Detection of receptor ligands by monitoring selective stabilization of a Renilla luciferase-tagged, constitutively active mutant, G-protein-coupled receptor

A Systematic Review of Inverse Agonism at Adrenoceptor Subtypes

As many, if not most, ligands at G protein-coupled receptor antagonists are inverse agonists, we systematically reviewed inverse agonism at the nine adrenoceptor subtypes. Except for β₃-adrenoceptors, inverse agonism has been reported for each of the adrenoceptor subtypes, most often for β₂-adrenoceptors, including endogenously expressed receptors in human tissues. As with other receptors, the detection and degree of inverse agonism depend on the cells and tissues under investigation, i.e., they are greatest when the model has a high intrinsic tone/constitutive activity for the response being studied. Accordingly, they may differ between parts of a tissue, for instance, atria vs. ventricles of the heart, and within a cell type, between cellular responses. The basal tone of endogenously expressed receptors is often low, leading to less consistent detection and a lesser extent of observed inverse agonism. Extent inverse agonism depends on specific molecular properties of a compound, but inverse agonism appears to be more common in certain chemical classes. While inverse agonism is a fascinating facet in attempts to mechanistically understand observed drug effects, we are skeptical whether an a priori definition of the extent of inverse agonism in the target product profile of a developmental candidate is a meaningful option in drug discovery and development.

A key mammalian cholesterol synthesis enzyme, squalene monooxygenase, is allosterically stabilized by its substrate

Cholesterol biosynthesis is a high-cost process and, therefore, tightly regulated by both transcriptional and posttranslational negative feedback mechanisms in response to the level of cellular cholesterol. Squalene monooxygenase (SM, also known as squalene epoxidase or SQLE) is a rate-limiting enzyme in the cholesterol biosynthetic pathway and catalyzes epoxidation of squalene. The stability of SM is negatively regulated by cholesterol via its N-terminal regulatory domain (SM-N100). In this study, using a SM-luciferase fusion reporter cell line, we performed a chemical genetics screen that identified inhibitors of SM itself as up-regulators of SM. This effect was mediated through the SM-N100 region, competed with cholesterol-accelerated degradation, and required the E3 ubiquitin ligase MARCH6. However, up-regulation was not observed with statins, well-established cholesterol biosynthesis inhibitors, and this pointed to the presence of another mechanism other than reduced cholesterol synthesis. Further analyses revealed that squalene accumulation upon treatment with the SM inhibitor was responsible for the up-regulatory effect. Using photoaffinity labeling, we demonstrated that squalene directly bound to the N100 region, thereby reducing interaction with and ubiquitination by MARCH6. Our findings suggest that SM senses squalene via its N100 domain to increase its metabolic capacity, highlighting squalene as a feedforward factor for the cholesterol biosynthetic pathway.

Non-allosteric enzyme switches possess larger effector-induced changes in thermodynamic stability than their non-switch analogs

The ability to regulate cellular protein activity offers a broad range of biotechnological and biomedical applications. Such protein regulation can be achieved by modulating the specific protein activity or through processes that regulate the amount of protein in the cell. We have previously demonstrated that the nonhomologous recombination of the genes encoding maltose binding protein (MBP) and TEM1 β-lactamase (BLA) can result in genes that confer maltose-dependent resistance to β-lactam antibiotics even though the encoded proteins are not allosteric enzymes. We showed that these phenotypic switches-named based on their conferral of a switching phenotype to cells-resulted from a specific interaction with maltose in the cell that increased the switches cellular accumulation. Since phenotypic switches represent an important class of engineered proteins for basic science and biotechnological applications in vivo, we sought to elucidate the phenomena behind the increased accumulation and switching properties. Here, we demonstrate the key role for the linker region between the two proteins. Experimental evidence supports the hypothesis that in the absence of their effector, some phenotypic switches possess an increased rate of unfolding, decreased conformational stability, and increased protease susceptibility. These factors alone or in combination serve to decrease cellular accumulation. The effector functions to increase cellular accumulation by alleviating one or more of these defects. This perspective on the mechanism for phenotypic switching will aid the development of design rules for switch construction for applications and inform the study of the regulatory mechanisms of natural cellular proteins.

