The N-terminus of CXCR4 splice variants determines expression and functional properties

Abstract

C-X-C motif chemokine ligand 12(CXCL12) is an essential chemokine for organ development and homeostasis in multiple tissues. Its receptor, C-X-C chemokine receptor type 4(CXCR4), is expressed on the surface of target cells. The chemokine and receptor are expressed almost ubiquitously in human tissues and cells throughout life, and abnormal expression of CXCL12 and CXCR4 is observed in pathological conditions, such as inflammation and cancer. CXCR4 is reportedly translated into five splicing variants of different lengths, which each have different amino acids in the N-terminus. As the N-terminus is the first recognition site for chemokines, CXCR4 variants may respond differently to CXCL12. Despite these differences, the molecular and functional properties of CXCR4 variants have not been thoroughly described or compared. Here, we explored the expression of CXCR4 variants in cell lines and analyzed their roles in cellular responses using biochemical approaches. RT-PCR revealed that most cell lines express more than one CXCR4 variant. When expressed in HEK293 cells, the CXCR4 variants differed in protein expression efficiency and cell surface localization. Although variant 2 demonstrated the strongest expression and cell surface localization, variants 1, 3, and 5 also mediated chemokine signaling and induced cellular responses. Our results demonstrate that the N-terminal sequences of each CXCR4 variant determine the expression of the receptor and affect ligand recognition. Functional analyses revealed that CXCR4 variants may also affect each other or interact during CXCL12-stimulated cellular responses. Altogether, our results suggest that CXCR4 variants may have distinct functional roles that warrant additional investigation and could contribute to future development of novel drug interventions.