Conventional Agonists Research Articles

A neutrophil inhibitory pepducin derived from FPR1 expected to target FPR1 signaling hijacks the closely related FPR2 instead

Pepducins constitute a unique class of G-protein coupled receptor (GPCR) modulating lipopeptides. Pepducins with inhibitory effects on neutrophils could potentially be developed into anti-inflammatory pharmaceuticals. A pepducin with a peptide sequence identical to the third intracellular loop of FPR1 was found to inhibit neutrophil functions including granule mobilization and superoxide production. This FPR1-derived pepducin selectively inhibited signaling and cellular responses through FPR2, but not FPR1 as expected. Binding to the neutrophil surface of a conventional FPR2 agonist is also inhibited. The fatty acid is essential for inhibition and pepducins with shorter peptides lose in potency. In summary, a pepducin designed to target FPR1 was found to hijack FPR2 and potently inhibit neutrophil functions.

Targeted Delivery of LXR Agonist Using a Site-Specific Antibody-Drug Conjugate.

Liver X receptor (LXR) agonists have been explored as potential treatments for atherosclerosis and other diseases based on their ability to induce reverse cholesterol transport and suppress inflammation. However, this therapeutic potential has been hindered by on-target adverse effects in the liver mediated by excessive lipogenesis. Herein, we report a novel site-specific antibody-drug conjugate (ADC) that selectively delivers a LXR agonist to monocytes/macrophages while sparing hepatocytes. The unnatural amino acid para-acetylphenylalanine (pAcF) was site-specifically incorporated into anti-CD11a IgG, which binds the α-chain component of the lymphocyte function-associated antigen 1 (LFA-1) expressed on nearly all monocytes and macrophages. An aminooxy-modified LXR agonist was conjugated to anti-CD11a IgG through a stable, cathepsin B cleavable oxime linkage to afford a chemically defined ADC. The anti-CD11a IgG-LXR agonist ADC induced LXR activation specifically in human THP-1 monocyte/macrophage cells in vitro (EC50-27 nM), but had no significant effect in hepatocytes, indicating that payload delivery is CD11a-mediated. Moreover, the ADC exhibited higher-fold activation compared to a conventional synthetic LXR agonist T0901317 (Tularik) (3-fold). This novel ADC represents a fundamentally different strategy that uses tissue targeting to overcome the limitations of LXR agonists for potential use in treating atherosclerosis.

Nucleotides Acting at P2Y Receptors: Connecting Structure and Function.

Eight G protein-coupled P2Y receptor (P2YR) subtypes are important physiologic mediators. The human P2YRs are fully activated by ATP (P2Y2 and P2Y11), ADP (P2Y1, P2Y12, and P2Y13), UTP (P2Y2 and P2Y4), UDP (P2Y6 and P2Y14), and UDP glucose (P2Y14). Their structural elucidation is progressing rapidly. The X-ray structures of three ligand complexes of the Gi-coupled P2Y12R and two of the Gq-coupled P2Y1Rs were recently determined and will be especially useful in structure-based ligand design at two P2YR subfamilies. These high-resolution structures, which display unusual binding site features, complement mutagenesis studies for probing ligand recognition and activation. The structural requirements for nucleotide agonist recognition at P2YRs are relatively permissive with respect to the length of the phosphate moiety, but less so with respect to base recognition. Nucleotide-like antagonists and partial agonists are also known for P2Y1, P2Y2, P2Y4, and P2Y12Rs. Each P2YR subtype has the ability to be activated by structurally bifunctional agonists, such as dinucleotides, typically, dinucleoside triphosphates or tetraphosphates, and nucleoside polyphosphate sugars (e.g., UDP glucose) as well as the more conventional mononucleotide agonists. A range of dinucleoside polyphosphates, from triphosphates to higher homologs, occurs naturally. Earlier modeling predictions of the P2YRs were not very accurate, but recent findings have provided much detailed structural insight into this receptor family to aid in the rational design of new drugs.

