 receptors, bolster glutamate release and NMDA receptors to assist increasing synaptic plasticity in the 'activated' synapse; the parallel engagement of the astrocytic syncytium releases adenosine further inhibiting neighboring synapses, thus sharpening the encoded plastic change. Brain insults trigger a large outflow of adenosine and ATP, as a danger signal. A₁ R are a hurdle for damage initiation, but they desensitize upon prolonged activation. However, if the insult is near-threshold and/or of short-duration, A₁ R trigger preconditioning, which may limit the spread of damage. Brain insults also up-regulate A_2A R, probably to bolster adaptive changes, but this heightens brain damage since A_2A R blockade affords neuroprotection in models of epilepsy, depression, Alzheimer's, or Parkinson's disease. This initially involves a control of synaptotoxicity by neuronal A_2A R, whereas astrocytic and microglia A_2A R might control the spread of damage. The A_2A R signaling mechanisms are largely unknown since A_2A R are pleiotropic, coupling to different G proteins and non-canonical pathways to control the viability of glutamatergic synapses, neuroinflammation, mitochondria function, and cytoskeleton dynamics. Thus, simultaneously bolstering A₁ R preconditioning and preventing excessive A_2A R function might afford maximal neuroprotection. The main physiological role of the adenosine modulation system is to sharp the salience of information encoding through a combined action of adenosine A_2A receptors (A_2A R) in the synapse undergoing an alteration of synaptic efficiency with an increased inhibitory action of A₁ R in all surrounding synapses. Brain insults trigger an up-regulation of A_2A R in an attempt to bolster adaptive plasticity together with adenosine release and A₁ R desensitization; this favors synaptotocity (increased A_2A R) and decreases the hurdle to undergo degeneration (decreased A₁ R). Maximal neuroprotection is expected to result from a combined A_2A R blockade and increased A₁ R activation. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".

Can residence time offer a useful strategy to target agonist drugs for sustained GPCR responses?

Residence time describes the how long a ligand is bound to its target, and is attracting interest in drug discovery as a potential means of improving clinical efficacy by increasing target coverage. This concept, as originally applied to antagonists, is more complicated for G-protein-coupled receptor (GPCR) agonists because of the transiency of receptor responses (via desensitization and internalization). However, in some cases sustained GPCR agonist responses have been observed, with evidence consistent with a role for slow binding kinetics. We propose a model to explain our understanding of how residence time and rebinding might influence sustained signaling by internalized receptors. We also highlight the anticipated benefit for drug discovery of fully understanding and exploiting these phenomena to target desirable receptor response profiles selectively.

Inhibition of Gαs/cAMP Signaling Decreases TCR-Stimulated IL-2 transcription in CD4(+) T Helper Cells

BACKGROUND

The role of cAMP in regulating T cell activation and function has been controversial. cAMP is generally known as an immunosuppressant, but it is also required for generating optimal immune responses. As the effect of cAMP is likely to depend on its cellular context, the current study investigated whether the mechanism of activation of Gαs and adenylyl cyclase influences their effect on T cell receptor (TCR)-stimulated interleukin-2 (IL-2) mRNA levels.

METHODS

The effect of blocking Gs-coupled receptor (GsPCR)-mediated Gs activation on TCR-stimulated IL-2 mRNA levels in CD4(+) T cells was compared with that of knocking down Gαs expression or inhibiting adenylyl cyclase activity. The effect of knocking down Gαs expression on TCR-stimulated cAMP accumulation was compared with that of blocking GsPCR signaling.

RESULTS

ZM-241385, an antagonist to the Gs-coupled A2A adenosine receptor (A2AR), enhanced TCR-stimulated IL-2 mRNA levels in primary human CD4(+) T helper cells and in Jurkat T cells. A dominant negative Gαs construct, GαsDN3, also enhanced TCR-stimulated IL-2 mRNA levels. Similar to GsPCR antagonists, GαsDN3 blocked GsPCR-dependent activation of both Gαs and Gβγ. In contrast, Gαs siRNA and 2',5'-dideoxyadenosine (ddA), an adenylyl cyclase inhibitor, decreased TCR-stimulated IL-2 mRNA levels. Gαs siRNA, but not GαsDN3, decreased TCR-stimulated cAMP synthesis. Potentiation of IL-2 mRNA levels by ZM-241385 required at least two days of TCR stimulation, and addition of ddA after three days of TCR stimulation enhanced IL-2 mRNA levels.

