Holistic Methods for the Analysis of cNMP Effects

Agonist-induced phosphorylation of orthologues of the orphan receptor GPR35 functions as an activation sensor

G protein-coupled receptor 35 (GPR35) is poorly characterized but nevertheless has been revealed to have diverse roles in areas including lower gut inflammation and pain. The development of novel reagents and tools will greatly enhance analysis of GPR35 functions in health and disease. Here, we used mass spectrometry, mutagenesis, and [³²P] orthophosphate labeling to identify that all five hydroxy-amino acids in the C-terminal tail of human GPR35a became phosphorylated in response to agonist occupancy of the receptor and that, apart from Ser²⁹⁴, each of these contributed to interactions with arretin-3, which inhibits further G protein-coupled receptor signaling. We found that Ser³⁰³ was key to such interactions; the serine corresponding to human GPR35a residue 303 also played a dominant role in arrestin-3 interactions for both mouse and rat GPR35. We also demonstrated that fully phospho-site–deficient mutants of human GPR35a and mouse GPR35 failed to interact effectively with arrestin-3, and the human phospho-deficient variant was not internalized from the surface of cells in response to agonist treatment. Even in cells stably expressing species orthologues of GPR35, a substantial proportion of the expressed protein(s) was determined to be immature. Finally, phospho-site–specific antisera targeting the region encompassing Ser³⁰³ in human (Ser³⁰¹ in mouse) GPR35a identified only the mature forms of GPR35 and provided effective sensors of the activation status of the receptors both in immunoblotting and immunocytochemical studies. Such antisera may be useful tools to evaluate target engagement in drug discovery and target validation programs.

Methamphetamine Blocks Adenosine A2A Receptor Activation via Sigma 1 and Cannabinoid CB1 Receptors

Methamphetamine is, worldwide, one of the most consumed drugs of abuse. One important side effect is neurodegeneration leading to a decrease in life expectancy. The aim of this paper was to check whether the drug affects one of the receptors involved in neurodegeneration/neuroprotection events, namely the adenosine A2A receptor (A2AR). First, we noticed that methamphetamine does not affect A2A functionality if the receptor is expressed in a heterologous system. However, A2AR becomes sensitive to the drug upon complexes formation with the cannabinoid CB1 receptor (CB1R) and the sigma 1 receptor (σ1R). Signaling via both adenosine A2AR and cannabinoid CB1R was affected by methamphetamine in cells co-expressing the two receptors. In striatal primary cultures, the A2AR-CB1R heteromer complex was detected and methamphetamine not only altered its expression but completely blocked the A2AR- and the CB1R-mediated activation of the mitogen activated protein kinase (MAPK) pathway. In conclusion, methamphetamine, with the participation of σ1R, alters the expression and function of two interacting receptors, A2AR, which is a therapeutic target for neuroprotection, and CB1R, which is the most abundant G protein-coupled receptor (GPCR) in the brain.

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

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

Cocaine Blocks Effects of Hunger Hormone, Ghrelin, Via Interaction with Neuronal Sigma-1 Receptors

Despite ancient knowledge on cocaine appetite-suppressant action, the molecular basis of such fact remains unknown. Addiction/eating disorders (e.g., binge eating, anorexia, bulimia) share a central control involving reward circuits. However, we here show that the sigma-1 receptor (σ1R) mediates cocaine anorectic effects by interacting in neurons with growth/hormone/secretagogue (ghrelin) receptors. Cocaine increases colocalization of σ1R and GHS-R1a at the cell surface. Moreover, in transfected HEK-293T and neuroblastoma SH-SY5Y cells, and in primary neuronal cultures, pretreatment with cocaine or a σ1R agonist inhibited ghrelin-mediated signaling, in a similar manner as the GHS-R1a antagonist YIL-781. Results were similar in G protein-dependent (cAMP accumulation and calcium release) and in partly dependent or independent (ERK1/2 phosphorylation and label-free) assays. We provide solid evidence for direct interaction between receptors and the functional consequences, as well as a reliable structural model of the macromolecular σ1R-GHS-R1a complex, which arises as a key piece in the puzzle of the events linking cocaine consumption and appetitive/consummatory behaviors.

