Pharmacology and structure of isolated conformations of the adenosine A₂A receptor define ligand efficacy

Molecular and Structural Insight into Adenosine A2A Receptor in Neurodegenerative Disorders: A Significant Target for Efficient Treatment Approach

All biological tissues and bodily fluids include the autacoid adenosine. The P1 class of purinergic receptors includes adenosine receptors. Four distinct G-protein-coupled receptors on the cellular membrane mediate the effects of adenosine, whose cytoplasmic content is regulated by producing/degrading enzymes and nucleoside transporters. A2A receptor has received a great deal of attention in recent years because it has a wide range of potential therapeutic uses. A2B and, more significantly, A2A receptors regulate numerous physiological mechanisms in the central nervous system (CNS). The inferior targetability of A2B receptors towards adenosine points that they might portray a promising medicinal target since they are triggered only under pharmacological circumstances (when adenosine levels rise up to micromolar concentrations). The accessibility of specific ligands for A2B receptors would permit the exploration of such a theory. A2A receptors mediate both potentially neurotoxic and neuroprotective actions. Hence, it is debatable to what extent they play a role in neurodegenerative illnesses. However, A2A receptor blockers have demonstrated clear antiparkinsonian consequences, and a significant attraction exists in the role of A2A receptors in other neurodegenerative disorders. Amyloid peptide extracellular accumulation and tau hyperphosphorylation are the pathogenic components of AD that lead to neuronal cell death, cognitive impairment, and memory loss. Interestingly, in vitro and in vivo research has shown that A2A adenosine receptor antagonists may block each of these clinical symptoms, offering a crucial new approach to combat a condition for which, regrettably, only symptomatic medications are currently available. At least two requirements must be met to determine whether such receptors are a target for diseases of the CNS: a complete understanding of the mechanisms governing A2A-dependent processes and the availability of ligands that can distinguish between the various receptor populations. This review concisely summarises the biological effects mediated by A2A adenosine receptors in neurodegenerative disorders and discusses the chemical characteristics of A2A adenosine receptor antagonists undergoing clinical trials. Selective A2A receptor blocker against neurodegenerative disorders.

A2A adenosine receptor agonists, antagonists, inverse agonists and partial agonists

The Gs-coupled A2A adenosine receptor (A2AAR) has been explored extensively as a pharmaceutical target, which has led to numerous clinical trials. However, only one selective A2AAR agonist (regadenoson, Lexiscan) and one selective A2AAR antagonist (istradefylline, Nouriast) have been approved by the FDA, as a pharmacological agent for myocardial perfusion imaging (MPI) and as a cotherapy for Parkinson's disease (PD), respectively. Adenosine is widely used in MPI, as Adenoscan. Despite numerous unsuccessful clinical trials, medicinal chemical activity around A2AAR ligands has accelerated recently, particularly through structure-based drug design. New drug-like A2AAR antagonists for PD and cancer immunotherapy have been identified, and many clinical trials have ensued. For example, imaradenant (AZD4635), a compound that was designed computationally, based on A2AAR X-ray structures and biophysical mapping. Mixed A2AAR/A2BAR antagonists are also hopeful for cancer treatment. A2AAR antagonists may also have potential as neuroprotective agents for treatment of Alzheimer's disease.

Safety review of current pharmacotherapies for levodopa-treated patients with Parkinson's disease

INTRODUCTION

Levodopa remains the gold standard for treatment of Parkinson's disease (PD). Patients develop complications with disease progression, necessitating adjunctive therapy to control fluctuations in motor and non-motor symptoms and dyskinesia. Knowledge of medication safety and tolerability is critical to ascertain the benefit-risk ratio and select an adjunctive therapy that provides the highest chance for medication adherence. Posing a challenge are the sheer abundance of options, stemming from the development of several new drugs in recent years, as well as differences in commercial drug availability worldwide.

AREAS COVERED

This review evaluates the efficacy, safety, and tolerability of current US FDA-approved pharmacotherapies for levodopa-treated PD patients, including dopamine agonists, monoamine oxidase type-B inhibitors, catechol-O-methyltransferase inhibitors, the N-methyl-D-aspartate receptor antagonist amantadine, and the adenosine receptor antagonist istradefylline. Data were taken from pivotal phase III randomized controlled and post-surveillance studies, when available, that directly led to FDA-approval.

EXPERT OPINION

No strong evidence exists to support use of a specific adjunctive treatment for improving Off time. Only one medication has demonstrated improvement in dyskinesia in levodopa-treated PD patients; however, every patient cannot tolerate it and therefore adjunctive therapy should be tailored to an individual's symptoms and risk for specific adverse effects.

Anti-parkinsonian activity of the adenosine A2A receptor antagonist/inverse agonist KW-6356 as monotherapy in MPTP-treated common marmosets

KW-6356 is a novel adenosine A_2A receptor antagonist/inverse agonist that not only blocks binding of adenosine to adenosine A_2A receptor but also inhibits the constitutive activity of adenosine A_2A receptor. The efficacy of KW-6356 as both monotherapy and an adjunct therapy to L-3,4-dihydroxyphenylalanine (L-DOPA)/decarboxylase inhibitor in Parkinson's disease (PD) patients has been reported. However, the first-generation A_2A antagonist istradefylline, which is approved for use as an adjunct treatment to L-DOPA/decarboxylase inhibitor in adult PD patients experiencing OFF episodes, has not shown statistically significant efficacy as monotherapy. In vitro pharmacological studies have shown that the pharmacological properties of KW-6356 and istradefylline at adenosine A_2A receptor are markedly different. However, the anti-parkinsonian activity and effects on dyskinesia of KW-6356 in PD animal models and the differences in the efficacy between KW-6356 and istradefylline are unknown. The present study investigated the anti-parkinsonian activity of KW-6356 as monotherapy in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated common marmosets, and its efficacy was directly compared with that of istradefylline. In addition, we investigated whether or not repeated administration of KW-6356 induced dyskinesia. Oral administration of KW-6356 reversed motor disability in a dose-dependent manner up to 1 mg/kg in MPTP-treated common marmosets. The magnitude of anti-parkinsonian activity induced by KW-6356 was significantly greater than that of istradefylline. Repeated administration of KW-6356 induced little dyskinesia in MPTP-treated common marmosets primed to exhibit dyskinesia by prior exposure to L-DOPA. These results indicate that KW-6356 can be a novel non-dopaminergic therapy as monotherapy without inducing dyskinesia in PD patients.

Sub-millisecond conformational dynamics of the A2A adenosine receptor revealed by single-molecule FRET

The complex pharmacology of G-protein-coupled receptors (GPCRs) is defined by their multi-state conformational dynamics. Single-molecule Förster Resonance Energy Transfer (smFRET) is well suited to quantify dynamics for individual protein molecules; however, its application to GPCRs is challenging. Therefore, smFRET has been limited to studies of inter-receptor interactions in cellular membranes and receptors in detergent environments. Here, we performed smFRET experiments on functionally active human A_2A adenosine receptor (A_2AAR) molecules embedded in freely diffusing lipid nanodiscs to study their intramolecular conformational dynamics. We propose a dynamic model of A_2AAR activation that involves a slow (>2 ms) exchange between the active-like and inactive-like conformations in both apo and antagonist-bound A_2AAR, explaining the receptor's constitutive activity. For the agonist-bound A_2AAR, we detected faster (390 ± 80 µs) ligand efficacy-dependent dynamics. Our work establishes a general smFRET platform for GPCR investigations that can potentially be used for drug screening and/or mechanism-of-action studies.