Rotigotine is a potent agonist at dopamine D1 receptors as well as at dopamine D2 and D3 receptors.

Rotigotine acts as a dopamine receptor agonist with high affinity for the dopamine D2, D3, D4 and D5 receptors but with a low affinity for the dopamine D1 receptor. We have investigated this further in radioligand binding and functional studies and compared the profile of rotigotine with that of other drugs used in the treatment of Parkinson's disease (PD). The binding of rotigotine to human dopamine D1, D2, D3, D4 and D5 receptors was determined in radioligand binding studies using [(3)H]rotigotine and compared with that of standard antagonist radioligands. Functional interactions of rotigotine with human dopamine receptors was also determined. [(3)H]rotigotine can be used as an agonist radioligand to label all dopamine receptor subtypes and this can be important to derive agonist affinity estimates. Rotigotine maintains this high affinity in functional studies at all dopamine receptors especially D1, D2 and D3 receptors and, to a lesser extent, D4 and D5 receptors. Rotigotine, like apomorphine but unlike ropinirole and pramipexole, was a potent agonist at all dopamine receptors. Rotigotine is a high-potency agonist at human dopamine D1, D2 and D3 receptors with a lower potency at D4 and D5 receptors. These studies differentiate rotigotine from conventional dopamine D2 agonists, used in the treatment of PD, such as ropinirole and pramipexole which lack activity at the D1 and D5 receptors, but resembles that of apomorphine which has greater efficacy in PD than other dopamine agonists but has suboptimal pharmacokinetic properties.

A pepducin derived from the third intracellular loop of FPR2 is a partial agonist for direct activation of this receptor in neutrophils but a full agonist for cross-talk triggered reactivation of FPR2.

We recently described a novel receptor cross-talk mechanism in neutrophils, unique in that the signals generated by the PAF receptor (PAFR) and the ATP receptor (P2Y2R) transfer formyl peptide receptor 1 (FPR1) from a desensitized (non-signaling) state back to an actively signaling state (Forsman H et al., PLoS One, 8:e60169, 2013; Önnheim K, et al., Exp Cell Res, 323∶209, 2014). In addition to the G-protein coupled FPR1, neutrophils also express the closely related receptor FPR2. In this study we used an FPR2 specific pepducin, proposed to work as an allosteric modulator at the cytosolic signaling interface, to determine whether the cross-talk pathway is utilized also by FPR2. The pepducin used contains a fatty acid linked to a peptide sequence derived from the third intracellular loop of FPR2, and it activates as well as desensensitizes this receptor. We now show that neutrophils desensitized with the FPR2-specific pepducin display increased cellular responses to stimulation with PAF or ATP. The secondary PAF/ATP induced response was sensitive to FPR2-specific inhibitors, disclosing a receptor cross-talk mechanism underlying FPR2 reactivation. The pepducin induced an activity in naïve cells similar to that of a conventional FPR2 agonist, but with lower potency (partial efficacy), meaning that the pepducin is a partial agonist. The PAF- or ATP-induced reactivation was, however, much more pronounced when neutrophils had been desensitized to the pepducin as compared to cells desensitized to conventional agonists. The pepducin should thus in this respect be classified as a full agonist. In summary, we demonstrate that desensitized FPR2 can be transferred back to an actively signaling state by receptor cross-talk signals generated through PAFR and P2Y2R, and the difference in agonist potency with respect to pepducin-induced direct receptor activation and cross-talk reactivation of FPR2 puts the concept of functional selectivity in focus.