CONCLUSIONS

GsPCRs play an inhibitory role in the regulation of TCR-stimulated IL-2 mRNA levels whereas Gαs and cAMP can play a stimulatory one. Additionally, TCR-dependent activation of Gαs does not appear to involve GsPCRs. These results suggest that the context of Gαs/cAMP activation and the stage of T cell activation and differentiation determine the effect on TCR-stimulated IL-2 mRNA levels.

A two-state model for the diffusion of the A2A adenosine receptor in hippocampal neurons: agonist-induced switch to slow mobility is modified by synapse-associated protein 102 (SAP102)

The A_2A receptor is a class A/rhodopsin-like G protein-coupled receptor. Coupling to its cognate protein, G_s, occurs via restricted collision coupling and is contingent on the presence of cholesterol. Agonist activation slows diffusion of the A_2A adenosine receptor in the lipid bilayer. We explored the contribution of the hydrophobic core and of the extended C terminus by examining diffusion of quantum dot-labeled receptor variants in dissociated hippocampal neurons. Single particle tracking of the A_2A receptor(1–311), which lacks the last 101 residues, revealed that agonist-induced confinement was abolished and that the agonist-induced decrease in diffusivity was reduced substantially. A fragment comprising the SH3 domain and the guanylate kinase domain of synapse-associated protein 102 (SAP102) was identified as a candidate interactor that bound to the A_2A receptor C terminus. Complex formation between the A_2A receptor and SAP102 was verified by coimmunoprecipitation and by tracking its impact on receptor diffusion. An analysis of all trajectories by a hidden Markov model was consistent with two diffusion states where agonist activation reduced the transition between the two states and, thus, promoted the accumulation of the A_2A receptor in the compartment with slow mobility. Overexpression of SAP102 precluded the access of the A_2A receptor to a compartment with restricted mobility. In contrast, a mutated A_2A receptor (with ³⁸³DVELL³⁸⁷ replaced by RVRAA) was insensitive to the action of SAP102. These observations show that the hydrophobic core per se does not fully account for the agonist-promoted change in mobility of the A_2A receptor. The extended carboxyl terminus allows for regulatory input by scaffolding molecules such as SAP102.

Segregation and crosstalk of D1 receptor-mediated activation of ERK in striatal medium spiny neurons upon acute administration of psychostimulants

The convergence of corticostriatal glutamate and dopamine from the midbrain in the striatal medium spiny neurons (MSN) triggers synaptic plasticity that underlies reinforcement learning and pathological conditions such as psychostimulant addiction. The increase in striatal dopamine produced by the acute administration of psychostimulants has been found to activate not only effectors of the AC5/cAMP/PKA signaling cascade such as GluR1, but also effectors of the NMDAR/Ca(2+)/RAS cascade such as ERK. The dopamine-triggered effects on both these cascades are mediated by D1R coupled to Golf but while the phosphorylation of GluR1 is affected by reductions in the available amount of Golf but not of D1R, the activation of ERK follows the opposite pattern. This segregation is puzzling considering that D1R-induced Golf activation monotonically increases with DA and that there is crosstalk from the AC5/cAMP/PKA cascade to the NMDAR/Ca(2+)/RAS cascade via a STEP (a tyrosine phosphatase). In this work, we developed a signaling model which accounts for this segregation based on the assumption that a common pool of D1R and Golf is distributed in two D1R/Golf signaling compartments. This model integrates a relatively large amount of experimental data for neurons in vivo and in vitro. We used it to explore the crosstalk topologies under which the sensitivities of the AC5/cAMP/PKA signaling cascade to reductions in D1R or Golf are transferred or not to the activation of ERK. We found that the sequestration of STEP by its substrate ERK together with the insensitivity of STEP activity on targets upstream of ERK (i.e. Fyn and NR2B) to PKA phosphorylation are able to explain the experimentally observed segregation. This model provides a quantitative framework for simulation based experiments to study signaling required for long term potentiation in MSNs.

Adenosine receptors and Huntington's disease

Adenosine regulates important pathophysiological functions via four distinct adenosine receptor subtypes (A1, A2A, A2B, and A3). The A1 and A2A adenosine receptors (A1R and A2AR) are major targets of caffeine and have been extensively investigated. Huntington's disease (HD) is a dominant neurodegenerative disease caused by an abnormal CAG expansion in the Huntingtin gene. Since the first genetic HD model was created almost two decades ago, tremendous progress regarding the function of the adenosine receptors in HD has been made. Chronic intake of caffeine was recently shown to be positively associated with the disease onset of HD. Moreover, genetic polymorphism of A2AR is believed to impact the age of onset. Given the importance of adenosine receptors as drug targets for human diseases, this review highlights the recent findings that delineate the roles of adenosine receptors in HD and discusses their potential for serving as drug targets and/or biomarkers for HD. Adenosine is a purine nucleoside that regulates important physiological functions via four different adenosine receptors (A1, A2A, A2B, and A3). These adenosine receptors have seven transmembrane domains and belong to the G protein-coupled receptor family. The functions of the A1 adenosine receptor (A1R) and A2A adenosine receptor (A2AR) have been investigated relative to HD. In this review, we summarize the recent findings regarding the role of adenosine receptors in HD and discuss the potential application of adenosine receptors as drug targets and biomarkers for HD.