Alleviation of Cocaine Withdrawal and Pertinent Interactions between Salvinorin-Based Antagonists and Kappa Opioid Receptor

The kappa opioid receptor (KOR) is involved in the regulation of both the reward and mood processes. Recent reports find that the use of drugs of abuse increases the production of dynorphin and the overall activation of KOR. Long-acting KOR antagonists, such as norbinaltorphimine (nor-BNI), JDTic, and 5'-guanidinonaltrindole (GNTI), have been shown to stop depressive and anxiety-related disorders, which are the common side effects of withdrawal that can lead to a relapse in drug use. Unfortunately, these prototypical KOR antagonists are known to induce selective KOR antagonism that is delayed by hours and extremely prolonged, and their use in humans comes with serious safety concerns because they possess a large window for potential drug-drug interactions. Furthermore, their persistent pharmacodynamic activities can hinder the ability to reverse unanticipated side effects immediately. Herein, we report our studies of the lead selective, salvinorin-based KOR antagonist (1) as well as nor-BNI on C57BL/6N male mice for spontaneous cocaine withdrawal. Assessment of pharmacokinetics showed that 1 is a short-acting compound with an average half-life of 3.75 h across different compartments (brain, spinal cord, liver, and plasma). Both 1 (5 mg/kg) and nor-BNI (5 mg/kg) were shown to reduce spontaneous withdrawal behavior in mice, with 1 producing additional anti-anxiety-like behavior in a light-dark transition test (however, no mood-related effects of 1 or nor-BNI were observed at the current dosing in an elevated plus maze or a tail suspension test). Our results support the study of selective, short-acting KOR antagonists for the treatment of psychostimulant withdrawal and the associated negative mood states that contribute to relapse. Furthermore, we identified pertinent interactions between 1 and KOR via computational studies, including induced-fit docking, mutagenesis, and molecular dynamics simulations, to gain insight into the design of future selective, potent, and short-acting salvinorin-based KOR antagonists.

Presynaptic adenosine receptor heteromers as key modulators of glutamatergic and dopaminergic neurotransmission in the striatum

Adenosine plays a very significant role in modulating striatal glutamatergic and dopaminergic neurotransmission. In the present essay we first review the extensive evidence that indicates this modulation is mediated by adenosine A₁ and A_2A receptors (A₁Rs and A_2ARs) differentially expressed by the components of the striatal microcircuit that include cortico-striatal glutamatergic and mesencephalic dopaminergic terminals, and the cholinergic interneuron. This microcircuit mediates the ability of striatal glutamate release to locally promote dopamine release through the intermediate activation of cholinergic interneurons. A₁Rs and A_2ARs are colocalized in the cortico-striatal glutamatergic terminals, where they form A₁R-A_2AR and A_2AR-cannabinoid CB₁ receptor (CB₁R) heteromers. We then evaluate recent findings on the unique properties of A₁R-A_2AR and A_2AR-CB₁R heteromers, which depend on their different quaternary tetrameric structure. These properties involve different allosteric mechanisms in the two receptor heteromers that provide fine-tune modulation of adenosine and endocannabinoid-mediated striatal glutamate release. Finally, we evaluate the evidence supporting the use of different heteromers containing striatal adenosine receptors as targets for drug development for neuropsychiatric disorders, such as Parkinson's disease and restless legs syndrome, based on the ability or inability of the A_2AR to demonstrate constitutive activity in the different heteromers, and the ability of some A_2AR ligands to act preferentially as neutral antagonists or inverse agonists, or to have preferential affinity for a specific A_2AR heteromer.

Cancer-Associated Mutations of the Adenosine A2A Receptor Have Diverse Influences on Ligand Binding and Receptor Functions

The adenosine A_2A receptor (A_2AAR) is a class A G-protein-coupled receptor (GPCR). It is an immune checkpoint in the tumor micro-environment and has become an emerging target for cancer treatment. In this study, we aimed to explore the effects of cancer-patient-derived A_2AAR mutations on ligand binding and receptor functions. The wild-type A_2AAR and 15 mutants identified by Genomic Data Commons (GDC) in human cancers were expressed in HEK293T cells. Firstly, we found that the binding affinity for agonist NECA was decreased in six mutants but increased for the V275A mutant. Mutations A165V and A265V decreased the binding affinity for antagonist ZM241385. Secondly, we found that the potency of NECA (EC₅₀) in an impedance-based cell-morphology assay was mostly correlated with the binding affinity for the different mutants. Moreover, S132L and H278N were found to shift the A_2AAR towards the inactive state. Importantly, we found that ZM241385 could not inhibit the activation of V275A and P285L stimulated by NECA. Taken together, the cancer-associated mutations of A_2AAR modulated ligand binding and receptor functions. This study provides fundamental insights into the structure–activity relationship of the A_2AAR and provides insights for A_2AAR-related personalized treatment in cancer.

Single Stabilizing Point Mutation Enables High-Resolution Co-Crystal Structures of the Adenosine A2A Receptor with Preladenant Conjugates

The G protein-coupled adenosine A_2A receptor (A_2A AR) is an important new (potential) drug target in immuno-oncology, and for neurodegenerative diseases. Preladenant and its derivatives belong to the most potent A_2A AR antagonists displaying exceptional selectivity. While crystal structures of the human A_2A AR have been solved, mostly using the A_2A -StaR2 protein that bears 9 point mutations, co-crystallization with Preladenant derivatives has so far been elusive. We developed a new A_2A AR construct harboring a single point mutation (S91^3.39 K) which renders it extremely thermostable. This allowed the co-crystallization of two novel Preladenant derivatives, the polyethylene glycol-conjugated (PEGylated) PSB-2113, and the fluorophore-labeled PSB-2115. The obtained crystal structures (2.25 Å and 2.6 Å resolution) provide explanations for the high potency and selectivity of Preladenant derivatives. They represent the first crystal structures of a GPCR in complex with PEG- and fluorophore-conjugated ligands. The applied strategy is predicted to be applicable to further class A GPCRs.

4-Heteroaryl Substituted Amino-3,5-Dicyanopyridines as New Adenosine Receptor Ligands: Novel Insights on Structure-Activity Relationships and Perspectives

A new set of amino-3,5-dicyanopyridines was synthesized and biologically evaluated at the adenosine receptors (ARs). This chemical class is particularly versatile, as small structural modifications can influence not only affinity and selectivity, but also the pharmacological profile. Thus, in order to deepen the structure–activity relationships (SARs) of this series, different substituents were evaluated at the diverse positions on the dicyanopyridine scaffold. In general, the herein reported compounds show nanomolar binding affinity and interact better with both the human (h) A₁ and A_2A ARs than with the other subtypes. Docking studies at hAR structure were performed to rationalize the observed affinity data. Of interest are compounds 1 and 5, which can be considered as pan ligands as binding all the ARs with comparable nanomolar binding affinity (A₁AR: 1, K_i = 9.63 nM; 5, K_i = 2.50 nM; A_2AAR: 1, K_i = 21 nM; 5, Ki = 24 nM; A₃AR: 1, Ki = 52 nM; 5, Ki = 25 nM; A_2BAR: 1, EC₅₀ = 1.4 nM; 5, EC₅₀ = 1.12 nM). Moreover, these compounds showed a partial agonist profile at all the ARs. This combined AR partial agonist activity could lead us to hypothesize a potential effect in the repair process of damaged tissue that would be beneficial in both wound healing and remodeling.

