In vivo structure/function and expression analysis of the CX3C chemokine fractalkine

The CX3C chemokine family is composed of only one member, CX(3)CL1, also known as fractalkine, which in mice is the sole ligand of the G protein-coupled, 7-transmembrane receptor CX(3)CR1. Unlike classic small peptide chemokines, CX(3)CL1 is synthesized as a membrane-anchored protein that can promote integrin-independent adhesion. Subsequent cleavage by metalloproteases, either constitutive or induced, can generate shed CX(3)CL1 entities that potentially have chemoattractive activity. To study the CX3C interface in tissues of live animals, we generated transgenic mice (CX(3)CL1(cherry): CX(3)CR1(gfp)), which express red and green fluorescent reporter genes under the respective control of the CX(3)CL1 and CX(3)CR1 promoters. Furthermore, we performed a structure/function analysis to differentiate the in vivo functions of membrane-tethered versus shed CX(3)CL1 moieties by comparing their respective ability to correct established defects in macrophage function and leukocyte survival in CX(3)CL1-deficient mice. Specifically, expression of CX(3)CL1(105 Delta), an obligatory soluble CX(3)CL1 isoform, reconstituted the formation of transepithelial dendrites by intestinal macrophages but did not rescue circulating Ly6C(lo) CX(3)CR1(hi) blood monocytes in CX(3)CR1(gfp/gfp) mice. Instead, monocyte survival required the full-length membrane-anchored CX(3)CL1, suggesting differential activities of tethered and shed CX(3)CL1 entities. (Blood. 2011; 118(22): e156-e167)