Adenosine receptor control of cognition in normal and disease

Adenosine and adenosine receptors (ARs) are increasingly recognized as important therapeutic targets for controlling cognition under normal and disease conditions for its dual roles of neuromodulation as well as of homeostatic function in the brain. This chapter first presents the unique ability of adenosine, by acting on the inhibitory A1 and facilitating A2A receptor, to integrate dopamine, glutamate, and BNDF signaling and to modulate synaptic plasticity (e.g., long-term potentiation and long-term depression) in brain regions relevant to learning and memory, providing the molecular and cellular bases for adenosine receptor (AR) control of cognition. This led to the demonstration of AR modulation of social recognition memory, working memory, reference memory, reversal learning, goal-directed behavior/habit formation, Pavlovian fear conditioning, and effort-related behavior. Furthermore, human and animal studies support that AR activity can also, through cognitive enhancement and neuroprotection, reverse cognitive impairments in animal models of Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease, and schizophrenia. Lastly, epidemiological evidence indicates that regular human consumption of caffeine, the most widely used psychoactive drug and nonselective AR antagonists, is associated with the reduced cognitive decline in aging and AD patients, and with the reduced risk in developing PD. Thus, there is a convergence of the molecular studies revealing AR as molecular targets for integrating neurotransmitter signaling and controlling synaptic plasticity, with animal studies demonstrating the strong procognitive impact upon AR antagonism in normal and disease brains and with epidemiological and clinical evidences in support of caffeine and AR drugs for therapeutic modulation of cognition. Since some of adenosine A2A receptor antagonists are already in phase III clinical trials for motor benefits in PD patients with remarkable safety profiles, additional animal and human studies to better understand the mechanism underlying the AR-mediated control of cognition under normal and disease conditions will provide the required rationale to stimulate the necessary clinical investigation to rapidly translate adenosine and AR drug as a novel strategy to control memory impairment in neuropsychiatric disorders.

The stimulation of adenosine A2A receptors ameliorates the pathological phenotype of fibroblasts from Niemann-Pick type C patients

Niemann-Pick type C1 (NPC1) disease is a rare neurovisceral disorder characterized by intracellular accumulation of unesterified cholesterol, sphingolipids, and other lipids in the lysosomal compartment. A deregulation of lysosomal calcium has been identified as one of the earliest steps of the degenerative process. Since adenosine A2A receptors (A2ARs) control lysosome trafficking and pH, which closely regulates lysosomal calcium, we hypothesized a role for these receptors in NPC1. The aim of this study was to evaluate the effects of the A2AR agonist CGS21680 on human control and NPC1 fibroblasts. We show that CGS21680 raises lysosomal calcium levels and rescues mitochondrial functionality (mitochondrial inner membrane potential and expression of the complex IV of the mitochondrial respiratory chain), which is compromised in NPC1 cells. These effects are prevented by the selective blockade of A2ARs by the antagonist ZM241385. The effects of A2AR activation on lysosomal calcium are not mediated by the cAMP/PKA pathway but they appear to involve the phosphorylation of ERK1/2. Finally, CGS21680 reduces cholesterol accumulation (Filipin III staining), which is the main criterion currently used for identification of a compound or pathway that would be beneficial for NPC disease, and such an effect is prevented by the Ca(2+) chelator BAPTA-AM. Our findings strongly support the hypothesis that A2AR agonists may represent a therapeutic option for NPC1 and provide insights on their mechanisms of action.