The Pharmacological Potential of Adenosine A2A Receptor Antagonists for Treating Parkinson's Disease

The adenosine A_2A receptor subtype is recognized as a non-dopaminergic pharmacological target for the treatment of neurodegenerative disorders, notably Parkinson’s disease (PD). The selective A_2A receptor antagonist istradefylline is approved in the US and Japan as an adjunctive treatment to levodopa/decarboxylase inhibitors in adults with PD experiencing OFF episodes or a wearing-off phenomenon; however, the full potential of this drug class remains to be explored. In this article, we review the pharmacology of adenosine A_2A receptor antagonists from the perspective of the treatment of both motor and non-motor symptoms of PD and their potential for disease modification.

Brain Iron Deficiency Changes the Stoichiometry of Adenosine Receptor Subtypes in Cortico-Striatal Terminals: Implications for Restless Legs Syndrome

Brain iron deficiency (BID) constitutes a primary pathophysiological mechanism in restless legs syndrome (RLS). BID in rodents has been widely used as an animal model of RLS, since it recapitulates key neurochemical changes reported in RLS patients and shows an RLS-like behavioral phenotype. Previous studies with the BID-rodent model of RLS demonstrated increased sensitivity of cortical pyramidal cells to release glutamate from their striatal nerve terminals driving striatal circuits, a correlative finding of the cortical motor hyperexcitability of RLS patients. It was also found that BID in rodents leads to changes in the adenosinergic system, a downregulation of the inhibitory adenosine A₁ receptors (A₁Rs) and upregulation of the excitatory adenosine A_2A receptors (A_2ARs). It was then hypothesized, but not proven, that the BID-induced increased sensitivity of cortico-striatal glutamatergic terminals could be induced by a change in A₁R/A_2AR stoichiometry in favor of A_2ARs. Here, we used a newly developed FACS-based synaptometric analysis to compare the relative abundance on A₁Rs and A_2ARs in cortico-striatal and thalamo-striatal glutamatergic terminals (labeled with vesicular glutamate transporters VGLUT1 and VGLUT2, respectively) of control and BID rats. It could be demonstrated that BID (determined by measuring transferrin receptor density in the brain) is associated with a selective decrease in the A₁R/A_2AR ratio in VGLUT1 positive-striatal terminals.

New fused pyrroles with rA1/A2A antagonistic activity as potential therapeutics for neurodegenerative disorders

In a pilot study, eleven pyrrolopyridine and pyrrolopyrimidine derivatives (specifically, 7-azaindole and 7-deazapurine derivatives) were synthesised by Suzuki cross-coupling reactions and evaluated via radioligand binding assays as potential adenosine receptor (AR) antagonists in order to further investigate the structure-activity relationships of these compounds. 6-Chloro-4-phenyl-1H-pyrrolo[2,3-b]pyridine, with a 7-azaindole scaffold, was identified as a selective A₁ AR antagonist with a rA₁K_i value of 0.16 µM, and interestingly, the addition of a N-atom to the aforementioned fused heterocyclic ring system, creating corresponding 7-deazapurines, led to a dual A₁/A_2A AR ligand (2-chloro-4-phenyl-7H-pyrrolo[2,3-d]pyrimidine: rA₁K_i: 0.19 ± 0.02 µM; rA_2AK_i: 0.43 ± 0.01 µM). Introducing an additional N-atom into the heterocyclic ring system was tolerable for rA₁ AR affinity and also led to rA_2A AR affinity. This pilot study concluded that new 7-azaindole and 7-deazapurine derivatives represent interesting scaffolds for design of A₁ and/or A_2A AR antagonists.

Ligand modulation of the conformational dynamics of the A2A adenosine receptor revealed by single-molecule fluorescence

G protein-coupled receptors (GPCRs) are the largest class of transmembrane proteins, making them an important target for therapeutics. Activation of these receptors is modulated by orthosteric ligands, which stabilize one or several states within a complex conformational ensemble. The intra- and inter-state dynamics, however, is not well documented. Here, we used single-molecule fluorescence to measure ligand-modulated conformational dynamics of the adenosine A_2A receptor (A_2AR) on nanosecond to millisecond timescales. Experiments were performed on detergent-purified A₂R in either the ligand-free (apo) state, or when bound to an inverse, partial or full agonist ligand. Single-molecule Förster resonance energy transfer (smFRET) was performed on detergent-solubilized A_2AR to resolve active and inactive states via the separation between transmembrane (TM) helices 4 and 6. The ligand-dependent changes of the smFRET distributions are consistent with conformational selection and with inter-state exchange lifetimes ≥ 3 ms. Local conformational dynamics around residue 229^6.31 on TM6 was measured using fluorescence correlation spectroscopy (FCS), which captures dynamic quenching due to photoinduced electron transfer (PET) between a covalently-attached dye and proximal aromatic residues. Global analysis of PET-FCS data revealed fast (150-350 ns), intermediate (50-60 μs) and slow (200-300 μs) conformational dynamics in A_2AR, with lifetimes and amplitudes modulated by ligands and a G-protein mimetic (mini-G_s). Most notably, the agonist binding and the coupling to mini-G_s accelerates and increases the relative contribution of the sub-microsecond phase. Molecular dynamics simulations identified three tyrosine residues (Y112, Y288^7.53, and Y290^7.55) as being responsible for the dynamic quenching observed by PET-FCS and revealed associated helical motions around residue 229^6.31 on TM6. This study provides a quantitative description of conformational dynamics in A_2AR and supports the idea that ligands bias not only GPCR conformations but also the dynamics within and between distinct conformational states of the receptor.

How do adenosine A2A receptors regulate motor function?

Adenosine A_2A receptor antagonism is a new therapeutic strategy in the symptomatic treatment of Parkinson's disease (PD). This review addresses how adenosine A_2A receptors are involved with the control of motor function via the basal ganglia-thalamocortical circuit, and considers the anatomical localization and physiological function of the receptor, along with its ultrastructural localization in critical areas/neurons of the circuit. Based on this understanding of the functional significance of the adenosine A_2A receptor in the basal ganglia, the mode of action of A_2A receptor antagonists is explored in terms of the dynamic functioning of the basal ganglia and the activity of the internal circuits of the striatum in PD. Finally, the pathophysiological differences between the normal and PD states are examined to emphasize the importance of the adenosine A_2A receptor.