Agonist occupancy of a single monomeric element is sufficient to cause internalization of the dimeric beta2-adrenoceptor

A range of studies have indicated that many rhodopsin-like, family A G protein-coupled receptors, including the beta(2)-adrenoceptor, exist and probably function as dimers. It is less clear if receptors internalize as dimers and if agonist occupancy of only one element of a dimer is sufficient to cause internalization of a receptor dimer into the cell. We have used a chemogenomic approach to demonstrate that this is the case. Following expression of the wild type beta(2)-adrenoceptor, isoprenaline but not 1-(3''4'-dihydroxyphenyl)-3-methyl-1-butanone, which does not have significant affinity for the wild type receptor, caused receptor internalization. By contrast, 1-(3'4'-dihydroxyphenyl)-3-methyl-1-butanone, but not isoprenaline that does not have high affinity for the mutated receptor, caused internalization of Asp(113)Serbeta(2)-adrenoceptor. Following co-expression of wild type and Asp(113)Serbeta(2)-adrenoceptors each of isoprenaline and 1-(3'4'-dihydroxyphenyl)-3-methyl-1-butanone caused the co-internalization of both of these two forms of the receptor. Co-expressed wild type and Asp(113)Serbeta(2)-adrenoceptors were able to be co-immunoprecipitated and 1-(3'4'-dihydroxyphenyl)-3-methyl-1-butanone produced internalization of the wild type receptor that was not prevented by the beta-adrenoceptor antagonist propranolol that binds with high affinity only to the wild type receptor. These results demonstrate that agonist occupancy of either single binding site of the beta(2)-adrenoceptor dimer is sufficient to cause internalization of the dimer and that antagonist occupation of one of the two ligand binding sites is unable to prevent agonist-mediated internalization.

Green fluorescent proteins in receptor research: An emerging tool for drug discovery

In the last five years, green fluorescent protein (GFP) has emerged from being a mere curiosity to become a reliable tool for molecular pharmacological research. GFP produces an intense and stable green fluorescence noncatalytically by absorbing blue light maximally at 395 nm and emitting green light with a peak at 509 nm. It consists of 238 amino acids and its molecular mass is 27-30 kDa. GFP fluorescence occurs without cofactors and this property allows GFP fluorescence to be utilised in nonnative organisms, wherein it can be used as a reporter. This use of GFP permits real-time analysis of receptor dynamics. The emitted fluorescence can be used as a nontoxic marker and detected using fluorescence-activated cell sorting (FACS), thus avoiding any staining procedure, expensive mRNA analysis or hazardous radiolabeled binding assays. The potential value of GFP has also been recognized in orphan receptor research, where various GFP-tagged therapeutic proteins have been constructed in an attempt to identify the endogenous ligand(s). These chimeric proteins have been used to determine the site and time course of receptor expression and to relate receptor dynamics with therapeutic outcome. The preparation of new GFP constructs for identifying germ layer cells (endodermal, ectodermal, and mesodermal), as well as neuronal, haematopoietic, endothelial, and cartilage cells, has provided a useful battery of tissue/receptor-specific screening assays for new chemical entities. Genetically engineered cells with GFP expression have provided a valuable tool for automated analysis, and can be adapted for high-throughput systems. GFP is being increasingly utilised for the study of receptor dynamics, where, having already proved beneficial, it will likely continue to contribute towards the search for new classes of drugs, as well as to "de-orphaning" orphan receptors.

Ligand Specific Up-Regulation of aRenilla reniformisLuciferase-Tagged, Structurally Unstable Muscarinic M3Chimeric G Protein-Coupled Receptor

The rat muscarinic acetylcholine receptor subtype 3 was modified by swapping the third intracellular loop with the corresponding region of a constitutively active mutant human beta2-adrenergic receptor and attaching Renilla reniformis luciferase to its C terminus. The chimeric fusion receptor displayed constitutive Gq- and Gs-coupled activity as demonstrated in nuclear factor of activated T cell and cAMP response element reporter gene assays. The chimeric receptor displayed a pharmacological binding profile comparable with that of the wild-type receptor for agonists, antagonists, and inverse agonists but showed a large decrease in expression in both human embryonic kidney 293 and COS-7 cells. Long-term treatment of cells expressing the chimeric receptor with agonists, antagonists, and inverse agonists resulted in a concentration-dependent up-regulation in the steady-state levels that was not observed for the wild-type receptor. The EC50 of neutral antagonists and inverse agonists was significantly correlated to their binding affinities at the wild-type receptor, whereas agonists demonstrated greater EC50 values for the chimeric receptor. To validate the approach as a means of discovering novel receptor modulators, a cell-based, high-throughput screening assay was developed and used to screen a small molecule compound collection against the chimeric fusion receptor. Several novel hits were identified and confirmed by ligand binding assay and functional assays using the wild-type rat muscarinic acetylcholine receptor subtype 3.

