Cloning and Expression of CC-Chemokine-Family Orthologous Genes in Zebrafish

CC chemokines, which are distinguished by two adjacent cysteine residues in a conserved position, can be classified by certain spatial and functional characteristics. Many chemokines and their receptors have been previously demonstrated to be implicated in the emigration of leukocytes from the blood stream to inflammation sites. To date, the chemokine system has only been characterized in mouse and human, albeit fragmentary evidence from some non mammalian species suggests that the chemokine system came into existence early in vertebrate evolution. To gain a further understanding of the potential roles of CC chemokines in the development of a representative teleost, here we cloned the zebrafish orthologues of the mammalian ccl1, ccl21, and ccl25 genes, and described expression patterns of ccl21 and ccl25 in early larval and adult fish by whole mount in situ hybridization. Here, we report that the ccl21 transcripts were observed in various tissues such as pharyngeal pouches, pharynx, blood vessel, intestine, and cranial nerve in 7-dpf-larvae, and in spinal cord, blood vessels, blood cells, kidney, and liver in adult fish. On the other hand, expression of ccl25 was only detectable in the presumptive thymic primordium in the pharyngeal region as early as 36 hpf and persisted in later laval stages. Moreover, we found the recruitment of T-lymphoid progenitor cells was significantly abolished by the morpholino-mediated knockdown of ccl25, which suggests ccl25 is indispensable for colonization of thymus. Interestingly, ccl25 was also maternally transcribed in fish oocytes, and was categorized as cortex class mRNA, which localizes to the cortex of the oocyte during oogenesis. Taken together, our results provide the direct evidence to show the existence of CC chemokine orthologues in zebrafish. These temporally and anatomically restricted expression patterns provide basic foundation for the genetic dissection of their function in this model organism.