Potent and selective A3 adenosine receptor antagonists bearing aminoesters as heterobifunctional moieties

A₃ adenosine receptors were found to have a role in different pathological states, such as glaucoma, renal fibrosis, neuropathic pain and cancer. Consequently, it is important to utilize any molecular tool which could help to study these conditions. In the present study we continue our search for potent A₃ adenosine receptor ligands which could be successively conjugated to other molecules with the aim of obtaining more potent (e.g. allosteric ligand conjugation) or detectable ligands (e.g. fluorescent molecule or biotin conjugation). Specifically, different aminoester moieties were introduced at the 5 position of the pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine core. The ester functionalization represents the candidate for the subsequent conjugation. All the reported compounds are potent hA₃ adenosine receptor antagonists and some of them exhibited high selectivity against the other adenosine receptors. The main structural terms of ligand recognition and selectivity were disclosed by molecular modelling studies. Molecular docking results led to the characterization of an alternative binding mode for antagonists at the orthosteric binding site of the hA₃ adenosine receptor, evaluated and assessed by classical molecular dynamics simulations.

The Specificity of Downstream Signaling for A1 and A2AR Does Not Depend on the C-Terminus, Despite the Importance of This Domain in Downstream Signaling Strength

Recent efforts to determine the high-resolution crystal structures for the adenosine receptors (A₁R and A_2AR) have utilized modifications to the native receptors in order to facilitate receptor crystallization and structure determination. One common modification is a truncation of the unstructured C-terminus, which has been utilized for all the adenosine receptor crystal structures obtained to date. Ligand binding for this truncated receptor has been shown to be similar to full-length receptor for A_2AR. However, the C-terminus has been identified as a location for protein-protein interactions that may be critical for the physiological function of these important drug targets. We show that variants with A_2AR C-terminal truncations lacked cAMP-linked signaling compared to the full-length receptor constructs transfected into mammalian cells (HEK-293). In addition, we show that in a humanized yeast system, the absence of the full-length C-terminus affected downstream signaling using a yeast MAPK response-based fluorescence assay, though full-length receptors showed native-like G-protein coupling. To further study the G protein coupling, we used this humanized yeast platform to explore coupling to human-yeast G-protein chimeras in a cellular context. Although the C-terminus was essential for Gα protein-associated signaling, chimeras of A₁R with a C-terminus of A_2AR coupled to the A₁R-specific Gα (i.e., Gαi1 versus Gαs). This surprising result suggests that the C-terminus is important in the signaling strength, but not specificity, of the Gα protein interaction. This result has further implications in drug discovery, both in enabling the experimental use of chimeras for ligand design, and in the cautious interpretation of structure-based drug design using truncated receptors.

Allosteric Antagonism of the A2A Adenosine Receptor by a Series of Bitopic Ligands

Allosteric antagonism by bitopic ligands, as reported for many receptors, is a distinct modulatory mechanism. Although several bitopic A_2A adenosine receptor (A_2AAR) ligand classes were reported as pharmacological tools, their receptor binding and functional antagonism patterns, i.e., allosteric or competitive, were not well characterized. Therefore, here we systematically characterized A_2AAR binding and functional antagonism of two distinct antagonist chemical classes. i.e., fluorescent conjugates of xanthine amine congener (XAC) and SCH442416. Bitopic ligands were potent, weak, competitive or allosteric, based on the combination of pharmacophore, linker and fluorophore. Among antagonists tested, XAC, XAC245, XAC488, SCH442416, MRS7352 showed K_i binding values consistent with K_B values from functional antagonism. Interestingly, MRS7396, XAC-X-BY630 (XAC630) and 5-(N,N-hexamethylene)amiloride (HMA) were 9–100 times weaker in displacing fluorescent MRS7416 binding than radioligand binding. XAC245, XAC630, MRS7396, MRS7416 and MRS7322 behaved as allosteric A_2AAR antagonists, whereas XAC488 and MRS7395 antagonized competitively. Schild analysis showed antagonism slopes of 0.42 and 0.47 for MRS7396 and XAC630, respectively. Allosteric antagonists HMA and MRS7396 were more potent in displacing [³H]ZM241385 binding than MRS7416 binding. Sodium site D52N mutation increased and decreased affinity of HMA and MRS7396, respectively, suggesting possible preference for different A_2AAR conformations. The allosteric binding properties of some bitopic ligands were rationalized and analyzed using the Hall two-state allosteric model. Thus, fluorophore tethering to an orthosteric ligand is not neutral pharmacologically and may confer unexpected properties to the conjugate.

Insilico analysis of marine indole alkaloids for design of adenosine A2A receptor antagonist

Neurological disease is the disease associated with most of geriatric population in the world. The diseases like Alzheimer's disease and Parkinson's disease are associated with the change in the life style in current era. Treatment of these diseases normally focused on the agents which can able to manipulate the neurotransmitter release, so it is associated with severe side effects. Adenosine receptors are the upcoming targets for the inflammatory as well as neurological diseases as agents like istradefylline are in the clinical use. Marine natural products are the rich source of the valuable drug like substances, number marine alkaloids are known for their ability to pass blood brain barrier (BBB) which is major hurdle in the neurological drug discovery. Here, we report the virtual screening of some marine alkaloids for adenosine 2 receptor binding potential. Results indicated topsentin C, 6'-debromohamacanthin, 6-hydroxydiscodermindole and discodermindole are having excellent binding affinity towards the adenosine 2A receptor than other selected alkaloids.Communicated by Ramaswamy H. Sarma.

Ligand-induced conformational changes in a SMALP-encapsulated GPCR

The adenosine 2A receptor (A_2AR), a G-protein-coupled receptor (GPCR), was solubilised and purified encapsulated in styrene maleic acid lipid particles (SMALPs). The purified A_2AR-SMALP was associated with phospholipids characteristic of the plasma membrane of Pichia pastoris, the host used for its expression, confirming that the A_2AR-SMALP encapsulated native lipids. The fluorescence spectrum of the A_2AR-SMALP showed a characteristic broad emission peak at 330 nm, produced by endogenous Trp residues. The inverse agonist ZM241385 caused 30% increase in fluorescence emission, unusually accompanied by a red-shift in the emission wavelength. The emission spectrum also showed sub-peaks at 321 nm, 335 nm and 350 nm, indicating that individual Trp inhabited different environments following ZM241385 addition. There was no effect of the agonist NECA on the A_2AR-SMALP fluorescence spectrum. Substitution of two Trp residues by Tyr suggested that ZM241385 affected the environment and mobility of Trp246^6.48 in TM6 and Trp268^7.33 at the extracellular face of TM7, causing transition to a more hydrophobic environment. The fluorescent moiety IAEDANS was site-specifically introduced at the intracellular end of TM6 (residue 231^6.33) to report on the dynamic cytoplasmic face of the A_2AR. The inverse agonist ZM241385 caused a concentration-dependent increase in fluorescence emission as the IAEDANS moved to a more hydrophobic environment, consistent with closing the G-protein binding crevice. NECA generated only 30% of the effect of ZM241385. This study provides insight into the SMALP environment; encapsulation supported constitutive activity of the A_2AR and ZM241385-induced conformational transitions but the agonist NECA generated only small effects.

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Conformational changes in the A_2AR monitored in a nano-scale membrane disc (SMALP).

