Glycosaminoglycan binding to soluble CX3CL1 impacts monocyte migration in vitro

In addition to providing shear-resistant cell-cell adhesion in its natural membrane-bound form, the soluble variant of CX3CL1 (or fractalkine) promotes strong cell migration through signaling via its unique G-protein-coupled receptor CX3CR1 on monocytes, macrophages, T-cells, and NK-cells. To induce cell migration, most chemokines benefit from their interaction with the heterogenous group of glycosaminoglycans (GAGs), which act as coreceptors in chemotactic processes. While the interaction of many chemokines with their GAG counterparts has been investigated in detail, the potential interaction of CX3CL1 and GAGs has not yet received sufficient attention.

Here, we show that the bioactive N-terminal, soluble chemokine domain of CX3CL1 (cdCX3CL1) binds to heparan sulfate (HS) as well as to dermatan sulfate (DS or CS-B), exhibiting Kd-values in the mid nanomolar range. Moreover, the removal of monocyte-surface HS reduced cdCX3CL1-induced cell migration, thereby strongly indicating a potential biological relevance of CX3CL1 binding to GAGs. Interaction studies taking into account the extracellular receptor-peptide of CX3CR1 showed that, in addition to binding CX3CL1, CX3CR1 was found to bind to HS as well. Cross-linking cdCX3CL1 and the CX3CR1 peptide led to a ten-fold higher HS binding affinity compared to the isolated proteins.

These findings further strengthen the assumption of an extended interaction network, in which GPCR, chemokine, and GAGs affect each other simultaneously.