High-content assays for ligand regulation of G-protein-coupled receptors

High-content assays rely on the imaging of cellular events. They can be used to monitor the activation of G-protein-coupled receptors (or other receptors), their internalization into the cell, or alterations in their amount. In addition, multiplexed assays can provide further information about the characteristics of the receptor. Recent improvements in throughput using high-content screening platforms means that such assays are now an integral element of functional analysis in the drug discovery process.

Concerted stimulation and deactivation of pertussis toxin-sensitive G proteins by chimeric G protein-coupled receptor-regulator of G protein signaling 4 fusion proteins: analysis of the contribution of palmitoylated cysteine residues to the GAP activity of RGS4

Agonists stimulated high-affinity GTPase activity in membranes of HEK293 cells following coexpression of the alpha 2A-adrenoceptor and a pertussis toxin-resistant mutant of Go1 alpha. Enzyme kinetic analysis of Vmax and Km failed to detect regulation of the effect of agonist by a GTPase activating protein. This did occur, however, when cells were also transfected to express RGS4. Both elements of a fusion protein in which the N-terminus of RGS4 was linked to the C-terminal tail of the alpha 2A-adrenoceptor were functional, as it was able to provide concerted stimulation and deactivation of the G protein. By contrast, the alpha 2A-adrenoceptor-RGS4 fusion protein stimulated but did not enhance deactivation of a form of Go1 alpha that is resistant to the effects of regulator of G protein signaling (RGS) proteins. Employing this model system, mutation of Asn128 but not Asn88 eliminated detectable GTPase activating protein activity of RGS4 against Go1 alpha. Mutation of all three cysteine residues that are sites of post-translational acylation in RGS4 also eliminated GTPase activating protein activity but this was not achieved by less concerted mutation of these sites. These studies demonstrate that a fusion protein between a G protein-coupled receptor and an RGS protein is fully functional in providing both enhanced guanine nucleotide exchange and GTP hydrolysis of a coexpressed G protein. They also provide a direct means to assess, in mammalian cells, the effects of mutation of the RGS protein on function in circumstances in which the spatial relationship and orientation of the RGS to its target G protein is defined and maintained.

Regulated and constitutive activation of specific signalling pathways by the human S1P5 receptor

1 We tested the hypothesis, whether G Protein-coupled receptors (GPCRs) may differentially regulate specific signalling pathways by constitutive and agonist-induced activation using the human sphingosine 1-phosphate receptor S1P(5) as a model. 2 S1P(5) receptor-expressing HEK293 cells exhibited a high degree of basal activity for both inhibition of adenylyl cyclase and extracellular signal regulated kinase (ERK) when cultured in serum, which contains high levels of sphingosine 1-phosphate (S1P). However, basal activity was independent of the presence of S1P: (i) constitutive activity remained when cells were cultured in delipidated serum, (ii) addition of S1P to delipidated serum did not increase basal S1P(5) receptor signalling. 3 Conversely, constitutive inhibition of forskolin-stimulated adenylyl cyclase was further enhanced by S1P in S1P(5)-HEK293 cells. 4 Transfection of several mammalian cell lines (CHO-K1, HEK293, NIH-3T3, RH7777) with the S1P(5) receptor induced cell rounding, which was more pronounced in the presence of S1P-containing serum. Rounded cell morphology did not correlate with apoptotic cell death, but led to detachment of cells. 5 Cell surface ELISA assays showed that a fraction of plasma membrane S1P(5) receptors were dose-dependently internalized with S1P. 6 These data reveal that intrinsic inhibition of unstimulated adenylyl cyclase or ERK activity by the S1P(5) receptor is insensitive to ligand modulation. Conversely, effects on forskolin-stimulated adenylyl cyclase, cell morphology and internalization can be further augmented with S1P. Our results suggest that different signal transduction pathways are not equally activated through constitutively active GPCRs with promiscuous signalling characteristics.