Recruitment of a cytoplasmic chaperone relay by the A2A adenosine receptor

The adenosine A_2A receptor is a prototypical rhodopsin-like G protein-coupled receptor but has several unique structural features, in particular a long C terminus (of >120 residues) devoid of a palmitoylation site. It is known to interact with several accessory proteins other than those canonically involved in signaling. However, it is evident that many more proteins must interact with the A_2A receptor, if the trafficking trajectory of the receptor is taken into account from its site of synthesis in the endoplasmic reticulum (ER) to its disposal by the lysosome. Affinity-tagged versions of the A_2A receptor were expressed in HEK293 cells to identify interacting partners residing in the ER by a proteomics approach based on tandem affinity purification. The receptor-protein complexes were purified in quantities sufficient for analysis by mass spectrometry. We identified molecular chaperones (heat-shock proteins HSP90α and HSP70-1A) that interact with and retain partially folded A_2A receptor prior to ER exit. Complex formation between the A_2A receptor and HSP90α (but not HSP90β) and HSP70-1A was confirmed by co-affinity precipitation. HSP90 inhibitors also enhanced surface expression of the receptor in PC12 cells, which endogenously express the A_2A receptor. Finally, proteins of the HSP relay machinery (e.g. HOP/HSC70-HSP90 organizing protein and P23/HSP90 co-chaperone) were recovered in complexes with the A_2A receptor. These observations are consistent with the proposed chaperone/coat protein complex II exchange model. This posits that cytosolic HSP proteins are sequentially recruited to folding intermediates of the A_2A receptor. Release of HSP90 is required prior to recruitment of coat protein complex II components. This prevents premature ER export of partially folded receptors.

Reengineering the collision coupling and diffusion mode of the A2A-adenosine receptor: palmitoylation in helix 8 relieves confinement

The A(2A)-adenosine receptor undergoes restricted collision coupling with its cognate G protein G(s) and lacks a palmitoylation site at the end of helix 8 in its intracellular C terminus. We explored the hypothesis that there was a causal link between the absence of a palmitoyl moiety and restricted collision coupling by introducing a palmitoylation site. The resulting mutant A(2A)-R309C receptor underwent palmitoylation as verified by both mass spectrometry and metabolic labeling. In contrast to the wild type A(2A) receptor, the concentration-response curve for agonist-induced cAMP accumulation was shifted to the left with increasing expression levels of A(2A)-R309C receptor, an observation consistent with collision coupling. Single particle tracking of quantum dot-labeled receptors confirmed that wild type and mutant A(2A) receptor differed in diffusivity and diffusion mode; agonist activation resulted in a decline in mean square displacement of both receptors, but the drop was substantially more pronounced for the wild type receptor. In addition, in the agonist-bound state, the wild type receptor was frequently subject to confinement events (estimated radius 110 nm). These were rarely seen with the palmitoylated A(2A)-R309C receptor, the preferred diffusion mode of which was a random walk in both the basal and the agonist-activated state. Taken together, the observations link restricted collision coupling to diffusion limits imposed by the absence of a palmitoyl moiety in the C terminus of the A(2A) receptor. The experiments allowed for visualizing local confinement of an agonist-activated G protein-coupled receptor in an area consistent with the dimensions of a lipid raft.

Constitutive activity of the A2A adenosine receptor and compartmentalised cyclic AMP signalling fine-tune noradrenaline release

Neuroblastoma SH-SY5Y (SH) cells endogenously express A(2A) adenosine receptors and can be differentiated into a sympathetic neuronal phenotype, capable of depolarisation-dependent noradrenaline release. Using differentiated SH culture, we here explored the link between A(2A)-receptor signalling and neurotransmitter release. In response to the receptor agonist CGS21680, the cells produced cyclic AMP (cAMP), and when depolarised, they released increased amounts of noradrenaline. An A(2A)-receptor antagonist, XAC, as well as an inhibitor of cAMP-dependent protein kinase A (PKA), H89, depressed agonist-dependent release. In the presence of XAC or H89, noradrenaline release was found to be below basal values. This suggested that release facilitation also owes to constitutive receptor activity. We demonstrate that even in the absence of an agonist, the native A(2A)-receptor stimulated cAMP production, leading to the activation of PKA and enhanced noradrenaline release. Ancillary, non-cAMP-dependent effects of the receptor (i.e. phosphorylation of CREB, of Rabphilin3A) were refractory to constitutive activation. PKA-dependent facilitation of noradrenaline release was recapitulated with membrane-permeable 8-Br-cAMP; in addition to facilitation, 8-Br-cAMP caused marked inhibition of release, an effect not observed upon receptor activation. Inhibition by receptor-independent cAMP was likely due to suppression of voltage-dependent calcium current (VDCC) and increased activity of Src-family kinases. Receptor-mediated release facilitation was reproduced in the presence of tetrodotoxin (blocking action potentials); hence, the signalling occurred at the active zone comprising release sites. Our findings thus support (1) presynaptic localisation of the A(2A)-receptor and (2) suggest that compartmentalised pathways transmit cAMP signalling in order to facilitate depolarisation-dependent neurotransmitter release.

