The evolution and functional characterization of CXC chemokines and receptors in lamprey

Chemokines are a large family of soluble peptides guiding cell migration in development and immune defense. They interact with chemokine receptors and are essential for the coordination of cell migration in diverse physiological processes. The CXC subfamily is one of the largest groups in the chemokine family and consists of multiple members. In this study, we identified homologues of three chemokine ligands (CXCL8, CXCL_F5 and CXCL12) and two CXC receptor like molecules (CXCR_L1 and CXCR_L2) in lamprey. Sequence analysis revealed that they share the same genomic organization with their counterparts in jawed vertebrates but synteny was not conserved. Lamprey CXCL8 and CXCL12 have four conserved cysteine residues whilst the CXCL_F5 has two additional cysteine residues. In addition, CXCL_F5 is evolutionarily related to the fish specific CXC chemokine groups previously identified and contains multiple cationic aa residues in the extended C- terminal region. The two CXCRs possess seven transmembrane domains and conserved structural elements for receptor activation and signaling, including the DRYXXI(V)Y motif in TM2, the disulphide bond connecting ECL2 and TM3, the WXP motif in TM6 and NPXXY motif in TM7. The identified CXC chemokines and receptors were constitutively expressed in tissues including the liver, kidney, intestine, heart, gills, supraneural body and primary leukocytes, but exhibited distinct expression patterns. Relatively high expression was detected in the gills for CXCL8, CXCL_F5 and CXCR_L1 and in the supraneural body for CXCL12 and CXCR_L2. All the genes except CXCL12 were upregulated by stimulation with LPS, pokeweed and bacterial infection, and the CXCL8 and CXCL_F5 was induced by poly (I:C). Functional analysis showed that the CXCL8 and CXCL_F5 specifically interacted with CXCR_L1 and CXCR_L2, respectively. Our results demonstrate that the CXC chemokine system had diversified in jawless fish.