https://orcid.org/0000-0001-5692-5975
Key Points
Heparan sulfate-bound CXCL12γ, but not CXCL12α, activates CXCR4 in B cells without receptor internalization or desensitization
After stimulation by heparan sulfate proteoglycan-presented CXCL12γ B cells retain the ability to migrate towards CXCL12α
CXCL12-CXCR4 signaling is involved in a wide variety of homeostatic and pathological processes, but the role of specific CXCL12 isoforms has remained largely unexplored. We have recently shown that the CXCL12γ isoform, which holds an exceptionally high affinity for heparan sulfate (HS), is produced by human bone marrow stromal cells (BMSCs) and remains cell-surface immobilized by HS proteoglycans (HSPGs). This HS-bound CXCL12γ is critical for the adhesion of multiple myeloma cells to BMSCs and for BMSC-mediated drug resistance. Here, we investigated how CXCL12γ activates and regulates CXCR4, by employing a variety of biosensors in HEK293T cells and endogenous CXCR4-expressing B-lymphoma and myeloma cell lines as well as primary B cells. We show that CXCL12γ and CXCL12α bind CXCR4 with a similar affinity and that the cumulative activation of CXCR4 over time is equal for both ligands, although CXCL12α activates CXCR4 more rapidly. Whereas non-bound CXCL12γ and CXCL12α equally induce CXCR4 internalization, cell- or heparin-bound CXCL12γ hardly induces CXCR4 internalization or desensitization. CXCL12γ presented by HSPGs on the membrane of human bone marrow endothelial cells (HBMECs) induces potent cell adhesion to the endothelium under physiological flow, while, upon encountering HBMEC-bound CXCL12γ, cells retain the ability to migrate towards CXCL12α. Taken together, our data demonstrate that CXCL12γ and CXCL12α differentially modulate CXCR4 trafficking and that CXCL12γ, immobilized and presented by HSPGs on the cell surface of HBMECs, can efficiently arrest circulating cells without causing CXCR4 internalization or desensitization, thus allowing subsequent cell migration towards a CXCL12α gradient.