Fluorescence/bioluminescence resonance energy transfer techniques to study G-protein-coupled receptor activation and signaling

Fluorescence and bioluminescence resonance energy transfer (FRET and BRET) techniques allow the sensitive monitoring of distances between two labels at the nanometer scale. Depending on the placement of the labels, this permits the analysis of conformational changes within a single protein (for example of a receptor) or the monitoring of protein-protein interactions (for example, between receptors and G-protein subunits). Over the past decade, numerous such techniques have been developed to monitor the activation and signaling of G-protein-coupled receptors (GPCRs) in both the purified, reconstituted state and in intact cells. These techniques span the entire spectrum from ligand binding to the receptors down to intracellular second messengers. They allow the determination and the visualization of signaling processes with high temporal and spatial resolution. With these techniques, it has been demonstrated that GPCR signals may show spatial and temporal patterning. In particular, evidence has been provided for spatial compartmentalization of GPCRs and their signals in intact cells and for distinct physiological consequences of such spatial patterning. We review here the FRET and BRET technologies that have been developed for G-protein-coupled receptors and their signaling proteins (G-proteins, effectors) and the concepts that result from such experiments.

Clozapine, atypical antipsychotics, and the benefits of fast-off D2 dopamine receptor antagonism

Drug-receptor interactions are traditionally quantified in terms of affinity and efficacy, but there is increasing awareness that the drug-on-receptor residence time also affects clinical performance. While most interest has hitherto been focused on slow-dissociating drugs, D(2) dopamine receptor antagonists show less extrapyramidal side effects but still have excellent antipsychotic activity when they dissociate swiftly. Fast dissociation of clozapine, the prototype of the "atypical antipsychotics", has been evidenced by distinct radioligand binding approaches both on cell membranes and intact cells. The surmountable nature of clozapine in functional assays with fast-emerging responses like calcium transients is confirmatory. Potential advantages and pitfalls of the hitherto used techniques are discussed, and recommendations are given to obtain more precise dissociation rates for such drugs. Surmountable antagonism is necessary to allow sufficient D(2) receptor stimulation by endogenous dopamine in the striatum. Simulations are presented to find out whether this can be achieved during sub-second bursts in dopamine concentration or rather during much slower, activity-related increases thereof. While the antagonist's dissociation rate is important to distinguish between both mechanisms, this becomes much less so when contemplating time intervals between successive drug intakes, i.e., when pharmacokinetic considerations prevail. Attention is also drawn to the divergent residence times of hydrophobic antagonists like haloperidol when comparing radioligand binding data on cell membranes with those on intact cells and clinical data.

Long and short distance movements of β(2)-adrenoceptor in cell membrane assessed by photoconvertible fluorescent protein dendra2-β(2)-adrenoceptor fusion

Local movements of receptors in the plasma membrane have been extensively studied, as it is generally believed that the dynamics of membrane distribution of receptors regulate their functions. However, the properties of large-scale (>5μm) receptor movements in the membrane are relatively obscure. In the present study, we addressed the question as to whether the large-scale movement of receptor in the plasma membrane at the whole cell level can be explained quantitatively by its local diffusive properties. We used HEK 293 cells transfected with human β2-adrenoceptor fused to photoconvertible fluorescent protein dendra2 as a model system; and found that 1) functional integrity of the dendra2-tagged receptor remains apparently intact; 2) in a mesoscopic scale (~4μm), ~90% of the receptors are mobile on average, and receptor influx to, and out-flux from a membrane area can be symmetrically explained by a diffusion-like process with an effective diffusion coefficient of ~0.1μm(2)/s; 3) these mobility parameters are not affected by the activity state of the receptor (assessed by using constitutively active receptor mutants); 4) in the macroscopic scale (4-40μm), although a slowly diffusing fraction of receptors (with D<0.01μm(2)/s) is identifiable in some cases, the movement of the predominant fraction is perfectly explained by the same effective diffusion process observed in the mesoscopic scale, suggesting that the large scale structure of the cell membrane as felt by the receptor is apparently homogeneous in terms of its mesoscopic properties. We also showed that intracellular compartments and plasma membrane are kinetically connected even at steady-state.