Minireview: More Than Just a Hammer: Ligand “Bias” and Pharmaceutical Discovery

Conventional orthosteric drug development programs targeting G protein-coupled receptors (GPCRs) have focused on the concepts of agonism and antagonism, in which receptor structure determines the nature of the downstream signal and ligand efficacy determines its intensity. Over the past decade, the emerging paradigms of "pluridimensional efficacy" and "functional selectivity" have revealed that GPCR signaling is not monolithic, and that ligand structure can "bias" signal output by stabilizing active receptor states in different proportions than the native ligand. Biased ligands are novel pharmacologic entities that possess the unique ability to qualitatively change GPCR signaling, in effect creating "new receptors" with distinct efficacy profiles driven by ligand structure. The promise of biased agonism lies in this ability to engender "mixed" effects not attainable using conventional agonists or antagonists, promoting therapeutically beneficial signals while antagonizing deleterious ones. Indeed, arrestin pathway-selective agonists for the type 1 parathyroid hormone and angiotensin AT1 receptors, and G protein pathway-selective agonists for the GPR109A nicotinic acid and μ-opioid receptors, have demonstrated unique, and potentially therapeutic, efficacy in cell-based assays and preclinical animal models. Conversely, activating GPCRs in "unnatural" ways may lead to downstream biological consequences that cannot be predicted from prior knowledge of the actions of the native ligand, especially in the case of ligands that selectively activate as-yet poorly characterized G protein-independent signaling networks mediated via arrestins. Although much needs to be done to realize the clinical potential of functional selectivity, biased GPCR ligands nonetheless appear to be important new additions to the pharmacologic toolbox.

Small Molecule Agonists of PPAR-γ Exert Therapeutic Effects in Esophageal Cancer

The transcription factor PPAR-γ plays various roles in lipid metabolism, inflammation, cellular differentiation, and apoptosis. PPAR-γ agonists used to treat diabetes may have utility in cancer treatment. Efatutazone is a novel later generation PPAR-γ agonist that selectively activates PPAR-γ target genes and has antiproliferative effects in a range of malignancies. In this study, we investigated PPAR-γ status in esophageal squamous cell carcinoma (ESCC) and investigated the antiproliferative effects of efatutazone. PPAR-γ was expressed heterogeneously in ESCC, in which it exhibited an inverse relationship with Ki-67 expression. PPAR-γ expression was associated independently with good prognosis in ESCC. Efatutazone, but not the conventional PPAR-γ agonist troglitazone, inhibited ESCC cell proliferation in vitro and in vivo. Mechanistic investigations suggested that efatutazone acted by upregulating p21Cip1 protein in the nucleus through inactivation of the Akt pathway and dephosphorylation of p21Cip1 at Thr145 without affecting the transcriptional activity of p21Cip1. We also found that treatment with efatutazone led to phosphorylation of the EGF receptor and activation of the mitogen-activated protein kinase (MAPK) pathway. Accordingly, the combination of efatutazone with the antiepithelial growth factor receptor antibody cetuximab synergized to negatively regulate the phosphoinositide 3-kinase-Akt and MAPK pathways. Together, our results suggest that efatutazone, alone or in combination with cetuximab, may offer therapeutic effects in ESCC.

Comparison of gonadotropin-releasing hormone agonist with GnRH antagonist in polycystic ovary syndrome patients undergoing in vitro fertilization cycle: Retrospective analysis from a tertiary center and review of literature.

INTRODUCTION:Polycystic ovary syndrome (PCOS) is one of the most common infertility factor for which women are enrolled in in vitro fertilization (IVF) technique. In the recent years, gonadotropin releasing hormone antagonist protocol has emerged as the protocol of choice for controlled ovarian hyperstimulation in these patients.OBJECTIVES:The objective of the present study is to compare conventional long agonist protocol with fixed antagonist protocol in PCOS patients undergoing IVF cycle.MATERIALS AND METHODS:Retrospective analysis of 4 years data of a single center from northern India. Totally 81 patients who had long agonist protocol were compared with 36 patients with similar baseline characteristics who had antagonist protocol.RESULT:Total dose of gonadotropin required was significantly lower (P - 0.004) in the antagonist group. There was no significant difference in pregnancy rate or incidence of ovarian hyperstimulation syndrome between two groups. Cycle cancellation due to arrest of follicular growth was significantly higher in the antagonist group (P - 0.027).CONCLUSION:More randomized control trials and meta-analysis are required before replacing conventional long agonist protocol with antagonist protocol in patients with polycystic ovary syndrome.

