β-Arrestin–Dependent Activation of the Cofilin Pathway Is Required for the Scavenging Activity of the Atypical Chemokine Receptor D6

Abstract

Chemokines promote the recruitment of leukocytes to sites of infection and inflammation by activating conventional heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs). Chemokines are also recognized by a set of atypical chemokine receptors (ACRs), which cannot induce directional cell migration but are required for the generation of chemokine gradients in tissues. ACRs are presently considered "silent receptors" because no G protein-dependent signaling activity is observed after their engagement by cognate ligands. We report that engagement of the ACR D6 by its ligands activates a β-arrestin1-dependent, G protein-independent signaling pathway that results in the phosphorylation of the actin-binding protein cofilin through the Rac1-p21-activated kinase 1 (PAK1)-LIM kinase 1 (LIMK1) cascade. This signaling pathway is required for the increased abundance of D6 protein at the cell surface and for its chemokine-scavenging activity. We conclude that D6 is a signaling receptor that exerts its regulatory function on chemokine-mediated responses in inflammation and immunity through a distinct signaling pathway.

The Chemokine Receptor CCR1 Is Constitutively Active, Which Leads to G Protein-independent, β-Arrestin-mediated Internalization

“…Moreover, constitutive CCR1 internalization required ␤-arrestin. This mechanism shows parallels with that of the "professional" scavenging chemokine receptor, D6, which also utilizes a G protein-independent and ␤-arrestin-dependent pathway (95). Moreover, in FIGURE 10.…”

Mutational Analysis of Atypical Chemokine Receptor 3 (ACKR3/CXCR7) Interaction with Its Chemokine Ligands CXCL11 and CXCL12

“…36). It is possible that arrestin influences ACKR2 scavenging indirectly via modulation of receptor trafficking (37)(38)(39)(40). For ACKR3, a report from the zebrafish model that arrestin regulates intracellular receptor transport rather than endocytosis points into the same direction (41).…”

Overview and potential unifying themes of the atypical chemokine receptor family

“…The structural reasons for this differential behavior have not been defined. b-Arrestin coupling correlates with receptor trafficking properties and is required for the scavenger function of ACKR2 [152] and ACKR3 [151,166,167], whereas it appears dispensable for ACKR4 internalization and ligand uptake [126] (see Fig. 3C).…”

“…3A). Direct measurements of ligand-dependent Ga i activation has revealed no activity for ACKR1 [150] and ACKR3 [151], and measurements of intracellular cAMP levels have revealed no activity for ACKR3 [136] and only a minimal activity for ACKR2 [152] [132] and activates Ga i signaling upon CXCL12 but not CXCL11 stimulation in rodent astrocytes and human glioma cell lines [14]. Noteworthy, although ACKR4 is unable to activate Ga i , Ga s , or Ga q subunits, uncoupling of constitutively associated Ga i by pertussis toxin allows the receptor to slightly increase cAMP levels upon ligand engagement [156].…”