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Profile of phospholipids in A_2AR-SMALP similar to the plasma membrane.

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A partially-active conformation of A_2AR is supported in a SMALP.

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Inverse agonist induced dose-dependent conformational transitions in A_2AR-SMALP.

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In contrast to inverse agonist, agonist induced only small conformational changes.

Control of glutamate release by complexes of adenosine and cannabinoid receptors

Background

It has been hypothesized that heteromers of adenosine A_2A receptors (A2AR) and cannabinoid CB₁ receptors (CB1R) localized in glutamatergic nerve terminals mediate the integration of adenosine and endocannabinoid signaling involved in the modulation of striatal excitatory neurotransmission. Previous studies have demonstrated the existence of A2AR-CB1R heteromers in artificial cell systems. A dependence of A2AR signaling for the Gi protein-mediated CB1R signaling was described as one of its main biochemical characteristics. However, recent studies have questioned the localization of functionally significant A2AR-CB1R heteromers in striatal glutamatergic terminals.

Results

Using a peptide-interfering approach combined with biophysical and biochemical techniques in mammalian transfected cells and computational modeling, we could establish a tetrameric quaternary structure of the A2AR-CB1R heterotetramer. This quaternary structure was different to the also tetrameric structure of heteromers of A2AR with adenosine A₁ receptors or dopamine D₂ receptors, with different heteromeric or homomeric interfaces. The specific quaternary structure of the A2A-CB1R, which depended on intermolecular interactions involving the long C-terminus of the A2AR, determined a significant A2AR and Gs protein-mediated constitutive activation of adenylyl cyclase. Using heteromer-interfering peptides in experiments with striatal glutamatergic terminals, we could then demonstrate the presence of functionally significant A2AR-CB1R heteromers with the same biochemical characteristics of those studied in mammalian transfected cells. First, either an A2AR agonist or an A2AR antagonist allosterically counteracted Gi-mediated CB1R agonist-induced inhibition of depolarization-induced glutamate release. Second, co-application of both an A2AR agonist and an antagonist cancelled each other effects. Finally, a CB1R agonist inhibited glutamate release dependent on a constitutive activation of A2AR by a canonical Gs-Gi antagonistic interaction at the adenylyl cyclase level.

Conclusions

We demonstrate that the well-established cannabinoid-induced inhibition of striatal glutamate release can mostly be explained by a CB1R-mediated counteraction of the A2AR-mediated constitutive activation of adenylyl cyclase in the A2AR-CB1R heteromer.

Protein-peptide association kinetics beyond the seconds timescale from atomistic simulations

Understanding and control of structures and rates involved in protein ligand binding are essential for drug design. Unfortunately, atomistic molecular dynamics (MD) simulations cannot directly sample the excessively long residence and rearrangement times of tightly binding complexes. Here we exploit the recently developed multi-ensemble Markov model framework to compute full protein-peptide kinetics of the oncoprotein fragment 25-109Mdm2 and the nano-molar inhibitor peptide PMI. Using this system, we report, for the first time, direct estimates of kinetics beyond the seconds timescale using simulations of an all-atom MD model, with high accuracy and precision. These results only require explicit simulations on the sub-milliseconds timescale and are tested against existing mutagenesis data and our own experimental measurements of the dissociation and association rates. The full kinetic model reveals an overall downhill but rugged binding funnel with multiple pathways. The overall strong binding arises from a variety of conformations with different hydrophobic contact surfaces that interconvert on the milliseconds timescale.

Current and investigational non-dopaminergic agents for management of motor symptoms (including motor complications) in Parkinson's disease

INTRODUCTION

Parkinson's disease is characterized by a heterogeneous combination of motor and non motor symptoms. The nigrostriatal dopamine deficit is one of its essential pathophysiologic features. Areas covered: This invited narrative review provides an overlook over current available and future promising non dopaminergic therapeutics to modulate altered dopaminergic neurotransmission in Parkinson's disease. Current research strategies aim to proof clinical efficacy by amelioration of motor symptoms and preponderant levodopa related movement fluctuations. These so-called motor complications are characterized by involuntary movements as a result of an overstimulation of the nigrostriatal dopaminergic system or by temporary recurrence of motor symptoms, when beneficial effects of dopamine substituting drugs vane. Expert opinion: Non dopaminergic modulation of dopamine replacement is currently mostly investigated in well defined and selected patients with motor complications to get approval. However, the world of daily maintenance of patients with its individually adapted, so-called personalised, therapy will determine the real value of these therapeutics. Here the clinical experience of the treating neurologists and the courage to use unconventional drug combinations are essential preconditions for successful treatments of motor and associated non motor complications in cooperation with the patients and their care giving surroundings.

Evidence for the heterotetrameric structure of the adenosine A2A-dopamine D2 receptor complex

Heteromers of G-protein-coupled receptors (GPCRs) have emerged as potential novel targets for drug development. Accumulating evidence indicates that GPCRs can form homodimers and heteromers, with homodimers being the predominant species and oligomeric receptors being formed as multiples of dimers. Recently, heterotetrameric structures have been proposed for dopamine D1receptor (D1R)-dopamine D3receptor (D3R) and adenosine A2Areceptor (A2AR)-dopamine D2receptor (D2R) heteromers. The structural model proposed for these complexes is a heteromer constituted by two receptor homodimers. The existence of GPCR homodimers and heteromers provides a structural basis for inter-protomer allosteric mechanisms that might account for a multiplicity of unique pharmacological properties. In this review, we focus on the A2AR-D2R heterotetramer as an example of an oligomeric structure that is key in the modulation of striatal neuronal function. We also review the interfaces involved in this and other recently reported heteromers of GPCRs. Furthermore, we discuss several published studies showing theex vivoexpression of A2AR-D2R heteromers. The ability of A2AR agonists to decrease the affinity of D2R agonists has been reported and, on the basis of this interaction, A2AR antagonists have been proposed as potential drugs for the treatment of Parkinson's disease. The heterotetrameric structure of the A2AR-D2R complex offers a novel model that can provide new clues about how to adjust the drug dosage to the expected levels of endogenous adenosine.

A2AR Binding Kinetics in the Ligand Depletion Regime

Ligand binding plays a fundamental role in stimulating the downstream signaling of membrane receptors. Here, ligand-binding kinetics of the full-length human adenosine A_2A receptor (A_2AR) reconstituted in detergent micelles were measured using a fluorescently labeled ligand via fluorescence anisotropy. Importantly, to optimize the signal-to-noise ratio, these experiments were conducted in the ligand depletion regime. In the ligand depletion regime, the assumptions used to determine analytical solutions for one-site binding models for either one or two ligands in competition are no longer valid. We therefore implemented a numerical solution approach to analyze kinetic binding data as experimental conditions approach the ligand depletion regime. By comparing the results from the numerical and the analytical solutions, we highlight the ligand-receptor ratios at which the analytical solution begins to lose predictive accuracy. Using the numerical solution approach, we determined the kinetic rate constants of the fluorescent ligand, FITC-APEC, and those for three unlabeled ligands using competitive association experiments. The association and dissociation rate constants of the unlabeled ligands determined from the competitive association experiments were then independently validated using competitive dissociation data. Based on this study, a numerical solution is recommended to determine kinetic ligand-binding parameters for experiments conducted in the ligand-depletion regime.

