Biased and g protein-independent signaling of chemokine receptors

Abstract

Biased signaling or functional selectivity occurs when a 7TM-receptor preferentially activates one of several available pathways. It can be divided into three distinct forms: ligand bias, receptor bias, and tissue or cell bias, where it is mediated by different ligands (on the same receptor), different receptors (with the same ligand), or different tissues or cells (for the same ligand-receptor pair). Most often biased signaling is differentiated into G protein-dependent and β-arrestin-dependent signaling. Yet, it may also cover signaling differences within these groups. Moreover, it may not be absolute, i.e., full versus no activation. Here we discuss biased signaling in the chemokine system, including the structural basis for biased signaling in chemokine receptors, as well as in class A 7TM receptors in general. This includes overall helical movements and the contributions of micro-switches based on recently published 7TM crystals and molecular dynamics studies. All three forms of biased signaling are abundant in the chemokine system. This challenges our understanding of "classic" redundancy inevitably ascribed to this system, where multiple chemokines bind to the same receptor and where a single chemokine may bind to several receptors - in both cases with the same functional outcome. The ubiquitous biased signaling confers a hitherto unknown specificity to the chemokine system with a complex interaction pattern that is better described as promiscuous with context-defined roles and different functional outcomes in a ligand-, receptor-, or cell/tissue-defined manner. As the low number of successful drug development plans implies, there are great difficulties in targeting chemokine receptors; in particular with regard to receptor antagonists as anti-inflammatory drugs. Un-defined and putative non-selective targeting of the complete cellular signaling system could be the underlying cause of lack of success. Therefore, biased ligands could be the solution.

Keywords: 7TM structure–function; 7TM-receptor; G protein coupling; b-arrestin recruitment; biased signaling; chemokine system; ligand/receptor/tissue bias; pathway-specific drug development.

Figure 1

Overview of the promiscuity of the human chemokine system. Chemokine ligands are listed vertically, while chemokine receptors are listed horizontally. A circle indicates interaction between the receptor and ligand. No receptors are reported for CCL18, CXCL4, and CXCL14. Atypical chemokine receptors are boxed in black on the right. The diagram is constructed based on (144) and updated to match the database from IUPHAR (2) and the NCBI gene bank.

Figure 2

Overview of different variations of biased signaling. Biased signaling describes a situation in which a receptor preferentially activates one signaling pathway over another. (Left) Ligand bias is used to differentiate between two ligands acting on the same receptor, where ligand A favorably activates pathway 1, whereas ligand B activates pathway 2. (Center) In receptor bias, the same ligand binds to two different receptors, and activates pathway 1 via receptor A, but pathway 2 through receptor B. (Right) In tissue (or cell) bias, the same ligand: receptor-complex is activated in two different tissues or cell types (or different species), and in tissue A pathway 1 is preferentially activated, whereas pathway 2 is more likely to be activated in tissue B.

Figure 3

Activation of a 7TM receptor. The active β₂-adrenergic receptor with TM-5 and -6 or -7 highlighted in blue according to their importance for receptor interaction with G protein or β-arrestin, respectively. The structures are visualized with Molsoft Browser Pro©.

Figure 4

Biased activity in CCR5. (A,B) Dose–response curves showing the aplaviroc-mediated activation and inhibition of CCR5 WT (purple), [L203F]-CCR5 (dark blue), and [G286F]-CCR5 (light blue) in G protein and β-arrestin-signaling. Aplaviroc was tested with (full lines) or without (dotted lines) the endogenous ligand CCL3. (C,D) Computational models of the WT and mutant receptors depicting the difference in side chain conformations of important amino acids. (E,F) Helical wheel diagrams showing the positions of relevant amino acids.