Mechanisms of G protein activation via the D2 dopamine receptor: evidence for persistent receptor/G protein interaction after agonist stimulation

Agonist stimulation at human μ opioid receptors in a [(35)S]GTPγS incorporation assay: observation of "bell-shaped" concentration-response relationships under conditions of strong receptor G protein coupling

CONTEXT

The appearance of "bell"- (or "inverted U"-) shaped agonist concentration-response curves (CRCs) in in vitro pharmacological experiments is a frequently observed but poorly communicated phenomenon. In the context of G protein coupled receptor research, it is commonly attributed to the recruitment of secondary targets or to desensitization or feedback processes, but the concrete background of these observations often remains intriguing.

OBJECTIVE

Here, we addressed the subject of bell-shaped agonist CRCs at the µ opioid receptor (µOR) by testing the impact of experimental conditions favoring G protein coupling.

METHODS

G protein activation by recombinant human µORs heterologously expressed in CHO cells was assessed in [(35)S]GTPγS binding assays using the opioid ligands DAMGO, morphine, fentanyl and naloxone. Experimental conditions were varied by changing the NaCl (10-300 mM) and the GDP concentration (0.3-30 µM).

RESULTS

Both the sodium and the GDP concentration were inversely related to G protein coupling, as evident by an increase in basal [(35)S]GTPγS incorporation at low sodium and low GDP levels and by the concomitant appearance of the partial agonist activity of the µOR antagonist, naloxone. Bell-shaped CRCs were observed for the efficacious agonists DAMGO, fentanyl and morphine, and this phenomenon was promoted by low sodium as well as by low GDP concentrations.

CONCLUSION

µOR agonist CRCs show a non-monotonic behavior with a decline of maximal stimulation under conditions of strong receptor-G protein coupling, and this behavior is visible at the level of G protein activation itself.

Pathway-selective antagonism of proteinase activated receptor 2

BACKGROUND AND PURPOSE

Proteinase activated receptor 2 (PAR2) is a GPCR associated with inflammation, metabolism and disease. Clues to understanding how to block PAR2 signalling associated with disease without inhibiting PAR2 activation in normal physiology could be provided by studies of biased signalling.

EXPERIMENTAL APPROACH

PAR2 ligand GB88 was profiled for PAR2 agonist and antagonist properties by several functional assays associated with intracellular G-protein-coupled signalling in vitro in three cell types and with PAR2-induced rat paw oedema in vivo.

KEY RESULTS

In HT29 cells, GB88 was a PAR2 antagonist in terms of Ca(2+) mobilization and PKC phosphorylation, but a PAR2 agonist in attenuating forskolin-induced cAMP accumulation, increasing ERK1/2 phosphorylation, RhoA activation, myosin phosphatase phosphorylation and actin filament rearrangement. In CHO-hPAR2 cells, GB88 inhibited Ca(2+) release, but activated G(i/o) and increased ERK1/2 phosphorylation. In human kidney tubule cells, GB88 inhibited cytokine secretion (IL6, IL8, GM-CSF, TNF-α) mediated by PAR2. A rat paw oedema induced by PAR2 agonists was also inhibited by orally administered GB88 and compared with effects of locally administered inhibitors of G-protein coupled pathways.

CONCLUSIONS AND IMPLICATIONS

GB88 is a biased antagonist of PAR2 that selectively inhibits PAR2/G(q/11)/Ca(2+)/PKC signalling, leading to anti-inflammatory activity in vivo, while being an agonist in activating three other PAR2-activated pathways (cAMP, ERK, Rho) in human cells. These findings highlight opportunities to design drugs to block specific PAR2-linked signalling pathways in disease, without blocking beneficial PAR2 signalling in normal physiology, and to dissect PAR2-associated mechanisms of disease in vivo.

Use of the GTPγS ([35S]GTPγS and Eu-GTPγS) binding assay for analysis of ligand potency and efficacy at G protein-coupled receptors

In this review I consider assays for G protein-coupled receptor (GPCR) activity based on the binding of labelled analogues of GTPγS ([(35) S]GTPγS or Eu-GTPγS) to G proteins in tissues (GTPγS binding assays). Such assays provide convenient measures of GPCR activity close to the receptor in the signalling cascade. In order to set up a GTPγS binding assay, the requirements of the assay must be considered. These are tissue source, GTPγS analogue, G protein, GDP, Mg(2+) /Na(+) ions, saponin, incubation time. The assay, once optimized, can be used to generate concentration/response curves for GPCRs signalling via G(i/o) proteins (or to other G proteins with a modified assay) and actions of agonists, inverse agonists and antagonists may, in principle, be assessed. For agonists and inverse agonists, data for the maximal agonist effect, the concentration of ligand giving a half-maximal response and the Hill coefficient may be derived. For antagonists, data for the equilibrium dissociation constant can be obtained. The mechanistic basis of the assay is considered. Although the assay can be used to profile ligands, under the conditions it is used, it may not be measuring the same event that determines GPCR action in cells.

LINKED ARTICLES

This article is part of a themed section on Analytical Receptor Pharmacology in Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2010.161.issue-6

Mechanisms underlying agonist efficacy

Agonist efficacy is a measure of how well an agonist can stimulate a response system linked to a receptor. Efficacy can be assessed in functional assays and various parameters (E(max), K(A)/EC(50), E(max).K(A)/EC(50)) determined. The E(max).K(A)/EC(50) parameter provides a good estimate of efficacy across the full range of efficacy. A convenient assay for the efficacy of agonists for some receptors is provided by the [(35)S]GTP[S] (guanosine 5'-[gamma-[(35)S]thio]triphosphate)-binding assay. In this assay, the normal GTP-binding event in GPCR (G-protein-coupled receptor) activation is replaced by the binding of the non-hydrolysable analogue [(35)S]GTP[S]. This assay may be used to profile ligands for their efficacy, and an example here is the D(2) dopamine receptor where an efficacy scale has been set up using this assay. The mechanisms underlying the assay have been probed. The time course of [(35)S]GTP[S] binding follows a pseudo-first-order reaction with [(35)S]GTP[S] binding reaching equilibrium after approx. 3 h. The [(35)S]GTP[S]-binding event is the rate-determining step in the assay. Agonists regulate the maximal level of [(35)S]GTP[S] bound, rather than the rate constant for binding. The [(35)S]GTP[S]-binding assay therefore determines agonist efficacy on the basis of the amount of [(35)S]GTP[S] bound rather than the rate of binding.