Functional selectivity of adenosine receptor ligands

Adenosine receptors are plasma membrane proteins that transduce an extracellular signal into the interior of the cell. Basically every mammalian cell expresses at least one of the four adenosine receptor subtypes. Recent insight in signal transduction cascades teaches us that the current classification of receptor ligands into agonists, antagonists, and inverse agonists relies very much on the experimental setup that was used. Upon activation of the receptors by the ubiquitous endogenous ligand adenosine they engage classical G protein-mediated pathways, resulting in production of second messengers and activation of kinases. Besides this well-described G protein-mediated signaling pathway, adenosine receptors activate scaffold proteins such as β-arrestins. Using innovative and sensitive experimental tools, it has been possible to detect ligands that preferentially stimulate the β-arrestin pathway over the G protein-mediated signal transduction route, or vice versa. This phenomenon is referred to as functional selectivity or biased signaling and implies that an antagonist for one pathway may be a full agonist for the other signaling route. Functional selectivity makes it necessary to redefine the functional properties of currently used adenosine receptor ligands and opens possibilities for new and more selective ligands. This review focuses on the current knowledge of functionally selective adenosine receptor ligands and on G protein-independent signaling of adenosine receptors through scaffold proteins.

Ligand-based peptide design and combinatorial peptide libraries to target G protein-coupled receptors

G protein-coupled receptors (GPCRs) are considered to represent the most promising drug targets; it has been repeatedly said that a large fraction of the currently marketed drugs elicit their actions by binding to GPCRs (with cited numbers varying from 30-50%). Closer scrutiny, however, shows that only a modest fraction of (≈60) GPCRs are, in fact, exploited as drug targets, only ≈20 of which are peptide-binding receptors. The vast majority of receptors in the humane genome have not yet been explored as sites of action for drugs. Given the drugability of this receptor class, it appears that opportunities for drug discovery abound. In addition, GPCRs provide for binding sites other than the ligand binding sites (referred to as the "orthosteric site"). These additional sites include (i) binding sites for ligands (referred to as "allosteric ligands") that modulate the affinity and efficacy of orthosteric ligands, (ii) the interaction surface that recruits G proteins and arrestins, (iii) the interaction sites of additional proteins (GIPs, GPCR interacting proteins that regulate G protein signaling or give rise to G protein-independent signals). These sites can also be targeted by peptides. Combinatorial and natural peptide libraries are therefore likely to play a major role in identifying new GPCR ligands at each of these sites. In particular the diverse natural peptide libraries such as the venom peptides from marine cone-snails and plant cyclotides have been established as a rich source of drug leads. High-throughput screening and combinatorial chemistry approaches allow for progressing from these starting points to potential drug candidates. This will be illustrated by focusing on the ligand-based drug design of oxytocin (OT) and vasopressin (AVP) receptor ligands using natural peptide leads as starting points.

Differential trafficking of adenosine receptors in hippocampal neurons monitored using GFP- and super-ecliptic pHluorin-tagged receptors

Adenosine receptors (ARs) modulate many cellular and systems-level processes in the mammalian CNS. However, little is known about the trafficking of ARs in neurons, despite their importance in controlling seizure activity and in neuroprotection in cerebral ischaemia. To address this we examined the agonist-dependent internalisation of C-terminal GFP-tagged A(1)Rs, A(2A)Rs and A(3)Rs in primary hippocampal neurons. Furthermore, we developed a novel super-ecliptic pHluorin (SEP)-tagged A(1)R which, via the N-terminal SEP tag, reports the cell-surface expression and trafficking of A(1)Rs in real-time. We demonstrate the differential trafficking of ARs in neurons: A(3)Rs internalise more rapidly than A1Rs, with little evidence of appreciable A(2A)R trafficking over the time-course of the experiments. Furthermore, the novel SEP-A(1)R construct revealed the time-course of internalisation and recovery of cell-surface expression to occur within minutes of agonist exposure and removal, respectively. These observations highlight the labile nature of A(1)R and A(3)Rs when expressed at the neuronal plasma membrane. Given the high levels of adenosine in the brain during ischaemia and seizures, internalisation of the inhibitory A(1)R may result in hyperexcitability, increased brain damage and the development of chronic epileptic states.

Adenosine receptors and membrane microdomains

Adenosine receptors are a member of the large family of seven transmembrane spanning G protein coupled receptors. The four adenosine receptor subtypes-A(1), A(2a), A(2b), A(3)-exert their effects via the activation of one or more heterotrimeric G proteins resulting in the modulation of intracellular signaling. Numerous studies over the past decade have documented the complexity of G protein coupled receptor signaling at the level of protein-protein interactions as well as through signaling cross talk. With respect to adenosine receptors, the activation of one receptor subtype can have profound direct effects in one cell type but little or no effect in other cells. There is significant evidence that the compartmentation of subcellular signaling plays a physiological role in the fidelity of G protein coupled receptor signaling. This compartmentation is evident at the level of the plasma membrane in the form of membrane microdomains such as caveolae and lipid rafts. This review will summarize and critically assess our current understanding of the role of membrane microdomains in regulating adenosine receptor signaling.

