Understanding GPCRs - from orphan receptors to novel drugs

High-throughput molecular pathology in human tissues as a method for driving drug discovery

To facilitate prioritization of potential drug targets, gene expression can be localized to individual cell types in normal and diseased tissues. Given the complexity of molecular physiology and pathology, the creation of large-scale molecular pathology databases collating data obtained from human tissues is a challenging marriage of old and new technologies, particularly when considering the many issues that preclude easy access to substantial quantities of human tissues. Molecular pathology databases are powerful tools and are essential for early-stage drug discovery, enabling informed decisions to be made with respect to scientific direction and follow-up research.

Synthesis and Biophysical Characterization of a Multidomain Peptide from a Saccharomyces cerevisiae G Protein-coupled Receptor

We attached peptides corresponding to the seventh transmembrane domain (TMD7) of the alpha-mating factor receptor (Ste2p) of Saccharomyces cerevisiae to a hydrophilic, 40-residue fragment of the carboxyl terminus of this G protein-coupled receptor. Peptides corresponding to (a) the 40-residue portion of the carboxyl tail (T-40), (b) the tail plus a part of TMD7 (M7-12-T40), and (c) to the tail plus the full TMD7 (M7-24-T40) were chemically synthesized and purified. The molecular mass and primary sequence of these peptides were confirmed by mass spectrometry and tandem mass spectrometry procedures. Circular dichroism (CD) revealed that T-40 was disordered in phosphate buffer and in the presence of 1,2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-[phospho-racemic-(1-glycerol)] bilayers. In contrast, M7-12-T40 and M7-24-T40 peptides were partially helical in the presence of vesicles, and difference CD spectroscopy showed that the transmembrane regions of these peptides were 42 and 94% helical, respectively. CD analysis also demonstrated that M7-24-T40 retained its secondary structure in the presence of 1-palmitoyl-2-hydroxy-sn-glycero-3-[phospho-racemic-(1-glycerol)] micelles at 0.5 mm concentration. Thus, the tail and the transmembrane domain of the multidomain 64-amino acid residue peptide manifest individual conformational preferences. Measurement of tryptophan fluorescence indicated that the transmembrane domain integrated into bilayers in a manner similar to that expected for this region in the native state of the receptor. This study demonstrated that the tail of Ste2p can be used as a hydrophilic template to study transmembrane domain structure using techniques such as CD and NMR spectroscopy.

Design of selective peptidomimetic agonists for the human orphan receptor BRS-3

New tool substances may help to unravel the physiological role of the human orphan receptor BRS-3 and its possible use as a drug target for the treatment of obesity and cancer. In continuation of our work on BRS-3, the solid- and solution-phase synthesis of a library of low molecular weight peptidomimetic agonists based on the recently developed short peptide agonist 4 is described. Functional potencies of the compounds were determined measuring calcium mobilization in a fluorometric imaging plate reader (FLIPR) assay. Focusing on the N-terminus, the d-Phe-Gln moiety of 4 was modified in a combinatorial SAR-oriented medicinal chemistry approach. With the incorporation of N-arylated glycine and alanine building blocks azaglycine, piperazine, or piperidine and the synthesis of semicarbazides and semicarbazones, a number of highly potent and selective compounds with a reduced number of peptide bonds were obtained, which also should have enhanced metabolic stability.

LSSIG is a novel murine leukocyte-specific GPCR that is induced by the activation of STAT3

G-protein-coupled receptors (GPCRs) transduce the signal of a wide variety of chemokines, cytokines, neurotransmitters, hormones, odorants, and others to regulate the biologic homeostasis, including hematopoiesis and immunity. Here we report the molecular cloning of leukocyte-specific STAT-induced GPCR (LSSIG), which is a novel murine orphan GPCR with the highest homology to human GPR43. The mRNA expression of LSSIG was clearly induced in M1 leukemia cells during the leukemia inhibitory factor (LIF)-induced differentiation to macrophages, and the induction was evidently signal transducers and activators of transcription 3 (STAT3)-dependent. GPR43 expression was also strongly induced in HL-60 and U937 leukemia cells during the differentiation to monocytes. Further analysis showed that the expression of both LSSIG and GPR43 is highly restricted in hematopoietic tissues. Cytokine-stimulation induced LSSIG and GPR43 in bone marrow cells, and monocytes and neutrophils, respectively. These results suggest that LSSIG and GPR43 might play pivotal roles in differentiation and immune response of monocytes and granulocytes.

Residues in the first extracellular loop of a G protein-coupled receptor play a role in signal transduction

The Saccharomyces cerevisiae pheromone, alpha-factor (WHWLQLKPGQPMY), and Ste2p, its G protein-coupled receptor, were used as a model system to study ligand-receptor interaction. Cys-scanning mutagenesis on each residue of EL1, the first extracellular loop of Ste2p, was used to generate a library of 36 mutants with a single Cys residue substitution. Mutation of most residues of EL1 had only negligible effects on ligand affinity and biological activity of the mutant receptors. However, five mutants were identified that were either partially (L102C and T114C) or severely (N105C, S108C, and Y111C) compromised in signaling but retained binding affinities similar to those of wild-type receptor. Three-dimensional modeling, secondary structure predictions, and subsequent circular dichroism studies on a synthetic peptide with amino acid sequence corresponding to EL1 suggested the presence of a helix corresponding to EL1 residues 106 to 114 followed by two short beta-strands (residues 126 to 135). The distinctive periodicity of the five residues with a signal-deficient phenotype combined with biophysical studies suggested a functional involvement in receptor activation of a face on a 3(10) helix in this region of EL1. These studies indicate that EL1 plays an important role in the conformational switch that activates the Ste2p receptor to initiate the mating pheromone signal transduction pathway.