Analysis of functional selectivity through G protein-dependent and -independent signaling pathways at the adrenergic α(2C) receptor

Adrenoceptor Desensitization: Current Understanding of Mechanisms

G-protein coupled receptors (GPCRs) transduce a wide range of extracellular signals. They are key players in the majority of biologic functions including vision, olfaction, chemotaxis, and immunity. However, as essential as most of them are to body function and homeostasis, overactivation of GPCRs has been implicated in many pathologic diseases such as cancer, asthma, and heart failure (HF). Therefore, an important feature of G protein signaling systems is the ability to control GPCR responsiveness, and one key process to control overstimulation involves initiating receptor desensitization. A number of steps are appreciated in the desensitization process, including cell surface receptor phosphorylation, internalization, and downregulation. Rapid or short-term desensitization occurs within minutes and involves receptor phosphorylation via the action of intracellular protein kinases, the binding of β-arrestins, and the consequent uncoupling of GPCRs from their cognate heterotrimeric G proteins. On the other hand, long-term desensitization occurs over hours to days and involves receptor downregulation or a decrease in cell surface receptor protein level. Of the proteins involved in this biologic phenomenon, β-arrestins play a particularly significant role in both short- and long-term desensitization mechanisms. In addition, β-arrestins are involved in the phenomenon of biased agonism, where the biased ligand preferentially activates one of several downstream signaling pathways, leading to altered cellular responses. In this context, this review discusses the different patterns of desensitization of the α 1-, α 2- and the β adrenoceptors and highlights the role of β-arrestins in regulating physiologic responsiveness through desensitization and biased agonism. SIGNIFICANCE STATEMENT: A sophisticated network of proteins orchestrates the molecular regulation of GPCR activity. Adrenoceptors are GPCRs that play vast roles in many physiological processes. Without tightly controlled desensitization of these receptors, homeostatic imbalance may ensue, thus precipitating various diseases. Here, we critically appraise the mechanisms implicated in adrenoceptor desensitization. A better understanding of these mechanisms helps identify new druggable targets within the GPCR desensitization machinery and opens exciting therapeutic fronts in the treatment of several pathologies.

How New Developments in Pharmacology Receptor Theory Are Changing (Our Understanding of) Hypertension Therapy

BACKGROUND

Many hypertension therapeutics were developed prior to major advances in drug receptor theory. Moreover, newer drugs may take advantage of some of the newly understood modalities of receptor function.

GOAL

The goal of this review is to provide an up-to-date summary of drug receptor theory. This is followed by a discussion of the drug classes recognized for treating hypertension to which new concepts in receptor theory apply.

RESULTS

We raise ideas for mechanisms of potential new antihypertensive drugs and whether they may take advantage of new theories in drug-receptor interaction.

Single cell imaging of ERK and Akt activation dynamics and heterogeneity induced by G protein-coupled receptors

Kinases play key roles in signaling networks that are activated by G-protein-coupled receptors (GPCRs). Kinase activities are generally inferred from cell lysates, hiding cell-to-cell variability. To study the dynamics and heterogeneity of ERK and Akt proteins, we employed high-content biosensor imaging with kinase translocation reporters. The kinases were activated with GPCR ligands. We observed ligand concentration-dependent response kinetics to histamine, α2-adrenergic and S1P receptor stimulation. By using G-protein inhibitors, we observed that Gq mediated the ERK and Akt responses to histamine. In contrast, Gi was necessary for ERK and Akt activation in response to α2-adrenergic receptor activation. ERK and Akt were also strongly activated by S1P, showing high heterogeneity at the single-cell level, especially for ERK. Cluster analysis of time series derived from 68,000 cells obtained under the different conditions revealed several distinct populations of cells that display similar response dynamics. ERK response dynamics to S1P showed high heterogeneity, which was reduced by the inhibition of Gi. To conclude, we have set up an imaging and analysis strategy that reveals substantial cell-to-cell heterogeneity in kinase activity driven by GPCRs.

Summary: High-content biosensor imaging with kinase translocation reporters reveals substantial cell-to-cell heterogeneity in kinase activity driven by G-protein-coupled receptors.

Conformational Basis of G Protein-Coupled Receptor Signaling Versatility

G protein-coupled receptors (GPCRs) are privileged structural scaffolds in biology that have the versatility to regulate diverse physiological processes. Interestingly, many GPCR ligands exhibit significant 'bias' - the ability to preferentially activate subsets of the many cellular pathways downstream of these receptors. Recently, complementary information from structural and spectroscopic approaches has made significant inroads into understanding the mechanisms of these biased ligands. The consistently emerging theme is that GPCRs are highly dynamic proteins, and ligands with varying pharmacological properties differentially modulate the equilibrium among multiple conformations. Biased signaling and other recently appreciated complexities of GPCR signaling thus appear to be a natural consequence of the conformational heterogeneity of GPCRs and GPCR-transducer complexes.