Bifunctional Ligands Allow Deliberate Extrinsic Reprogramming of the Glucocorticoid Receptor

Therapies based on conventional nuclear receptor ligands are extremely powerful, yet their broad and long-term use is often hindered by undesired side effects that are often part of the receptor's biological function. Selective control of nuclear receptors such as the glucocorticoid receptor (GR) using conventional ligands has proven particularly challenging. Because they act solely in an allosteric manner, conventional ligands are constrained to act via cofactors that can intrinsically partner with the receptor. Furthermore, effective means to rationally encode a bias for specific coregulators are generally lacking. Using the (GR) as a framework, we demonstrate here a versatile approach, based on bifunctional ligands, that extends the regulatory repertoire of GR in a deliberate and controlled manner. By linking the macrolide FK506 to a conventional agonist (dexamethasone) or antagonist (RU-486), we demonstrate that it is possible to bridge the intact receptor to either positively or negatively acting coregulatory proteins bearing an FK506 binding protein domain. Using this strategy, we show that extrinsic recruitment of a strong activation function can enhance the efficacy of the full agonist dexamethasone and reverse the antagonist character of RU-486 at an endogenous locus. Notably, the extrinsic recruitment of histone deacetylase-1 reduces the ability of GR to activate transcription from a canonical GR response element while preserving ligand-mediated repression of nuclear factor-κB. By providing novel ways for the receptor to engage specific coregulators, this unique ligand design approach has the potential to yield both novel tools for GR study and more selective therapeutics.

A novel modified ultra-long agonist protocol improves the outcome of high body mass index women with polycystic ovary syndrome undergoing IVF/ICSI

In an attempt to evaluate the effectiveness of a novel modified ultra-long agonist (ULA) protocol on polycystic ovary syndrome (PCOS) patients undergoing in vitro fertilisation (IVF)/intracytoplasmic sperm injection (ICSI), a retrospective study of 499 women employed with either ULA or conventional long agonist (LA) protocol was analyzed. In high BMI group (>25 kg/m2), the ULA protocol yielded significant higher clinic pregnancy rate (PR) (70.2% versus 50.8%, p < 0.05), implantation rate (52.7% versus 35.7%, p < 0.05) and live birth rate (63.8% versus 39.0%, p < 0.05) when compared with LA protocol. In low BMI group (≤25 kg/m2), the ULA protocol also demonstrated a higher clinic PR (70.8% versus 59.5%, p < 0.05) whereas implantation rate and live birth rate are comparable. Within ULA protocol, the clinic PR, implantation rate and live birth rate are similar between high and low BMI patients. Similarly, the clinic PR and live birth rate demonstrated no significant difference within LA group but there is a significant lower implantation rate (35.7% versus 63.9%, p < 0.05) observed in high BMI patients. No difference in miscarriage rate and severe OHSS rate was found among all groups. In conclusion, ULA protocol benefits the IVF outcomes of PCOS patients with high BMI status.

Comparison of the Effects and Safety of Mild Ovarian Stimulation and Conventional Ovarian Stimulation with A Long GnRH Agonist Protocol on IVF Outcomes

Objective To conduct a Meta-analysis of studies that compared the efficacies of mild ovarian stimulation and conventional long GnRH agonist protocol in patients undergoing IVF or intracytoplasmic sperm injection (ICSI). Methods Meta-analysis was performed. All studies were published by July 2012 with data related to outcomes following mild ovarian stimulation compared with the conventional protocol. Odds ratios (ORs) and weighted / standardized mean difference (WMD/SMD) from individual study were pooled in fixed and random effect models. Main outcome measure was the efficacy of mild ovarian stimulation. Results Six articles were included in this Meta-analysis. The number of oocytes retrieved was lower, the cycle cancellation rate was higher and the incidence of ovarian hyperstimulation syndrome (OHSS) was lower in the mild stimulation group than in conventional ovarian stimulation group. Clinical pregnancy rates were similar in both mild and conventional stimulation groups. Conclusions The level of evidence supporting the use of mild stimulation protocols in IVF is low, considering the fewer oocytes retrieved and the higher rates of cycle cancellation.

