Heparan sulfate differentially controls CXCL12α- and CXCL12γ-mediated cell migration through differential presentation to their receptor CXCR4

Chemokines stimulate signals in cells by binding to G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors. These chemoattractant cytokines also interact with heparan sulfate (HS), which provides positional information within tissues in the form of haptotactic gradients along which cells can migrate directionally. To investigate the mechanism by which HS modulates chemokine functions, we used the CXC chemokine CXCL12, which exists in different isoforms that all signal through CXCR4 but have distinct HS-binding domains. In experiments with both cell-associated and solubilized CXCR4, we found that although CXCL12 gamma bound to CXCR4 with a higher affinity than did CXCL12 alpha, CXCL12 gamma displayed reduced signaling and chemotactic activities. These properties were caused by the specific carboxyl-terminal region of CXCL12 gamma, which, by interacting with CXCR4 sulfotyrosines, mediated high-affinity, but nonproductive, binding to CXCR4. HS prevented CXCL12 gamma from interacting with the CXCR4 sulfotyrosines, thereby functionally presenting the chemokine to its receptor such that its activity was similar to that of CXCL12 alpha. HS had no effects on the binding of CXCL12 alpha to CXCR4 or its biological activity, suggesting that this polysaccharide controls CXCL12 in an isoform-specific manner. These data suggest that the HS-dependent regulation of chemokine functions extends beyond the simple process of immobilization and directly modulates receptor ligation and activation.