Adenosine A2A receptor signaling and golf assembly show a specific requirement for the gamma7 subtype in the striatum

The adenosine A(2A) receptor (A(2A)R) is increasingly recognized as a novel therapeutic target in Parkinson disease. In striatopallidal neurons, the G-protein α(olf) subtype is required to couple this receptor to adenylyl cyclase activation. It is now well established that the βγ dimer also performs an active role in this signal transduction process. In principal, sixty distinct βγ dimers could arise from combinatorial association of the five known β and 12 γ subunit genes. However, key questions regarding which βγ subunit combinations exist and whether they perform specific signaling roles in the context of the organism remain to be answered. To explore these questions, we used a gene targeting approach to specifically ablate the G-protein γ(7) subtype. Revealing a potentially new signaling paradigm, we show that the level of the γ(7) protein controls the hierarchial assembly of a specific G-protein α(olf)β(2)γ(7) heterotrimer in the striatum. Providing a probable basis for the selectivity of receptor signaling, we further demonstrate that loss of this specific G-protein heterotrimer leads to reduced A(2A)R activation of adenylyl cyclase. Finally, substantiating an important role for this signaling pathway in pyschostimulant responsiveness, we show that mice lacking the G-protein γ(7) subtype exhibit an attenuated behavioral response to caffeine. Collectively, these results further support the A(2A)R G-protein α(olf)β(2)γ(7) interface as a possible therapeutic target for Parkinson disease.

Modulation of brain-derived neurotrophic factor (BDNF) actions in the nervous system by adenosine A(2A) receptors and the role of lipid rafts

In this paper we review some novel aspects related to the way adenosine A(2A) receptors (A(2A)R) modulate the action of BDNF or its high-affinity receptors, the TrkB receptors, on synaptic transmission and plasticity, as well as upon cholinergic currents and GABA transporters. Evidence has been accumulating that adenosine A(2A)Rs are required for most of the synaptic actions of BDNF. In some cases, where A(2A)Rs are constitutively activated (e.g. by endogenous extracellular adenosine), the need for A(2A)R activation for the maintenance of the synaptic influences of BDNF can be envisaged from the loss of BDNF effects upon blockade of adenosine A(2A)Rs or upon removal of extracellular adenosine with adenosine deaminase. In some other cases, it is necessary to enhance extracellular adenosine levels (e.g. depolarization) or to further activate A(2A)Rs (e.g. with selective agonists) to trigger a BDNF neuromodulatory role at the synapses. Age- and cell-dependent differences may determine the above two possibilities, but in all cases it is quite clear that there is close interplay between adenosine A(2A)Rs and BDNF TrkB receptors at synapses. The role of lipid rafts in this cross-talk will be discussed. This article is part of a Special Issue entitled: "Adenosine Receptors".

Ecto-5'-nucleotidase (CD73) attenuates allograft airway rejection through adenosine 2A receptor stimulation

There are multiple drivers of leukocyte recruitment in lung allografts that contribute to lymphocytic bronchitis (LB) and bronchiolitis obliterans (BO). The innate mechanisms driving (or inhibiting) leukocyte trafficking to allografts remain incompletely understood. This study tested the hypothesis that CD73 (ecto-5'nucleotidase), an enzyme that catalyzes the conversion of AMP to adenosine, is a critical negative regulator of LB and BO. Implantation of tracheal allografts from wild type (WT) mice into CD73(-/-) recipients revealed a striking increase in airway luminal obliteration at 7 d (62 +/- 4% and 47 +/- 5% for CD73(-/-) and WT allograft recipients, respectively; p = 0.046). There was also a concordant increase in CD3(+) lymphocytic infiltration (523 +/- 41 cells and 313 +/- 43 cells for CD73(-/-) and WT allograft recipients, respectively; p = 0.013). Because real-time PCR revealed a 43-fold upregulation of mRNA for the adenosine A2A receptor (A2AR) in WT allografts compared with WT isografts (p = 0.032), additional experiments were performed to determine whether the protective effect of CD73 was due to generation of adenosine and its stimulation of the A2AR. Treatment of WT recipients with an A2AR agonist significantly reduced CD3(+) lymphocyte infiltration and airway luminal obliteration; similar treatment of CD73(-/-) recipients rescued them from LB and airway obliteration. These data implicate CD73 acting through adenosine generation and its stimulation of the A2AR as a critical negative modulator of lymphocyte recruitment into airway allografts. The CD73/adenosine axis might be a new therapeutic target to prevent BO.

