Our group has a long-standing interest in targeting protein-protein interactions, which play key roles in countless biological processes. The majority of our work has focused on developing α/β-peptides that antagonize, or inhibit, these interactions, to prevent some activity. Activating, or agonizing, protein-based activity also occurs via protein-protein interactions, and requires both binding and a subsequent conformational change in the target protein for activity. We are exploring the ability of α/β-peptides to act as agonists of protein-protein interactions by targeting various peptide-binding G-protein coupled receptors (GPCRs). GPCRs are cell-surface receptors that relay information from the extracellular space to the cell through their interactions with various extracellular ligands. We have found that both the parathyroid hormone receptor (PTHR) and glucagon-like peptide-1 receptor (GLP-1R) can be effectively activated by α/β-peptide analogues of the endogenous agonists (PTH and GLP-1, respectively), and that these α/β-peptides have potent, long-lasting activity in vivo (1, 2).
Activation of GPCRs by their cognate agonists leads to recruitment of several proteins, including G-proteins and β-arrestins, to the intracellular domain of the receptor. Each of these recruited proteins can activate different signaling pathways within the cell. We use cell-based assays to measure the activation of G-protein signaling and recruitment of β-arrestins-1 and -2. We have discovered that certain αàβ amino acid replacements in the sequence of GLP-1 or PTH lead to peptides that can selectively activate G-protein signaling over β-arrestin signaling. An extension of the ‘biased agonism’ concept is to achieve the selectivity between different signaling states within a single pathway. For PTH1R signaling, we have found that the αàβ amino acid replacement strategy can be used to tune ligand selectivity between two different states (R0 and RG states) of the PTH1R, which correspond, both through G-protein mediated signaling, to prolonged endosomal signaling and transient cell-membrane signaling, respectively. We are currently exploring the mechanistic details of these different modes of selectivity. We are also probing the use of these selective agonists in isolating specific signaling pathways to determine the phenotypic outcomes of these signaling pathways.