Second Extracellular Loop of Human Glucagon-like Peptide-1 Receptor (GLP-1R) Differentially Regulates Orthosteric but Not Allosteric Agonist Binding and Function

Cassandra Koole,Denise Wootten,John Simms,Emilia E Savage,Laurence J Miller,Arthur Christopoulos,Patrick M Sexton

doi:10.1074/jbc.m111.309369

Abstract

The glucagon-like peptide-1 receptor (GLP-1R) is a prototypical family B G protein-coupled receptor that exhibits physiologically important pleiotropic coupling and ligand-dependent signal bias. In our accompanying article (Koole, C., Wootten, D., Simms, J., Miller, L. J., Christopoulos, A., and Sexton, P. M. (2012) J. Biol. Chem. 287, 3642-3658), we demonstrate, through alanine-scanning mutagenesis, a key role for extracellular loop (ECL) 2 of the receptor in propagating activation transition mediated by GLP-1 peptides that occurs in a peptide- and pathway-dependent manner for cAMP formation, intracellular (Ca(2+)(i)) mobilization, and phosphorylation of extracellular signal-regulated kinases 1 and 2 (pERK1/2). In this study, we examine the effect of ECL2 mutations on the binding and signaling of the peptide mimetics, exendin-4 and oxyntomodulin, as well as small molecule allosteric agonist 6,7-dichloro-2-methylsulfonyl-3-tert-butylaminoquinoxaline (compound 2). Lys-288, Cys-296, Trp-297, and Asn-300 were globally important for peptide signaling and also had critical roles in governing signal bias of the receptor. Peptide-specific effects on relative efficacy and signal bias were most commonly observed for residues 301-305, although R299A mutation also caused significantly different effects for individual peptides. Met-303 was more important for exendin-4 and oxyntomodulin action than those of GLP-1 peptides. Globally, ECL2 mutation was more detrimental to exendin-4-mediated Ca(2+)i release than GLP-1(7-36)-NH(2), providing additional evidence for subtle differences in receptor activation by these two peptides. Unlike peptide activation of the GLP-1R, ECL2 mutations had only limited impact on compound 2 mediated cAMP and pERK responses, consistent with this ligand having a distinct mechanism for receptor activation. These data suggest a critical role of ECL2 of the GLP-1R in the activation transition of the receptor by peptide agonists.

Highlights

The ECL2 of the glucagon-like peptide-1 receptor (GLP-1R) is critical for GLP-1 peptide-mediated selective signaling
We examine the effect of ECL2 mutations on the binding and signaling of the peptide mimetics, exendin-4 and oxyntomodulin, as well as small molecule allosteric agonist 6,7-dichloro-2-methylsulfonyl-3-tert-butylaminoquinoxaline
Cell Surface Expression of Human GLP-1R ECL2 Alanine Mutants—As reported in our accompanying article (18), cell surface expression determined through detection of the N-terminal double c-Myc epitope label was reduced for mutant receptors D293A, C296A, W297A, S301A, N304A, Y305A, and L307A, and it increased for N300A and M303A, no significant changes were observed for the remaining mutants (Table 1)

Summary

Introduction

The ECL2 of the GLP-1R is critical for GLP-1 peptide-mediated selective signaling. Results: Mutation of most ECL2 residues to alanine results in changes in binding and/or efficacy of oxyntomodulin and exendin-4 but not allosteric agonists. 287, 3642–3658), we demonstrate, through alanine-scanning mutagenesis, a key role for extracellular loop (ECL) 2 of the receptor in propagating activation transition mediated by GLP-1 peptides that occurs in a peptide- and pathway-dependent manner for cAMP formation, intracellular (Ca2؉i) mobilization, and phosphorylation of extracellular signal-regulated kinases 1 and 2 (pERK1/2). Unlike peptide activation of the GLP-1R, ECL2 mutations had only limited impact on compound 2 mediated cAMP and pERK responses, consistent with this ligand having a distinct mechanism for receptor activation. These data suggest a critical role of ECL2 of the GLP-1R in the activation transition of the receptor by peptide agonists

Methods

Results

Conclusion