Effects of ifenprodil on the antidepressant-like activity of NMDA ligands in the forced swim test in mice

Multiple pre-clinical and clinical studies clearly displayed implication of the NMDA receptors in development of depressive disorders since a variety of NMDA receptor antagonists exhibit an antidepressant-like effect. The main aim of our study was to assess the influence of ifenprodil - an allosteric modulator selectively binding at the NR2B subunit on the performance in the forced swim test in mice of various NMDA receptor ligands interacting with distinct components of the NMDA receptor complex. Ifenprodil at a dose of 10mg/kg enhanced the antidepressant-like effect of CGP 37849 (a competitive NMDA receptor antagonist, 0.312mg/kg), L-701,324 (an antagonist at glycine site, 1mg/kg), MK-801 (a non-competitive antagonist, 0.05mg/kg) and d-cycloserine (a partial agonist of a glycine site, 2.5mg/kg) but it did not shorten the immobility time of animals which concurrently received an inorganic modulator of the NMDA receptor complex, such as Zn(2+) (2.5mg/kg) or Mg(2+) (10mg/kg). On the other hand, the antidepressant-like effect of ifenprodil (20mg/kg) was reversed by N-methyl-d-aspartic acid (an agonist at the glutamate site, 75mg/kg) or d-serine (an agonist at the glycine site, 100nmol/mouse). In conclusion, the antidepressant-like potential of ifenprodil given concomitantly with NMDA ligands was either reinforced (in the case of both partial agonist and antagonists, except for magnesium and zinc) or diminished (in the case of conventional full agonists).

SAT0035 Toll-like receptor-mediated expression of profibrotic mediators by circulating monocytes in systemic sclerosis

BackgroundSystemic sclerosis (SSc) is an autoimmune disease characterised by fibrosis, vascular dysfunction and abnormal activation of immune cells including monocytes. It was suggested that monocytes along with fibroblasts play an...

Emergent biological properties of arrestin pathway-selective biased agonism

Our growing appreciation of the pluridimensionality of G protein-coupled receptor (GPCR) signaling, combined with the phenomenon of orthosteric ligand “bias”, has created the possibility of drugs that selectively modulate different aspects of GPCR function for therapeutic benefit. When viewed from the short-term perspective, e.g. changes in receptor conformation, effector coupling or second messenger generation, biased ligands appear to activate a subset of the response profile produced by a conventional agonist. Yet when examined in vivo, the limited data available suggest that biased ligand effects can diverge from their conventional counterparts in ways that cannot be predicted from their in vitro efficacy profile. What is currently missing, at least with respect to G protein and arrestin pathway-selective ligands, is a rational framework for relating the in vitro efficacy of a “biased” agonist to its in vivo actions that will enable drug screening programs to identify ligands with the desired biological effects.

Bifunctional epitope-agonist ligands of the bradykinin B2 receptor

Two bradykinin (BK) B(2) receptor agonists N-terminally extended with the myc epitope were synthesized and evaluated: myc-KPG-BK and myc-KGP-B-9972. The latter was modeled on the inactivation-resistant agonist B-9972 (D-Arg(0), Hyp(3), Igl(5), Oic(7), Igl(8)-BK) and is also resistant to endosomal inactivation. Despite a large loss of affinity relative to the parent peptide, the tagged analogs are conventional agonists in the umbilical vein contractility assay and compete for [(3)H]BK binding at the rabbit B(2) receptor. Endocytosed myc-KGP-B-9972 most effectively carried AlexaFluor-488-conjugated anti-myc monoclonal antibodies into intact cells expressing the B(2) receptor. Results support the prospects of functionally-active cargoes entering cells in a pharmacologically controlled manner.