Nondopaminergic therapy of motor and nonmotor symptoms in Parkinson's disease: a clinician's perspective

Patients with Parkinson's disease suffer from impaired motor behavior due to the dopaminergic striatal deficit and nonmotor symptoms, which also result from nondopaminergic neuronal death. This review provides a personal opinion on treatment strategies for symptoms, resulting at least partially from nondopaminergic neurodegeneration, and on therapeutic modulation of dopaminergic neurotransmission. Patient-tailored treatment regimes on the basis of an individual risk benefit ratio as essential precondition try to balance all these symptoms. Individually varying heterogeneity of symptoms, nonlinear disease progression, treatment response, acceptance, tolerability and safety of applied therapies demand a close, consistent relationship between patient and treating physician. Daily maintenance of patients does not ask for too detailed treatment guidelines.

A mutagenesis and screening strategy to generate optimally thermostabilized membrane proteins for structural studies

The thermostability of an integral membrane protein (MP) in detergent solution is a key parameter that dictates the likelihood of obtaining well-diffracting crystals that are suitable for structure determination. However, many mammalian MPs are too unstable for crystallization. We developed a thermostabilization strategy based on systematic mutagenesis coupled to a radioligand-binding thermostability assay that can be applied to receptors, ion channels and transporters. It takes ∼6-12 months to thermostabilize a G-protein-coupled receptor (GPCR) containing 300 amino acid (aa) residues. The resulting thermostabilized MPs are more easily crystallized and result in high-quality structures. This methodology has facilitated structure-based drug design applied to GPCRs because it is possible to determine multiple structures of the thermostabilized receptors bound to low-affinity ligands. Protocols and advice are given on how to develop thermostability assays for MPs and how to combine mutations to make an optimally stable mutant suitable for structural studies. The steps in the procedure include the generation of ∼300 site-directed mutants by Ala/Leu scanning mutagenesis, the expression of each mutant in mammalian cells by transient transfection and the identification of thermostable mutants using a thermostability assay that is based on binding of an (125)I-labeled radioligand to the unpurified, detergent-solubilized MP. Individual thermostabilizing point mutations are then combined to make an optimally stable MP that is suitable for structural biology and other biophysical studies.

The use of conformationally thermostabilised GPCRs in drug discovery: application to fragment, structure and biophysical techniques

Recent developments in receptor stabilisation have facilitated major advances in G protein-coupled receptor (GPCR) research, notably structural biology, over the past eight years. Here we review the application of fragment, structure and biophysical techniques using stabilised GPCRs (StaR proteins), and their impact in the drug discovery process. These techniques have, most recently, been utilised in the discovery of the non-alkyne mGlu₅ negative allosteric modulator HTL14242, in addition to the dual orexin receptor antagonist HTL6641, with differentiated residence time kinetics.

Simulation and Comparative Analysis of Different Binding Modes of Non-nucleoside Agonists at the A2A Adenosine Receptor

Non-nucleoside agonists of adenosine receptors were analysed at the A2A adenosine receptor to simulate and compare their possible binding modes. The docking studies were performed by using different arrangements of the binding cavity and various docking tools. Mutagenesis results reported in literature were used as reference data for the assessment of the different ligand arrangements observed in this study. The results suggest four possible binding modes, two of which appear compatible with an agonist activity and in agreement with the mutagenesis data. This study provides useful information for the design of new simplified compounds presenting agonist activity at the A2A adenosine receptor.

Structurally Enabled Discovery of Adenosine A2A Receptor Antagonists

Over the past decade there has been a revolution in the field of G protein-coupled receptor (GPCR) structural biology. Many years of innovative research from different areas have come together to fuel this significant change in the fortunes of this field, which for many years was characterized by the paucity of high-resolution structures. The determination to succeed has been in part due to the recognized importance of these proteins as drug targets, and although the pharmaceutical industry has been focusing on these receptors, it can be justifiably argued and demonstrated that many of the approved and commercially successful GPCR drugs can be significantly improved to increase efficacy and/or reduce undesired side effects. In addition, many validated targets in this class remain to be drugged. It is widely recognized that application of structure-based drug design approaches can help medicinal chemists a long way toward discovering better drugs. The achievement of structural biologists in providing high-resolution insight is beginning to transform drug discovery efforts, and there are a number of GPCR drugs that have been discovered by use of structural information that are in clinical development. This review aims to highlight the key developments that have brought success to GPCR structure resolution efforts and exemplify the practical application of structural information for the discovery of adenosine A_2A receptor antagonists that have potential to treat multiple conditions.

Ligand-Independent Adenosine A2B Receptor Constitutive Activity as a Promoter of Prostate Cancer Cell Proliferation

Aberrant ligand-independent G protein-coupled receptor constitutive activity has been implicated in the pathophysiology of a number of cancers. The adenosine A2B receptor (A2BAR) is dynamically upregulated under pathologic conditions associated with a hypoxic microenvironment, including solid tumors. This, in turn, may amplify ligand-independent A2BAR signal transduction. The contribution of A2BAR constitutive activity to disease progression is currently unknown yet of fundamental importance, as the preferred therapeutic modality for drugs designed to reduce A2BAR constitutive activity would be inverse agonism as opposed to neutral antagonism. The current study investigated A2BAR constitutive activity in a heterologous expression system and a native 22Rv1 human prostate cancer cell line exposed to hypoxic conditions (2% O2). The A2BAR inverse agonists, ZM241385 [4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol] or PSB-603 (8-(4-(4-(4-chlorophenyl)piperazide-1-sulfonyl)phenyl)-1-propylxanthine), mediated a concentration-dependent decrease in baseline cAMP levels in both cellular systems. Proliferation of multiple prostate cancer cell lines was also attenuated in the presence of PSB-603. Importantly, both the decrease in baseline cAMP accumulation and the reduction of proliferation were not influenced by the addition of adenosine deaminase, demonstrating that these effects are not dependent on stimulation of A2BARs by the endogenous agonist adenosine. Our study is the first to reveal that wild-type human A2BARs have high constitutive activity in both model and native cells. Furthermore, our findings demonstrate that this ligand-independent A2BAR constitutive activity is sufficient to promote prostate cancer cell proliferation in vitro. More broadly, A2BAR constitutive activity may have wider, currently unappreciated implications in pathologic conditions associated with a hypoxic microenvironment.

Protein conformational plasticity and complex ligand-binding kinetics explored by atomistic simulations and Markov models

Understanding the structural mechanisms of protein–ligand binding and their dependence on protein sequence and conformation is of fundamental importance for biomedical research. Here we investigate the interplay of conformational change and ligand-binding kinetics for the serine protease Trypsin and its competitive inhibitor Benzamidine with an extensive set of 150 μs molecular dynamics simulation data, analysed using a Markov state model. Seven metastable conformations with different binding pocket structures are found that interconvert at timescales of tens of microseconds. These conformations differ in their substrate-binding affinities and binding/dissociation rates. For each metastable state, corresponding solved structures of Trypsin mutants or similar serine proteases are contained in the protein data bank. Thus, our wild-type simulations explore a space of conformations that can be individually stabilized by adding ligands or making suitable changes in protein sequence. These findings provide direct evidence of conformational plasticity in receptors.

