A single amino acid substitution in CXCL12 confers functional selectivity at the beta-arrestin level
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Antonella Rigo1,*, Isacco Ferrarini1,*, Giulio Innamorati2 and Fabrizio Vinante1
1Section of Hematology, Cancer Research & Cell Biology Laboratory, Department of Medicine, University of Verona, Verona, Italy
2Laboratory of Translational Surgery, Department of Surgical Sciences, Dentistry, Gynecology and Pediatrics, University of Verona, Verona, Italy
*A. Rigo and I. Ferrarini share first authorship and contributed equally to this article
Isacco Ferrarini, email: firstname.lastname@example.org
Keywords: CXCL12; [N33A]CXCL12; G protein-coupled receptors; biased agonism; β-arrestin
Received: March 01, 2018 Accepted: May 13, 2018 Published: June 22, 2018
CXCL12/CXCR4 axis relies on both heterotrimeric Gi protein and β-arrestin coupling to trigger downstream responses. G protein activation allows for calcium flux, chemotaxis and early extracellular-signal regulated kinases 1/2 (ERK1/2) phosphorylation, whereas β-arrestin recruitment leads to late signaling, receptor desensitization and internalization. Together they may regulate the balance between transactivation and transinhibition of epithelial growth factor receptor 1 (HER1). Since we have previously noted significant differences between CXCL12 and its structural variant [N33A]CXCL12 in CXCR4 signaling, we sought to better characterize them by performing cAMP inhibition and β-arrestin recruitment assays, as well as functional tests that separately investigate G protein and β-arrestin-induced responses. [N33A]CXCL12 showed reduced potency both in Gαi coupling and β-arrestin recruitment as compared to the wild type chemokine, acting as an unbiased ligand. While these findings translated into reduced potency within Gαi-dependent functions, β-arrestin-dependent modules were affected in a more peculiar way. Unlike CXCL12, the mutant analogue did not restore HB-EGF-stimulated HER1 from CXCR4-induced transinhibition, and did not trigger the late wave of ERK1/2 phosphorylation. Instead, CXCR4 internalization was not impaired upon [N33A]CXCL12 stimulation. These differences highlight the novel opportunity to dissect CXCL12 signaling within the β-arrestin layer, in which the mutant chemokine clearly favors the internalization module over the other pathways. Such functional selectivity has an impact on HER1 activation status and may play a relevant part in the crosstalk between tyrosine kinase and seven transmembrane receptors.
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