Generation and analysis of constitutively active and physically destabilized mutants of the human beta(1)-adrenoceptor

Constitutive activity of wild-type and mutant forms of human beta(1)- and beta(2)-adrenoceptors was measured by guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding assays using fusion proteins between these receptors and G(s)alpha. Constitutive activity of the beta(1)-adrenoceptor is enhanced by mutation of Leu(322). The ability of ligands to suppress receptor instability and produce up-regulation is often associated with constitutively active mutants. Leu(322)Lysbeta(1)-adrenoceptor, but not wild type, was up-regulated by exposure to the beta(1)-adrenoceptor selective blocker betaxolol. More extensive sequence alterations of the beta(1)-adrenoceptor were generated to mimic the initially described constitutively active mutant (CAM) of the beta(2)-adrenoceptor that is up-regulated strongly by betaxolol. Substitution of amino acids 316 to 324 of the beta(1)-adrenoceptor with the equivalent alpha(1b)-adrenoceptor sequence did not result in up-regulation by betaxolol. However, these forms of both beta(1)- and beta(2)-adrenoceptors displayed substantial and equivalent constitutive activity. The addition of the Leu(322)Lys mutation into the alpha(1b)-adrenoceptor substituted beta(1)-adrenoceptor to produce the CAMKbeta(1)-adrenoceptor allowed substantially greater levels of up-regulation by betaxolol without enhancement of constitutive [(35)S]GTPgammaS binding. Arg(156)Alabeta(1)-adrenoceptor was up-regulated strongly by betaxolol but displayed lower constitutive activity than did other mutants. Binding of [(35)S]GTPgammaS binding to all the fusion proteins was increased substantially by isoprenaline. Despite the ability of betaxolol to cause up-regulation of many mutants, only for the CAMbeta(2)-adrenoceptor-G(s)alpha and CAMKbeta(1)-adrenoceptor-G(s)alpha fusion proteins was the basal binding of [(35)S]GTPgammaS decreased by betaxolol. Clear resolution between receptor constitutive activity and ligand suppression of receptor instability can be obtained for mutant beta-adrenoceptors, and potential inverse agonists do not function equally at phenotypically apparently equivalent CAM receptors.

Homo- and hetero-oligomeric interactions between G-protein-coupled receptors in living cells monitored by two variants of bioluminescence resonance energy transfer (BRET): hetero-oligomers between receptor subtypes form more efficiently than between less closely related sequences

Homo- and hetero-oligomerization of G-protein-coupled receptors (GPCRs) were examined in HEK-293 cells using two variants of bioluminescence resonance energy transfer (BRET). BRET(2) (a variant of BRET) offers greatly improved separation of the emission spectra of the donor and acceptor moieties compared with traditional BRET. Previously recorded homo-oligomerization of the human delta-opioid receptor was confirmed using BRET(2). Homo-oligomerization of the kappa-opioid receptor was observed using both BRET techniques. Both homo- and hetero-oligomers, containing both delta- and kappa-opioid receptors, were unaffected by the presence of receptor ligands. BRET detection of opioid receptor homo- and hetero-oligomers required expression of 50,000-100,000 copies of the receptor energy acceptor construct per cell. The effectiveness of delta-kappa-opioid receptor hetero-oligomer formation was as great as for homomeric interactions. The capacity of the two opioid receptors to form oligomeric complexes with the beta(2)-adrenoceptor was also assessed. Although such interactions were detected, at least 250,000 copies per cell of the energy acceptor were required. Requirement for high levels of receptor expression was equally pronounced in attempts to measure hetero-oligomer formation between the kappa-opioid receptor and the thyrotropin-releasing hormone receptor-1. These studies indicate that constitutively formed homo- and hetero-oligomers of opioid receptor subtypes can be detected in living cells containing less than 100,000 copies of the receptors. However, although hetero-oligomeric interactions between certain less closely related GPCRs can be detected, they appear to be of lower affinity than homo- or hetero-oligomers containing closely related sequences. Interactions recorded between certain GPCR family members in heterologous expression systems are likely to be artefacts of extreme levels of overexpression.

Protein-protein interactions at G-protein-coupled receptors

The basic module of signal transduction that involves G-protein-coupled receptors is usually portrayed as comprising a receptor, a heterotrimeric G protein and an effector. It is now well established that regulated interactions between receptors and arrestins, and between G proteins and regulators of G-protein signalling alter the effectiveness and kinetics of information transfer. However, more recent studies have begun to identify a host of other proteins that interact selectively with individual receptors at both the intracellular and extracellular face of the membrane. Although the functional relevance of many of these interactions is only beginning to be understood, current information indicates that these interactions might determine receptor properties, such as cellular compartmentalization or signal selection, and can promote protein scaffolding into complexes that integrate function.