Extension of Drosophila melanogaster life span with a GPCR peptide inhibitor

Abstract

G protein-coupled receptors (GPCRs) mediate signaling from extracellular ligands to intracellular signal transduction proteins 1 . Methuselah (Mth) is a class B (secretin-like) GPCR, a family typified by their large, ligand-binding, N-terminal extracellular domains2. Down-regulation of mth increases the lifespan of Drosophila melanogaster 3 -inhibitors of Mth signaling would thus be expected to enhance longevity. We used mRNA display selection 4,5 to identify high affinity (K D = 15 to 30 nM) peptide ligands that bind to the N-terminal ectodomain of Mth. The selected peptides are potent antagonists of Mth signaling, and structural studies suggest that they perturb the interface between the Mth ecto-and transmembrane (TM) domains. Flies constitutively expressing a Mth antagonist peptide exhibit a robust lifespan extension, suggesting that the peptides inhibit Mth signaling in vivo. Our work thus provides novel lifespan-extending ligands for a metazoan and a general approach for the design of modulators of this important class of GPCRs.Because of their participation in numerous cell processes, GPCRs are the targets of approximately half of marketed drugs, and new GPCR ligands continue to be pursued and developed 6 . Naïve approaches toward GPCR ligand identification typically involve highthroughput screening of a molecular library (10 2 to 10 5 unique members) in functional, cellbased assays [6][7][8] . A powerful, alternative approach for the rapid isolation of novel ligands is in vitro peptide selection using mRNA display, which allows access to very high library complexities (>10 13 ) in a robust format 4 . High affinity peptide ligands for RNA, small molecule, and protein targets have been identified by mRNA display selection 5 .The crystal structure of the hexahistidine-tagged Mth ectodomain was previously determined, demonstrating that the mature, N-terminal extracellular domain of Mth is a stably folded, glycosylated protein of 195 residues 9 . Since the ectodomains of other class B GPCRs maintain recognition of their cognate ligands independently of their TM cores 10,11 , we targeted the Mth ectodomain for in vitro selection to isolate putative modulators of Mth signaling. We expressed and purified a specifically biotinylated construct of the Mth ectodomain 12 to avoid using the weak hexahistidine epitope as an immobilization tag for selection 13 . Hence, the Mth ectodomain was homogeneously presented, perhaps mimicking the juxtaposition of the ectodomain and TM domain in the context of the full-length receptor.We constructed a random, 27-mer peptide mRNA display library. After eight rounds of selection, with the final four rounds including preclearing steps on matrix without target and specific elution with free, non-biotinylated Mth, we obtained a final 8 th round pool that exhibited high activity for Mth and negligible non-specific binding (Fig. 1). DNA sequencing of individual clones from the final selection round revealed a highly conserved consensus, [R/ P]xxWxxR, which we term the RWR motif ...

In vitro selection of a peptide antagonist of growth hormone secretagogue receptor using cDNA display

“…Because ligand-binding sites of class B GPCRs are in the extracellular domain, purified extracellular domains immobilized on beads can be used as a binding target. Ja et al (20) screened the peptide ligand of Mth, a class B GPCR, via mRNA display with the extracellular domain of Mth. However, because ligand-binding sites of Class A GPCRs include transmembrane regions composed of multiple transmembrane α-helices, the use of complete receptors is required to recreate molecular structures accurately.…”

“…Alternatively, in vitro display techniques, such as mRNA display (16,17) and ribosome display (18,19), provide advantages over phage display in available library size and selection rapidity. In fact, novel peptide (20) or proteins (21)(22)(23)(24)(25)(26) binding to a cell surface receptor were also selected by in vitro display techniques, some of which (20,22,24) are physiologically active. However, bioactive peptides that bind to class A GPCRs, which constitute a major GPCR family (27), have not been selected via in vitro display techniques.…”

The Drosophila G protein‐coupled receptor, Methuselah, exhibits a promiscuous response to peptides

“…9 We subsequently used mRNA display selection to identify novel RWR motif-containing peptides that bind with high affinity to Mth and act as inhibitors of Sun-mediated Mth activation. 7 For two of the peptide antagonists, R8-01 and R8-12, we synthesized randomly scrambled mutants for use as negative controls. As expected, the scrambled R8-12 peptide exhibited no activity on Mth.…”

“…7 Data analysis and background subtraction were performed with Softmax Pro 4.8 (Molecular Devices, Sunnyvale, CA) and sigmoidal fits were calculated using Origin 6.0 Professional (OriginLab Corp., Northampton, MA). Assays of mth-B and mth-like (mthl) receptors were also performed using either transiently transfected (Lipofectamine, Invitrogen, Carlsbad, CA) or isogenic stable expression (Flp-In system, Invitrogen) HEK 293 cell lines generated with cDNA clones from the Drosophila Genomics Resource Center (mth-B, SD05804; mthl1, AT18671; mthl2, RH57551; mthl3, GM02553; and mthl5, RE31350).…”

“…6 We previously isolated antagonists (RWR motif peptides) of Mth that extend lifespan when over-expressed in vivo. 7 The interaction site and dynamics of peptide binding to Mth were modeled, describing how these antagonists interact with the Mth ectodomain. 8 Further studies of activators and inhibitors of Mth signaling might lead to a better understanding of GPCR-agonist interactions.…”