Conformational plasticity influences several aspects of protein function. Here the authors combine extensive MD simulations with Markov state models—using trypsin as model—to reveal new mechanistic details of how conformational plasticity influence ligand-receptors interactions.

From G Protein-coupled Receptor Structure Resolution to Rational Drug Design

A number of recent technical solutions have led to significant advances in G protein-coupled receptor (GPCR) structural biology. Apart from a detailed mechanistic view of receptor activation, the new structures have revealed novel ligand binding sites. Together, these insights provide avenues for rational drug design to modulate the activities of these important drug targets. The application of structural data to GPCR drug discovery ushers in an exciting era with the potential to improve existing drugs and discover new ones. In this review, we focus on technical solutions that have accelerated GPCR crystallography as well as some of the salient findings from structures that are relevant to drug discovery. Finally, we outline some of the approaches used in GPCR structure based drug design.

Allosteric interactions between agonists and antagonists within the adenosine A2A receptor-dopamine D2 receptor heterotetramer

Adenosine A2A receptor (A2AR)-dopamine D2 receptor (D2R) heteromers are key modulators of striatal neuronal function. It has been suggested that the psychostimulant effects of caffeine depend on its ability to block an allosteric modulation within the A2AR-D2R heteromer, by which adenosine decreases the affinity and intrinsic efficacy of dopamine at the D2R. We describe novel unsuspected allosteric mechanisms within the heteromer by which not only A2AR agonists, but also A2AR antagonists, decrease the affinity and intrinsic efficacy of D2R agonists and the affinity of D2R antagonists. Strikingly, these allosteric modulations disappear on agonist and antagonist coadministration. This can be explained by a model that considers A2AR-D2R heteromers as heterotetramers, constituted by A2AR and D2R homodimers, as demonstrated by experiments with bioluminescence resonance energy transfer and bimolecular fluorescence and bioluminescence complementation. As predicted by the model, high concentrations of A2AR antagonists behaved as A2AR agonists and decreased D2R function in the brain.

Biosensor-based affinities and binding kinetics of small molecule antagonists to the adenosine A(2A) receptor reconstituted in HDL like particles

The options for investigating solubilised G protein-coupled receptors (GPCRs) by biophysical techniques have long been hampered by their instability. A thermostabilised adenosine A2A receptor expressed in insect cells, purified in detergent and reconstituted into high-density lipoprotein (HDL) particles was immobilised onto a Surface Plasmon Resonance sensor chip. This allowed measurement of affinities and kinetics for A2A antagonists with affinities ranging from 50 pM to almost 2 μM. Compared with other formats, reproduction of affinities, and dissociation and association rate constants are good, reasonable and poor respectively, indicating stabilised receptors in HDL particles are useful for investigating specific aspects of GPCR-ligand interactions.

Molecular Determinants of CGS21680 Binding to the Human Adenosine A2A Receptor

The adenosine A2A receptor (A(2A)R) plays a key role in transmembrane signaling mediated by the endogenous agonist adenosine. Here, we describe the crystal structure of human A2AR thermostabilized in an active-like conformation bound to the selective agonist 2-[p-(2-carboxyethyl)phenylethyl-amino]-5'-N-ethylcarboxamido adenosine (CGS21680) at a resolution of 2.6 Å. Comparison of A(2A)R structures bound to either CGS21680, 5'-N-ethylcarboxamido adenosine (NECA), UK432097 [6-(2,2-diphenylethylamino)-9-[(2R,3R,4S,5S)-5-(ethylcarbamoyl)-3,4-dihydroxy-tetrahydrofuran-2-yl]-N-[2-[[1-(2-pyridyl)-4-piperidyl]carbamoylamino]ethyl]purine-2-carboxamide], or adenosine shows that the adenosine moiety of the ligands binds to the receptor in an identical fashion. However, an extension in CGS21680 compared with adenosine, the (2-carboxyethyl)phenylethylamino group, binds in an extended vestibule formed from transmembrane regions 2 and 7 (TM2 and TM7) and extracellular loops 2 and 3 (EL2 and EL3). The (2-carboxyethyl)phenylethylamino group makes van der Waals contacts with side chains of amino acid residues Glu169(EL2), His264(EL3), Leu267(7.32), and Ile274(7.39), and the amine group forms a hydrogen bond with the side chain of Ser67(2.65). Of these residues, only Ile274(7.39) is absolutely conserved across the human adenosine receptor subfamily. The major difference between the structures of A(2A)R bound to either adenosine or CGS21680 is that the binding pocket narrows at the extracellular surface when CGS21680 is bound, due to an inward tilt of TM2 in that region. This conformation is stabilized by hydrogen bonds formed by the side chain of Ser67(2.65) to CGS21680, either directly or via an ordered water molecule. Mutation of amino acid residues Ser67(2.65), Glu169(EL2), and His264(EL3), and analysis of receptor activation either in the presence or absence of ligands implicates this region in modulating the level of basal activity of A(2A)R.

Real-time monitoring of binding events on a thermostabilized human A2A receptor embedded in a lipid bilayer by surface plasmon resonance

Membrane proteins (MPs) are prevalent drug discovery targets involved in many cell processes. Despite their high potential as drug targets, the study of MPs has been hindered by limitations in expression, purification and stabilization in order to acquire thermodynamic and kinetic parameters of small molecules binding. These bottlenecks are grounded on the mandatory use of detergents to isolate and extract MPs from the cell plasma membrane and the coexistence of multiple conformations, which reflects biochemical versatility and intrinsic instability of MPs. In this work ,we set out to define a new strategy to enable surface plasmon resonance (SPR) measurements on a thermostabilized and truncated version of the human adenosine (A2A) G-protein-coupled receptor (GPCR) inserted in a lipid bilayer nanodisc in a label- and detergent-free manner by using a combination of affinity tags and GFP-based fluorescence techniques. We were able to detect and characterize small molecules binding kinetics on a GPCR fully embedded in a lipid environment. By providing a comparison between different binding assays in membranes, nanodiscs and detergent micelles, we show that nanodiscs can be used for small molecule binding studies by SPR to enhance the MP stability and to trigger a more native-like behaviour when compared to kinetics on A2A receptors isolated in detergent. This work provides thus a new methodology in drug discovery to characterize the binding kinetics of small molecule ligands for MPs targets in a lipid environment.

The safety of istradefylline for the treatment of Parkinson's disease

INTRODUCTION

Antagonism of the A2A receptor improves motor behavior in patients with Parkinson's disease (PD), according to results of clinical studies which confirm findings of previous experimental research. The xanthine derivative, istradefylline , has the longest half-life out of the available A2A receptor antagonists. Istradefylline easily crosses the blood-brain barrier and shows a high affinity to the human A2A receptor.

AREAS COVERED

This narrative review aims to discuss the safety and tolerability of istradefylline against the background of the currently available drug portfolio for the treatment of PD patients.

