Nicole Merten

Discovery of Potent and Selective Agonists for the Free Fatty Acid Receptor 1 (FFA1/GPR40), a Potential Target for the Treatment of Type II Diabetes

A series of 4-phenethynyldihydrocinnamic acid agonists of the free fatty acid receptor 1 (FFA(1)) has been discovered and explored. The preferred compound 20 (TUG-424, EC(50) = 32 nM) significantly increased glucose-stimulated insulin secretion at 100 nM and may serve to explore the role of FFA(1) in metabolic diseases such as diabetes or obesity.

Applying label-free dynamic mass redistribution technology to frame signaling of G protein-coupled receptors noninvasively in living cells

Label-free dynamic mass redistribution (DMR) is a cutting-edge assay technology that enables real-time detection of integrated cellular responses in living cells. It relies on detection of refractive index alterations on biosensor-coated microplates that originate from stimulus-induced changes in the total biomass proximal to the sensor surface. Here we describe a detailed protocol to apply DMR technology to frame functional behavior of G protein–coupled receptors that are traditionally examined with end point assays on the basis of detection of individual second messengers, such as cAMP, Ca2+ or inositol phosphates. The method can be readily adapted across diverse cellular backgrounds (adherent or suspension), including primary human cells. Real-time recordings can be performed in 384-well microtiter plates and be completed in 2 h, or they can be extended to several hours depending on the biological question to be addressed. The entire procedure, including cell harvesting and DMR detection, takes 1–2 d.

The experimental power of FR900359 to study Gq-regulated biological processes

Despite the discovery of heterotrimeric αβγ G proteins ∼25 years ago, their selective perturbation by cell-permeable inhibitors remains a fundamental challenge. Here we report that the plant-derived depsipeptide FR900359 (FR) is ideally suited to this task. Using a multifaceted approach we systematically characterize FR as a selective inhibitor of Gq/11/14 over all other mammalian Gα isoforms and elaborate its molecular mechanism of action. We also use FR to investigate whether inhibition of Gq proteins is an effective post-receptor strategy to target oncogenic signalling, using melanoma as a model system. FR suppresses many of the hallmark features that are central to the malignancy of melanoma cells, thereby providing new opportunities for therapeutic intervention. Just as pertussis toxin is used extensively to probe and inhibit the signalling of Gi/o proteins, we anticipate that FR will at least be its equivalent for investigating the biological relevance of Gq.

Decoding Signaling and Function of the Orphan G Protein–Coupled Receptor GPR17 with a Small-Molecule Agonist

Activation of GPR17 prevents oligodendrocyte maturation and reveals that inhibiting GPR17 may be a therapeutic strategy to treat multiple sclerosis. Overcoming a Myelination Maturity Block Demyelinating diseases, such as multiple sclerosis (MS), are characterized by the failure of oligodendrocytes to mature and produce myelin, the protective sheaths surrounding axons. The role of the orphan G protein (heterotrimeric guanine nucleotide–binding protein)–coupled receptor GPR17 in this process is debated. Hennen et al. identified a GPR17-selective small-molecule agonist and showed that application of this agonist induced G protein–mediated signaling that prevented maturation of cultured oligodendrocytes. The findings establish an inhibitory role for GPR17 in the cellular maturation process that enables remyelination of injured axons and suggest that GPR17 may be pharmacologically targeted to treat MS. Replacement of the lost myelin sheath is a therapeutic goal for treating demyelinating diseases of the central nervous system (CNS), such as multiple sclerosis (MS). The G protein (heterotrimeric guanine nucleotide–binding protein)–coupled receptor (GPCR) GPR17, which is phylogenetically closely related to receptors of the “purinergic cluster,” has emerged as a modulator of CNS myelination. However, whether GPR17-mediated signaling positively or negatively regulates this critical process is unresolved. We identified a small-molecule agonist, MDL29,951, that selectively activated GPR17 even in a complex environment of endogenous purinergic receptors in primary oligodendrocytes. MDL29,951-stimulated GPR17 engaged the entire set of intracellular adaptor proteins for GPCRs: G proteins of the Gαi, Gαs, and Gαq subfamily, as well as β-arrestins. This was visualized as alterations in the concentrations of cyclic adenosine monophosphate and inositol phosphate, increased Ca2+ flux, phosphorylation of extracellular signal–regulated kinases 1 and 2 (ERK1/2), as well as multifeatured cell activation recorded with label-free dynamic mass redistribution and impedance biosensors. MDL29,951 inhibited the maturation of primary oligodendrocytes from heterozygous but not GPR17 knockout mice in culture, as well as in cerebellar slices from 4-day-old wild-type mice. Because GPCRs are attractive targets for therapeutic intervention, inhibiting GPR17 emerges as therapeutic strategy to relieve the oligodendrocyte maturation block and promote myelin repair in MS.

