The overall objective of this application is to better understand the molecular basis of ligand-induced activation states of the secretin receptor, a prototypic class B G protein-coupled receptor. These insights should fill key gaps in current knowledge and facilitate the ultimate development of receptor-active drugs. This receptor family includes established targets for treatment of diabetes, obesity, osteoporosis, migraine, anxiety, and depression;however, development of small molecule agonists acting at these targets has been particularly challenging. Recently solved crystal structures demonstrating uniquely open helical bundle domains of two family members have helped to explain this challenge. This domain has, therefore, become the focal point of the current proposal, with component aims directed at the impact of helical bundle interactions with (i) biologically active ligands, (ii) th receptor amino terminus, and (iii) associated membrane proteins. The CENTRAL HYPOTHESIS is that conformational changes in this domain that initiate a spectrum of intracellular signaling events can be differentially affected by various ligands that interact with it directly, or indirecly by binding to the receptor amino terminus, and by interactions with other membrane proteins within quaternary complexes. New recognition of secretin receptor involvement in a unique cross-class hetero-receptor complex with the angiotensin 1a receptor provides insights relevant to physiology and therapeutics, and there is also strong rationale for use of a secretin receptor agonist to manage related major health problems, including obesity, diabetes, hypertension, and heart failure, as well as cholestasis.
Our SPECIFIC AIMS will test three hypotheses: (i) the open space high in the helical bundle that is devoid of traditional docking pockets where the orthosteric agonist acts can be effectively mapped;(ii) amino-terminal and core domains interact in a specific and dynamic manner that affects the state of activation of the receptor;and (iii) key molecular interactions exist within a cross-class hetero-receptor complex that can be utilized to selectively modulate important biological activities.
The first aim will explore the molecular determinants of agonist activity by utilizing cysteine trapping of residues at the top of transmembrane segments and within extracellular loops using cysteines in positions comprising the helix N-capping motif, and evaluating potential candidate pharmacophores directed to the natural activation pocket by attachment to a ligand that can be applied as a site-selective anchor that binds with high affinity to the receptor amino terminus.
The second aim will gain insights into the receptor holo- structure by utilizing biochemical and mutagenesis strategies to define and disrupt or stabilize the interfaces between the secretin receptor amino terminus and core, and to examine functional implications.
The third aim will explore the physical association between secretin and angiotensin 1a receptors, defining the structural basis and functional implications of their association, the impact of the full spectrum of types of drugs acting at each receptor, and developing strategies to take advantage of this unique hetero-receptor complex.

Public Health Relevance

This project is designed to better understand how drugs can interact with and activate the secretin receptor, a class B G protein-coupled receptor with a variety of gastrointestinal, metabolic, and cardiovascular functions. A key application of a secretin agonist would be management of components of metabolic syndrome, including obesity, diabetes, and cardiovascular disease, as well as the itching associated with cholestasis.

National Institute of Health (NIH)
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
High Priority, Short Term Project Award (R56)
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Clinical, Integrative and Molecular Gastroenterology Study Section (CIMG)
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Serrano, Jose
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Mayo Clinic, Arizona
United States
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Harikumar, Kaleeckal G; Augustine, Mary Lou; Lee, Leo T O et al. (2016) Structure and Function of Cross-class Complexes of G Protein-coupled Secretin and Angiotensin 1a Receptors. J Biol Chem 291:17332-44
Dong, Maoqing; Lam, Polo C-H; Orry, Andrew et al. (2016) Use of Cysteine Trapping to Map Spatial Approximations between Residues Contributing to the Helix N-capping Motif of Secretin and Distinct Residues within Each of the Extracellular Loops of Its Receptor. J Biol Chem 291:5172-84
Lee, Leo T O; Ng, Stephanie Y L; Chu, Jessica Y S et al. (2014) Transmembrane peptides as unique tools to demonstrate the in vivo action of a cross-class GPCR heterocomplex. FASEB J 28:2632-44
Dong, M; Koole, C; Wootten, D et al. (2014) Structural and functional insights into the juxtamembranous amino-terminal tail and extracellular loop regions of class B GPCRs. Br J Pharmacol 171:1085-101
Koole, Cassandra; Savage, Emilia E; Christopoulos, Arthur et al. (2013) Minireview: Signal bias, allosterism, and polymorphic variation at the GLP-1R: implications for drug discovery. Mol Endocrinol 27:1234-44
Miller, Laurence J (2013) Molecular basis of peptide activation of the GLP-1 receptor. Mol Metab 2:60-1
Ke, Jiyuan; Harikumar, Kaleeckal G; Erice, Clara et al. (2013) Structure and function of Norrin in assembly and activation of a Frizzled 4-Lrp5/6 complex. Genes Dev 27:2305-19
Dong, Maoqing; Lam, Polo C-H; Pinon, Delia I et al. (2011) Molecular basis of secretin docking to its intact receptor using multiple photolabile probes distributed throughout the pharmacophore. J Biol Chem 286:23888-99
Miller, Laurence J; Chen, Quan; Lam, Polo C-H et al. (2011) Refinement of glucagon-like peptide 1 docking to its intact receptor using mid-region photolabile probes and molecular modeling. J Biol Chem 286:15895-907
Dong, Maoqing; Lam, Polo C-H; Pinon, Delia I et al. (2010) Secretin occupies a single protomer of the homodimeric secretin receptor complex: insights from photoaffinity labeling studies using dual sites of covalent attachment. J Biol Chem 285:9919-31

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