Biased agonism at chemokine receptors

Abstract

In the human chemokine system, interactions between the approximately 50 known endogenous chemokine ligands and 20 known chemokine receptors (CKRs) regulate a wide range of cellular functions and biological processes including immune cell activation and homeostasis, development, angiogenesis, and neuromodulation. CKRs are a family of G protein-coupled receptors (GPCR), which represent the most common and versatile class of receptors in the human genome and the targets of approximately one third of all Food and Drug Administration-approved drugs. Chemokines and CKRs bind with significant promiscuity, as most CKRs can be activated by multiple chemokines and most chemokines can activate multiple CKRs. While these ligand-receptor interactions were previously regarded as redundant, it is now appreciated that many chemokine:CKR interactions display biased agonism, the phenomenon in which different ligands binding to the same receptor signal through different pathways with different efficacies, leading to distinct biological effects. Notably, these biased responses can be modulated through changes in ligand, receptor, and or the specific cellular context (system). In this review, we explore the biochemical mechanisms, functional consequences, and therapeutic potential of biased agonism in the chemokine system. An enhanced understanding of biased agonism in the chemokine system may prove transformative in the understanding of the mechanisms and consequences of biased signaling across all GPCR subtypes and aid in the development of biased pharmaceuticals with increased therapeutic efficacy and safer side effect profiles.

Keywords: Biased agonism; Chemokine System; G protein-coupled receptors.

FIGURE 1:. The complexity of the human chemokine system.

Chemokine receptors fall into five categories: CCRs, CXCRs, ACKRs, XCRs and CX₃CRs. Chemokine ligands fall into four categories: CCLs, CXCLs, XCLs, CX₃CLs. Lines connecting chemokine receptors to chemokines are colored for clarity.

FIGURE 2:. Biased agonism at G Protein-Coupled Receptors.

Signaling at G Protein-Coupled Receptors can be driven by ligands that are (A) balanced and equally activate both G protein and β-arrestins, (B) those that signal primarily through heterotrimeric G proteins (G protein-biased), or (C) those that signal primarily through β-arrestin adapter proteins (β-arrestin-biased). Biased agonism can be achieved through (D) biased ligands, (E) biased receptors, or (F) biased systems

FIGURE 3:. Phosphorylation barcode promotes differential downstream signaling.

Distinct patterns of G Protein-Coupled Receptor phosphorylation can promote differential downstream signaling cascades. (A) A phosphorylation barcode which preferentially drives β-arrestin activation of with effector “A” while (B) demonstrates a phosphorylation barcode which preferentially drives β-arrestin mediated activation of effector “B”.