Evidence for biased-ligand signaling induced by endogenous ligands for the urotensin II receptor (UIIR)
Warren, Taylor Kathe
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The urotensin II receptor (UIIR) is a G protein-coupled receptor of which the physiological role is currently unknown. Its two biological ligands, urotensin II (UII) and urotensin II-related peptide (URP), primarily activate the Gq-coupled pathway. It has been shown that these peptides are expressed in the same neurons at the same time, and have equivalent binding affinities (Kd) as well as the same EC 50 in calcium mobilization assays. Their identical cyclic regions have been fully conserved from invertebrates to mammals, but UII has an additional short n-terminal tail containing at least one charged residue. I hypothesize that the peptides produce differential post-activation signaling cascades that are correlative to the presence or absence of a tail region containing charged residues. Concentration-dependent calcium mobilization was monitored following additions of UII, URP, and truncated versions of UII to CHO cells stably expressing rUIIR. Pre-treatment of 10pM UII resulted in reduction in maximal response when a subsequent ceiling concentration of 10nM UII was introduced. Under the same conditions of equimolar URP, no significant decrease was seen. Surprisingly, a truncated form of UII containing the cyclic region and a conserved charged residue demonstrated an indistinguishable desensitization pattern from URP. Cell viability was assayed after 24-hour incubation with increasing concentrations of UII or URP. UII also concentration-dependently and significantly increased cell viability compared to control and URP incubation. Peptide truncations PE-URP and E-URP minimally increased viability comparable to control. Phosphorylated extracellular signal regulated kinase (ERK) levels were compared following a time course incubation with peptide. UII and URP (10pM) additionally exhibited differential dynamics in phosphorylation of ERK. We conclude that UII and URP are physiological examples of biased ligands, but the exact pathways that exhibit this phenomenon are largely unknown. Further exploration of this biased-signaling activation story will contribute to UIIR as another GPCR drug target, with the additional ability to select a particular downstream pathway utilizing biased-ligand signaling by use of small molecules.