The G Protein-Coupled Receptor UT of the Neuropeptide Urotensin II Displays Structural and Functional Chemokine Features

Abstract

The urotensinergic system was previously considered as being linked to numerous physiopathological states, including atherosclerosis, heart failure, hypertension, pre-eclampsia, diabetes, renal disease, as well as brain vascular lesions. Thus, it turns out that the actions of the urotensin II (UII)/G protein-coupled receptor UT system in animal models are currently not predictive enough in regard to their effects in human clinical trials and that UII analogs, established to target UT, were not as beneficial as expected in pathological situations. Thus, many questions remain regarding the overall signaling profiles of UT leading to complex involvement in cardiovascular and inflammatory responses as well as cancer. We address the potential UT chemotactic structural and functional definition under an evolutionary angle, by the existence of a common conserved structural feature among chemokine receptorsopioïdergic receptors and UT, i.e., a specific proline position in the transmembrane domain-2 TM2 (P2.58) likely responsible for a kink helical structure that would play a key role in chemokine functions. Even if the last decade was devoted to the elucidation of the cardiovascular control by the urotensinergic system, we also attempt here to discuss the role of UII on inflammation and migration, likely providing a peptide chemokine status for UII. Indeed, our recent work established that activation of UT by a gradient concentration of UII recruits Gαi/o and Gα13 couplings in a spatiotemporal way, controlling key signaling events leading to chemotaxis. We think that this new vision of the urotensinergic system should help considering UT as a chemotactic therapeutic target in pathological situations involving cell chemoattraction.

Keywords: G protein-coupled receptor; UT; chemokine; migration; proline; urotensin II.

Figure 1

Schematic representation of the structure of human UT. The amino acids represented in yellow represent highly conserved residues within class A/Rhodopsin G protein-coupled receptor of which the UT is belonging. It concerns two N-glycosylation sites in the N-terminal part (Nterm), a NLxxxD motif in TM2, a ERY motif at the cytoplasmic end of the TM3, the CFxP motif in TM6, and NPxxY within the TM7. The key proline in position 2.58 appears in red within the TM2. The two cysteine residues involved in the disulfide bridge between the extracellular end of the TM3 and the e2 loop appear in blue. A nuclear localization motif (NLM) sequence (in pink) was also identified in i3 loop. In addition to these consensus motifs, the C-terminal tail of UT exhibits Serine phosphorylation sites (in green) potentially involved in β-arrestin 1 and 2 anchoring, cysteine, palmitoylation site and plasma membrane anchor (black) sites, as well as two polyproline type I and II motifs (in violet) extracted from analysis by Scansite (http://scansite3.mit.edu#home). Inset, the alignment of the UT C-terminal (C-term) sequence shows that the prolyproline motif allowing interaction with SH3 protein domain, is specifically conserved in hominoids.

Figure 2

Classification of the different G protein-coupled receptor (GPCR) sub-families according to the multidimensional scaling (MDS) analysis and focus on the proline position in TM2 of receptors from the G0 and G1 groups. (A) In the MDS representation of Rhodopsin-like GPCRs, receptors are visualized as points, with the distances between points as close as possible to the distance in the identity matrix [from Ref. (76)]. The points cluster into four groups, highlighted by ellipses. The color code indicates receptor sub-families and is given in the Figure along with the group the sub-family belongs to. Examples of receptors with the position of the TM2 proline are shown for the G0 and G1 groups. The arrow indicates the position of UT [modified from Ref. (76)]. (B) Cartoon view of the PEP receptor OX2 (P2.59, PDB access number: 4S0V, left panel) and of the CHEM receptor CXCR4 (P2.58, PDB access number: 3ODU, right panel). TM2 is slate. The TM2 proline (green) and the preceding oxygen (red) are shown as spheres. In CXCR4, P2.58 is close to the carbonyl groups at positions −3 and −4 (proline kink). In OX2, P2.59 is close to the carbonyl groups at positions −4 and −5 (proline bulge). Thus, according to the position 2.58 or 2.59 of the TM2 proline, the structure of TM2 presents a kink or a bulge.

Figure 3

A hypothetical outline of chemokine signaling cascade relayed by the urotensinergic system inducing cell migration. Illustration of a pathophysiological situation involving directional migration/invasion of cells expressing UT in response to a urotensin II (UII) gradient concentration. It is proposed that mobile high-affinity UT coupled to both Gαi/o and Gα13 is activated by a low concentration of UII, would promote the formation of protrusions and adhesions at the front of the cell through PI3K/PIP3/GEF/Rac/Cdc42 signaling cascade. At the back of the migrating cell, concomitant activation of G13, likely allows actomyosin contraction via the Rho/ROCK/MLCK pathway. To favor cell progression toward the emission source of UII, mobile or engaged UT coupled to Gi/o in lipid rafts may activate proteins responsible for the formation and maturation of focal adhesions composed of αv integrins and vinculin. Together, this pleiotropic UT associated signaling events represents a prototypic chemokine-mediated mechanism shared by P2.58 GPCRs allowing chemotactic migration. Cdc42, cell division control protein 42; GEF, guanine nucleotide exchange factor; MAP1A, microtubule-associated protein 1A; MLCK, myosin light-chain kinase; PI3K, phosphatidylinositol-3 kinase; PIP3, phosphatidylinositol 4,5-trisphosphate; ROCK, rho-associated protein kinase [from Ref. (154)].