Receptor Subtype-specific Docking of Asp6.59 with C-terminal Arginine Residues in Y Receptor Ligands

Y receptors (YRs) are G protein-coupled receptors whose Y1R, Y2R, and Y5R subtypes preferentially bind neuropeptide Y (NPY) and peptide YY, whereas mammalian Y4Rs show a higher affinity for pancreatic polypeptide (PP). Comparison of YR orthologs and paralogs revealed Asp6.59 to be fully conserved throughout all of the YRs reported so far. By replacing this conserved aspartic acid residue with alanine, asparagine, glutamate, and arginine, we now show that this residue plays a crucial role in binding and signal transduction of NPY/PP at all YRs. Sensitivity to distinct replacements is, however, receptor subtype-specific. Next, we performed a complementary mutagenesis approach to identify the contact site of the ligand. Surprisingly, this conserved residue interacts with two different ligand arginine residues by ionic interactions; although in Y2R and Y5R, Arg33 is the binding partner of Asp6.59, in Y1R and Y4R, Arg35 of human PP and NPY interacts with Asp6.59. Furthermore, Arg25 of PP and NPY is involved in ligand binding only at Y2R and Y5R. This suggests significant differences in the docking of YR ligands between Y1/4R and Y2/5R and provides new insights into the molecular binding mode of peptide agonists at GPCRs. Furthermore, the proposed model of a subtype-specific binding mode is in agreement with the evolution of YRs.

The C-terminal Tail of CRTH2 Is a Key Molecular Determinant That Constrains Gαi and Downstream Signaling Cascade Activation

Prostaglandin D2 activation of the seven-transmembrane receptor CRTH2 regulates numerous cell functions that are important in inflammatory diseases, such as asthma. Despite its disease implication, no studies to date aimed at identifying receptor domains governing signaling and surface expression of human CRTH2. We tested the hypothesis that CRTH2 may take advantage of its C-tail to silence its own signaling and that this mechanism may explain the poor functional responses observed with CRTH2 in heterologous expression systems. Although the C terminus is a critical determinant for retention of CRTH2 at the plasma membrane, the presence of this domain confers a signaling-compromised conformation onto the receptor. Indeed, a mutant receptor lacking the major portion of its C-terminal tail displays paradoxically enhanced Gαi and ERK1/2 activation despite enhanced constitutive and agonist-mediated internalization. Enhanced activation of Gαi proteins and downstream signaling cascades is probably due to the inability of the tail-truncated receptor to recruit β-arrestin2 and undergo homologous desensitization. Unexpectedly, CRTH2 is not phosphorylated upon agonist-stimulation, a primary mechanism by which GPCR activity is regulated. Dynamic mass redistribution assays, which allow label-free monitoring of all major G protein pathways in real time, confirm that the C terminus inhibits Gαi signaling of CRTH2 but does not encode G protein specificity determinants. We propose that intrinsic CRTH2 inhibition by its C terminus may represent a rather unappreciated strategy employed by a GPCR to specify the extent of G protein activation and that this mechanism may compensate for the absence of the classical phosphorylation-dependent signal attenuation.

Conjugated linoleic acids mediate insulin release through islet G protein coupled receptor FFA1/GPR40

Among dietary components, conjugated linoleic acids (CLAs) have attracted considerable attention as weight loss supplements in the Western world because they reduce fat stores and increase muscle mass. However, a number of adverse effects are also ascribed to the intake of CLAs such as aggravation of insulin resistance and the risk of developing diabetes. However, the mechanisms accounting for the effects of CLAs on glucose homeostasis are incompletely understood. Herein we provide evidence that CLAs specifically activate the cell surface receptor FFA1, an emerging therapeutic target to treat type 2 diabetes. Using different recombinant cellular systems engineered to stably express FFA1 and a set of diverse functional assays including the novel, label-free non-invasive dynamic mass redistribution technology (Corning® Epic® biosensor), both CLA isomers cis-9, trans-11-CLA and trans-10, cis-12-CLA were found to activate FFA1 in vitro at concentrations sufficient to also account for FFA1 activation in vivo. Each CLA isomer markedly increased glucose-stimulated insulin secretion in insulin-producing INS-1E cells that endogenously express FFA1 and in primary pancreatic β-cells of wild type but not FFA1−/− knock-out mice. Our findings establish a clear mechanistic link between CLAs and insulin production and identify the cell surface receptor FFA1 as a molecular target for CLAs, explaining their acute stimulatory effects on insulin secretion in vivo. CLAs are also revealed as insulinotropic components in widely used nutraceuticals, a finding with significant implication for development of FFA1 modulators to treat type 2 diabetes.