β-Arrestin-Selective G Protein-Coupled Receptor Agonists Engender Unique Biological Efficacy in Vivo

Biased G protein-coupled receptor agonists are orthosteric ligands that possess pathway-selective efficacy, activating or inhibiting only a subset of the signaling repertoire of their cognate receptors. In vitro, D-Trp(12),Tyr(34)-bPTH(7-34) [bPTH(7-34)], a biased agonist for the type 1 PTH receptor, antagonizes receptor-G protein coupling but activates arrestin-dependent signaling. In vivo, both bPTH(7-34) and the conventional agonist hPTH(1-34) stimulate anabolic bone formation. To understand how two PTH receptor ligands with markedly different in vitro efficacy could elicit similar in vivo responses, we analyzed transcriptional profiles from calvarial bone of mice treated for 8 wk with vehicle, bPTH(7-34) or hPTH(1-34). Treatment of wild-type mice with bPTH(7-34) primarily affected pathways that promote expansion of the osteoblast pool, notably cell cycle regulation, cell survival, and migration. These responses were absent in β-arrestin2-null mice, identifying them as downstream targets of β-arrestin2-mediated signaling. In contrast, hPTH(1-34) primarily affected pathways classically associated with enhanced bone formation, including collagen synthesis and matrix mineralization. hPTH(1-34) actions were less dependent on β-arrestin2, as might be expected of a ligand capable of G protein activation. In vitro, bPTH(7-34) slowed the rate of preosteoblast proliferation, enhanced osteoblast survival when exposed to an apoptotic stimulus, and stimulated cell migration in wild-type, but not β-arrestin2-null, calvarial osteoblasts. These results suggest that bPTH(7-34) and hPTH(1-34) affect bone mass in vivo through predominantly separate genomic mechanisms created by largely distinct receptor-signaling networks and demonstrate that functional selectivity can be exploited to change the quality of G protein-coupled receptor efficacy.

Clomiphene Citrate plus Modified GnRH Antagonist Protocol for Women with Poor Ovarian Response Undergoing ICSI Treatment Cycles: Randomized Controlled Trial

Objective: To compare Clomiphene Citrate (CC) and lower dose of Human chorionic Gonadotropins (HMG) plus Gonadotropins Releasing Hormone (GnRH) antagonist with conventional mid-luteal long agonist GnRH with higher dose of HMG in women with poor ovarian response, undergoing Intracytoplasmic Sperm Injection (ICSI). Poor ovarian response was defined as the development of less than 3 follicles ≥ 17 mm on HCG day in a previous midluteal GnRH agonist-IVF/ICSI cycle or women with age >35 years old. Method: Pilot non blind two arms parallel randomised controlled. Seventy women with a history of previous poor ovarian response undergoing controlled ovarian hyperstimulation (COH) for ICSI. Interventions: The control group (n=35) received a conventional mid-luteal long GnRH agonist treatment with daily injections of HP HMG, starting on day 2 of the cycle at a dose of 300 IU /day. In the study group (n=35), pre-treatment with luteal E2 supplementation was administrated followed by clomiphene citrate for 5 days, starting on day 2 of the cycle followed by daily injections of 225 IU HP HMG and GnRH antagonist from cycle day 6 onward. Results: There was no evidence of statistically significant difference between both groups regarding the clinical pregnancy rate per women randomized (5/35 (14%) versus 3/35 (9%); 95% CI: 0.39-8.09, p=0.43). The cost of ovarian stimulation per cycle worked out to be USD 208, 8 and 557.3 for CC ovarian stimulation group and conventional ovarian stimulation group respectively. Conclusion: CC and a lower dose of HMG plus GnRH antagonist preceded by mid-luteal E2 supplementation is as effective as the conventional ovarian stimulation in producing similar ICSI outcomes at lower cost in women with poor ovarian response.

