The Role of SDF‐1 in Signaling Differences Between CXCR4 and Atypical ACKR3 Chemokine Receptors

Abstract

SDF‐1 is a ligand that plays a critical role in cancer metastases and is secreted in specialized somatic cells in the body. Its chemoattractant properties attract and induce migration of tumor cells located in other tissues in the body to the site in which SDF‐1 is secreted. This attraction and migration of tumor cells allows a new tumor to form at secondary locations within the body. Although the understanding of the function of SDF‐1 being secreted in certain regions of the body is not clear, we seek to understand the binding patterns of this ligand with its chemokine receptors CXCR4 and ACKR3 (also called CXCR7). Chemokine receptors are a subclass of G‐protein coupled receptors, which are multi‐pass transmembrane proteins with 7 TM domains. GPCR activation initiates signaling cascades within the targeted cell. Understanding which signaling pathway is favored in CXCR4 and ACKR3 can help us understand the function of SDF‐1 and the mechanism of secondary tumor formation and development. ACKR3 is considered an atypical receptor as it has been experimentally shown to not couple to Gi/o protein in many cells whereas CXCR4 can couple to either Gi/oprotein or β‐arrestin. ACKR3 strongly binds to SDF‐1 and can control the local levels of SDF‐1 available for binding to CXCR4. Both receptors can couple to β‐arrestins, which are proteins responsible for receptor internalization and potential G‐protein‐independent signaling. To understand the interplay between these two receptors in the context of their interactions with SDF‐1, Gi protein, and β‐arrestin, we have successfully generated structural models of these two receptors in apo form, as heterodimers (bound to only SDF‐1, only Gi protein, or only β‐arrestin), and as heterotrimers (bound to both SDF‐1 and Gi protein, or both SDF‐1 and β‐arrestin). These structural models have been relaxed in their physiological environment using a series of molecular dynamics (MD) simulations. Binding free energy analysis of the heterodimers is beginning to shed light on the signaling differences between the two receptors, what identifies ACKR3 as an atypical receptor, and how it sequesters excess SDF‐1 to modulate CXCR4 signaling.