Innovative Bioluminescence Resonance Energy Transfer Assay Reveals Differential Agonist-Induced D2 Receptor Intracellular Trafficking and Arrestin-3 Recruitment

The dopamine D2 receptor (D2R) mediates ligand-biased signaling with potential therapeutic implications. However, internalization, choice of endocytic routes, and degradation of the D2R in lysosomes may also participate in agonist-directed trafficking. We developed bioluminescence resonance energy transfer (BRET) assays that measure relative distances between Renilla luciferase8-tagged D2R and green fluorescent protein 2 (GFP2)-tagged K-Ras (plasma membrane marker), and between luciferase8-tagged D2R and GFP2-Rab5 (early), GFP2-Rab4 (recycling), or GFP2-Rab7 (late) endosomal markers. The BRET signal between D2R-Luc and GFP2-K-Ras was robustly diminished after receptor internalization induced by dopamine, with subsequent BRET signals increasing when luciferase8-tagged D2R approached GFP2-Rab proteins in endosomal compartments. All BRET signals were blocked by the selective D2R antagonist haloperidol and were decreased by low temperature and high sucrose blocks, two parameters interfering with internalization. Some antipsychotic drugs, such as aripiprazole, are less efficacious in internalizing D2R than most of the antiparkinsonian agents. However, antipsychotics were nearly as efficacious as antiparkinsonians in directing the D2R toward early and recycling endosomes. The Rab7 marker for the late endosome/lysosome route was also capable of discriminating between D2R compounds. We could show that some drugs engaged the D2R either to interact preferentially with arrestin-3 or to internalize. Our study revealed that D2R trafficking in cells was differentially regulated by antipsychotic and antiparkinsonian drugs. Taken together, the BRET assays reported here could further help decipher D2R ligand-induced arrestin-3 recruitment and trafficking, with potentially more selective therapeutic profiles and fewer undesired side effects.

Functional Characterization of a Novel Series of Biased Signaling Dopamine D3 Receptor Agonists

Dopamine receptors play an integral role in controlling brain physiology. Importantly, subtype selective agonists and antagonists of dopamine receptors with biased signaling properties have been successful in treating psychiatric disorders with a low incidence of side effects. To this end, we recently designed and developed SK609, a dopamine D3 receptor (D3R) selective agonist that has atypical signaling properties. SK609 has shown efficacy in reversing akinesia and reducing L-dopa-induced dyskinesia in a hemiparkinsonian rats. In the current study, we demonstrate that SK609 has high selectivity for D3R with no binding affinity on D2R high- or low-affinity state when tested at a concentration of 10 μM. In addition, SK609 and its analogues do not induce desensitization of D3R as determined by repeated agonist treatment response in phosphorylation of ERK1/2 functional assay. Most significantly, SK609 and its analogues preferentially signal through the G-protein-dependent pathway and do not recruit β-arrestin-2, suggesting a functional bias toward the G-protein-dependent pathway. Structure-activity relationship (SAR) studies using analogues of SK609 demonstrate that the molecules bind at the orthosteric site by maintaining the conserved salt bridge interactions with aspartate 110 on transmembrane 3 and aryl interactions with histidine 349 on transmembrane 6, in addition to several hydrophobic interactions with residues from transmembranes 5 and 6. The compounds follow a strict SAR with reference to the three pharmacophore elements: substituted phenyl ring, length of the linker connecting phenyl ring and amine group, and orientation and hydrophobic branching groups at the amine among SK609 analogues for efficacy and functional selectivity. These features of SK609 and the analogues suggest that biased signaling is an inherent property of this series of molecules.

Spinophilin Is Indispensable for the α2B Adrenergic Receptor-Elicited Hypertensive Response

The α₂ adrenergic receptor (AR) subtypes are important for blood pressure control. When activated, the α_2A subtype elicits a hypotensive response whereas the α_2B subtype mediates a hypertensive effect that counteracts the hypotensive response by the α_2A subtype. We have previously shown that spinophilin attenuates the α_2AAR-dependent hypotensive response; in spinophilin null mice, this response is highly potentiated. In this study, we demonstrate that spinophilin impedes arrestin-dependent phosphorylation and desensitization of the α_2BAR subtype by competing against arrestin binding to this receptor subtype. The Del301-303 α_2BAR, a human variation that shows impaired phosphorylation and desensitization and is linked to hypertension in certain populations, exhibits preferential interaction with spinophilin over arrestin. Furthermore, Del301-303 α_2BAR-induced ERK signaling is quickly desensitized in cells without spinophilin expression, showing a profile similar to that induced by the wild type receptor in these cells. Together, these data suggest a critical role of spinophilin in sustaining α_2BAR signaling. Consistent with this notion, our in vivo study reveals that the α_2BAR-elicited hypertensive response is diminished in spinophilin deficient mice. In arrestin 3 deficient mice, where the receptor has a stronger binding to spinophilin, the same hypertensive response is enhanced. These data suggest that interaction with spinophilin is indispensable for the α_2BAR to elicit the hypertensive response. This is opposite of the negative role of spinophilin in regulating α_2AAR-mediated hypotensive response, suggesting that spinophilin regulation of these closely related receptor subtypes can result in distinct functional outcomes in vivo. Thus, spinophilin may represent a useful therapeutic target for treatment of hypertension.