IVF comparison of ART outcomes in infertile PCOS women; in vitro maturation (IVM) vs. GnRH agonist vs. GnRH antagonist cycles

In women with PCOS, exaggerated ovarian response to COH result in increased risk of ovarian hyperstimulation syndrome(OHSS) and multiple gestation.This study was to compare ART outcomes among IVM and conventional IVF, GnRH agonist and GnRH antagonist cycles for infertile women with PCOS.

Activation of α7 Nicotinic Receptors by Orthosteric and Allosteric Agonists: Influence on Single-Channel Kinetics and Conductance

Nicotinic acetylcholine receptors (nAChRs) are oligomeric transmembrane proteins in which five subunits coassemble to form a central ion channel pore. Conventional agonists, such as acetylcholine (ACh), bind to an orthosteric site, located at subunit interfaces in the extracellular domain. More recently, it has been demonstrated that nAChRs can also be activated by ligands binding to an allosteric transmembrane site. In the case of α7 nAChRs, ACh causes rapid activation and almost complete desensitization. In contrast, allosteric agonists such as 4-(4-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c] quin oline-8-sulfonamide (4BP-TQS) activate α7 nAChRs more slowly and cause only low levels of apparent desensitization. In the present study, single-channel patch-clamp recording has been used to investigate differences in the mechanism of activation of α7 nAChRs by ACh and 4BP-TQS. The most striking difference between activation by ACh and 4BP-TQS is in single-channel kinetics. In comparison with activation by ACh, single-channel open times and burst lengths are substantially longer (~160-800-fold, respectively), and shut times are shorter (~8-fold) when activated by 4BP-TQS. In addition, coapplication of ACh and 4BP-TQS results in a further increase in single-channel burst lengths. Mean burst lengths seen when the two agonists are coapplied (3099 ± 754 ms) are ~2.5-fold longer than with 4BP-TQS alone and ∼370-fold longer than with ACh alone. Intriguingly, the main single-channel conductance of α7 nAChRs, was significantly larger when activated by 4BP-TQS (100.3 ± 2.4 pS) than when activated by ACh (90.0 ± 2.7 pS), providing evidence that activation by allosteric and orthosteric agonists results in different α7 nAChRs open-channel conformations.

Biased Agonism at the Parathyroid Hormone Receptor: A Demonstration of Functional Selectivity in Bone Metabolism

'Biased agonism' refers to the ability of a ligand to selectively recruit different intracellular signaling proteins to elicit distinct phenotypic effects in cells. While conventional G protein-coupled receptor (GPCR) agonism and antagonism can be regarded as modulating the quantity of efficacy, functionally selective or 'biased' ligands qualitatively change the trafficking of information flowing across the plasma membrane. The concept of ligand directed signaling fundamentally raises the potential of pharmacologic agents with novel therapeutic profiles possessing improved therapeutic efficacy or reduced side effects. Currently, there is little experimental evidence that biased ligands offer advantages over conventional agonists/antagonists in vivo. Recent work examining biased agonism at the type I parathyroid hormone receptor (PTH1R) demonstrates that selective activation of G protein-independent arrestin-mediated signaling pathways elicits a physiologic response in bone distinct from that induced by the conventional PTH1R agonist PTH(1-34). While intermittent (daily) administration of PTH(1-34) (teriparitide) is effective in increasing bone formation, PTH(1-34) administration is also associated with increases in bone resorption and a propensity to promote hypercalcemia/hypercalcuria. In contrast, D-Trp12,Tyr34-bPTH(7-34) (PTH-βarr), an arrestin pathway-selective agonist for the PTH1R, induces anabolic bone formation independent of classic G protein-coupled signaling mechanisms. Unlike PTH(1-34), PTH-βarr appears to 'uncouple' the anabolic effects of PTH1R activation from its catabolic and calcitropic effects. Such findings offer evidence that arrestin pathway-selective GPCR agonists can elicit potentially beneficial effects in vivo that cannot be achieved using conventional agonist or antagonist ligands.