Structure−Activity Study of Dihydrocinnamic Acids and Discovery of the Potent FFA1 (GPR40) Agonist TUG-469

The free fatty acid 1 receptor (FFA1 or GPR40), which is highly expressed on pancreatic β-cells and amplifies glucose-stimulated insulin secretion, has emerged as an attractive target for the treatment of type 2 diabetes. Several FFA1 agonists containing the para-substituted dihydrocinnamic acid moiety are known. We here present a structure-activity relationship study of this compound family suggesting that the central methyleneoxy linker is preferable for the smaller compounds, whereas the central methyleneamine linker gives higher potency to the larger compounds. The study resulted in the discovery of the potent and selective full FFA1 agonist TUG-469 (29).

Lack of evidence for AT1R/B2R heterodimerization in COS-7, HEK293, and NIH3T3 cells: how common is the AT1R/B2R heterodimer?

It has been suggested previously ( AbdAlla, S., Lother, H., and Quitterer, U. (2000) Nature 407, 94-98 ) that the angiotensin II type 1 receptor (AT1R) and the bradykinin B2 receptor (B2R) form constitutive heterodimers. Furthermore they demonstrate that AT1R signaling significantly increases in the presence of the B2R. These findings suggest that heterodimerization and potentiation of AT1R signaling is a universal phenomenon that occurs as a natural consequence of simultaneous expression of the two receptors. Hence this potential interaction is of great pharmacological and biological interest that adds an additional layer of complexity to the understanding of the cross-talk between the renin-angiotensin and kallikrein-kinin systems. Given the remarkable significance of this finding, scientists from four independent research groups have set out to reproduce and further examine the potential AT1R/B2R interaction. We have investigated functional potentiation by the B2R of AT1R signaling in three different cell lines using multiple assays including phosphoinositide hydrolysis, ERK activation, β-arrestin recruitment, and receptor selection and amplification technology, and we have examined dimerization using bioluminescence resonance energy transfer and regulated secretion/aggregation technology. However, although both the AT1Rs and B2Rs were functional in our systems and the systems were fine tuned to detect small changes in receptor function, we failed to detect any functional modulation by or physical interaction between the two receptor proteins. In contrast to the previous observations, our data collectively suggest that AT1R/B2R heterodimerization does not occur as a natural consequence of their simultaneous expression in the same cell nor does the B2R influence the AT1R signaling.

The Third Intracellular Loop Stabilizes the Inactive State of the Neuropeptide Y1 Receptor

Constitutively active G-protein-coupled receptors (GPCRs) can signal even in the absence of ligand binding. Most Class I GPCRs are stabilized in the resting conformation by intramolecular interactions involving transmembrane domain (TM) 3 and TM6, particularly at loci 6.30 and 6.34 of TM6. Signaling by Gi/Go-coupled receptors such as the Neuropeptide Y1 receptor decreases already low basal metabolite levels. Thus, we examined constitutive activity using a biochemical assay mediated by a Gi/Gq chimeric protein and a more direct electrophysiological assay. Wild-type (WT-Y1) receptors express no measurable, agonist-independent activation, while μ-opioid receptors (MOR) and P2Y12 purinoceptors showed clear evidence of constitutive activation, especially in the electrophysiological assay. Neither point mutations at TM6 (T6.30A or N6.34A) nor substitution of the entire TM3 and TM6 regions from the MOR into the Y1 receptor increased basal WT-Y1 activation. By contrast, chimeric substitution of the third intracellular loop (ICL3) generated a constitutively active, Y1-ICL3-MOR chimera. Furthermore, the loss of stabilizing interactions from the native ICL3 enhanced the role of surrounding residues to permit basal receptor activation; because constitutive activity of the Y1-ICL3-MOR chimera was further increased by point mutation at locus 6.34, which did not alter WT-Y1 receptor activity. Our results indicate that the ICL3 stabilizes the Y1 receptor in the inactive state and confers structural properties critical for regulating Y receptor activation and signal transduction. These studies reveal the active participation of the ICL3 in the stabilization and activation of Class I GPCRs.