Cannabigerol Action at Cannabinoid CB1 and CB2 Receptors and at CB1-CB2 Heteroreceptor Complexes

Cannabigerol (CBG) is one of the major phytocannabinoids present in Cannabis sativa L. that is attracting pharmacological interest because it is non-psychotropic and is abundant in some industrial hemp varieties. The aim of this work was to investigate in parallel the binding properties of CBG to cannabinoid CB₁ (CB₁R) and CB₂ (CB₂R) receptors and the effects of the compound on agonist activation of those receptors and of CB₁–CB₂ heteroreceptor complexes. Using [³H]-CP-55940, CBG competed with low micromolar K_i values the binding to CB₁R and CB₂R. Homogeneous binding in living cells, which is only technically possible for the CB₂R, provided a 152 nM K_i value. Also interesting, CBG competed the binding of [³H]-WIN-55,212-2 to CB₂R but not to CB₁R (K_i: 2.7 versus >30 μM). The phytocannabinoid modulated signaling mediated by receptors and receptor heteromers even at low concentrations of 0.1–1 μM. cAMP, pERK, β-arrestin recruitment and label-free assays in HEK-293T cells expressing the receptors and treated with endocannabinoids or selective agonists proved that CBG is a partial agonist of CB₂R. The action on cells expressing heteromers was similar to that obtained in cells expressing the CB₂R. The effect of CBG on CB₁R was measurable but the underlying molecular mechanisms remain uncertain. The results indicate that CBG is indeed effective as regulator of endocannabinoid signaling.

cCMP and cUMP Across the Tree of Life: From cCMP and cUMP Generators to cCMP- and cUMP-Regulated Cell Functions

The cyclic purine nucleotides cAMP and cGMP are well-established second messenger molecules that are generated by distinct nucleotidyl cyclases (NCs) and regulate numerous cell functions via specific effector molecules. In contrast, the existence of the cyclic pyrimidine nucleotides cCMP and cUMP has been controversial for many years. The development of highly specific and sensitive mass spectrometry methods has enabled the unequivocal detection and quantitation of cCMP and cUMP in biological systems. These cNMPs occur broadly in numerous mammalian cell lines and primary cells. cCMP has also been detected in mouse organs, and both cCMP and cUMP occur in various developmental stages of the zebrafish Danio rerio. So far, the soluble guanylyl cyclase (sGC) and soluble adenylyl cyclase (sAC) have been identified as cCMP and cUMP generators. Dissociations in the expression patterns of sAC and sGC relative to cCMP and cUMP abundance may point to the existence of hitherto unidentified cCMP- and cUMP-generating NCs. The broad occurrence of cCMP and cUMP in vertebrates and the distinct cNMP patterns suggest specific roles of these cNMPs in the regulation of numerous cell functions.

Label-Free Dynamic Mass Redistribution and Bio-Impedance Methods for Drug Discovery

Label-free biosensors are increasingly employed in drug discovery. Cell-based biosensors provide valuable insights into the biological consequences of exposing cells and tissues to chemical agents and the underlying molecular mechanisms associated with these effects. Optical biosensors based on the detection of dynamic mass redistribution (DMR) and impedance biosensors using cellular dielectric spectroscopy (CDS) capture changes of the cytoskeleton of living cells in real time. Because signal transduction correlates with changes in cell morphology, DMR and CDS biosensors are exquisitely suited for recording integrated cell responses in an unbiased, yet pathway-specific manner without the use of labels that may interfere with cell function. Described in this unit are several experimental approaches utilizing optical label-free system capturing dynamic mass redistribution (DMR) in living cells (Epic System) and an impedance-based CDS technology (CellKey). In addition, potential pitfalls associated with these assays and alternative approaches for overcoming such technical challenges are discussed. © 2017 by John Wiley & Sons, Inc.