β-Arrestin-biased signaling of PTH analogs of the type 1 parathyroid hormone receptor

Parathyroid hormone (PTH) is an anabolic agent that mediates bone formation through activation of the Gα(s)-, Gα(q)- and β-arrestin-coupled parathyroid hormone receptor type 1 (PTH1R). Pharmacological evidence based on the effect of PTH(7-34), a PTH derivative that is said to preferentially activate β-arrestin signaling through PTH1R, suggests that PTH1R-activated β-arrestin signaling mediates anabolic effects on bone. Here, we performed a thorough evaluation of PTH(7-34) signaling behaviour using quantitative assays for β-arrestin recruitment, Gα(s)- and Gα(q)-signaling. We found that PTH(7-34) inhibited PTH-induced cAMP accumulation, but was unable to induce β-arrestin recruitment, PTH1R internalization and ERK1/2 phosphorylation in HEK293, CHO and U2OS cells. Thus, the β-arrestin bias of PTH(7-34) is not apparent in every cell type examined, suggesting that correlating in vivo effects of PTH(7-34) to in vitro pharmacology should be done with caution.

Pharmacological characterization of receptor redistribution and beta-arrestin recruitment assays for the cannabinoid receptor 1

Receptor redistribution and beta-arrestin recruitment assays provide a G-protein-subtype-independent method to measure ligand-stimulated activation of G-protein-coupled receptors. In particular beta-arrestin assays are becoming an increasingly popular tool for drug discovery. The authors have compared a high-content-imaging-based Redistribution assay and 2 nonimaging-based beta-arrestin recruitment assays, Tango and PathHunter, for the cannabinoid receptor 1. Inasmuch as all 3 assays use receptors that are modified at the C-terminus, the authors verified their pharmacology via detection of Galpha(i) coupling of the receptor in cAMP assays using reference ligands. The potencies and efficacies of the cannabinoid receptor agonists CP55,940 and WIN55,212-2 correlated well between the 3 assays, and are comparable with the measured ligand binding affinities. The inverse agonist SR141716 decreased basal signal in all 3 assays, but only in the Tango bla assay a reliable EC50 could be determined for this compound, suggesting that Tango is the most suitable assay for the identification of new inverse agonists. Both the Redistribution and the PathHunter assay could discriminate partial agonists from full agonists, whereas in the Tango assay partial agonists behaved as full agonists. Only the PathHunter cells allowed detection of cannabinoid receptor activation via beta-arrestin recruitment and Galpha(i)-protein-mediated inhibition of cAMP, thus enabling the identification of biased ligands that differ in these cellular effects. The characteristics and limitations of the different assays are discussed.

High-Throughput Screening with Immobilized Metal Ion Affinity-Based Fluorescence Polarization Detection, A Homogeneous Assay for Protein Kinases

Protein kinases are one of the most important target classes in high-throughput screening today. The use of generic assay technologies facilitates assay development for new targets and decreases the time needed for implementation of assays in robotic screening. For tyrosine kinases, several generic assay technology platforms are available. These technologies make use of high-affinity antibodies that discriminate between phosphorylated tyrosines and non-phosphorylated tyrosines. Similar generic antibodies specific for phosphoserine or phosphothreonine are lacking. Recently, a non-antibody-based fluorescence polarization assay for protein kinases has become available, called IMAP (Molecular Devices, Sunnyvale, CA). In this assay, a fluorescently labeled peptide substrate that is phosphorylated by kinase is captured on metal-derivatized nanoparticles. We have evaluated IMAP in high-throughput screening, and compared this technology with a competition fluorescence polarization immunoassay based on an antibody specific for a phosphorylated peptide substrate. A random collection of >250000 compounds was screened with the two assays. Fluorescent library compounds were identified by calculation of fluorescence intensity values from the screening data, and by assaying in the absence of fluorescent reagents. Fluorescence polarization artifacts were filtered out further by testing in an ELISA-based kinase assay. Our data show that IMAP is a robust technology for high-throughput screening of kinase targets, and suggest that it is less susceptible to fluorescence polarization artifacts than the competition fluorescence polarization immunoassay.

Amyloid-beta-induced chemokine production in primary human macrophages and astrocytes

In Alzheimer's disease (AD), chemotaxis might be responsible for attracting glial cells towards the neuritic plaque. Using primary monocyte-derived macrophages and primary adult astrocytes as a model, amyloid-beta (Abeta) (1-42) was able to stimulate the production, as measured by RT-PCR, of MIP-1alpha and MIP-1beta mRNA in macrophages and MCP-1 in astrocytes. Cocultures showed in unstimulated as well as in Abeta-stimulated cells an increase in MIP-1alpha, MIP-1beta and MCP-1 mRNA. ELISAs of supernatant samples of stimulated macrophages and astrocytes also showed an increase in MIP-1alpha and MIP-1beta in macrophages and MCP-1 in astrocytes. Stimulated cocultures showed an increase in MIP-1alpha, MIP-1beta and MCP-1 protein levels in contrast to unstimulated cocultures.