EXPERT OPINION

Istradefylline was safe and well tolerated in clinical trials, which have focused on l-DOPA-treated PD patients. The future of istradefylline as a complementary drug for modulation of the dopaminergic neurotransmission also relies on its potential to act like an l-DOPA plus dopamine agonist sparing future treatment alternative and to reduce the risk of predominant l-DOPA-related onset of motor complications in addition to its direct ameliorating effect on motor symptoms. Dopamine-substituting drugs may dose-dependently produce systemic side effects, particularly onset of hypotension and nausea by peripheral dopamine receptor stimulation. Istradefylline does not interfere with these peripheral receptors and therefore shows a good safety and tolerability profile.

Purification of Stabilized GPCRs for Structural and Biophysical Analyses

G protein-coupled receptors (GPCRs) are of particular importance for drug discovery, being the targets of many existing drugs, and being linked to many diseases where new therapies are required. However, as integral membrane proteins, they are generally unstable when removed from their membrane environment, precluding them from the wide range of structural and biophysical techniques which can be applied to soluble proteins such as kinases. Through the use of protein engineering methods, mutations can be identified which both increase the thermostability of GPCRs when purified in detergent, as well as biasing the receptor toward a specific physiologically relevant conformational state. The resultant stabilized receptor (known as a StaR) can be purified in multiple-milligram quantities, whilst retaining correct folding, thus enabling the generation of reagents suitable for a broad range of structural and biophysical studies. Example protocols for the purification of StaR proteins for analysis, ligand screening with the thiol-specific fluorochrome N-[4-(7-diethylamino-4-methyl-3-coumarinyl)phenyl]maleimide (CPM), surface plasmon resonance (SPR), and crystallization for structural studies are presented.

Isothermal chemical denaturation to determine binding affinity of small molecules to G-protein coupled receptors

The determination of accurate binding affinities is critical in drug discovery and development. Several techniques are available for characterizing the binding of small molecules to soluble proteins. The situation is different for integral membrane proteins. Isothermal chemical denaturation has been shown to be a valuable biophysical method to determine, in a direct and label-free fashion, the binding of ligands to soluble proteins. In this study, the application of isothermal chemical denaturation was applied to an integral membrane protein, the A2a G-protein coupled receptor. Binding affinities for a set of 19 small molecule agonists/antagonists of the A2a receptor were determined and found to be in agreement with data from surface plasmon resonance and radioligand binding assays previously reported in the literature. Therefore, isothermal chemical denaturation expands the available toolkit of biophysical techniques to characterize and study ligand binding to integral membrane proteins, specifically G-protein coupled receptors in vitro.

Uncovering caffeine's adenosine A2A receptor inverse agonism in experimental parkinsonism

Caffeine, the most consumed psychoactive substance worldwide, may have beneficial effects on Parkinson’s disease (PD) therapy. The mechanism by which caffeine contributes to its antiparkinsonian effects by acting as either an adenosine A_2A receptor (A_2AR) neutral antagonist or an inverse agonist is unresolved. Here we show that caffeine is an A_2AR inverse agonist in cell-based functional studies and in experimental parkinsonism. Thus, we observed that caffeine triggers a distinct mode, opposite to A_2AR agonist, of the receptor’s activation switch leading to suppression of its spontaneous activity. These inverse agonist-related effects were also determined in the striatum of a mouse model of PD, correlating well with increased caffeine-mediated motor effects. Overall, caffeine A_2AR inverse agonism may be behind some of the well-known physiological effects of this substance both in health and disease. This information might have a critical mechanistic impact for PD pharmacotherapeutic design.

Human monocyte recognition of adenosine-based cyclic dinucleotides unveils the A2a Gαs protein-coupled receptor tonic inhibition of mitochondrially induced cell death

Cyclic dinucleotides are important messengers for bacteria and protozoa and are well-characterized immunity alarmins for infected mammalian cells through intracellular binding to STING receptors. We sought to investigate their unknown extracellular effects by adding cyclic dinucleotides to the culture medium of freshly isolated human blood cells in vitro. Here we report that adenosine-containing cyclic dinucleotides induce the selective apoptosis of monocytes through a novel apoptotic pathway. We demonstrate that these compounds are inverse agonist ligands of A2a, a Gαs-coupled adenosine receptor selectively expressed by monocytes. Inhibition of monocyte A2a by these ligands induces apoptosis through a mechanism independent of that of the STING receptors. The blockade of basal (adenosine-free) signaling from A2a inhibits protein kinase A (PKA) activity, thereby recruiting cytosolic p53, which opens the mitochondrial permeability transition pore and impairs mitochondrial respiration, resulting in apoptosis. A2a antagonists and inverse agonist ligands induce apoptosis of human monocytes, while A2a agonists are antiapoptotic. In vivo, we used a mock developing human hematopoietic system through NSG mice transplanted with human CD34(+) cells. Treatment with cyclic di-AMP selectively depleted A2a-expressing monocytes and their precursors via apoptosis. Thus, monocyte recognition of cyclic dinucleotides unravels a novel proapoptotic pathway: the A2a Gαs protein-coupled receptor (GPCR)-driven tonic inhibitory signaling of mitochondrion-induced cell death.

Structure-based drug design for G protein-coupled receptors

Our understanding of the structural biology of G protein-coupled receptors has undergone a transformation over the past 5 years. New protein-ligand complexes are described almost monthly in high profile journals. Appreciation of how small molecules and natural ligands bind to their receptors has the potential to impact enormously how medicinal chemists approach this major class of receptor targets. An outline of the key topics in this field and some recent examples of structure- and fragment-based drug design are described. A table is presented with example views of each G protein-coupled receptor for which there is a published X-ray structure, including interactions with small molecule antagonists, partial and full agonists. The possible implications of these new data for drug design are discussed.

Vas deferens neuro-effector junction: from kymographic tracings to structural biology principles

The vas deferens is a simple bioassay widely used to study the physiology of sympathetic neurotransmission and the pharmacodynamics of adrenergic drugs. The role of ATP as a sympathetic co-transmitter has gained increasing attention and furthered our understanding of its role in sympathetic reflexes. In addition, new information has emerged on the mechanisms underlying the storage and release of ATP. Both noradrenaline and ATP concur to elicit the tissue smooth muscle contractions following sympathetic reflexes or electrical field stimulation of the sympathetic nerve terminals. ATP and adenosine (its metabolic byproduct) are powerful presynaptic regulators of co-transmitter actions. In addition, neuropeptide Y, the third member of the sympathetic triad, is an endogenous modulator. The peptide plus ATP and/or adenosine play a significant role as sympathetic modulators of transmitter's release. This review focuses on the physiological principles that govern sympathetic co-transmitter activity, with special interest in defining the motor role of ATP. In addition, we intended to review the recent structural biology findings related to the topology of the P2X1R based on the crystallized P2X4 receptor from Danio rerio, or the crystallized adenosine A2A receptor as a member of the G protein coupled family of receptors as prototype neuro modulators. This review also covers structural elements of ectonucleotidases, since some members are found in the vas deferens neuro-effector junction. The allosteric principles that apply to purinoceptors are also reviewed highlighting concepts derived from receptor theory at the light of the current available structural elements. Finally, we discuss clinical applications of these concepts.