Analysis of Fractalkine Receptor CX₃CR1 Function by Targeted Deletion and Green Fluorescent Protein Reporter Gene Insertion

TLDR

CX3CR1 is a seven‑transmembrane receptor for the chemokine fractalkine, whose membrane‑anchored and soluble forms mediate adhesive and chemoattractive signals and whose expression on activated endothelium and neurons makes it a candidate therapeutic target in inflammatory CNS diseases. The study aimed to elucidate the physiological role of CX3CR1–fractalkine interactions by generating a CX3CR1‑knockout mouse in which the gene was replaced with a GFP reporter. Replacing the CX3CR1 locus with GFP enabled the authors to map CX3CR1 expression to monocytes, subsets of NK and dendritic cells, and brain microglia. CX3CR1‑deficient mice showed that CX3CR1 is the sole murine fractalkine receptor, yet its absence does not impair monocyte extravasation, dendritic‑cell migration or differentiation, nor disrupt microglial responses to peripheral nerve injury.

Abstract

The seven-transmembrane receptor CX3CR1 is a specific receptor for the novel CX3C chemokine fractalkine (FKN) (neurotactin). In vitro data suggest that membrane anchoring of FKN, and the existence of a shed, soluble FKN isoform allow for both adhesive and chemoattractive properties. Expression on activated endothelium and neurons defines FKN as a potential target for therapeutic intervention in inflammatory conditions, particularly central nervous system diseases. To investigate the physiological function of CX3CR1-FKN interactions, we generated a mouse strain in which the CX3CR1 gene was replaced by a green fluorescent protein (GFP) reporter gene. In addition to the creation of a mutant CX3CR1 locus, this approach enabled us to assign murine CX3CR1 expression to monocytes, subsets of NK and dendritic cells, and the brain microglia. Analysis of CX3CR1-deficient mice indicates that CX3CR1 is the only murine FKN receptor. Yet, defying anticipated FKN functions, absence of CX3CR1 interferes neither with monocyte extravasation in a peritonitis model nor with DC migration and differentiation in response to microbial antigens or contact sensitizers. Furthermore, a prominent response of CX3CR1-deficient microglia to peripheral nerve injury indicates unimpaired neuronal-glial cross talk in the absence of CX3CR1.