Adenosine receptor desensitization and trafficking

As with the majority of G-protein-coupled receptors, all four of the adenosine receptor subtypes are known to undergo agonist-induced regulation in the form of desensitization and trafficking. These processes can limit the ability of adenosine receptors to couple to intracellular signalling pathways and thus reduce the ability of adenosine receptor agonists as well as endogenous adenosine to produce cellular responses. In addition, since adenosine receptors couple to multiple signalling pathways, these pathways may desensitize differentially, while the desensitization of one pathway could even trigger signalling via another. Thus, the overall picture of adenosine receptor regulation can be complex. For all adenosine receptor subtypes, there is evidence to implicate arrestins in agonist-induced desensitization and trafficking, but there is also evidence for other possible forms of regulation, including second messenger-dependent kinase regulation, heterologous effects involving G proteins, and the involvement of non-clathrin trafficking pathways such as caveolae. In this review, the evidence implicating these mechanisms is summarized for each adenosine receptor subtype, and we also discuss those issues of adenosine receptor regulation that remain to be resolved as well as likely directions for future research in this field.

From cradle to twilight: the carboxyl terminus directs the fate of the A(2A)-adenosine receptor

The extended carboxyl terminus of the A(2A)-adenosine receptor is known to engage several proteins other than those canonically involved in signalling by GPCRs (i.e., G proteins, G protein-coupled receptor kinases/GRKs, arrestins). The list includes the deubiquinating enzyme USP4, α-actinin, the guanine nucleotide exchange factor for ARF6 ARNO, translin-X-associated protein, calmodulin, the neuronal calcium binding protein NECAB2 and the synapse associated protein SAP102. However, if the fate of the A(2A)-receptor is taken into account - from its birthplace in the endoplasmic reticulum to its presumed site of disposal in the lysosome, it is evident that many more proteins must interact with the A(2A)-adenosine receptor. There are several arguments that support the conjecture that these interactions will preferentially occur with the carboxyl terminus of the A(2A)-adeonsine receptor: (i) the extended carboxyl terminus (of 122 residues=) offers the required space to accommodate companions; (ii) analogies can be drawn with other receptors, which engage several of these binding partners with their C-termini. This approach allows for defining the nature of the unknown territory. As an example, we posit a chaperone/coat protein complex-II (COPII) exchange model that must occur on the carboxyl terminus of the receptor. This model accounts for the observation that a minimum size of the C-terminus is required for correct folding of the receptor. It also precludes premature recruitment of the COPII-coat to a partially folded receptor.

Subtype-specific differences in corticotropin-releasing factor receptor complexes detected by fluorescence spectroscopy

G protein-coupled receptors have been proposed to exist in signalosomes subject to agonist-driven shifts in the assembly disassembly equilibrium, affected by stabilizing membrane lipids and/or cortical actin restricting mobility. We investigated the highly homologous corticotropin-releasing factor receptors (CRFRs), CRFR1 and -2, which are different within their hydrophobic core. Agonist stimulation of CRFR1 and CRFR2 gave rise to similar concentration-response curves for cAMP accumulation, but CRFR2 underwent restricted collision coupling. Both CRFR1 and CRFR2 formed constitutive oligomers at the cell surface and recruited beta-arrestin upon agonist activation (as assessed by fluorescence resonance energy transfer microscopy in living cells). However, CRFR2, but not CRFR1, failed to undergo agonist-induced internalization. Likewise, agonist binding accelerated the diffusion rate of CRFR2 only (detected by fluorescence recovery after photobleaching and fluorescence correlation spectroscopy) but reduced the mobile fraction, which is indicative of local confinement. Fluorescence intensity distribution analysis demonstrated that the size of CRFR complexes was not changed. Disruption of the actin cytoskeleton abolished the agonist-dependent increase in CRFR2 mobility, shifted the agonist concentration curve for CRFR2 to the left, and promoted agonist-induced internalization of CRFR2. Our observations are incompatible with an agonist-induced change in monomer-oligomer equilibrium, but they suggest an agonist-induced redistribution of CRFR2 into a membrane microdomain that affords rapid diffusion but restricted mobility and that is stabilized by the actin cytoskeleton. Our data show that membrane anisotropy can determine the shape and duration of receptor-generated signals in a subtype-specific manner.